Abstract
The idea that herbaceous plants have higher relative growth rates (RGRs) compared with woody plants is fundamental to many of the most influential theories in plant ecology. This difference in growth rate is thought to reflect systematic variation in physiology, allocation and leaf construction. Previous studies documenting this effect have, however, ignored differences in seed mass. As woody species often have larger seeds and RGR is negatively correlated with seed mass, it is entirely possible the lower RGRs observed in woody species is a consequence of having larger seeds rather than different growth strategies. Using a synthesis of the published literature, we explored the relationship between RGR and growth form, accounting for the effects of seed mass and study-specific effects (e.g. duration of study and pot volume), using a mixed-effects model. The model showed that herbaceous species do indeed have higher RGRs than woody species, and that the difference was independent of seed mass, thus at all seed masses, herbaceous species on average grow faster than woody ones.
Highlights
Relative growth rate (RGR) varies widely between species [1,2]
Both theories predict a link between RGR and growth form, with woody species having lower RGRs than herbaceous species
A meta-analysis of six studies [12] demonstrated the generality of the negative relationship between RGR and seed mass, suggesting that it is robust to differences in experimental protocol and the pool of species used
Summary
Relative growth rate (RGR) varies widely between species [1,2]. In a large screening experiment, Grime & Hunt [3] grew 130 species in standardized, ‘optimal’ conditions (no competition, plentiful light and nutrients), and found that even in this environment there was a sixfold variation in maximum RGR between species. A meta-analysis of six studies [12] demonstrated the generality of the negative relationship between RGR and seed mass, suggesting that it is robust to differences in experimental protocol and the pool of species used. As a result of this, it is possible that the relationship between RGR and growth form is a consequence of variation in seed mass, rather than the differences in growth strategy. We were unable to use this approach with published estimates of RGR, and so compare growth forms at a common seed mass. A linear mixed-effects model for variation in RGR was developed in R [21] using the lme package [22] These models allow the dependence on covariates and unmeasured study-specific factors to be explored. Markov chain Monte Carlo methods (based on the mcmcsamp function in lme and pvals.fnc in languageR)
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