Comparison of leaf traits regarding nitrogen use in congeneric pairs from sandy coastal plains (restingas) of Southeast Brazil

Background/Question/Methods

Restingas are a fragile vegetation complex peripheral to the Atlantic Rainforest that grow over sand ridges along the Brazilian coast. Their physiognomic, floristic and structural diversity contrasts with a large number of environmental limiting factors, including the availability of water and nutrients. Consequently, a series of morpho-anatomical and physiological adjustments are needed to ensure species development, maintenance and reproduction. We compared three restinga physiognomies that grow under different soil and climatic conditions along Southeast Brazil (dry, intermediate and wet restingas), using leaf traits related to nitrogen use – nitrogen (N) and phosphorus (P) concentration, elemental ratios C/N and N/P, leaf mass per area (LMA), and nitrogen isotopic composition (δ¹⁵N). We wanted to know which of these traits would best explain the degree of species differentiation along the studied environmental gradient. We also wanted to know if the relationships between such traits were able to describe plant responses to environmental variation. Comparisons were made within genera that occurred in at least two of the studied restingas, as well as between congeneric pairs from the two more divergent sites. In order to ensure data phylogenetic independence, paired t-tests were performed, and also analysis of phylogenetically independent contrasts (PICs).

Results/Conclusions

The congeners from wet restinga had higher N and P concentrations and δ¹⁵N values, as well as lower C/N ratio and LMA than those from dry and intermediate ones. The greatest leaf trait divergence was observed between congeneric pairs from dry and wet restingas. Leaf traits relationships of these two congeneric pair showed direct relationships between N and P concentrations, N concentration and δ¹⁵N, as well as between LMA and C/N ratio. Inverse relationships were observed between LMA and both N and P concentrations, and also between δ¹⁵N and P concentration. Additionally, we observed an inverse relationship between δ¹⁵N and LMA, suggesting that this relationship might also be used as a good descriptor of the strategies on the acquisition and use of nutrients. Such relationships remained significant when performing the analysis using PICs, reinforcing the distinctiveness of the wet restinga from the others when regarding leaf economy. Our results emphasize the importance of environmental conditions on the adjustment of the analyzed leaf traits, independent from the degree of species phylogenetic relatedness. Such adjustments affect species ecophysiological performance from the restingas here studied, and probably other physiognomies that are distributed along the wide range of the Brazilian coast.