The effect of soil fertility on tree community functional variation in a Panamanian lower montane forest

Background/Question/Methods

Soil nutrients play an important role in the structuring of tropical forest tree communities. More than half of all tropical tree species show a significant habitat association to one or more soil macronutrients, and dramatic compositional turnover along soil fertility gradients provides evidence of strong soil-based habitat filtering. However, it is unclear which tree functional traits habitat filtering acts on, or if the strength of filtering is the equivelant at both ends of the soil gradient. We hypothesized that if habitat filtering should be strongest at the low fertility sites where adaptations for stress tolerance of most critical, then there should be greater relative functional similarity among dominant trees at the nutrient-poor relative to nutrient-rich sites. To test these hypotheses, we measured 15 physical and chemical leaf and wood functional traits in eight 1-ha forest plots spanning a vast soil fertility gradient in a lower montane Panamanian forest. For each plot, we calculated an index of functional narrowness (K_ratio) for each trait based on the ratio of the abundance-weighted kurtosis of the distribution of species means (K_dominance) to the kurtosis of the unweighted distribution of species means (K_presence). Values larger than one indicate that the dominant species in a community converge on a similar functional trait value. We used linear regression models to determine the relationship between plot K_ratiofor each trait and soil pH, used as a metric of soil fertility.

Results/Conclusions

Of the 15 traits examined, only two traits, leaf toughness and leaf nitrogen, had significant negative relationship between K_ratio and soil pH. These results suggest that there is greater functional similarity in traits related to herbivory on low fertility relative to high fertility sites, indicating that herbivory may be more costly on low fertility soils. The lack of significant relationships overall indicates that habitat filtering on leaf and wood traits may not be as important as dispersal or filtering based on unmeasured belowground traits in determining compositional turnover along soil gradients.