Hydraulic constraints in the functional scaling of trees

I conducted a literature survey to assess the available information on relationships between size-expressed in terms of diameter and dry biomass-and hydraulic efficiency of woody structures at different scales, from stem segments to whole trees. Three data sets were constructed: the first described the relationship between segment diameter and hydraulic conductivity (k(h); kg in s(-1) MPa-1) in four species; the second, for the same four species, described the intraspecific trajectories of change in total hydraulic conductance (G; kg s(-1) MPa-1) during ontogeny, i.e., from saplings to mature trees, thereby providing a comparison between allometric scaling laws at the scales of segments and whole trees; the third comprised pooled means for nine species that described the interspecific trajectory of change in G with tree size. The scaling coefficients obtained were compared with predictions made with an architectural fractal-like model incorporating tissue-specific hydraulic architecture parameters (West et at. 1999). When data on segment kh were examined, the fractal-like model closely predicted the scaling of kh with segment diameter in four species. However, the model failed to predict accurately in all species the intraspecific scaling at the branch and whole-tree levels, and consistently overestimated the scaling coefficients. The results suggest that ontogenetic changes in tree size during the life cycle of one tree may result in tradeoffs between optimal hydraulic supply to the existing leaf area and maintenance costs of the supporting xylem tissue. The model of West et al. (1999) may be useful for understanding broad interspecific patterns, but not for understanding more subtle ontogenetic changes.