Wood density is a well-studied functional trait of plants that is known to be implicated in mechanical stability and drought tolerance and is correlated with growth rate and longevity. Wood density also varies along climatic gradients at different ecological scales. However, few studies have examined the relationship between wood density, allometry, and climate. We examined how climate and allometry related to variation in wood density for native California trees and shrubs. We compiled a large dataset for California woody species including wood density, allometric relationships (e.g. DBH to height ratio, canopy width to height ratio), growth form, climatic parameters, ecosystem type, and leaf traits. We tested the roles of climate and allometry in explaining variation in wood density using phylogenetically independent contrasts and linear mixed models.
Results/Conclusions
Plant allometry was often strongly related to wood density, though the effects varied across growth forms (trees versus shrubs). This result suggests that structural constraints on trees and shrubs differ substantially. Within both trees and shrubs, wood density was correlated with aridity and mean annual precipitation, with both growth forms showing denser wood in drier climates. Aridity and mean annual precipitation were more significant climatic parameters than temperature, which emphasizes the constraint of drought on California woody plants. The ratio of dry mass to wet mass in the leaves was correlated positively with wood density for both trees and shrubs, which further suggests that wood density is influenced by drought tolerance and that trees and shrubs have similar responses to drought. Our model suggests that that both structural demands and climatic variation play an important role in predicting wood density, allowing us to disentangle the confounding effects of the two that have complicated previous studies. A greater understanding of the structural versus hydraulic demands placed on wood density will facilitate the mechanistic incorporation of wood density into future vegetation models.