Variation in plant functional traits along a soil fertility gradient in temperate deciduous forests

Background/Question/Methods

Determining how functional traits vary along environmental gradients is a key step toward understanding the factors influencing the distribution and abundance of species, and may also provide critical insights to the assembly of ecological communities.  For plants, important functional traits include seed mass, leaf size and form, specific leaf area (SLA), plant height, and the production of herbaceous vs. woody growth.  These traits are predicted to be responsive to gradients in soil fertility, as they are likely to be correlates of plant growth rates and competitive abilities.  In this study, we hypothesized that variation in plant functional traits would be correlated with soil calcium content in Temperate Deciduous Forests, as calcium is a key indicator of soil nutrient status in this system.  We surveyed plant functional traits in twenty-five 0.1 ha plots in mature Temperate Deciduous Forest vegetation across New York and New England.  Within each plot, we sampled ground layer plants every 2 m along the central 50 m axis to document plant height, leaf size, SLA, and growth form.  We then examined correlations between these functional traits and key environmental factors.  We also surveyed seed mass in species pairs with divergent distributions along the calcium gradient.

Results/Conclusions

All of the plant functional traits examined exhibited some degree of significant correlation with the soil calcium gradient.  For example, soil calcium content was positively correlated with leaf area (p < 0.001), plant height (p < 0.01), and SLA (p < 0.05).  In addition, plant and leaf growth form also significant trends, with both percent herbaceous vs. woody growth in plots and percent of plants with compound leaves both being highly positively correlated with soil calcium (p < 0.001).  In the parallel comparative analysis of seed mass variation along the calcium gradient, we found that the dry seed masses of species associated with higher calcium soils were, on average, ~31% greater than the seed masses of related species associated with more acidic, low calcium soils (p < 0.05).  Overall, our results confirm the prediction that plant functional traits vary substantially along this important environmental gradient and suggest that above-ground competition for space and light may be more intense in forest plant communities on fertile, high-calcium soils.  Interestingly, species richness also correlated positively with the soil calcium gradient, despite the expectation that increased competition at higher soil fertility might lead to declines in diversity.