Leaf Age Matters in Remote Sensing: Taking Ground Truth for Spectroscopic Studies in Hemiboreal Deciduous Trees with Continuous Leaf Formation

Abstract

We examined the seasonal changes in biophysical, anatomical, and optical traits of young leaves, formed throughout the vegetative season due to sylleptic growth, and mature leaves formed by proleptic growth in spring. Leaf developmental categories contribute to the top-of-canopy reflectance and should be considered when taking ground truth for remote sensing studies (RS). Deciduous tree species, Betula pendula, Populus tremula, and Alnus incana, were sampled from May to October 2018 in an Estonian hemiboreal forest. Chlorophyll and carotenoid content were detected biochemically; leaf anatomical traits (leaf, palisade, and spongy mesophyll thickness) were measured on leaf cross-sections; leaf reflectance was measured by a spectroradiometer with an integrating sphere (350–2500 nm). Biophysical and anatomical leaf traits were related to 64 vegetation indices (VIs). Linear models based on VIs for all tested leaf traits were more robust if both juvenile and mature leaves were included. This study provides information on which VIs are interchangeable or independent. Pigment and leaf thickness sensitive indices formed PC1; water and structural trait related VIs formed an independent group associated with PC3. Type of growth and leaf age could affect the validation of biophysical and anatomical leaf trait retrieval from the optical signal. It is, therefore, necessary to sample both leaf developmental categories—young and mature—in RS, especially if sampling is only once within the vegetation season.

1. Introduction

Under ongoing climate change, plants are exposed to an ever-increasing load of environmental stress, which may affect the vegetative net primary production and carbon sequestration [1,2,3]. Forests are the most important of all terrestrial ecosystems, representing the greatest terrestrial carbon storage pool [4]. However, the ability of forests to sequester carbon is dependent on the physiological status of trees. Remote sensing provides a suitable tool for large-scale i.e., spatial temporal monitoring of tree and forest status, as well as carbon fluxes and pools at large scales [5].

Hemiboreal forests represent a transitional zone between boreal and temperate forest biomes [6]. The territory of hemiboreal forests is expected to expand north, gradually replacing boreal forests [7] due to climate change, particularly warming, which is more progressive in the northern boreal region. Moreover, hemiboreal forest stands are characterized by greater seasonal variability in forest microclimate, canopy structure, and plant activity when compared to boreal forests [7]. In all ecosystems where seasonality in phenology occurs, including hemiboreal forests, remote sensing must account for dynamic changes in optical signals during the vegetative season. Also, knowledge of seasonal variability in leaf-level traits which determine the optical signal of vegetation (such as photosynthetic pigment content, dry matter and water content, and leaf internal structure) is essential for scaling the optical signal to the canopy level and interpreting airborne and satellite-based data.

Crown architecture can be regarded as the integrated link between form and function in a tree [8]. There are two major patterns of tree crown branching: (a) prolepsis, a rhythmic branching process from buds formed before a period of dormancy, and (b) syllepsis, a continuous branching process during the vegetative season from incompletely formed lateral buds [9]. Prolepsis and syllepsis can both occur in the same tree [8]. Thus, in many woody species, two types of leaf development occur: (1) pre-formed leaves (also called early leaves) that originate from overwintering buds after a dormancy stage, and (2) neoformed leaves (late leaves) from buds without passing through the dormant period and instead developing entirely during the current growing season [10,11]. Sylleptically formed young leaves play a critical role in maximizing the carbon productivity of a tree’s crown [12], and are often found in fast-growing tree species. Usually, the pre-formed leaves grow on both short and long shoots [10] while the neoformed leaves develop on long shoots, as confirmed recently in silver birch (B. pendula) [13]. Neoformed leaves on sylleptic branches could form a substantial part of the upper- and external, sun-exposed crown layer [14] and, thus, they are thought to have a significant contribution to the top-of-canopy reflectance signal. However, based on our knowledge to date, not much attention has been paid to variation in leaf optical signal as a result of developmental origin in deciduous temperate and hemiboreal trees.

Regarding anatomical traits, leaves of different developmental origins—either proleptic or sylleptic—usually differ quantitatively [15,16]. Otherwise, the anatomy of both leaf developmental categories corresponds to a typical dorsiventral structure [17]. In dorsiventral leaves, dermal tissue is comprised of an upper (adaxial) and lower (abaxial) epidermis, ground tissue—photosynthetic mesophyll differentiated into palisade (toward the adaxial epidermis) and spongy (toward the abaxial epidermis) parenchyma (Figure 1) and vascular tissue formed by an isotropic net of vascular bundles (not shown in Figure 1). Elongated cylindrical palisade parenchyma cells are well adapted for capturing light and the palisade parenchyma thickness is strongly correlated with leaf thickness and photosynthetic capacity [18]. Spongy parenchyma cells are irregularly shaped and the thickness of the spongy parenchyma layer, alongside the proportion of intercellular spaces compared to the palisade parenchyma, affects the conductance of CO₂ inside the leaves [19].

Within-canopy variation of leaf structural and biophysical traits should be considered when designing spectroscopic studies at the leaf level [20] or taking ground truth for remote sensing data calibration. From this point of view, sun- and shade-acclimated leaves are usually taken into account and sampling designs are based on the vertical gradient within the canopy, reflecting differences in leaf anatomical, biophysical, and optical traits [21]. Many recent studies have shown that the within-canopy variation of leaf traits and optical properties is remarkable and should be taken into account for upscaling [13,22]. However, leaf traits related to optical properties are rarely studied regarding proleptic and sylleptic growth; only poplar species have been significantly studied from this perspective [10,12,15,23].

Tree phenology and leaf ontogenesis in deciduous trees are closely related to meteorological conditions during a particular vegetative season [24,25]. After bud burst originating from proleptically formed buds, leaf anatomical and biophysical traits gradually change to promote photosynthetic activity. Leaf thickness increases remarkably at the beginning of the season [26], remains constant during the mid-season [24,26] and then decreases during leaf senescence [21]. Photosynthetic pigments follow a similar trend as leaf structural traits during the growing season: their content increases in juvenile leaves, reaches a maximum in mature leaves in the middle of leaf lifespan [27], and then decreases during senescence [28,29,30].

Just as leaf traits change within a vegetative season, optical properties are dynamic over time in temperate deciduous trees [24,31]. Dillen et al. [32] observed that leaf foliar reflectance provided a good indicator of leaf photosynthesis and nitrogen content from early bud break to leaf fall in Quercus rubra L. and Betula papyrifera Marsh. A similar pattern of temporal dynamics in optical properties was observed in P. tremuloides and Populus balsamifera [21]. Changes in the leaf optical properties of European boreal deciduous tree species are especially significant at the beginning and end of the season [33,34]. Since leaf structure determines leaf optical properties, seasonal changes in leaf optical properties usually follow trends in leaf structural and biochemical traits: not only pigment content changes, but also in other traits such as leaf mass per area, nitrogen, and carbon content [31]. Furthermore, dynamics in optical properties at the leaf level affect the spectral signal at the canopy level [24,35,36]. Thus, interpreting remotely acquired optical information at larger spatial and temporal scales should account for factors such as leaf structural and biochemical traits influencing leaf-level optical properties [37].

