Wood density is a poor predictor of competitive ability among individuals of the same species

Competition between immediate neighbors has been regarded as one of the most important forces of natural selection that shapes the species composition, diversity and evolution of plant traits within a community. Here, I examined wood density (WD) variation in populations of two tree species, where individuals have contrasting access to resources. In five Nothofagus betuloides and Nothofagus pumilio even-aged, mixed second-growth forests in Patagonia (Chile), I sampled pairs of dominant (full access to resources) and the nearest-to-it suppressed individuals (constrained access to resources), with the following main objective: to assess whether WD is a good predictor of the competitive ability of individuals in tree populations. To accomplish this objective, I quantified, at the individual level, sapwood (WDsap) and heartwood densities (WDheart), sapwood proportion (Sapp), growth rates (BAI10) and growth efficiency (GE). Given that sapwood and heartwood differ in functionality, I used WDst, a standardized way of computing WD that considers both wood type proportions in the stem. In order to determine the relationship between growth rate (BAI10, a proxy of competitive ability) and WD at the intraspecific level, I analyzed the effect of contrasting access to resources (dominant versus suppressed individuals) and WD fitting mixed-effects correlation models (LMM). WDst showed very low variation (<6% of CV), and WDst did not differ between dominant and suppressed individuals for both species, nor did Sapp. Dominant individuals of both species had significantly higher BAI10 and GE values than suppressed individuals. I found a non-significant correlation between BAI10 and WDst. Independently of tree species and access to resources, WDheart was significantly higher than WDsap. In conclusion, my results are unequivocal in that WD is a poor predictor of competitive ability among individuals of the same species and thus interspecific trends found in the growth-WD relationship are not paralleled at the intraspecific level.