1. Introduction
During evolution, vegetation has developed alternative metabolism paths and defense reactions that enable it to grow in unfavorable conditions, as well as regenerate damage, which is referred to as resistance to stress, disturbance, and competition [
1,
2]. One of the crucial factors causing stress in vegetation is trampling [
3,
4], including crushing and defragmentation of organs [
5]. Adaptation of plants to difficult conditions is observable in their leaf construction, leaf resilience as well as stem flexibility and extended root system. This impacts the so-called tolerance to the trampling of a particular species [
6,
7]. Due to extensive environmental changes, in most cases, the passive protection of nature does not guarantee that the extinction of a species or ecosystem degradation will stop [
8]. Therefore, it is necessary to explore the measurement of plant parameters and properties in order to assess ecosystem functioning [
9,
10], especially in valuable natural and hard-to-reach high-mountain ecosystems.
Hyperspectral remote sensing involves analyzing numerous narrow bands of electromagnetic radiation that enables the repeated monitoring of vegetation in a quantitative and qualitative manner, but must be verified by biometric measurements such as chlorophyll content or chlorophyll fluorescence [
11,
12]. In fact, researchers refer to the measurement of fluorescence as the barcode of physiological properties of a specific plant, which may be used e.g., for taxonomic purposes [
13]. Fluorescence is the quantity of absorbed light (quanta) leading to excitation of chlorophyll particles that have not been used in photosynthesis due to stressors [
14]; both abiotic and biotic. Therefore, the measurement of chlorophyll fluorescence is believed to be the best photosynthesis efficiency index [
15] for most species, i.e., not exposed to stressors. Optimal fluorescence (F
v/F
m) rates are approximately 0.83 with adaptation to darkness [
16,
17]. High photosynthetically active light intensity is characteristic of alpine plants, causing stress [
18], similarly, a decrease in the air temperature to −2°C brings about a reduction of the F
v/F
m ratio [
19]. Even though alpine vegetation reduces the negative effects of strong sunlight exposure [
20] by numerous adaptations, still, a combination of several stressors (e.g., dryness) will result in browning of leaves, lower biomass production, poorer photosynthesis productivity [
21,
22] and a loss of turgidity [
23]; simultaneously, one may observe lower F
v/F
m rates (stress index).
In order to better explain the mechanism functioning in high-mountainous plants, and their true state of health, biophysical measurements and biometric values are commonly used for verifying spectral information. This can be through assessing the so-called APAR (Absorbed Photosynthetically Active Radiation) and fAPAR (fraction of Absorbed Photosynthetically Active Radiation) rates, moreover, a measurement of thermodynamic air temperature (
ta) and plant canopy radiation temperature (
ts), can assess water stress through a temperature ts-ta index rate and Chlorophyll Content Index (CCI). In addition, modern methods often use remote sensing indicators to model biometric values. For example, regression models have been used to perform quantitative analyses between spectral VIs and LAI [
24] or CCC (Canopy chlorophyll content) [
25] measured under different phenological stages. In such analysis models, such as PROSPECT and SAIL radiative transfer models (PROSAIL) [
26], multiple linear regression (SMLR; [
27]), partial least square regression (PLSR; [
28]) or multivariate adaptive regression splines (MARS; [
29]) are often used. In addition, the coefficient of determination (R
2) and RMSE are employed to evaluate the outputs. An advantage is these non-invasive methods can be shifted from ground-based to airborne [
30] or satellite monitoring [
11,
12], e.g., the ESA FLEX satellite mission [
31].
Research published about vegetation within the Tatra National Park has gradually evolved. Initially, the difference in trampled and reference vegetation was tested using only a ground-based spectrometer [
32] with spectral curves and remote sensing vegetation indices analyzed. In subsequent years, for the purpose of their verification, chlorophyll (CCI) and accumulated energy (fAPAR) measurements in the area of Kasprowy Peak and the surrounding area were added to broaden the scope to verification [
33]. Next, in order to check the methodology, the research was transferred to another TPN area—the Red Peaks, where the obtained test results were verified, and the methods were extended by applying chlorophyll fluorescence measurements to check what actual damage occurs during plant trampling [
34].
Assessing vegetation trampling has developed over several years [
32,
33,
34,
35] focusing on new verification methods as well as the application of research by park employees. The current paper presents an analysis based on field studies, with an emphasis on recultivated areas; not previously measured. The goals are an assessment of spectral ranges and remote sensing indices, which were carried out by dividing the swards into trampled and reference polygons. However, along with the development of tourism in the Park area, protective measures were applied to the most degraded areas that are also a focus of this article.
