Drought in tropical forests: the role of tree height and wood density for hydraulic efficiency, productivity and vulnerability to cavitation of trees along a lowland precipitation gradient (tropical moist to seasonally-dry)
Final Report Abstract
In the recent past, drought threatens tropical rainforests but the mechanisms leading to drought-induced tree mortality remain poorly understood. However, it appears that large and tall tropical trees are carrying a disproportionally higher risk for drought-induced tree mortality than smaller and understory trees, both in seasonally-dry and in perhumid tropical environments. Furthermore, tropical trees with light wood seem also to be at greater risk of drought-induced die-back, but the related findings are less consistent. Because both tree height and wood density are thought to be key functional traits influencing growth and survival, we selected a species sample covering steep gradients in both parameters at each of five sites across a precipitation gradient in Costa Rica. Major objectives were (i) to analyse the relation between hydraulic efficiency and productivity in dependence on tree height and wood density, (ii) to examine the influence of tree height, wood density and wood anatomical and hydraulic traits on branch embolism resistance, and (iii) to investigate the effect of tree size and dry season length on concentrations of non-structural carbohydrates in woody tissues. We could confirm that a high xylem hydraulic efficiency is necessary for high productivity in the tropical forest tree species of our sample, both at the stem and branch level. In support hereon, we observed a close relationship between tree height and daily water use. Consequently, tree height proved to be a good predictor for aboveground biomass increment. In contrast, wood density was not related to any of the wood anatomical traits, even not in trees growing in climates with a distinct dry season. Hence, light-wooded species did not show high rates of daily water use, and wood density was related to productivity at the two marginal sites of the gradient only. We assumed that tall tree species would show large xylem vessel, both at the stem base and in distal twigs, causing a high vulnerability to drought-induced hydraulic failure. Although the tallest trees indeed showed the largest vessel in both stem and branch wood, embolism resistance was unrelated to vessel diameter at the branch level. Instead of being most vulnerable to drought-induced hydraulic failure as speculated, tall tree species were most resistant. Although this contradicts recent findings by other groups, we meanwhile have sufficient evidence confirming that vessel diameter is not directly related to embolisms resistance. We therefore plan to investigate the thickness of branch xylem pit membranes, which should be thickest in tall trees. Because we observed a close negative relationship between embolism resistance and wood density in support of the existing literature, a connection with pit properties is likely. However, we originally speculated that stem xylem anatomical and derived hydraulic traits are suited as reliable indicators of branch embolism resistance. Ideally, these easily accessible, storable and measureable parameters could be used in combination with tree size to model a trees’ drought susceptibility. Indeed, although branch xylem properties were unrelated, vessel diameter and potential conductivity of the stem wood were positively related to branch embolism resistance, i.e. tree species with large vessels at the stem base were most vulnerable to drought-induced hydraulic failure at the canopy level. However, we assume that not vessel diameter at the stem base per se is the dominant driver of this observed relationship, but rather tree height and thus flow path length. Hence, it remains speculative whether hydraulic failure is the dominant cause for higher mortality rates in tall tropical trees. Future studies need to monitor minimum leaf water potential in order to estimate the hydraulic safety margin, but often this is not feasible at remote field sites with limited access to the canopy. We further assumed that tall trees require larger carbohydrate stores. Although the starch content of the stem wood was significantly related to tree height at two of the five sites, no climatic relationship existed. However, we observed a close negative relationship between the xylem pressure at 50% loss of hydraulic conductance and the stem wood starch content, a promising finding which deserves further attention, In conclusion, tall productive tree species possessing a highly efficient xylem along the flow path were most resistant against drought-induced hydraulic failure in contrast to general belief. Although wood density was unrelated to wood anatomical and hydraulic properties, a close relationship with xylem safety was observed. Future work should therefore focus on withinspecies variation in plant hydraulic properties in order to confirm or reject whether tall tropical trees are most vulnerable as recently claimed, and identify why wood density proved to be a good proxy.
Publications
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Size-bias in estimating vessel length distributions. Journal of Theoretical Biology 455: 329-341
Link R, Schuldt B, Choat B, Jansen S, Cobb A
