Transpiration is a main component of evaporation fluxes in vegetated areas [1
], in particular in the forested region with a dense vegetation cover [2
]. A global-scale study indicated that transpiration accounted for 60–80 % of the total evapotranspiration from the land surface [3
]. The transpiration process is rather complex and involves the biophysical properties of leaf stomata in response to multiple environmental stresses in terms of the availability of water, carbon, and energy [4
]. Transpiration is conducted through the opening leaf stomata, where carbon dioxide (CO2
) enters for photosynthesis. This process is supplied by water flux from the deep soil up to the root zone and includes sap flow and root water uptake [5
]. The transpiration rates are governed by the physiological behaviour of leaf stomata, which is dictated by the meteorological conditions and soil moisture [6
Quantification of the evapotranspiration rate in forest area is essential in hydrological and ecological studies. The Penman–Monteith equation is the most widely used evapotranspiration model in hydrological studies [8
]. Hydrologists often use an analytic expression of latent heat fluxes λE
is the latent heat for vaporization, in MJ kg−1
) to consider the evapotranspiration system as a single layer (single source) [10
]. Many previous studies showed that evapotranspiration calculated with the Penman–Monteith equation can match measurements with a satisfying accuracy in temperate humid areas with a dense vegetation cover [11
]. However, the influence of multiple environmental factors on physiological characteristics varies among vegetation species and the characteristics, therefore, need to be parameterised correctly with regard to a study site.
In general, energy transfer in a soil–vegetation–atmosphere system can be quantified with the electrical analogy method, in which the transpiration rate is dictated with a critical parameter named stomatal conductance (gc
, in m s−1
) or stomatal resistance (rc
, in s m−1
]. In a forest area, gc
for a certain tree species may be quantitatively determined by a traditional method using an inverse calculation of the simplified version of the Penman–Monteith equation using the in situ measured transpiration rate [15
]. In the last few decades, sap flow measurement technology has become the most common method to determine transpiration [16
]. The measured sap flow rate of individual trees can upscale to an experimental area to provide species-specific transpiration rates, which can be used to inversely estimate the response of gc
to multiple environmental stresses during a continuous time span. Kucera et al. [17
] used a novel approach where a direct parameterization of the Penman–Monteith equation was developed to compute the diurnal courses of stand canopy conductance from sap flow. Previous studies also showed that the inversely estimated gc
commonly shows complex patterns that are intimately related to meteorological variables (i.e., solar radiation, wind speed, the concentration of carbon dioxide in air, air humidity, and temperature), and soil moisture stress [12
]. Moreover, the distinct canopy characteristics (e.g., leaf area and leaf morphology) [21
] and stand characteristics (e.g., stand age and structure) [23
] also affect transpiration.
This study focused on the analysis of leaf stomatal behaviour based on the sap flow experimental data from an ecological changing area under natural disturbance. The study area is located in the upper Vydra basin (Czech Republic). Due to a bark beetle outbreak in the area, the spruce trees (Picea abies)
have dried up and trunks have fallen down, and new beech stands (Fagus sylvatica)
have been developed from the formal mixed forest stands mainly consisting of spruce and beech trees. Research studies, which have been focused on the bark beetle outbreak at the Šumava Mts., have mainly studied its impact on water regime [25
], water chemistry [28
], soil moisture or temperature [29
], or forest grow after the disturbance [31
]. In general, several studies also show changes in the water regime of mixed (spruce/beech) forest [32
] or comparisons between beech and spruce stands [35
]. After the forest disturbance, the study area experienced no change or trend of long-term water balance [26
], and stream geochemistry changed with long-lasting effects [27
]. However, there were detected shifts in the runoff generation processes, mainly in the root zone [27
]. For a better understanding of the mechanisms, detailed information on the evapotranspiration process in the area that is undergoing such an intense transition in the vegetation structure is needed.
This research study was thus aimed to assess the evapotranspiration process in the newly formed beech stands in the area affected by bark beetle outbreak. The key research questions were: (i) how to intensively quantify the transpiration rates for a newly formed beech stand in this locality, and (ii) why it is important to evaluate the stomatal behaviour when measuring sap flow.
As far as we know from the literature review, there is no similar study of beech stand transpiration in a location working with a newly formed beech stand as a principal factor of transpiration. A field experiment was set up aimed at the following objectives:
Quantifying stomatal conductance, gc, of the newly formed beech forest from a vegetative period to a deciduous period;
Determining the patterns of the diurnal variation of stomatal conductance for different vegetation periods;
Evaluating the impact of environmental factors on stomatal conductance.
A direct comparison with similar measurements at spruce stands could not be achieved due to the bark beetle outbreak. Therefore, in this study, we conducted a sap flow experiment in a plot covered by beech forest, varying in ages over the middle of summer and the beginning of autumn (day of year DOY 203–302, i.e., 23 July–30 October) in the year of 2015. In situ measurements of sap flow and meteorological forcing variables were used to inversely estimate stomatal conductance, gc.
Stomatal behaviour varies among tree species due to different parametrizations of gc
]. Many physiological models for describing the response of stomatal conductance, gc
, to environmental stress have been proposed, such as the Jarvis–Stewart model [43
], and the Ball–Berry model [49
]. For general vegetation types (e.g., broadleaf forests, needle leaf forests, shrub lands, croplands, grasslands, etc.), the parametrizations of gc
were available in a lookup table [5
]. These standard parametrizations have been used to estimate the moisture flux from land surface at a catchment scale or even at a global scale [51
]. However, stomatal behaviour commonly shows distinct properties even within a certain tree species [54
]. For a certain study site, general parametrization of one type of vegetation species may not be fully represented [42
]. In particular, for an area that has experienced severe changes in the distribution of tree species under climate change and insect-induced forest disturbance, it is necessary to inversely estimate gc
from sap flow measurement for special tree species.
