A facts based ranking of forest tree species' sensitivity to drought.
joint MATCHTREE-NCCR-climate (EcoWAT) project
A tree crown has in large only two ways to dissipate the energy it receives from the sun: heat convection or transpiration. If transpiration is reduced by limited water availability or by the limited ability of a certain species to reach moisture at great soil depth, the canopy must warm relative to trees that maintain transpirational flux. This will inevitably find expression in canopy surface temperature, and surface temperature becomes a proxy of tree water status. However, for surface temperatures to become conclusive, one needs to know soil moisture and sap flux. There is a possibility that tree crowns warm, simply because of vpd-related stomatal closure, without reflecting soil moisture (Körner 1985, Leuzinger and Körner 2007b). This is a well known physiological response among many trees to prevent cavitation at otherwise sufficient soil moisture. Hence, it is vital to obtain the relevant ground data to interprete canopy temperatures correctly. This is project addresses a rather timely issue of forest research as can be seen from two recent attempt s at exploring tree rersponses to drought (McMahon et al. 2009, Adams et al. 2009). Both papers are examples for the very limited tools applied to this question so far. The modelling approach (McMahon et al.) had hardly any facts (parameterization) to start from, and the experimental approach (Adams et al.) used data from small containerized trees which rapidly exhausted substrate moisture. Here we offer data obtained at the approriate scale by combining ground data for four selected site with six species with helicopter based remote sensing data.
Results show that canopy architecture has a consistent influence on canopy foliage temperature with 'dense canopy' species (A. pseudoplatanus, F. sylvatica and T. platyphyllos) being warmer (0.5-1.5 K) than 'open canopy' species (F. excelsior, P. avium and Q. petraea). While the canopy foliage was close to air temperature at the beginning of the drought period ( -0.1 to 0.7 K) it strongly increased with ongoing drought, especially at the two 'dry' sites with canopy warming of 3.5 to 5 K. The pronounced canopy foliage warming at the 'dry' sites was likely the result of a reduced transpiration rate as the sap flow was lowered by 20 to 35% in all species except F. excelsior and Q. petraea after 22 days of drought. Based on canopy foliage temperature and sap flow data we considered A. pseudoplatanus the most drought sensitive species followed by F. sylvatica, T. platyphyllos and P. avium and the two ring-porous species F. excelsior and Q. petraea being clearly the less sensitive ones. Our results are consistent with findings of other studies and proof that airborne thermal imagery is a trustworthy tool indentifying forest patches and species first/ most affected by prolonged drought.
Daniel Scherrer, Martin Bader, Christian Körner
Roman Zweifel (WSL), TREELIM
Regional forest services (Basel-Landschaft
Population dynamics at the high elevation limit of deciduous species in the Alps
joint MATCHTREE-TREELIM/ERC project
The causes of species-specific high elevation tree limits in common decidious species are not well understood. Once the uppermost position of each species has been identified, the question arises whether this range limit reflects past expansion events or active ongoing recruitment or even upslope migration. The aim of this project is to characterize population structure near the elevational cold limit of major broad-leaved tree species in Europe based on demographic data. These data will permit us to rank species by the extent their seedlings "explore" higher elevation terrain with possible links to recent climatic warming.
When the uppermost adult trees are taken as a reference, currently available data indicate that all species were represented by juvenile individuals in the vicinity of adults at their limit. However there was a pronounced species ranking. Tree recruitment of both seedlings and saplings was detected beyond adult limits in most of the species, up to 200 m above the current adult limit, which suggests that currently neither seed dispersal nor seedlings establishment constitute a serious limitation to extend the tree species' limits. Species clearly differed in their elevational limit, and regional differences (western vs eastern Switzerland) could be exhaustively explained by regional differences in growing season temperature.
This study clearly shows that the elevational limits of broad-leaved species in Swiss Alps were driven by temperature and that there are no recruitment limitations at the current adult tree limit under current temperatures. From the extent juvenile individuals have been found at elevations higher than current adult individuals, upward migration of the adult limit seems likely.
Yann Vitasse and the TREELIM team
, Jens Paulsen, Christian Körner
Niklaus Zimmermann (WSL)