Leaf:wood allometry and functional traits explain variation in growth rate of rainforest trees

Plant growth rates drive ecosystem productivity and are a central element of a species’ ecological strategy. For seedlings grown under controlled conditions, a large body of scientific literature has identified the functional traits responsible for driving interspecific variation in growth rate. However, the chief drivers of adult growth rates under field conditions are surprisingly little understood. Until recently it was widely assumed that the key trait drivers would be the same (e.g. specific leaf area), but an increasing number of papers has demonstrated that often this not the case.

In a recent Editor’s Choice article published in AoBP, Gray et al. quantified relationships between stem diameter growth rates and functional traits of adult woody plants for 41 species in an Australian tropical rainforest. The authors demonstrate that substantial variation in growth rate of tropical trees results from differences in relative mass allocation to leaves versus wood, and in a small number of tissue-specific properties. Specifically, there was a convincing negative relationship between SLA and stem diameter growth rate, a result which is well explained by theory, despite being opposite to that generally observed in seedlings. Leaf:sapwood mass ratios measured simply at the branch level also explained substantial variation in growth rates, suggesting that this easy-to-measure property should be included in future studies alongside traits such as SLA and photosynthetic rate. These results support new theory that strives to understand this topic from a cost-benefit perspective, potentially providing a way forward in a field that has largely reached an impasse.