This study tested the hypothesis that there is potential for using various hyperspectral vegetation indices for estimating, modeling chlorophyll fluorescence and chlorophyll for different types of alpine swards species. A key aspect of the research was finding the appropriate parts of the electromagnetic spectrum that can be used for ground, airborne and satellite level monitoring of plants. Another aspect was the correlation between remote sensing indices, plant physiology metrics (chlorophyll content and its fluorescence), absorption of accumulated energy for photosynthesis (APAR) and evapotranspiration. The precise determination of indicators and ranges of the spectrum for species will allow for the analysis of the condition and monitoring of individual protected species in the National Park.
2. Study Area and Research Objects
The Tatra Mountains are the highest mountain range between the Alps and the Caucasus. The Tatra National Park (Tatrzański Park Narodowy, TPN) covers 21,197 ha of the Polish part of Tatras, out of which 11,500 ha are under strict protection (all high-mountain ecosystems). The whole area of the Tatra Mountains includes two national parks: the TPN and the Tatranský Národný Park (TANAP), which jointly created the UNESCO Man & Biosphere nature reserve and met the criteria of the Special Protection Area (OSO) and Special Conservation Area (SOO). It is marked by a characteristic alpine landscape with a typical Central European climate with strong influences from Oceanic and Continental climates modified by altitude. Plant zones containing abundant flora (about 1000 species of vascular plants) and fauna include many species that are rare or endangered in Poland [
36]. Owing to the multitude of habitats and species, the area of the Tatra Mountains has been qualified as a Natura 2000 area. Alpine swards are exposed to high insolation and overheating and, simultaneously, strong and cold winds can cause considerable water loss. Thus, local vegetation has developed a range of protection mechanisms, such as succulent leaves and leaves covered with tomentum and wax. Due to the high altitude above sea level, the plants are smaller and less common. Their short height can protect them from unfavorable exposure to cold winds, with the whole plant able to survive under the snow. Alternatively, they may grow again from underground parts in the case of a shortened vegetation period [
37]. Also, some alpine plants have green leaves with a high chlorophyll content, which ensures rapid vegetation and blooming, allowing them to set fruit after the snow is gone.
The research area included the vicinity of the Kasprowy Peak, Beskid, Gąsienicowa Valley and Red Peaks (Kopa Kondracka, Ciemniak,
Figure 1), with the focus being vegetation growing in selected fragments of the alpine and subalpine floor termed alpine swards. Based on a list of precious flora kept by the Polish Chief Inspectorate for Environmental Protection, as well as maps of real vegetation [
38], dominant alpine sward species, i.e.,
Agrostis rupestris,
Festuca picta and
Luzula alpino-pilosa, were selected and subject to an in-depth analysis. These species grow on the most precious mountain areas where, from April to September, about 75% of the annual tourist traffic is concentrated. The observed phenomenon of strong anthropopressure leads to intense exploitation of the land, resulting in destruction and restructuring of the plant cover and soil erosion in the areas neighboring trails [
39,
40].
Agrostis rupestris is a narrow-leaved grass growing in tufts with 5–20 cm long raised blades [
41]. Its leaf blades are rolled, whereas the corymb may reach a length of 4 cm; the plant blooms in July and August and grows both on swards as well as on mountain pastures, having a typically scree-type nature [
42]. What is characteristic of the plant is its resistance to trampling, e.g., [
37,
43]. Mirek and Piękoś-Mirek [
44] described
Agrostis rupestris as a native species that has adapted to anthropogenic habitats.
Festuca picta belongs to the
Poaceae family and grows in thick tufts [
37,
45]. Its leaf blades are curved and filamentous, with a width of up to 0.7 cm, with the sole distinctive feature being the red-brown-violet color of inflorescences, which consist of numerous two-to-three-millimeter spikelets.
Festuca picta has very low habitat requirements.
Luzula alpino-pilosa, which belongs to the family of
Juncaceae, grows as dense fields on snow patch communities in humid couloirs, on gravels, screes, rocks, and banks of streams. This green plant, with leaves of approx. 2–7 mm width, will grow up to a height of 30 cm [
44] and blooms at the end of July and August. Its inflorescence is scattered and branched, whereas the flowers are inconspicuous and small. They grow on stalks, 2–3 on each, and have a dark brown, almost black, color [
44].
