project B4

Complementarity effects between 5 broad-leaved tree species on water consumption and drought resistance in temperate deciduous forests

The aim of my research is to identify effects of tree species identity & tree species diversity on canopy transpiration and drought tolerance in temperate deciduous forests. The species of interest are beech (Fagus sylvatica), ash (Fraxinus excelsior), lime (Tilia cordata), maple (Acer pseudoplatanus) and hornbeam (Carpinus betulus)

We assume that:
• Water use and crown transpiration varies between tree species with different functional traits.
• Mixed forest stands are able to exploit available soil water resources more efficiently than pure forest stands due to niche differentiation. In general this would cause higher canopy transpiration and higher seasonal variance in soil water content.
• Tree species with different functional traits differ in sensitivity to drought.
• Due to complementarity, mixed forest stands might be able to tolerate lower levels of soil water content than pure forest stands and could be therefore less sensitive to drought periods.

Within a pot experiment the effects of species identity and species diversity are investigated using small groups of juvenile trees. Different species combinations are compared within 3 levels of species diversity (1, 3 or 5 species) and the treatments are further divided into 2 levels of available soil water (wet, dry). The decrease in soil water content due to water consumption and evapotranspiration can be estimated gravimetrically. Furthermore, ecohydrological traits of tree species and treatments are characterized by morphological and physiological measurements.
In addition to the above experimental approach hydrophysiological traits of the considered trees species will be sampled in the forests of the Hainich NP. These data will provide an insight into how appropriate the results of the pot experiment for juvenile trees are for old grown forest ecosystems.
Furthermore, the pattern of soil water extraction in mixed forests due to tree species identity and species diversity will be investigated on a small scale by measurments of volumetric soil water content (TDR) and/or soil water potential (tensiometer). For this, a trenching-procedure will be tested.

Methods:
• Gravimetric estimation of soil water content, evapotranspiration and crown transpiration within a pot experiment.
• Photosynthesis (LiCor6400)
• Leaf analysis: C/N, δ13C, δ 18O
• Stomatal conductivity of leaves (porometry, LiCor1600)
• Hydraulic conductivity of xylem (Sperry, 1988)
• Leaf water potential (Scholander et al 1965)
• Pressure-volume curves (Scholander et al 1965)
• Vulnerability curves (cavitation resistance, Cochard, 2005)
• Mobile canopy lifter (max. 30m)
• Volumetric soil water content (TDR)
• Soil water potential (tensiometer)