Negative associations between nitrogen fixing trees and tropical forest biomass accumulation

Plant functional traits are thought to drive biomass production and biogeochemical cycling in tropical forests. Here, we find that local interactions between N-fixing and non-fixing trees' functional traits may negatively influence long-term carbon accumulation in tropical forests.

Plant functional traits (such as wood density, growth rate, leaf size, etc.) are thought to drive biomass production and biogeochemical cycling in tropical forests, but it remains unclear how nitrogen (N)‐fixing legumes influence the functional traits of neighboring trees and forest‐wide biomass dynamics.

Because N-fixers can convert atmospheric nitrogen into a plant-useable form, via their morality and turnover, they should theoretically promote forest growth. Recent studies have questioned this generality, and a wide gradient of findings have emerged, from net-negative to neutral to net-positive.

We speculated that this may partially be due to uncertainties regarding how N-fixers effect neighboring trees via density-dependency (i.e. strength in numbers versus a strong few), whether effects depend on stem size (i.e. larger effects from larger N-fixers), and/or the effects of the spatial scale of investigation.

To test our speculation, we examined 30‐years of stem demography data for ~20,000 trees in a lowland tropical forest in Trinidad that span a wide range of functional traits thought to drive aboveground biomass dynamics. We made use of 62 one-hectare plots that were subdivided in 25 smaller plots. These forests are also home to a dominant N-fixing species, Pentaclethra macroloba, which can either maintain almost monodominance or be completely absent in stands across the forest. This provides us with a useful gradient to simulate experimentally testing functional traits of neighboring trees across the gradient of N-fixer abundance.

We found that these forests show positive but decreasing long‐term net biomass accumulation (e.g. the strength of the carbon sink is decreasing over time) resulting from constant average productivity but increasing mortality of non‐fixing trees over time. Interestingly, we noted that high abundances of N‐fixing trees are associated with compositional shifts in non‐fixer functional traits that confer lower competitive performance and biomass accumulation. Across tree size classes, most interactions between N‐fixers and non‐fixers were negative, density dependent, and strongest at smaller spatial scales. Overall, our findings suggest that local trait‐based interactions between N‐fixing and non‐fixing trees can negatively influence long‐term carbon accumulation in tropical forests.

Map of the VMFR and a few key non-fixer functional traits across a gradient of N-fixer abundance.

Authors: Bryce Currey, Mike Oatham, Jack Brookshire