5. Discussion
Species differences for areas located next to tourist trails, experiencing trampling, were expected. The largest percentage of change (80–100%), for species across the whole electromagnetic spectrum, was for
Luzula alpino-pilosa and
Agrostis rupestris. In the case of
Luzula alpino-pilosa, it was related to the wide structure of the leaf blade as well as its thickness and size; implied considerable destruction from trampling. The leaves and stems of
Festuca picta are harder, so trampling led to a smaller percentage of change (50–60%), which was also true for
Agrostis rupestris. Similar conclusions were presented by Balcerkiewicz [
65], who reasoned that grazing caused the plants to grow in succession, with grassy vegetation transitioning to intermediate communities between grassy plants, alpine swards and matgrass swards.
Oprządek [
66] also observed that the closer to the trail, the more common species, e.g.,
Agrostis rupestris, which builds single-species patches along paths; negative and statistically significant correlations were achieved for three quantitative categories (total number of occurrences, quantity above 1 and >3). The strongest correlation (Rs = 0.902), was recorded for the last category (>3, i.e., above 50%), which indicated the dominance of this species along the trail and may indicate its greater resistance to trampling. This was confirmed by the observations of Mirek and Piękoś-Mirek [
44], who defined
Agrostis as a native species that has adapted to anthropogenic habitats and often becomes dominant. Its presence next to tourist trails was also noted by Górski [
67], who described the community of
Agrostis rupestris occurring on tourist paths and in other trampled places.
Remote sensing measurements made it possible to broadly categorize the state of the plants in terms of the effect of trampling. Based on the vegetation indices and biometrical variables, the resistance to trampling (from most to least resistant species) was:
Festuca picta,
Agrostis rupestris and then
Luzula alpino-pilosa.In the studies conducted by Oprządek [
66],
Luzula alpino-pilosa demonstrated a poor negative correlation (Rs = 0.382) to the distance from the trail. However, in contrast to
Agrostis rupestris, it does not create a hull community and instead finds favorable conditions through habitat (longer snow cover and the formation of gravels or screes on the edge of the path) that means it tends to form monogenous patches along the paths; confirmed by the observations of Mirek and Piękoś-Mirek [
44]. Another species that Oprządek [
66] referred to as negatively correlated (Rs = 0.793) with the distance from the trail within TPN, was
Festuca picta. This confirms our results and the validity of the methods used to monitor changes. The botanical studies conducted so far have not been sufficient, as they only quantify the percentage share of the species in the study area along the trail. Using remote sensing methods, especially chlorophyll analysis and chlorophyll fluorescence, a decrease in condition (by about 10–15%) in relation to reference polygons was determined for plants within trampled polygons.
Table 5 lists statistically significant ranges within the electromagnetic spectrum, which emphasize the differences between the spectral characteristics of the species studied in the three types of research polygons (trampled, reference, recultivated). This can be used to indicate significant wavelengths, with a list prepared on the basis of the wavelengths used within remote sensing vegetation indices, which have proved to be the most useful in showing differences and variability linked to species state. In this study, the greatest differences in species spectral characteristics were for the spectral ranges describing the absorption of photosynthetically active pigments (448–514, 531–707 nm), cell structures (1385–1556 nm), and water content (1801–1835, 1845–1862, 1879–2500 nm;
Table 5). These differences also result from specific adaptations to excessive radiation (change in the carotenoid and chlorophyll ratio).
In addition, these spectral ranges have previously been used to assess changes in the vegetation condition by authors such as Ruban et al. [
68], Gitelson and Merzlyak [
69] or Fourty and Baret [
79]. The bands used most frequently to analyze water content, and water stress included 950–970, 1150–1260, 1520–1540 nm [
81,
82]. The width of ranges (
Table 5) relevant for examining alpine swards condition points to the fact that swards may be monitored using satellite sensors as well as airborne hyperspectral imaging. For instance, WorldView-2 and 3 whose channels 2 (450–510 nm) and 5 (630–690 nm) overlap with the ranges deemed significant. Also, WorldView-4 has channels in the blue (450–510 nm) and red (655–690 nm), and similarly RapidEye (blue: 440–510 nm; red 630–685 nm), to which such environmental analyses may be applied.
