Functional differences between woodland savannas and seasonally dry forests from south-eastern Brazil: Evidence from 15N natural abundance studies

Abstract

Nitrogen availability and N-cycling dynamics across ecosystems play a critical role in plant functioning and species distribution. Measurements of 15N natural abundance provides a way to assess ecosystem N dynamics, and the range of nitrogen stable isotope values (δ15N) for plants in an ecosystem can indicate divergent strategies for N uptake. We tested the hypotheses that the N-rich seasonally dry forest would have higher soil and leaf δ15N and a smaller range of leaf δ15N values compared to the N-poor cerradao (savanna woodland). We measured N concentration and δ15N in two soil depths and leaves of 27 woody species in cerradao and 26 in seasonally dry forest. As expected, total soil N concentration decreased while soil δ15N value increased with soil depth. Regardless of soil depth, seasonally dry forest soils had higher δ15N and total N concentration compared to cerradao soils. Foliar δ15N values varied from −6.4‰ to 5.9‰ in cerradao and from −2.3‰ to 8.4‰ in seasonally dry forest plants. Phylogenetically independent contrasts analysis and comparisons of δ15N mean values of the most abundant species and species co-occurring in both sites confirmed the hypothesis of higher δ15N for seasonally dry forest in comparison to cerradao. These results corroborate the expectation of higher soil and leaf δ15N values in sites with higher soil N availability. However, except for the most abundant species, no across-site leaf–soil (δ15N leaf –δ15N soil) differences (Δδ15N) were found suggesting that differences in leaf δ15N between cerradao and seasonally dry forest are driven by differences in soil δ15N. Variation of leaf δ15N was large in both sites and only slightly higher in cerradao, suggesting high diversity of N use strategies for both cerradao and seasonally dry forest communities.