Highly variable bark-wood density relationships across tree species reflect tradeoffs in evolved tolerances to environmental stressors

Key message

Tree bark and wood density are highly variable and weakly positively correlated, with species having more or less dense bark than wood to adapt to different environmental stressors.

Tree bark is a complex, multifunctional structure and bark density varies widely across species. While wood density is recognized as a fundamental indicator of the functional ecology of trees, bark density has received much less attention as a key functional trait. Theoretically, bark and wood density should co-vary to some degree, but comprehensive examinations of this covariation are scarce. How do key functional traits of individual trees and species relate to bark and wood density variation/covariation? How does a tree’s life history and evolved tolerance to environmental stress shape variation/covariation in bark and wood density? This study draws from published literature and a large database of individual tree measurements of trees of diverse species and growing conditions, from forest ecosystems across the United States and Canada, to understand covariation between bark and wood density and its relationship to life-history traits and evolved tolerances to environmental stressors. The results of this study show a high tree-to-tree variation in both bark density and wood density, with inherited differences in tissue formation constraining the range of bark and wood densities. All analyses show that bark density was weakly, positively correlated with wood density. Mixed effects modeling showed a strong phylogenetic signal in variation in bark and wood density that was partially explained by the need for species to produce more or less -dense bark and wood to adapt to different environmental stressors (tolerance of drought, shade, frost, waterlogging and fire were all examined), with clearly different relationships for angiosperms versus gymnosperms.