This study explored the response of leaf stomata to multiple environmental factors of solar radiation, vapour pressure deficit, air temperature and soil moisture, and the key finding from this study was that the measured stomatal behaviour (see in Figure 7
) showed a large discrepancy compared with the typical parameter set of the deciduous forest which was often used. We found that the leaf stomatal conductance inferred from sap flow data did not show clear responses to solar radiation and soil moisture, and even show a less clear response to the air temperature. Stomatal conductance calculated from sap flow show lower values than it can be estimated from Zhou´s parameters [42
]. A lower stomatal conductance at midday can be explained by a limitation of photosynthesis due to the stomatal closure to prevent the water loss from intensive solar radiation and high temperatures. In general, energy supply theoretically limits evaporation during low radiation periods (when Rs <200 W m−2
), in contrast with high radiation periods (when Rs >200 W m−2
). However, the principle in forest areas and high latitude regions may be not the same. Köstner [56
] also found that on the daily basis, stomatal conductance and Rs for beech forest in Germany showed a near linear relation and available energy was not a limiting factor for transpiration considering only 40~75% of net radiation was used for beech transpiration.
The results of stomatal response showed that the soil moisture did not constrain transpiration considering its value was relatively high during the study period. Williams, et al. [57
] also found soil moisture was not a frequent stress factor in many forest stands including beech due to relatively abundant rainfall, which was consistent with the previous studies conducted in Central Europe that [56
]. The soil moisture content and sap flow of trees are less comparable in beech stand than in spruce stands. The reason for this could be the high rooting depth of beech tree stand. Spruces create shallow rooting zones (<50 cm). Therefore, they suck water from upper soil layers and can be comparable with the evaporation process from soil surface. On the other hand, beech stands receive non-negligible quantity of water from deeper horizons (from regolith) and therefore they dry less at soil surface or upper soil horizons. This could be a reason for why gc
does not perfectly fit to soil moisture. Notwithstanding that our measurements were determined during a period with less rainfall, the root system of a beech can reach a depth of several meters [60
], sucking water from lower layers. This study was focused in a soil profile up to the depth of 1 m which is drained generally by fine shallow root systems [60
]. On the other hand, we assumed lower evaporation from soil surface could be shaded by trees, covered with fallen leaves and dead wood. In addition, our data did not show any shifts in soil tension described by Or, et al. [62
From the previous study concerning subsurface flow mechanisms in this study site [37
], the dominant subsurface flow is biomat flow (i.e., shallow subsurface flow) and deep percolation. Biomat flow is mostly caused by stormflow events [63
], and deep percolation is connected with slow infiltration and long-term water storage (e.g., from snow melt). It is evident that each tree species (spruce x beech) is connected with different sources of water, and respectively different flow mechanisms in soil. If spruce stands were replaced by beech stands, it would have an impact on water storage in soil or regolith. It is possible to consider that beech stands together with decreasing snow cover can contribute to a change in runoff formation, respectively, to emptying deep water aquifers.
The response of gc
to vapour pressure deficit (VPD) demonstrated clear patterns. The increasing VPD shows similar trend with other studies [42
] that have found an exponential response. However, a linear relationship was also often adopted in many prior studies to describe the constrain of VPD on gc
]. The response of gc
to air temperature may be described with a quadratic function or a bell-shape function, and our study showed a slight scattered relation. Kučera, et al. [17
] mentioned that the estimated time lags between the sap flow and climate variables were 60 min for Rg and 30 min for D, and such hysteresis loops that we did not consider. This issue will influence the accuracy of the simulated timing.
This study focused on estimating the stomatal conductance using the measured sap flow at a newly formed beech stand, Šumava Mts. (the Czech Republic). Due to a bark beetle outbreak in the area, mixed forest stands (spruce and beech) have transformed into beech stands. From the differences of the rooting depth of each kind of tree, an impact on long-term water regime is expected. Trees can change soil moisture distribution or water storage in aquifers by transpiration. Therefore, our study was focused on the stomatal conductance of newly formed beech stands. The measured sap flow data were used to inversely estimate the stomatal conductance through the Penman–Monteith equation. The stomatal conductance reached the highest value at midday but, on some days, there was a sudden drop at midday. A drop of stomatal conductance at midday can be explained by a limitation of photosynthesis due to the stomatal closure to prevent the water loss from the most intensive solar radiation and higher temperature. We also found that the calculated stomatal conductance decreased dramatically in the deciduous period, as the estimation based on the Penman–Monteith equation did not account for the vegetation transition from the vegetative period to the deciduous period.
The parameterization of the Jarvis–Stewart model was used to describe the response of stomatal conductance under the varying environmental conditions of net radiation, vapour pressure deficit, temperature, and soil water content. The stomatal conductance showed a good relationship (connection) with vapour deficit but low correlation with soil moisture. The temperature showed a certain relation but not one as strong as the vapour deficit, which might be due to the smaller range during our study period that went without experiencing a wide spectrum of temperature. The soil moisture did not constrain transpiration considering its value is relatively high during the study period. Therefore, in the study area, vapour deficit and temperature are two key factors impacting the transpiration processes. The most important finding is that the parametrization of stress functions based on the typical deciduous forest does not perfectly represent the measured stomatal response of newly formed beech stands. Therefore, the sap flow results can provide valuable data to better understanding the evapotranspiration process in newly formed beech stands after the bark beetle outbreak in Central Europe.