The influence of leaf anatomy on leaf-level optical properties has been studied since the 1970s [38,39]. Nowadays, the opinion that leaf anatomical traits affect mainly optical properties in the near infra-red region (NIR) is widely accepted. Slaton et al. [40] observed that reflectance in NIR was positively correlated with leaf thickness, similar to other studies [41,42], which confirmed a positive relation between NIR reflectance and intercellular space-volume in spongy parenchyma. However, a recent study [43] showed that light scattering within the leaf is a result of tissue layer and cell arrangement, and thus could affect optical properties, not only in NIR but also in the visible part of the spectrum (VIS). Usually, investigations focused on the relation of leaf optical properties to anatomical traits aim to improve the detection of chlorophyll content as a non-specific physiological status marker [44]. Variation in leaf anatomy may complicate chlorophyll content estimates, even when using well-established spectroscopic methods for processing leaf optical data, such as vegetation indices (VIs) [30,45]. Our previous study, Lukeš et al. [24] pointed out that the asymmetric internal structure of dorsiventral leaves had a significant impact on top-of-canopy reflectance and, thus, should be taken into account when upscaling from leaf to canopy optical traits.

To date, various methods of biophysical traits retrieval from optical signal are available (reviewed by [46]). The parametric regression models are those with the longest tradition and they generally work with a limited number of spectral bands relating to a target biophysical variable. Beside single band reflectance at given wavelengths, many different spectral transformations, such as derivatives and vegetation indices, can be used [47,48,49]. The term “vegetation index” originates from the beginning of spaceborne vegetation remote sensing in the 1970s. In current usage, this term refers to any transformation of two or more discrete spectral bands, either at leaf or canopy level, for estimating vegetation properties [50]. By now, a vast number of different vegetation indices have been developed (review by [51]). The most widely used vegetation index is a Normalized Difference Vegetation Index (NDVI) [52,53] using reflectance at two bands: one band from the red spectral region and the other one in the NIR reflectance plateau. Another widely used group of vegetation indices describing the physiological status of vegetation uses the red edge region (700–730 nm) (e.g., [30,54]).

The main goals of the present study were to provide insight on how leaf developmental stage affects (1) biophysical, anatomical, and optical properties at the leaf level during the growing season; and (2) linear relationships of commonly used vegetation indices (VIs) in remote sensing to leaf biophysical and anatomical traits. We examined the seasonal changes in biophysical, anatomical, and optical traits of three hemiboreal deciduous tree species: silver birch (Betula pendula), black alder (Alnus incana), and Eurasian aspen (Populus tremula). All studied species exhibit both proleptic and sylleptic growth, which implies gradual leaf development and emergence of new neoformed leaves throughout the growing season. We hypothesized that there are systematic differences in biophysical, anatomical, and optical properties of young, sylleptically neoformed leaves compared to proleptically pre-formed leaves, which have regular phenological development following a vegetative season changes from spring to autumn.

First, we aimed to describe the seasonal course and variability in leaf biophysical, anatomical, and optical properties related to simultaneous occurrence of leaves in different developmental stage (mature pre-formed leaves and juvenile neoformed leaves) within a single crown. Second, we aimed to relate a broad range of commonly used VIs in remote sensing to leaf biophysical and anatomical traits and evaluate the effect of leaf developmental stage on these relations. It is necessary to emphasize that young neoformed leaves usually form the uppermost and external crown leaf layer, and thus, may have a significant contribution to the top-of-canopy reflectance measurements. We hypothesized that tighter relations of VIs to biophysical and anatomical traits could be achieved with inclusion of both pre-formed adult and neoformed juvenile leaves to the model. Our final aim was to evaluate the performance of various vegetation indices in biophysical and anatomical trait prediction. We hypothesized that vegetation indices strongly intercorrelate and form distinctive groups according to the predicted leaf traits.

2. Materials and Methods

2.1. Plant Sampling

Sampled deciduous trees, silver birch (Betula pendula), black alder (Alnus incana), and common aspen (Populus tremula) are the most abundant deciduous tree species in Estonian hemiboreal forests [55]. During the 2018 growing season, seven field trips for collecting leaf samples were conducted: 21 May, 6 June, 18 June, 2 July, 10 July, 10 September, and 1 October, corresponding to Day of Year (DOY) 141, 157, 169, 183, 191, 253, and 274, respectively. Sampling started in spring after bud burst, which occurred in the middle of April, 2018, when leaves had reached the minimum required size for optical measurements to cover the 1 cm diameter sample port of the integrating sphere. Sample collection campaigns were conducted five times over a two-month period—following regular leaf development of proleptically formed leaves from spring to summer when proleptically formed leaves reached full maturity. Each field collection lasted two days. For wider geographic coverage, the first day was spent sampling birch in the Tõravere park, adjacent to the Tartu Observatory (58°16′N 26°28′E, elevation 70 m). The next day, sampling of birch, alder and poplar was accomplished in the Järvselja Experimental Forest district (58°22′N, 27°20′E, elevation 38–40 m) (Figure 2). Only healthy leaves without apparent visual color changes or damage were sampled. Leaves were placed in plastic bags and stored at +4 °C. Leaf optical properties were measured within a few hours. All leaves from the first sampling date (DOY 141) were classified as juvenile leaves. Due to gradual leaf development caused by sylleptic growth, juvenile neoformed leaves were sampled from all studied tree species throughout the whole season. Mature (proleptically pre-formed) leaves represented typical foliage for a given canopy at a given time and were collected from short shoots and from the bases of the long shoots with high to moderate light availability due to their canopy positions. Juvenile (neoformed) leaves were located at the top of a crown at branch tips having high to excessive light availability. Sampling of juvenile leaves at later sampling dates was based primarily on their terminal position on the long shoots at the top of the crown. Taking into consideration that major anatomical adjustments occur during leaf formation while leaf senescence is dominated by pigment degradation, two additional smaller field campaigns were also conducted in the fall to describe pigment composition at the end of the growing season (DOYs 253 and 274). First senescent leaves were classified on DOY 253, based on visual color changes with prevailing yellow area on a leaf blade. There were still juvenile-looking leaves present on branch tips on DOY 253. The last sampling was conducted on DOY 274 when most remaining leaves were fully senescent, and a significant amount of foliage had already fallen. Leaves located on branch tips, where juvenile leaves were collected throughout the whole growing season, had reached the appearance of mature leaves and were classified as mature leaves on DOY 274 collection. Leaves located in the crown, where the mature leaf category was in previous collections, were classified as senescent leaves on DOY 274.

2.2. Measurement of Leaf Optical Properties

Leaf adaxial side hemispherical reflectance was measured in laboratory conditions. Leaf reflectance was measured with a spectroradiometer SVC HR-1024 (Spectra Vista Corporation, Poughkeepsie, NY, USA) equipped with an integrating sphere AvaSphere-50-REFL, and light source AvaLight-HAL (both Avantes, Apeldoorn, The Netherlands). The integrating sphere was coupled to the spectroradiometer and the light source with optical fibers. The SVC HR-1024 spectroradiometer has 1024 spectral bands covering the spectral range from 350 nm to 2500 nm. The spectral resolution is about 3.5 nm, 8.5 nm, and 2.5 nm in the spectral range 350–1000 nm, 1000–1850 nm, and 1850–2500 nm, respectively. The measurement time was set to 3 s, using automatic integration time and automatic subtraction of the dark signal. Reflectance spectra were measured with the integrating sphere using the substitution method. We measured the hemispherically integrated reflectance as the directional-hemispherical reflectance factor (DHRF) of the adaxial side of the leaf. The raw signal of the sample was compared to the raw signal from the working reference WS-2 (Avantes), which was calibrated against a reflectance standard SRT-99-050 (Labsphere Inc., Reflectance Calibration Laboratory, North Sutton, NH, USA). In addition to the sample and reference measurements, a signal from the light trap was recorded to account for the stray light inside the sphere caused by a fraction of the illuminating flux hitting the edge of the sample port instead of the sample. Raw data were processed with a custom script in statistical program R (3.5.0) [56] according to the theory of the integrating sphere [57]. During processing, the raw spectra were smoothed over 7 spectral bands of the spectroradiometer with a Hamming window (α0 = 0.54) [58]. Leaf spectra were quite noisy below 400 nm and above 2000 nm, therefore only the spectral range from 400 nm to 2000 nm was used for final analyses and visualization.

