Nitrogen-fixing trees are critical for forest carbon recovery during tropical forest secondary succession (Batterman etal 2013), although recent studies indicate a lack of – or negative – neighborhood effect of fixer abundance on forest carbon storage (Taylor etal 2017; Lai etal in review). Central to these studies is the idea that fixer abundance scales with ecosystem-level fixation rates. Yet whether this is the case in nature remains unclear. On the one hand, higher abundance of nitrogen-fixing trees should enable higher rates of nitrogen fixation via greater reducing power to fuel the fixation reaction. On the other hand, recent findings that tropical fixers utilize facultative fixation and that fixation down-regulates as forests mature suggest that the relationship between fixer abundance and ecosystem-level fixation rates may not scale linearly.
We here address this question and hypotheses using a chronosequence of 54 large forest plots in Panamanian lowland tropical rainforests that span over 50 years of forest recovery. Every tree has been measured for size, identified to species and evaluated for nitrogen fixation potential based on species identity. On a subset of 267 trees with fixation capacity, we quantify per-area and per-tree nodule biomass and apply a fixation rate based on measurements of nearby trees.
Results/Conclusions
We found little evidence that the abundance of nitrogen-fixing trees scaled with nitrogen fixation rates or biomass carbon storage during secondary succession in our forests. Across plots, fixation rates at both the individual tree and ecosystem levels did not increase with fixer abundance (measured as both absolute biomass and as a fraction of total forest biomass). Consistent with previous studies (Taylor etal 2017; Lai etal in review), we found a lack of positive effect of fixer abundance on forest biomass carbon storage. We conclude that the function of nitrogen-fixing trees is decoupled from their abundance in the forest throughout succession because of their use of a facultative fixation strategy. Our findings could explain the observations in previous studies of a lack of a positive fixer abundance effect. Furthermore they indicate that predictions of ecosystem nitrogen fixation cannot simply scale rates to nitrogen fixer abundance, and raise the question of what constrains nitrogen fixation activity and the ability of nitrogen fixers to support a carbon sink in tropical rainforests.