In this study, the median PRI rates for all species studied (in the case of reference and recultivated areas) are higher than the respective rates in trampled plants, with the latter ranging below the general median of 0.035. This means that alpine sward vegetation growing in trampled areas is exposed to additional stress that weakens it. In the literature, PRI values gradually increased from winter, through spring to summer [
83]. Species in trampled areas also showed a drop of 0.1 to 0.4 in WBI compared to reference and recultivated areas respectively. The species that demonstrated water deficiency included:
Agrostis rupestris and
Luzula alpino-pilosa in trampled areas (AR_T, LAP_T). Changes observable in the vegetation were confirmed by previous research where, due to trampling, significant differences in remote sensing vegetation indices (α = 0,001) for NDWI, NDII and WBI related to water content; ARVI related to general condition; PSRI and CAI related to the quantity of carbon in the dry cellulose matter and lignin as well as PRI related to the amount of light used in photosynthesis [
32]. Having determined the change in condition over two years, conclusions were drawn about the influence of weather conditions on vegetation’s state.
Fluorescence demonstrated a poorer condition for plants in trampled areas compared to those growing in reference or recultivated areas. For non-stressed plant material, the Fv/Fm value is expected to be 0.81–0.83 for leaf material dark-adapted for a minimum of 20 min [
84]. For reference and recultivated plants, the median rates measured for F
v/F
m fell within the range of 0.700–0.755. When subject to trampling, the measured values were significantly lower and below 0.700 with the median for all species being 0.653 while Fv’/Fm’ (without adaptation) was 0.398. In the literature, values lower by more or less than 0.100 were observed for Fv’/Fm’ compared to Fv/Fm [
85]. Therefore, the measurements confirmed the dependence, namely that plants exposed to abiotic and biotic stress show lower than 0.830 or 0.832 ± 0.004 F
v/F
m values [
86,
87], with a decrease of F
v/F
m and mechanic damage to plant tissues causing disturbed transport within the plant and a drop in biomass and overall robustness [
88]. However, high photosynthetically active light intensity can also lead to stress in plants, with a drop in the rate of F
v/F
m [
89]. Therefore, in shady sites, trampling can lead to less vegetation damage and a smaller drop in biodiversity [
90]. The resulting rates are reduced compared to the optimum F
v/F
m, amounting to 0.780–0.865 [
91] or 0.780–0.840 [
16]. Overall, measurement of chlorophyll fluorescence allows researchers to detect stress prior to the appearance of visible external symptoms, including poorer growth [
92], withering, necrosis and chlorosis [
93].
A high correlation coefficient of F
v/F
m with the indices describing the use of light for photosynthesis allows for long-term monitoring of vegetation. In this study, the F
v/F
m index demonstrated a high correlation with indices measuring chlorophyll content and the use of light for photosynthesis e.g., the species of alpine swards growing in reference areas were marked by a high correlation between F
v/F
m and PRI:
Festuca picta (Rs = 0.65). The rates of F
v’/F
m’ demonstrated the highest correlation with PRI (Rs = 0.40 for
Festuca picta from trampled areas; Rs = 0.60 for
Festuca picta in reference areas); and CAI (Rs = 0.51
Agrostis rupestris from trampled areas). Studies which may serve as an example include those conducted by Tan et al. [
94], in which the SIPI index strongly correlated with F
v/F
m (R = 0.88) and was used to measure the variability of condition of and damage to maize. Studies of the reaction of maize to volatile levels of hydration were demonstrated by a high correlation between the
Photochemical Reflectance Index 570 (PRI570) and F
v/F
m’ (R
2 = 0.76); this index turned out to be linked to water stress at an early stage, prior to the occurrence of structural changes in the plant [
95]. PRI was most often correlated with actual photochemical efficiency ΔFv/Fm’ and explained 17–90% of its variability; median
R2 was higher for conifers (0.50) than for other species groups (Broadleaf 0.40, Herbaceous and crop 0.48 [
83]. PRI explained 44–74% of the variability of the actual quantum yield for broadleaved and herbaceous/crop plants but accounted for only 1–40% of the variability of ΔFv/Fm’ for mixed forests [
83]. Panigada et al. [
96] reported that PRI570 and PRI586 correlated with ΔF/Fm′ (
R2 = 0.49 and 0.51, respectively) for cereal crops under water stress better than other PRIs [
96]. Compared with PRI, the reflectance ratio R686/R630 also yielded a slightly higher average R
2 when related to Chl-Fs and Fv/Fm parameters. The daily R
2 values for PRI and R686/R630 was varied between 0.27–0.78 and 0.20–0.70, respectively, such a high variation might be related to the high heterogeneity of leaf angle distribution [
97]. This justifies the interpretation of data gathered about the vegetation spectral properties, e.g., the use of light for photosynthesis (PRI) or WBI, being the indicator reflecting water content. PRI was significantly correlated with Fv/Fm, RWC (relative water content), mean
R2 between 0.58 and 0.86 [
83,
98]. The research literature indicates that most stressors affecting the photosynthesis apparatus may reduce the F
v/F
m ratio; some authors confirmed its constant rate during drought conditions [
99,
100].