2.3. Biophysical and Anatomical Analysis

After measuring the leaf optical properties on the leaf blade, excluding the midrib if possible, leaves from each field collection (seven total) were sampled for pigment content from the same leaf blade: one disk from each leaf, 0.8 cm in diameter, was sampled and plunged into a vial with DMF (dimethylformamide). After 7 days of DMF extraction, leaf pigment content—chlorophyll a + b (Chl) and carotenoids (Car)—were determined using spectroradiometric methods. The DMF extracts were measured in 3.5 mL cuvettes (CV10Q3500, Thorlabs Inc., NJ, USA) in a cuvette holder (CVH100, Thorlabs), illuminated with an incandescent light source (Avantes AvaLight-HAL, Apeldoorn, The Netherlands) and the transmitted radiation was measured with a spectroradiometer (SVC HR-1024, Spectra Vista Corporation, Poughkeepsie, NY, USA). Pure DMF was used as a reference. The reference measurement with pure DMF was made only once. However, a second cuvette with pure DMF was occasionally measured to correct for any potential drifts due to instability of the lamp or the spectroradiometer. Chl and Car contents were calculated based on absorbance at specific wavelengths according to Wellburn (1994) [59].

For anatomical analysis, approximately 0.5 cm² segments of selected leaves (five times in the 2018 season DOYs 141, 157, 169, 183, and 191) were fixed in FAA (70% ethanol, 40% formalin, and acetic acid in a volume ratio 90:5:5). Leaf cross-sections (about 50–80 µm thick) were prepared using a hand microtome and stained with toluidine blue. Light microscope images (five per leaf segment) were acquired using an Olympus BX40 microscope equipped with camera (EOS100D, Canon Inc., Tokyo, Japan) and processed in ImageJ image analysis software (https://imagej.net/, accessed on 31 March 2021). Leaf anatomical traits (leaf thickness, mesophyll thickness, palisade and spongy parenchyma thickness (PP, SP), adaxial and abaxial epidermis thickness) were measured three times on each image in regular intervals and the palisade to spongy parenchyma thickness ratio (PP/SP) was calculated.

Finally, fresh leaves were weighed, scanned for leaf area, and kept in an oven at 60 °C for three days; after drying, samples were weighed again. Leaf area was obtained from scanned images using ImageJ. Leaf area, and leaf fresh and dry weights were used to calculate the following leaf traits: leaf mass per area (LMA), fresh leaf mass per area (fLMA), water content per leaf area (WCLA) and dry weight to fresh weight ratio (DW/FW). Total nitrogen (N) and carbon (C) contents of the oven-dried samples were determined by the dry combustion method on a vario MAX CNS elemental analyzer (ELEMENTAR, Langenselbold, Germany).

We classify leaf traits into two groups (a) biophysical traits: Chl, Car, N, and C contents and there we also included traits involving structural parameters: LMA, fLMA, WCLA, and DW/FW; and (b) anatomical traits: traits based on solely quantitative anatomical parameters—leaf thickness, palisade, and spongy parenchyma thickness (PP and SP, respectively), palisade and spongy parenchyma ratio (PP/SP).

2.4. Statistical Processing of Optical, Biophysical, and Anatomical Leaf Traits

One-way analysis of variance (ANOVA) was performed to identify the differences in leaf anatomical, biophysical, and optical properties (represented as reflectance at particular wavelengths—R551, R673, R705, R800, R1242, R1511) among sampling dates (DOYs 141–274) and leaf development stages. To identify significant differences among groups after ANOVA application, multiple comparison tests were used: if the distribution was normal then the Tukey-Kramer test was used, if not the non-parametric Kruskal-Wallis test was used. Differences of reflectance in selected wavelengths, biophysical, and anatomical traits between juvenile and mature leaves during the season were tested with a Student’s t-test. All variability tests (Student’s t-test, ANOVA, multiple comparison tests) were performed with NCSS 9 software (NCSS, LCC Kaysville, Utah, USA).

To evaluate the effect of leaf developmental stage and phenology (DOY of sampling) principal component analysis (PCA) was applied on leaf reflectance spectra. PCA was based on the NIPALS algorithm [60], and the model was cross-validated with random 20 segments. Leaf developmental categories (juvenile, mature, or senescent) and DOY were treated as categorical variables. PCA of leaf spectra was performed in Unscrambler 11 (CAMO Analytics, Oslo, Norway).

Based on the literature, we analyzed 64 vegetation indices (VIs) and related them with biophysical and anatomical traits, see in Supplementary material Table S1. For visualization purposes, the relationships of VIs with biophysical and anatomical traits, coefficients of determination (R²) were summed and the “Summarized coefficient of determination” (SumR²) was created. All values of 64 VIs for all leaf samples across the vegetation season were also analyzed by PCA to reveal intercorrelation or independence of vegetation indices. Relation of VIs with biophysical and anatomical traits, and PCA on VIs were processed in R (4.0.3) using the ggfortify package [61].

3. Results

Results are presented in the following sequence: first, seasonal course of anatomical and biophysical traits for three developmental leaf categories separately: juvenile, mature, and senescent; second, leaf traits compared among the juvenile neoformed leaves and mature physiologically active leaves in relation to the time in the vegetation season (characterized by DOY); finally, seasonal course of anatomical and biophysical traits for all sampled leaves together to demonstrate overall canopy trends in studied leaf traits.

3.1. Seasonal Course of Anatomical and Biophysical Traits of Juvenile, Mature, and Senescent Leaves

Figure 3a–c shows the seasonal course of selected anatomical traits: palisade parenchyma thickness (PP), spongy parenchyma thickness (SP), and palisade/spongy parenchyma thickness ratio (PP/SP) of both juvenile and mature leaves for all three studied tree species (B. pendula, A. incana, P. tremula). Based on the comparisons of PP and SP separately in juvenile and mature leaves, no differences were observed within juvenile and mature leaves in sampling dates (DOYs 141–191). However, the increasing trend of the palisade to spongy parenchyma ratio in mature leaves was observed in leaves sampled in DOYs 169–191 (Figure 3c).

Later in the season only the biophysical traits were assessed till leaf senescence (DOYs 254 and 274), while anatomical traits were not studied due to technical reasons. In the two last sampling dates, senescent leaves were also included. Pigment contents (Chl and Car) and their ratios during the season for all three developmental leaf categories (juvenile, mature, and senescent) are separately shown in (Figure 3d–g). We did not observe any differences in pigment content or ratio in juvenile leaves from DOYs 141–274 and the data variability, particularly in Chl a/b and Car, was remarkable. In contrast, mature leaves showed a typical seasonal dynamic in Chl content with a peak on DOY 191. Ratios of Chl a/b and Car/Chl slightly decreased toward the end of the season, meanwhile the Car content was stable over the studied period. For senescent leaves (DOYs 253–274), values of Chl, Chl a/b, and Car were lower than the rest of the leaves except the Car/Chl ratio, which exhibited a sharp increase in values on DOY 274 and extreme variability compared to juvenile and mature leaves (Figure 3d–g).

