Hydraulic variability of three temperate broadleaf tree species along a water availability gradient in central Europe.

Plant hydraulic traits are key for understanding and predicting tree drought responses. Information about the degree of the traits' intra-specific variability may guide the selection of drought-resistant genotypes and is crucial for trait-based modelling approaches. For the three temperate minor broadleaf tree species Acer platanoides, Carpinus betulus and Tilia cordata, we measured xylem embolism resistance (P50 ), leaf turgor loss point (PTLP ), specific hydraulic conductivity (KS ), Huber values (HVs), and hydraulic safety margins in adult trees across a precipitation gradient. We further quantified trait variability on different organizational levels (inter-specific to within-canopy variation), and analysed its relationship to climatic and soil water availability. Although we observed a certain intra-specific trait variability (ITV) in safety-related traits (P50 , PTLP ) with higher within-tree and between-tree than between populations variability, the magnitude was small compared to inter-specific differences, which explained 78.4% and 58.3% of the variance in P50 and PTLP , respectively. In contrast, efficiency-related traits (KS , HV) showed a high ITV both within populations and within the crowns of single trees. Surprisingly, the observed ITV of all traits was neither driven by climatic nor soil water availability. In conclusion, the high degree of conservatism in safety-related traits highlights their potential for trait-based modelling approaches.

Soil moisture regime and palm height influence embolism resistance in oil palm.

With the prospect of climate change and more frequent El Niño-related dry spells, the drought tolerance of oil palm (Elaeis guineensis Jacq.), one of the most important tropical crop species, is of major concern. We studied the influence of soil water availability and palm height on the plasticity of xylem anatomy of oil palm fronds and their embolism resistance at well-drained and seasonally flooded riparian sites in lowland Sumatra, Indonesia. We found overall mean P12 and P50 values, i.e., the xylem pressures at 12% or 50% loss of hydraulic conductance, of -1.05 and - 1.86 MPa, respectively, indicating a rather vulnerable frond xylem of oil palm. This matches diurnal courses of stomatal conductance, which in combination with the observed low xylem safety evidence a sensitive water loss regulation. While the xylem anatomical traits vessel diameter (Dh), vessel density and potential hydraulic conductivity (Kp) were not different between the sites, palms in the moister riparian plots had on average by 0.4 MPa higher P50 values than plants in the well-drained plots. This could largely be attributed to differences in palm height between systems. As a consequence, palms of equal height had 1.3 MPa less negative P50 values in the moister riparian plots than in the well-drained plots. While palm height was positively related to P50, Dh and Kp decreased with height. The high plasticity in embolism resistance may be an element of the drought response strategy of oil palm, which, as a monocot, has a relatively deterministic hydraulic architecture. We conclude that oil palm fronds develop a vulnerable water transport system, which may expose the palms to increasing drought stress in a warmer and drier climate. However, the risk of hydraulic failure may be reduced by considerable plasticity in the hydraulic system and the environmental control of embolism resistance, and a presumably large stem capacitance.