By applying the MARS method to this study, the following prediction rates were established: R
2 from 0.71–0.96 and RMSE from 2–8% for F
v/F
m, with the R
2 falling to 0.68–0.93 and RMSE to 4–7% for CCI. During verification of the Red Peaks model, the following rates were detected: respectively, R
2 from 0.58–0.72 RMSE from 12–19% for F
v/F
m, and R
2 from 0.54 to 0.75 and RMSE from 11–16% for CCI. Nawar et al. [
29] modelled soil salinity and arrived at similar average model accuracies for MARS – R
2 = 0.73, RMSE = 6.53 or for content of clay and organic matter was (R
2 = 0.81, RMSE = 7.7; R
2 = 0.89, RMSE = 6.9; R
2 = 0.73, RMSE = 0.34; [
101].
6. Conclusions
The present study confirmed the usability of hyperspectral remote sensing for evaluating the condition of vegetation, with the dominant species of alpine swards characterized by a set of unique properties that can be measured using hyperspectral sensors and fluorescence measurements. The most significant properties included plant pigments (and their relative quantity), protective elements such as the leaf shape and structure or leaf cover (especially the wax cover). In addition, this paper represents an interdisciplinary approach, including both remote sensing (by spectrometric and bioradiometric measurements as well as the option of transmitting information to a different level) with ecophysiology (features of leaves) and ecology (management of protected areas); the methods in question combine spectral characteristics and vegetation indices providing data on fluorescence and, hence, allowing a detailed analysis. Hyperspectral measurements, in turn, were used to confirm statistically significant differences in the spectral characteristics of trampled, reference and recultivated vegetation. These observations were confirmed by the chlorophyll content, fluorescence and the ts-ta index; the Fv/Fm fluorescence ratio was used to evaluate chlorophyll condition and the process of photosynthesis. Studies, such as the one presented in this paper, may be repeated temporally to monitor the condition over time; each vegetation season is characterized by varied weather conditions that affect the development of vegetation. Also, the remaining natural and anthropogenic factors vary, which can be captured through monitoring conducted using remote sensing methods. Estimation of the condition of vegetation on recultivated polygons is important from the point of reasonableness the protection of plant cover used by employees of the Park; good condition of vegetation in these areas confirms the validity of recultivated methods of degraded areas.
Summing up, the condition of reference species was good, but the trampled vegetation had undergone visible changes. For this reason, based on statistically relevant changes in all the studied indices, a distribution of species was developed, starting from those that showed the lowest sensitivity to the trampling stressor to those that presented the greatest variability and sensitivity Festuca picta, Agrostis rupestris and then Luzula alpino-pilosa. This demonstrated the usability of hyperspectral remote sensing to evaluate the condition of alpine sward species growing on protected high-mountain areas. The application of non-invasive field measurements allowed for detailed in situ examinations of individual species, especially endemic flora, which is particularly valuable. The rates of spectral reflectance in vegetation growing in reference and recultivated areas were similar, which implies that recultivation is appropriate. The species under analysis reacted in various ways to trampling; this is due to their different structure and adaptations to difficult environmental conditions. As far as trampled plants are concerned, there was a drop in the amount of chlorophyll and water content, as well as a poorer condition of cell structures and decreased photosynthetic productivity.
Spectral width (full width at half maximum, FWHM) is significant when studying the condition of alpine sward vegetation; swards may be successfully monitored using hyperspectral airborne imaging, as well as satellite imaging. Examples of such satellites are WorldView-2 and WorldView-3, where channels 2 and 5 provide between 450–510 nm and 630–690 nm, are deemed significant in this study. Also, WorldView-4, featuring multi-spectral blue (450–510 nm) and red (655–690 nm) channels or RapidEye (blue: 440–510 nm; red 630–685 nm) may be used. However, one should bear in mind the role of the data’s spatial resolution and the impact of soil reflectance, especially in the case of pixels located near trails where the vegetation cover is not complete.