Structural traits LMA and DW/FW showed a gradual increase during the season for juvenile leaves, compared to the mature leaves, for which both traits remained stable; and for senescent leaves, LMA and DW/FW exhibited a rapid increase from DOYs 253 to 274. The WCLA was stable for juvenile, mature, and senescent leaves from DOYs 141 to 274 (Figure 3h–j).

3.2. Differences in Anatomical and Biophysical Traits between Juvenile and Mature Leaves during the Season

We compared three anatomical traits: the thickness of palisade parenchyma, spongy parenchyma, and the ratio of palisade/spongy parenchyma thickness (PP, SP, PP/SP, respectively, on juvenile and mature leaves of pooled data for all three species B. pendula, A. incana, P. tremula; (Figure 4a–c).

In the first sampling (DOY 141), all leaves were classified as juvenile, however, from DOY 157 we distinguished the fully expanded leaves as mature, while leaves from sylleptic growth, which showed the same phenotype as on DOY 141 were classified as juvenile. When comparing juvenile and mature leaves during the season, the PP was significantly higher in mature leaves than in juvenile leaves in all sampling dates (Figure 4a). The SP showed the same pattern with an exception later in the season (DOY 191), when mature and juvenile leaves did not significantly differ (Figure 4b). The difference between the juvenile and mature leaves in the PP/SP was significant only on DOY 169 and DOY 191 (Figure 4c).

Comparing the pigment content and their ratios in all sampling days (Figure 4d–f), a significantly higher Chl content was observed in mature leaves compared to juvenile leaves from DOYs 157 to 253 (Figure 4d). Both pigment ratios (Chl a/b and Car/Chl) differed significantly between mature and juvenile leaves starting from DOY 157. The Chl a/b ratio was higher for mature than for juvenile leaves; the values for the Car/Chl ratio were lower for mature than for juvenile leaves (Figure 4e,f).

Dry-mass and water-related traits (LMA, WCLA, and DW/FW; (Figure 4g–i)) showed high variances, particularly in juvenile leaves compared to the mature ones. Therefore, the differences in these traits were not significant with a few exceptions: higher LMA and WCLA in juvenile leaves than mature ones in the peak season (DOYs 183 and 191) and lower FW/DW ratio in juvenile leaves compared to mature ones in DOY 169 (Figure 4i). Juvenile leaves were naturally more folded than fully mature leaves. Thus, it was not possible to fully stretch these leaves either during optical measurements or during area-based sampling for pigment contents and LMA and implies possible small error into those measurement values. Nevertheless, the pigment content per unit of “non-stretched” leaf area was significantly lower in juvenile leaves. Despite the considerable variability in LMA, there was an apparent trend of increase from spring to peak season in both juvenile and mature leaves. There was also a trend of increasing DW/FW ratio from DOY 141 to 169 for both juvenile and mature leaves (Figure 4i).

3.3. Seasonal Course of Anatomical and Biophysical Traits of Pooled Leaves

Finally, the data for all leaf developmental categories were pooled together and general seasonal courses of studied anatomical and biophysical leaf traits were described (Figure 5). The PP increased from May to June (DOYs 141 and 157) and then remained stable until the end of the season (Figure 5a). In contrast, the SP remained unchanged during the season and it was fully developed since the first sampling (DOY 141) (Figure 5b). Thus, the moderately rising PP/SP (Figure 5c) resulted from increasing thickness of PP (Figure 5a).

Similar to the anatomical traits, the seasonal changes in pigment content and their ratios (Chl a/b, Car/Chl) in pooled juvenile, mature, and senescent leaves of all three studied species were analyzed. We observed an increasing trend of Chl content from the beginning of the season till the maximum in DOY 191 and then the significant drop during senescence between DOY 254 and DOY 274 (Figure 5d). The Chl a/b ratio was quite stable during the season, only showing larger variance in last sampling DOY 274 (Figure 5e). Similarly, Car/Chl was also stable until the DOY 274, when a sharp increase was caused by the seasonal drop of chlorophyll content during leaf senescence (Figure 5f).

Finally, the seasonal trends in structural traits were evaluated for all leaves pooled together (Figure 5g–i). The LMA showed seasonal dynamics, increasing from DOY 141 with the peak of values on DOY 183 and decreasing toward the end of the season (Figure 5g), the seasonal LMA pattern was similar as shown for separated leaf developmental categories (Figure 4g). The WCLA seasonal course for all leaf developmental categories was the most stable with the highest values on DOYs 183 and 191 and significant drop at the end of the season (Figure 5h). The DW/FW followed an increasing trend toward the season peak (DOY 183), then remained stable only with increasing variability in data (Figure 5i), which agrees with the pattern shown for the DW/FW of both juvenile and adult leaves analyzed separately (Figure 4i).

3.4. Phenology of Leaf Reflectance Related to Leaf Developmental Category

Figure 6 shows the leaf reflectance during the growing season accounting for the presence of leaves in different developmental stage within a tree crown. The spectra for the first sampling (DOY 141) are not shown, as all leaves in the canopy were classified as juvenile. Between DOYs 157 and 253 there are apparent differences between juvenile and mature leaves’ reflectance, particularly in green, red, NIR and SWIR spectral regions. In the last sampling (DOY 274), the difference between juvenile and mature leaves was less pronounced (Figure 6f). Senescent leaves were detected first on DOY 253 and their spectral signal is obviously different from juvenile and mature leaves in VIS, and from juvenile leaves also in NIR and SWIR spectral regions.

In Figure 6, grey vertical lines highlight the reflectance in representative wavelengths for the visible (VIS = 400–700 nm), near-infrared (NIR = 700–1000 nm), and the shortwave infrared region (SWIR = 1000–2000 nm), being of basic interest for vegetation spectroscopy. As representative wavelengths for given wavelength intervals, we present the nearest wavelengths measured by our spectroradiometer: green = 551 nm; red = 673 nm; red edge = 705 nm [45]; near infra-red = 800 nm, related to structural absorption features [40]; water absorption features = 1242 nm [62]; nitrogen absorption features = 1511 nm [63]. Differences among sampling dates and leaf developmental categories in these wavelengths are presented in (Figures S5–S7).

Seasonal trends in described wavelengths for juvenile, mature, and senescent leaves are shown in (Figure S5). In juvenile leaves, the seasonal effect on values of reflectance at 551 nm, 673 nm, 800 nm, and 1242 nm was absent; however, the reflectance at 705 nm exhibited an increasing trend toward the peak and end of the season; the reflectance in the SWIR spectral region showed a decreasing trend toward the end of the season. Reflectance of mature leaves was less stable: reflectance at 673 nm, 800 nm, 1242 nm, and 1511 nm did not change during the growing season, whereas reflectance at 551 nm and 705 nm decreased from DOY 157 to 253 and increased in DOY 274. Senescent leaves showed high variance of reflectance at 551 nm, 673 nm, and 705 nm at the end of the season (DOY 274); while variance of reflectance at 800 nm, 1242 nm, and 1511 nm was small at the end of the season, (Figure S5a–f).

