Leaf:wood allometry and functional traits together explain substantial growth rate variation in rainforest trees.

D S Falster,M G Bradford,A S D Eller,C E R Lehmann,L A Cernusak,I J Wright,E F Gray

doi:10.1093/aobpla/plz024

D S Falster, M G Bradford + Show 5 more

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https://doi.org/10.1093/aobpla/plz024

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Abstract

Plant growth rates drive ecosystem productivity and are a central element of plant ecological strategies. For seedlings grown under controlled conditions, a large literature has firmly identified the functional traits that drive interspecific variation in growth rate. For adult plants, the corresponding knowledge is surprisingly poorly understood. Until recently it was widely assumed that the key trait drivers would be the same (e.g. specific leaf area, or SLA), but an increasing number of papers has demonstrated this not to be the case, or not generally so. New theory has provided a prospective basis for understanding these discrepancies. Here we quantified relationships between stem diameter growth rates and functional traits of adult woody plants for 41 species in an Australian tropical rainforest. From various cost-benefit considerations, core predictions included that: (i) photosynthetic rate would be positively related to growth rate; (ii) SLA would be unrelated to growth rate (unlike in seedlings where it is positively related to growth); (iii) wood density would be negatively related to growth rate; and (iv) leaf mass:sapwood mass ratio (LM:SM) in branches (analogous to a benefit:cost ratio) would be positively related to growth rate. All our predictions found support, particularly those for LM:SM and wood density; photosynthetic rate was more weakly related to stem diameter growth rates. Specific leaf area was convincingly correlated to growth rate, in fact negatively. Together, SLA, wood density and LM:SM accounted for 52 % of variation in growth rate among these 41 species, with each trait contributing roughly similar explanatory power. That low SLA species can achieve faster growth rates than high SLA species was an unexpected result but, as it turns out, not without precedent, and easily understood via cost-benefit theory that considers whole-plant allocation to different tissue types. Branch-scale leaf:sapwood ratio holds promise as an easily measurable variable that may help to understand growth rate variation. Using cost-benefit approaches teamed with combinations of leaf, wood and allometric variables may provide a path towards a more complete understanding of growth rates under field conditions.

Highlights

Terrestrial primary productivity is a key factor controlling rates of land-atmosphere CO2 exchange (Beer et al 2010)
From various cost-benefit considerations, core predictions included that: (i) photosynthetic rate would be positively related to growth rate; (ii) specific leaf area (SLA) would be unrelated to growth rate; (iii) wood density would be negatively related to growth rate; and (iv) leaf mass:sapwood mass ratio (LM:SM) in branches would be positively related to growth rate
We investigate three hypotheses related to leaf tissue traits. (i) Regardless of plant size, higher light-saturated photosynthetic rate (Aarea) should drive faster growth rates, because faster photosynthesis increases the rate of biomass production (Gibert et al 2016). (ii) We hypothesized that higher leaf N and leaf P concentrations would be associated with faster growth rates

Summary

Introduction

Terrestrial primary productivity is a key factor controlling rates of land-atmosphere CO2 exchange (Beer et al 2010). Tropical forests account for 34 % of global terrestrial primary productivity, a disproportionate percentage considering they cover 7–10 % of the global land surface (Lewis et al 2009; Beer et al 2010). Two spectra of variation in leaf and wood functional traits organize species along a continuum of low to high tissue construction costs (Wright et al 2004; Chave et al 2009). The benefit of low tissue construction costs (low wood density) trades off against high whole-plant mortality rates (Chave et al 2009; Wright et al 2010). In general there is an expectation that low tissue construction costs should promote fast growth rates (Grime and Hunt 1975; Poorter and Remkes 1990; Garnier 1992; Lambers and Poorter 1992; Wright and Westoby 2001)

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