Natural abundance (δ¹5N) indicates shifts in nitrogen relations of woody taxa along a savanna-woodland continental rainfall gradient.

Water and nitrogen (N) interact to influence soil N cycling and plant N acquisition. We studied indices of soil N availability and acquisition by woody plant taxa with distinct nutritional specialisations along a north Australian rainfall gradient from monsoonal savanna (1,600-1,300 mm annual rainfall) to semi-arid woodland (600-250 mm). Aridity resulted in increased 'openness' of N cycling, indicated by increasing δ(15)N(soil) and nitrate:ammonium ratios, as plant communities transitioned from N to water limitation. In this context, we tested the hypothesis that δ(15)N(root) xylem sap provides a more direct measure of plant N acquisition than δ(15)N(foliage). We found highly variable offsets between δ(15)N(foliage) and δ(15)N(root) xylem sap, both between taxa at a single site (1.3-3.4 ‰) and within taxa across sites (0.8-3.4 ‰). As a result, δ(15)N(foliage) overlapped between N-fixing Acacia and non-fixing Eucalyptus/Corymbia and could not be used to reliably identify biological N fixation (BNF). However, Acacia δ(15)N(root) xylem sap indicated a decline in BNF with aridity corroborated by absence of root nodules and increasing xylem sap nitrate concentrations and consistent with shifting resource limitation. Acacia dominance at arid sites may be attributed to flexibility in N acquisition rather than BNF capacity. δ(15)N(root) xylem sap showed no evidence of shifting N acquisition in non-mycorrhizal Hakea/Grevillea and indicated only minor shifts in Eucalyptus/Corymbia consistent with enrichment of δ(15)N(soil) and/or decreasing mycorrhizal colonisation with aridity. We propose that δ(15)N(root) xylem sap is a more direct indicator of N source than δ(15)N(foliage), with calibration required before it could be applied to quantify BNF.

Climate change threatens native potential agroforestry plant species in Brazil.

Climate change is one of the main drivers of species extinction in the twentyfirst-century. Here, we  (1) quantify potential changes in species' bioclimatic area of habitat (BAH) of 135 native potential agroforestry species from the Brazilian flora, using two different climate change scenarios (SSP2-4.5 and SSP5-8.5) and dispersal scenarios, where species have no ability to disperse and reach new areas (non-dispersal) and where species can migrate within the estimated BAH (full dispersal) for 2041-2060 and 2061-2080. We then (2) assess the preliminary conservation status of each species based on IUCN criteria. Current and future potential habitats for species were predicted using MaxEnt, a machine-learning algorithm used to estimate species' probability distribution. Future climate is predicted to trigger a mean decline in BAH between 38.5-56.3% under the non-dispersal scenario and between 22.3-41.9% under the full dispersal scenario for 135 native potential agroforestry species. Additionally, we found that only 4.3% of the studied species could be threatened under the IUCN Red List criteria B1 and B2. However, when considering the predicted quantitative habitat loss due to climate change (A3c criterion) the percentages increased between 68.8-84.4% under the non-dispersal scenario and between 40.7-64.4% under the full dispersal scenario. To lessen such threats, we argue that encouraging the use of these species in rural and peri-urban agroecosystems are promising, complementary strategies for their long-term conservation.

Nitrogen and phosphorus additions negatively affect tree species diversity in tropical forest regrowth trajectories.

Nutrient enrichment is increasingly affecting many tropical ecosystems, but there is no information on how this affects tree biodiversity. To examine dynamics in vegetation structure and tree species biomass and diversity, we annually remeasured tree species before and for six years after repeated additions of nitrogen (N) and phosphorus (P) in permanent plots of abandoned pasture in Amazonia. Nitrogen and, to a lesser extent, phosphorus addition shifted growth among woody species. Nitrogen stimulated growth of two common pioneer tree species and one common tree species adaptable to both high- and low-light environments, while P stimulated growth only of the dominant pioneer tree Rollinia exsucca (Annonaceae). Overall, N or P addition reduced tree assemblage evenness and delayed tree species accrual over time, likely due to competitive monopolization of other resources by the few tree species responding to nutrient enrichment with enhanced establishment and/or growth rates. Absolute tree growth rates were elevated for two years after nutrient addition. However, nutrient-induced shifts in relative tree species growth and reduced assemblage evenness persisted for more than three years after nutrient addition, favoring two nutrient-responsive pioneers and one early-secondary tree species. Surprisingly, N + P effects on tree biomass and species diversity were consistently weaker than N-only and P-only effects, because grass biomass increased dramatically in response to N + P addition. The resulting intensified competition probably prevented an expected positive N + P synergy in the tree assemblage. Thus, N or P enrichment may favor unknown tree functional response types, reduce the diversity of coexisting species, and delay species accrual during structurally and functionally complex tropical rainforest secondary succession.