Abstract
Given the importance of forest ecosystems, the availability of reliable, spatially explicit information about the site-specific climate sensitivity of tree species is essential for implementing suitable adaptation strategies. In this study, airborne hyperspectral data were used to assess the response of deciduous species (dominated by European beech and Sessile and Pedunculate oak) to water stress during a summery dry spell. After masking canopy gaps, shaded crown areas and non-deciduous species, potentially indicative spectral indices, the Photochemical Reflectance Index (PRI), Moisture Stress Index (MSI), Normalized Difference Water Index (NDWI), and Chlorophyll Index (CI), were analyzed with respect to available maps of site-specific soil moisture regimes. PRI provided an important indication of site-specific photosynthetic stress on leaf level in relation to limitations in soil water availability. The CI, MSI and NDWI revealed statistically significant differences in total chlorophyll and water concentration at the canopy level. However, after reducing the canopy effects by normalizing these indices with respect to the structure-sensitive simple ratio (SR) vegetation index, it was not yet possible to identify site-specific concentration differences in leaf level at this early stage of the drought. The selected indicators were also tested with simulated EnMAP and Sentinel-2 data (derived from the original airborne data set). While PRI proved to be useful also in the spatial resolution of EnMAP (GSD = 30 m), this was not the case with Sentinel-2, owing to the lack of adequate spectral bands; the remaining indicators (MSI, CI, SR) were also successfully produced with Sentinel-2 data at superior spatial resolution (GSD = 10 m). The study confirms the importance of using earth observation systems for supplementing traditional ecological site classification maps, particularly during dry spells and heat waves when ecological gradients are increasingly reflected in the spectral response at the tree crown level. It also underlined the importance of using Sentinel-2 and EnMAP in synergy, as soon as both systems become available.
Highlights
Climatic changes seemed to have a generally positive impact on forest productivity when water was not limiting [1]
The directional area scattering factor” (DASF) and simple ratio (SR) are highly correlated and both exhibit a significant relationship with LAI; the latter is true for the Reduced Simple Ratio (RSR) but the correlation with
It was demonstrated that hyperspectral imaging can provide indication on drought- and heat-induced stress levels, which are consistent with the existing site condition maps, both with respect to the long-term impact on water and chlorophyll concentration at the canopy level and the immediate response of already stressed trees to water limitation and excess heat and light conditions
Summary
Climatic changes seemed to have a generally positive impact on forest productivity when water was not limiting [1]. Examples of recent die-offs have been reported from southern and central parts of Europe (e.g., [5]), and it is expected that forests will become increasingly affected by drought, either during exceptional events or under a long-term drift towards more arid conditions, such as which already prevails in Mediterranean areas. Extreme climatic events, such as the heat wave and drought episode experienced during summer 2003, are expected to occur at increased frequencies [6,7]. These combined effects bear the risk of a reduction of important ecosystem services, such as carbon sequestration, water retention, soil protection, biodiversity and, last but not least, economic value [8,10,11,12]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
