Competitive interactions between forest trees are driven by species' trait hierarchy, not phylogenetic or functional similarity: implications for forest community assembly

Abstract

The relative importance of competition vs. environmental filtering in the assembly of communities is commonly inferred from their functional and phylogenetic structure, on the grounds that similar species compete most strongly for resources and are therefore less likely to coexist locally. This approach ignores the possibility that competitive effects can be determined by relative positions of species on a hierarchy of competitive ability. Using growth data, we estimated 275 interaction coefficients between tree species in the French mountains. We show that interaction strengths are mainly driven by trait hierarchy and not by functional or phylogenetic similarity. On the basis of this result, we thus propose that functional and phylogenetic convergence in local tree community might be due to competition-sorting species with different competitive abilities and not only environmental filtering as commonly assumed. We then show a functional and phylogenetic convergence of forest structure with increasing plot age, which supports this view.

Plant functional traits have globally consistent effects on competition

“…To include the effects of traits on the parameters of the growth model, we build on previous studies that explored the role of traits for tree performances and tree competition 8,9,11 . We modelled the effect of traits, one trait at a time.…”

“…Data on species functional traits were extracted from existing sources. We focused on wood density, species SLA and maximum height, because these traits have previously been related to competitive interactions and are available for large numbers of species 8,9,11,15,30 (see Extended Data Tables 3 and 4 for trait coverage and their correlations). Where available, we used data collected locally (references for the local trait data used in this analysis include refs 15, 51, 60-62); otherwise we sourced data from the TRY trait data base 18 (references for the data extracted from the TRY database used in this analysis include refs 2, 3, 15, 63-130).…”

“…3 for maximum height). Given that the long-term outcomes of competition at the population level may be more influenced by tolerance of competition than by the competitive effect 16 , SLA might be less influential in succession.Coordination between trait values conferring a strong competitive effect and trait values conferring a high tolerance of competition has been widely expected 9,16 , but rarely documented 16,28 . Only wood density showed such coordination, as it was correlated with both competitive effect and tolerance of competition in the same direction (Fig.…”

“…Coordination between trait values conferring a strong competitive effect and trait values conferring a high tolerance of competition has been widely expected 9,16 , but rarely documented 16,28 . Only wood density showed such coordination, as it was correlated with both competitive effect and tolerance of competition in the same direction (Fig.…”

“…A long-standing hypothesis is that the intensity of competition decreases as two species diverge in trait values 7 (trait dissimilarity). The few studies [8][9][10][11][12][13] that have explored links between traits and competition have shown that linkages were more complex than this, as particular trait values may also confer competitive advantage independently from trait dissimilarity 9,13,14 . This distinction is fundamental for species coexistence and the local mixture of traits.…”