Differences in reflectance in selected wavelengths between juvenile and mature leaves during the season are shown in Figure S6. The difference between juvenile and mature leaf reflectance in green and red spectral regions was unstable with large variance, particularly for the reflectance of juvenile leaves, with differences in mid-summer season (Figure S6a,b). In general, juvenile leaves exhibited higher reflectance than mature leaves in the VIS region, most profoundly in the red-edge spectral region (Figure S6c) where juvenile leaves had significantly higher reflectance at 705 nm compared to mature leaves throughout the whole growing season. At longer wavelengths (800 nm, 1242 nm, and 1511 nm), mature leaves consistently showed significantly higher reflectance than the juvenile leaves during the period from DOY 157 to 253 (Figure S6c–f).

Finally, Figure 7 shows the seasonal trends in reflectance and its variability (standard deviations) for all leaf developmental categories pooled together. In the VIS spectral region, the main differences are obvious at the green peak (R551) and also in the red region (R673), where the increase of reflectance in DOYs 253 and 257 corresponds to chlorophyll content decrease in the end on the season. The variability in reflectance in VIS and red edge spectral regions also increases toward the end of the season. In the NIR, the lowest reflectance is apparent at the beginning of the season (DOY 141) when the leaves were not fully developed. Between DOYs 157 and 253, the reflectance was rather stable. An increase in NIR and SWIR reflectance was also observed during late fall (DOY 274) when senescent leaves prevailed in the canopy. The local maximum of reflectance standard deviation around 1400 nm probably corresponds with water loss before leaf abscission.

Figure S7 shows the seasonal trends in reflectance in selected wavelengths. Despite significant seasonal effects in the green and red-edge spectral regions, leaf reflectance kept similar values until DOY 253 and then sharply increased on DOY 274 Figure S7a,c. In the red spectral region, there was no significant difference during the season except for the DOY 274, when reflectance rose sharply (Figure S7). In NIR- and SWIR-specific wavelengths, we observed significant differences in reflectance at the beginning of the season (DOY 141) (Figure S7d–f) and at the end of the season (Figure S7e,f) compared to the peak of the season, where reflectance at 800, 1242, and 1511 nm was stable.

The principal component analysis was applied on the whole spectral range of leaf reflectance (400–2000 nm). The first component (PC1) explained 53% variability and the second component (PC2) explained 38% of the total variability in leaf spectra (Figure 8). The model was constructed with four principal components in total, explaining 96% of the total variability in leaf reflectance. Using leaf developmental category and leaf phenology (DOY of sampling) as categorical variables, there were apparent separation trends in spectral data. Sample scores were diagonally spread along both the main components (PC1 and PC2 axes); however, the juvenile and mature leaves formed distinctive clusters—triangles and dots, respectively. Mature leaves cluster more compactly (dots in a dark green ellipse), closer to the coordinates center in comparison to looser cluster of juvenile leaves with higher variability in spectra (triangles in larger light green ellipse in Figure 8). Senescent leaves mostly formed the third cluster (squares) with a few sample scores overlapping with the mature leaves’ cluster.

In case of leaf phenology (DOY), the separation trend was much less pronounced. Probably due to continuous leaf development, there were not very distinctive clusters for individual sampling dates (Figure 8). Only senescent leaves sampled at the end of the season (DOY 274) were partly separated from other sampling dates. Results of PCA of leaf spectra showed similar trends as recorded for biophysical and anatomical traits—the effect of leaf developmental stage was strong during the whole season and the effect of leaf phenology (DOY) was more apparent toward the end of the season.

3.5. Relation of VIs with Biophysical and Anatomical Traits

We tested the correlations of leaf biophysical and anatomical traits with 64 selected vegetation indices calculated from the leaf adaxial reflectance (for all coefficients of determination see Table S1). For each vegetation index, values of R² were summed to so-called “Summarized R²” for better visualization purposes. According to the strength of relationship defined by the coefficient of determination (R²) we detected two groups of leaf traits: (1) Leaf pigment-related traits (Chl, Chl a/b, Car and Car/Chl) and anatomical traits involving palisade parenchyma thickness—PP and PP/SP—related with the same vegetation indices mainly based on reflectance in wavelengths from 419.8 nm up to 925 nm (Figure 9a); and (2) leaf structural traits (dry and fresh LMA, water content leaf area, and nitrogen and carbon content) related with vegetation indices involving reflectance in wavelengths from 800 nm to 1724.7 nm (Figure 9b).

Figure 10, Figure 11, Figure 12 and Figure 13 show the relations of selected leaf biophysical and anatomical traits with the best-performing vegetation index for all leaf developmental categories together, and for juvenile, mature, and senescent leaves separately. The strongest relation for pooled leaf developmental categories was observed between VI Vogelmann and the contents of biochemically assessed chlorophyll a (R² = 0.88), total chlorophyll content (R² = 0.87), and chlorophyll b (R² = 0.78), see in Table S1. Samples for juvenile and mature leaves formed two partially overlapping clusters along the regression line (Figure 10a,b). Relation of total chlorophyll content to VI Vogelmann differed among the leaf developmental categories, according to R² values: 64%, 62%, and 99% of variability in chlorophyll content was explained in juvenile, mature, and senescent leaves, respectively (Figure 10a). For carotenoids, the highest correlation was observed with VI mND705 (R² = 0.6) for all leaf developmental categories; the relation explained 20%, 32%, and 77% of carotenoid content variability in juvenile, mature, and senescent leaves, respectively (Figure 10b). Surprisingly, for both pigments the indices showed the highest R² for senescent leaves (Chl R² = 0.99; Car R² = 0.77). The Car/Chl related the best to PSRI (R² = 0.85) if all leaves were pooled. However, for juvenile and mature leaves the explained proportion of variability was almost zero. For senescent leaves, the relation explained 69% of variability in Car/Chl (Figure 10c).

Leaf thickness showed the strongest relation to MCARI2 index (R² = 0.49) if juvenile and mature leaves were pooled (Figure 11a). For juvenile leaves alone, the relation between leaf thickness and MCARI2 was absent (R² = 0.01); for mature leaves, the correlation was weak (R² = 0.26). Among the anatomical leaf traits, PP correlated strongest with VI Datt2 (R² = 0.6) for juvenile and mature leaves pooled together. In contrast, there was no relation between PP and Datt2 if the juvenile and mature leaves were treated separately (Figure 11b).

Regarding the traits derived from structural parameters and water content, LMA, fLMA, DW/FW, and WCLA, relationships were looser than in pigment-related traits (Table S1). The DW/FW ratio did not correlate meaningfully with any of our tested indices: the maximum coefficient of determination for LMA was with vegetation index DLAI (R² = 0.58) for all leaf developmental stages together. The relation to DLAI index explained 67%, 60%, and 32% variability in LMA for juvenile, mature, and senescent leaves, respectively (Figure 12a). If all leaf developmental categories were pooled together, the fLMA correlated with NDII index quite well (R² = 0.64); however, for separated juvenile, mature, and senescent leaves the relation explained 56%, 79%, and 72% of variability, respectively (Figure 12b). The WCLA showed the best relation to MSI index (R² = 0.55) for all samples together; the variability explained by this relationship was different for juvenile, mature, and senescent leaves: 39%, 56%, and 79%, respectively (Figure 12c). The N content correlated loosely with VI Rre (R² = 0.43) for all leaf categories pooled together. If leaf developmental stages were separated, the relation was looser for juvenile leaves (R² = 0.29); absent for mature (R² = 0.01); and the strongest for senescent ones (R² = 0.82; (Figure 13a)). The C content did not meaningfully correlate to any index with exception of VI NDNI (R² = 0.14), where the correlation was significant but R² very low for all leaf developmental categories together and separated (Figure 13b).

