Wood density relates negatively to maximum plant height across major angiosperm and gymnosperm orders

Wood density is a crucial plant functional trait related to plant life history strategies. Its ecological importance in small-stature growth forms (e.g., shrubs) has not been extensively examined. Given that hydraulic conduit dimensions vary positively with plant height and that there is a negative relationship between conduits' diameter and wood density, I hypothesized an also negative relationship between wood density and plant height. Knowing that bark and pith proportions are significant in small-diameter stems, I additionally disentangled the contribution of wood, bark, and pith to stem density.

I determined density in small-diameter stems across 153 species spanning all major angiosperm and gymnosperm orders by considering a diversity of growth forms (trees, treelets, shrubs, vines, and hemiparasites). Stem cross sections were dissected to consider the densities of wood with bark and pith; wood with pith and without bark; wood with bark and no pith; and wood without bark and pith. Secondary growth was also measured.

Trees showed similar wood densities as non-self-supporting vines, and both showed significantly less dense wood than treelets, shrubs, and hemiparasites. General comparisons showed that wood was significantly denser than all other tissues, and these differences did not depend on growth form. Wood density was significantly and negatively related to growth rate and pith area proportions but not to bark thickness proportion.

An implicit negative relationship between maximum plant height and stem density emerges as a property of plants likely linked to hydraulic conductive size.