ALBERT

Description: Using an infrared camera, we measured the leaf temperature across different canopy positions of a 23-m-tall deciduous forest tree ( Fagus sylvatica L.) including typical sun and shade leaves as well as intermediate leaf forms, which differed significantly in specific leaf area (SLA). We calculated a temperature index ( I G ) and a crop water stress index (CWSI) using the surface temperatures of wet and dry reference leaves. Additional indices were computed using air temperature plus 5 °C ( I G + 5, CWSI + 5) as dry references. The minimum temperature of the wet leaf and the maximum temperature of the dry leaf proved to be most suitable as reference values. We correlated the temperature indices with leaf area-related conductance to water vapor ( g L ) using porometry at the leaf level and using xylem sap flow at the branch level. At the leaf and at the branch level, I G and CWSI were equally well suited as proxies of g L , whereas the relationships of I G + 5 and CWSI + 5 with g L were only weak or even insignificant. At the leaf level, the correlations of I G and CWSI with g L were significant in all parts of the crown. The slopes of g L vs. I G and CWSI did not differ significantly among the crown parts; this indicates that they were not influenced by SLA or irradiance. At the branch level, close correlations ( r 〉 0.8) were found between temperature indices and g L across the crown. These results demonstrate that satisfactory relationships between temperature indices and g L can be established in tall trees even in those canopy parts that are exposed to relatively low levels of irradiance and exhibit relatively low values of g L .

Description: Tree functional traits and their link to patterns of growth and demography are central to informing trait-based analyses of forest communities, and mechanistic models of forest dynamics. However, few data are available on how functional traits in trees vary through ontogeny, particularly in tropical species; and less is known about how patterns of size-dependent changes in traits may differ across species of contrasting life-history strategies. Here we describe size-dependent variation in seven leaf functional traits and four wood chemical traits, in two Dominican rainforest tree species ( Dacryodes excelsa Vahl. and Miconia mirabilis (Aubl.) L.O. Williams), ranging from small saplings to the largest canopy trees. With one exception, all traits showed pronounced variation with tree size (diameter at breast height, DBH). Leaf mass per area (LMA), thickness and tissue density increased monotonically with DBH in both species. Leaf area, leaf nitrogen (N) and carbon (C) : nitrogen (N) ratios also varied significantly with DBH; however, these patterns were unimodal, with peak trait values preceding the DBH at reproductive onset in both species. Size-dependent changes in leaf structural traits (LMA and leaf thickness) were generally similar in both species, while traits associated with leaf-level investment in C gain (leaf area, leaf C : N ratio) showed contrasting ontogenetic trends between species. Wood starch concentration varied with DBH in both species, also showing unimodal patterns with peaks preceding size at reproductive onset. Wood C concentration increased linearly with DBH in both species, though significantly only in M . mirabilis . Size-dependent patterns in wood chemical traits were similar between both species. Our data demonstrate pronounced variation in functional traits through tree ontogeny, probably due to a combination of environmental factors and shifts in resource allocation. Such ontogenetic variation is comparable in magnitude with interspecific variation, and so should be accounted for in trait-based studies of forest dynamics, structure and function.