Shoot growth of woody trees and shrubs is predicted by maximum plant height and associated traits

Sean M. Gleason, Andrea E. A. Stephens, Wade C. Tozer, Chris J. Blackman, Don W. Butler, Yvonne Chang, Alicia M. Cook, Julia Cooke, Claire A. Laws, Julieta A. Rosell, Stephanie A. Stuart, Mark Westoby

Plant species differ in the rate of elongation and thickening of individual branches (shoots).  This variation is important because it influences the outcome of competition between plants.  Although growth can be measured relatively easily, it is poorly understood how plant characteristics (traits), such as leaf size, water transport and height influence growth rate, as well as how the influence of these traits on growth might change from one environment to another.  For example, although leaves are important for capturing sunlight and converting atmospheric carbon dioxide into plant matter, it is less well understood if larger leaves result in faster growth than smaller leaves, or if the influence of leaf size might be more important in some environments (e.g. tropical forest) than in others.

In this study, we compared rates of shoot growth across 44 species from tropical, warm temperate, and cool temperate forests of eastern Australia.  We found that the growth of shoots was aligned with many traits, including the potential height of the species, the capacity of wood to transport water, the size of leaves, and wood density.  Among these traits, maximum plant height was the most important predictor of growth rate (larger species grew faster), and was itself aligned with several of the other measured traits.

Statistical models of growth revealed that the traits we measured could be clearly separated into three different groups – traits relating to the transport of water, traits relating to the functioning and efficiency of leaves, and traits relating to the strength and flexibility of wood.  Importantly, these three categories of traits each influenced growth, but were also strongly aligned with the maximum height of the species.  This result suggests that the size of plants and the traits that enable plants to grow quickly have been closely aligned by natural selection.  This alignment was evident across species occurring within individual forests, as well as across forests differing in precipitation and temperature.  Trait differences between species growing together in same forest were likely associated with differences in access to sunlight and water, whereas trait differences across forests were likely engendered by differences in climate.

 

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