In all the scatterplots (Figure 10, Figure 11, Figure 12 and Figure 13), we used different colors for individual sampling dates (DOYs) as well as different symbol size for leaf developmental categories (juvenile, mature, and senescent leaves), to be able to distinguish the patterns along the regression lines. For biophysical leaf traits, Chl, Car, fLMA, WCLA, and N content, there were three clusters representing the leaf developmental categories more distinctly. Similarly, for anatomical traits, leaf thickness and PP, two clusters were present along regression line corresponding to the juvenile and mature leaves. The Car/Chl ratio relation to vegetation index PSRI was mainly driven by senescent leaves showing high variability in values in contrast to a cluster of juvenile and mature leaves with low Car/Chl ratio variability (Figure 10c). The LMA showed the most homogeneous distribution of points along the regression line (Figure 12a). The nitrogen content (Figure 13a) exhibited similar pattern to the Car/Chl ratio variability.

Figure 14 shows the linear relationship between chlorophyll content and palisade parenchyma thickness for juvenile and mature leaves together (R² = 0.43).

3.6. Intercorrelation of Vegetation Indices and Their Clustering

The relations among the selected VIs were tested by principal component analysis. Although many different VIs have been developed, these are often strongly correlated with each other and actually tend not to give much independent information due to their formulas and similar wavelengths used. PCA was conducted to characterize the major groups of VIs and to evaluate how much independent information for the pooled data across all phenological stages (juvenile, mature, and senescent leaves pooled) are provided by VIs included to our study. The first principal component explained 53% of total variability. The first two components together described 76% and three components 83% of total variability captured by all 64 VIs in our study.

The first axis of PCA was related to all the various indices usually computed from red and red-edge wavelengths and most of them were primarily designed as chlorophyll indices. Those indices formed a big cluster toward the negative values of PC1 axis Cluster A1 in (Figure 15a). The best performing indices for chlorophyll and carotenoid prediction (Vogelmann and mND705, respectively, (Figure 10a,b)) belong to this cluster. The smaller Cluster A2 in (Figure 15a) formed toward the positive values of PC1 and contained indices related to leaf damage (N705, N715, N725) and leaf level chlorophyll (Gitelson2 and SR736/751). The majority of indices with high Summarized R² for pigments and mesophyll based anatomical traits (Figure 9a) belonged to the Clusters A1 and A2. The distribution of VIs among three principal components correspond well with the summarized R² of VIS and leaf traits. On the one hand, the pigment-related indices included into the PC1 and PC2 exhibited high SumR² with pigment contents and anatomical parameters (PP, SP, and PP/SP). On the other hand, VIs contributing to PC3 were related to water and dry mass contents and showed high SumR² with those structural traits derived from water and dry mass contents.

The second axis (PC2) was related to VIs that used reflectance around 670 nm, close to the chlorophyll absorption maximum, wavelengths close to 700 nm, and partly NIR (800, 900, 1200, and 1600 nm). Indices separated by PC2 formed two clusters located opposite each other along the PC2 axis: Clusters B1 and B2, (Figure 15a), almost independent of PC1. VIs from Clusters B1 and B2 were mostly designed for canopy-level parameters, such as LAI, canopy chlorophyll, canopy nitrogen, lignin, and water content Table S1. Some indices from Clusters B (DLAI, SRWI, NDWI, NDII, MSI, Ncont1510) also showed high summarized R² for structural and water-related traits (Figure 9b). The last one, a rather loose Cluster C, was distinguished equally by PC1 and PC2 (Figure 15a) and contained various indices designed primarily for detection of Car/Chl ratio, leaf level stress, and water content. The PSRI index from this Cluster C related best with Car/Chl ratio (Figure 10c).

Although PC3 explained only 7.37% of total variability in VIs values, together with PC2 it distinguished several VIs marked as Clusters D1 and D2 (Figure 15b). Most indices from Cluster D were associated with NIR and SWIR wavelengths and sensitive to water and lignin content, similarly, as described for Clusters B1 and 2. The best predictors of fresh mass and water related traits fLMA and WCLA (NDII and MSI, respectively) belonged to the Cluster D. The DLAI index (marked by cyan ellipse in (Figure 15a,b)) did not strongly correlate with any of the described clusters and contributed mainly to PC3. The DLAI was the best predictor of LMA. VIs in Clusters marked with identical letter are determined based on the same traits and correlate with each other.

4. Discussion

4.1. Seasonal Course and Variability in Biophysical and Anatomical Traits Related to Leaf Developmental Category

We are aware that to some extent, differences are present in leaf biophysical, anatomical, and optical traits among temperate trees [24,34,64]. Those differences are usually connected to shade tolerance [64], however the species in the present study are all shade-intolerant, early succession species. Therefore, we paid increased attention to the effect of leaf developmental stages and phenology, and in accordance with other spectroscopic studies [24,31,37], all analyses were conducted for biophysical and anatomical properties of three studied species pooled together. We observed leaf developmental asynchrony in all studied species, which was described earlier [10,13,65]. In all species, juvenile leaves were present on sylleptic branches during the whole season until the 1 October. Those neoformed leaves kept their juvenile character in most studied leaf traits (palisade and spongy parenchyma thickness and their ratio, chlorophyll content and pigment ratios), significantly differing from mature leaves in the canopy (Figure 4a–i). Liu et al. [66] confirmed the differences among young and mature leaves in various leaf structural traits (LMA, leaf thickness, dry mass) in three Chinese temperate deciduous species and highlighted the effect of leaf phenology on leaf traits. However, the effect of sylleptic and proleptic leaf growth was not considered in their study. Our results confirm previously described differences in Chl and Car between juvenile (neoformed) and mature (pre-formed) leaves (Figure 4d–f) throughout the whole season [67]. The exceptions were the first and the last samplings, where we observed that leaves were all juvenile or mature, respectively, regardless their developmental origin by either proleptic or sylleptic growth giving origin to neoformed leaves during the whole season. Our findings of thicker mature (pre-formed) leaves with thicker palisade and spongy parenchyma in comparison to neoformed ones was confirmed in Populus tremula [23].

Although we did not quantify the proportion of leaf developmental categories within the crowns of studied trees, we analyzed the seasonal courses of leaf traits for all developmental leaf categories pooled together (Figure 5). The trend in palisade parenchyma thickness and PP/SP increased early in the season or later (between 21 May and 6 June157; between 21 May and 2 July, respectively) which is in accordance with [24,68] (Figure 5a,c). Seasonal changes in chlorophylls and carotenoids followed typical courses for temperate broadleaves [24,29,30,31]: pigment accumulation until the peak season (from the 2 July to the 10 September) and later decrease during the leaf senescence (the 1 October). Increasing variance in chlorophyll content (Figure 5d) toward the end of the season was probably a result of different nitrogen intake and metabolism management by studied species. A. incana is a nitrogen-fixing species, thus, nitrogen is not much reabsorbed from senescent leaves in comparison to non-fixing species [69]. This corresponds with relatively high nitrogen and chlorophyll contents in alder senescent leaves in comparison to birch and poplar leaves (data not shown).

