N ₂ fixation is linked to the ability to encroach in African savanna trees

Abstract

Abstract Encroachment is a globally ubiquitous phenomenon, characterised by increasing indigenous tree densities in savanna and grassland. Encroachment has been attributed to rising atmospheric CO 2 concentrations fertilising tree growth and shifting the competitive balance between trees and grasses. However, only a subset of savanna tree species are currently described as encroachers, raising the hypothesis that CO 2 responsiveness differs among species. Within southern African savannas, encroachment is driven primarily by nitrogen (N 2 )‐fixing species, implying the CO 2 response may be mediated via traits that enhance plant‐available N. Using an open‐topped chamber system, we experimentally manipulated atmospheric CO 2 concentrations and soil moisture for 12 savanna tree species (six encroachers and six non‐encroachers) under ambient (a)CO 2 (~397.9 ppm) or elevated (e)CO 2 (~545.1 ppm) treatments and water limited or well‐watered soil moisture treatments. We measured N‐dynamics traits including nodule mass fraction (NMF), leaf δ 15 N, stem δ 15 N, and percentage of N derived from fixation (%Ndfa). We found that encroachers and non‐encroachers differ in short‐term N‐dynamics but share similar long‐term N allocation strategies. Encroachers exhibited lower leaf δ 15 N, indicating greater utilisation of N 2 fixation products to meet immediate short‐term protein synthesis. Long‐term N allocation strategies (NMF, stem δ 15 N, and %Ndfa) were similar between encroachers and non‐encroachers, with plants fixing more N 2 under the eCO 2 and well‐watered treatment. In encroachers, leaf and stem δ¹⁵N were unrelated, in contrast to the positive relationship in non‐encroachers, pointing to distinct tissue‐level N allocation, possibly reflecting differential N utilisation and retention. We demonstrate the significance of N 2 fixation in mediating the CO 2 responsiveness of encroaching savanna trees. N 2 fixation increases plant‐available N, likely enabling encroachers to meet immediate N demands even under water limitation and increasing CO 2 . The potential feedback loop, where eCO 2 enhances photosynthesis, facilitating greater C allocation for N 2 fixation, helps to explain the ecological success of the subset of species driving encroachment under increased atmospheric CO 2 . Read the free Plain Language Summary for this article on the Journal blog.