Pigment ratios and their seasonal dynamics give information about changing photosynthetic capacity and light acclimation. Although pooled leaves did not exhibit a significant seasonal pattern in Chl a/b ratio (Figure 5e), this trait was consistently higher for mature than juvenile leaves until the 10 July. Similarly, Car/Chl ratio was rather stable, with exception of significant increase in absolute value and variability in fall (the 1 October; Figure 5f). Carotenoids have dual role participating in (1) light harvesting by absorbing green-blue light and transferring energy to chlorophylls, and (2) dissipation of excess energy [70,71,72,73]. Higher Car/Chl ratio of high light exposed juvenile leaves in the peripheral crown (Figure 4f) can be also interpreted as protection against excess irradiance.

The phenological course of LMA for pooled juvenile and mature leaves (Figure 5g) was typical for mixed temperate stand [31]. Juvenile leaves showed a more pronounced seasonal pattern than mature leaves (Figure 3h). Similar LMA of juvenile and mature leaves (Figure 4g) may have resulted partly from overestimating LMA in juvenile leaves due to their folding. The most likely explanation for the increased LMA would be an accumulation of photosynthetic products in juvenile leaves [74]. Variability in LMA of temperate broadleaves at regional scales is largely attributed to leaf anatomical traits, mainly palisade parenchyma thickness [64]. Our observations confirm this relationship, as the trend in palisade parenchyma thickness of pooled leaves (Figure 5a) follows a similar seasonal course as LMA (Figure 5g).

A dilution effect (decrease of mass-based nitrogen content—(N%)) due to the accumulation of carbon-based metabolites could explain the drop from spring to summer in juvenile leaves (Figure S4a), similarly to [31]. Similar N% pattern of both juvenile and mature leaves (Figure S4c), suggests comparable photosynthetic capacity of both developmental categories of leaves.

Due to the time demanding and technically challenging field collections (e.g., sampling of sun-exposed leaves of high mature trees) for biophysical, anatomical, and optical traits at the leaf level, it is reasonable to pool samples of studied trees together in remote sensing studies dealing with coarser spatial resolution. In studies focused on mixed broadleaved forests, the species are usually pooled together as e.g., Demarez [37] dealt with hornbeam, oak, and beech with comparable LMA and leaf thickness. At the canopy level, both leaf developmental categories—mature pre-formed and juvenile neoformed leaves—contribute to the top-of-canopy reflectance. It has been reported that neoformed leaves on sylleptic branches may contribute with over 50% to total LAI in various poplar genotypes [14] and thus, should be taken into account from remote sensing point of view.

4.2. Seasonal Course of Leaf Optical Properties Related to Leaf Developmental Category

The juvenile leaves growing on branch tips were generally brighter in the VIS and darker in NIR and SWIR spectral regions compared to the mature leaves within one canopy (Figure 6a–c). The generally higher reflectance of these high-light exposed leaves in VIS [51,75,76] could be explained by lower chlorophyll content of juvenile leaves (Figure 4d). Therefore, the lower chlorophyll content and higher reflectance of upper-most juvenile leaves represent the strategy to avoid damage from excess irradiance [77,78]. Previous studies confirmed dominant leaf scattering from leaf internal structure in NIR [40,41,42], which explains why significantly thicker mature leaves have higher NIR reflectance than the thinner juvenile ones (Figure 6).

From a remote sensing perspective, different species and canopy positions of leaves contribute simultaneously to the spectral signal, which justifies pooling all leaves together for a generalization of patterns of leaf optical properties. For all species and leaf phenological phases pooled, leaf optical properties had some seasonal variation (Figure 8) in accordance with previous studies [24,31,34,79]. Due to stronger chlorophyll absorbance in the red spectral region, the reflectance signal can saturate in this region and therefore reflectance in the green and red-edge parts of spectrum are often preferred for chlorophyll estimation [45,51]. The seasonal course of reflectance in green (Figure S7a) and red-edge (Figure S7c) spectral regions followed better the seasonal course of Chl content (Figure 5d) than the reflectance in the red region (Figure 6b) for pooled leaves across species and phenological phases in our study as well.

4.3. Performance of Vegetation Indices in Leaf Traits Prediction—Implications for Interpretation of Canopy Level Remote Sensing Signal

4.3.1. Effect of Phenology and Leaf Developmental Category

We have presented examples of linear relationships of best performing indices to key leaf biophysical and anatomical traits (Figure 9, Figure 10 and Figure 11). We focused on the effect of leaf developmental category and leaf phenology on VI linear model performance. The effect of leaf age on modelling of biophysical traits from spectra is currently a topical theme for evergreen conifers bearing several needle age classes at once [20,36,80]; and also tropical tree species with leaf developmental asynchrony [81,82]. Moreover, in some tropical trees with asynchronous leaf development and long leaf lifespan (e.g., 360 days), even the leaf age could be modelled from leaf spectra [83].

In the case of chlorophyll and carotenoid content prediction, the best predictors were the vegetation indices Vogelmann and mND705, respectively, with best relation of senescent leaves. The regressions for senescent leaves were driven by Alnus incana samples, maintaining high pigment contents until the end of the season. Most of the temperate deciduous species reutilize the biochemical compounds and nutrients during the leaf senescence before abscission [27] and, thus, pigment and nitrogen content decreases in senescing leaves. Nitrogen reutilization and chlorophyll decay in the fall are suppressed in Alnus species due to their ability of nitrogen fixing. If chlorophyll and carotenoid contents were modelled for juvenile and mature leaves separately, R² remarkably decreased. If all developmental categories were pooled together, the indices for Chl and Car estimation performed very well (Chl R² = 0.87; Car R² = 0.60) approaching to partial least squares regression (PLSR) modelling (Chl R² = 073.; Car R² = 0.71) in [31] or chlorophyll modelling from vegetation indices (Chl R² = 0.80) in [84]. A similar finding applies to the Car/Chl ratio related to PSRI index, where the relationships were absent for juvenile and mature leaves separately, and the best performing model was for all developmental categories pooled (R² = 0.85). From the remote sensing perspective, the regression for all leaf developmental categories pooled together is more realistic, because it covers the full range of pigment contents and, thus, provides a more robust model. Similar findings were reported for PLSR modelling of chlorophyll and carotenoids contents in Norway spruce using several needle age classes [20].

Assessing the leaf anatomical traits derived from microscopy images is more time- and labor-demanding than assessing the routinely measured biophysical traits, which could be the reason why not many spectroscopic studies focus on anatomical trait prediction. However, Ref. [85] recently showed that anatomical traits such as spongy parenchyma to leaf thickness ratio and palisade to spongy parenchyma thickness ratio correlate to forest water use efficiency and gross primary production at regional scales. Therefore, we are convinced that modelling anatomical traits from spectral signal may contribute to understanding ecosystem functioning. We evaluated linear relationships of leaf anatomical traits to 64 vegetation indices and we achieved good relation for leaf and palisade parenchyma thickness (Figure 9). Both anatomical traits are influenced by developmental status and the relationships were absent for juvenile and mature leaves treated separately. Same pattern was observed in linear relationship between chlorophyll content and palisade parenchyma thickness (Figure 14). Despite being weaker, this relation demonstrates that the correlation of anatomical traits with vegetation indices presented here, is mediated primarily by photosynthetic pigment content and then by cellular structure affecting reflectance in longer wavelengths. Nevertheless, both leaf developmental categories occur within the crown for the long period of vegetative season (between 6 of June and 10 September; being assessed for anatomical traits only until the 10 July) and thus should be considered if taking a ground truth for remote sensing studies. Both indices Datt and MCARI2 are primarily designed as chlorophyll indices, however, they both calculate with NIR wavelengths (849 nm and 749 nm, respectively) and, thus, may relate well to leaf cellular structure. Palisade parenchyma is usually very dense tissue with only a small proportion of intercellular spaces and a high density of chloroplasts compared to spongy parenchyma, thus, a strong correlation with chlorophyll content can be expected. Total mesophyll thickness and leaf blade thickness followed a similar pattern.

Structural traits LMA, fLMA, and WCLA were best related to DLAI, NDII, and MSI indices, respectively (Figure 12). Correlation of LMA and DLAI (VI developed for LAI estimation) due to significant covariance of both structural traits (LMA and LAI), manifested mutual correlation of both structural traits during the season [86]. All traits were modelled with moderate accuracy if all leaf developmental categories were pooled (LMA R² = 0.58; fLMA R² = 0.64; and WCLA R² = 0.55). Separate models for juvenile, mature, and senescent leaves showed some variability in R² see (Figure 12), but not as remarkable as for carotenoids, Car/Chl ratios, and anatomical traits. Because estimated traits were coupled with water content (WCLA, fLMA), mature and senescent leaves contributed considerably more to the model than juvenile leaves. Yang et al. [31] retrieved LMA by PLSR with better results if data from the whole season were used (R² = 0.85), compared to only a spring (R² = 0.13) or summer sampling data (R² = 0.71). This confirms our results that the estimation of LMA is better from wider seasonal period than from only one sampling term. Parameters based on dry matter content (LMA) are useful inputs for various simulations at a landscape scale and for radiative transfer models [86].

Due to the strong covariance of nitrogen content with other leaf compounds, such as chlorophyll and proteins, it is not reasonable to estimate the nitrogen content using VIs since many spectral bands can be used for N estimation as summarized in Homolová et al. [87]. Even radiative transfer models at the leaf level are not the best way to predict nitrogen content—it is incorporated only in RTM LIBERTY [88], which is not as widely used as RTM PROSPECT [89]. For that reason, nitrogen content is usually well estimated by PLSR from hyperspectral data with results R² = 0.81–0.96 [90].

4.3.2. Effect of Vegetation Indices Intercorrelation

A vast number of different vegetation indices have been developed to estimate plant traits, such as pigment content, from reflectance data [84]. Similar to a previous study [51], we confirmed that vegetation indices using the red-edge spectral region, perform the best in estimating total chlorophyll content. Indices using this spectral region have an even stronger correlation with chlorophyll a than with total chlorophylls a + b Table S1. Total carotenoid content also varies together with chlorophyll content and therefore the same indices that perform well for estimating chlorophyll content are also good predictors for carotenoids. These pigment-sensitive indices formed the first axis of PCA, and leaf thickness was also related to the same group of Vis, see (Figure 9a and Figure 11a).

The Vis sensitive to leaf water, as well as fresh and dry mass contents formed another independent group associated with the PC3 Clusters D1 and 2, (Figure 15b), which also proved linear models (Figure 9b). Due to the strong covariation of different leaf traits, it is quite common that Vis originally developed for predicting one trait turn out to be good estimators for another related foliar trait, (e.g., estimation of nitrogen content [87]). For example, NDNI, which was developed for estimating foliar nitrogen, was primarily related better to leaf and palisade thickness in our data Table S1. Nitrogen content itself was then estimated better by red-edge indices developed originally for chlorophyll predictions. However, nitrogen content is known to be strongly correlated with chlorophyll content, thus, used in remote sensing studies, e.g., using hyperspectral data [91]. Canopy level indices developed to remove soil signal and extract vegetation may also perform well at the leaf level, such as DLAI to estimate LMA from leaf level reflectance measurements in our data. The Car/Chl ratio of senescent foliage produces a clear unique signal Cluster C, (Figure 15a), which is the best captured by indices designed to detect senescence [92]. A similar approach of evaluating redundancy and independence of various fluorescence parameters—indices extracted from chlorophyll a fluorescence fast kinetics (i.e., OJIP transient, or JIP-test) was used by [93].

The 64 vegetation indices used in the present study covered a broad spectral range from the visible spectrum to SWIR (1800 nm). Therefore, it is not surprising that PCA applied on values of 64 VIS gives similar score distribution as the PCA applied on the whole spectral range (Figure 8 and Figure 15a,b). With the current wide availability of hyperspectral data at leaf and canopy levels, the multivariate approaches to leaf trait modelling, such as PLSR, exploiting the whole spectral range, are preferentially used [20,46,81]. Although our source data were hyperspectral, we built the present study on vegetation indices, which are still widely used due to rise of various affordable multispectral sensors [80,94,95,96] or satellite multispectral data [97]. On a large scale, starting in 2015 thanks to a NASA initiative, the Harmonized Landsat and Sentinel-2 (HLS) surface reflectance data set represents a novel type of multispectral satellite data [98,99] and vegetation indices derived from HLS have potential to monitor land surface phenology [100].

5. Conclusions

We presented differences in leaf biophysical, anatomical, and optical traits and their seasonal dynamics among leaves of different developmental category: pre-formed mature and neoformed juvenile leaves in three common hemiboreal tree species. In general, juvenile leaves exhibited higher reflectance than mature leaves in the VIS. The difference was even more pronounced in the red-edge spectral region (705 nm) within the season, during which chlorophyll content correlated well with the palisade parenchyma thickness.

Both leaf developmental categories—juvenile and mature—contribute to the top-of-canopy reflectance and should be considered when taking ground truth (i.e., leaf biophysical and anatomical traits). The linear models of leaf traits from vegetation indices usually performed better if all leaf developmental categories (juvenile, mature, and senescent) were included. From a remote sensing perspective, the prediction models constructed using all leaf developmental categories composing the monitored tree canopy pooled together are more realistic, because they cover the full range of leaf trait values and, thus, provide a more robust model. Structural and anatomical traits relate to plant physiological processes at leaf and canopy levels and serve as inputs to radiative transfer models. This justifies why modelling of anatomical traits from leaf and canopy spectra should not be neglected.

Our study provides information on which indices are good predictors of particular leaf traits and how these indices are interchangeable or independent when dealing with temperate or hemiboreal deciduous trees. VIs are still a valuable approach regarding current increase of affordable multispectral cameras combined with unmanned aerial systems with very high spatial resolution, or HLS product, and can be used for precision agriculture, forestry, or vegetation monitoring.

The take home message of this study is that even in temperate deciduous forests, the type of growth and leaf formation of target species should be regarded when taking ground truth for calibrating and validating models for biophysical and anatomical leaf trait retrieval from optical signal. When dealing with species with continuous- or semi-continuous of leaf formation (such as Salix spp. or young Alnus spp., Betula spp., and Populus spp. [65], it is necessary to sample both leaf developmental categories, pre-formed and neoformed leaves, especially if the sampling is conducted only once within the vegetation season. The same approach, to construct the models represented by different leaf age groups, was recommended by [81] for ecosystems with unsynchronized phenology (e.g., evergreen tropical forests) and by [20,36] for evergreen conifers.