Higher survival drives the success of nitrogen-fixing trees through succession in Costa Rican rainforests
Trees capable of symbiotic nitrogen (N) fixation trees (hereafter, “N fixers,” even if they are not actively fixing) are abundant in many tropical forests, and are major players in both local and global biogeochemical cycles. In temperate forests, it is well known that N fixers specialize in early-successional niches, but in tropical forests, successional trends of N-fixing species are poorly understood. We use a long-term census study of regenerating and old-growth tropical rainforests in Costa Rica, along with an individual-based model, to ask three questions about successional patterns of N-fixing versus non-fixing species (“non-fixers”). (1) What are the successional trends of N-fixing versus non-fixing tree abundance, basal area, and size in these sites? (2) How do growth, recruitment, and mortality rates of N-fixing trees compare to non-fixing trees across forest age? (3) What are the relative contributions of these three demographic rates to successional trends in abundance, basal area, and size of N-fixing trees? We also evaluated the extent to which our results were driven by one particularly dominant N-fixing species, Pentaclethra macroloba.
N fixers increase in relative basal area during succession, rising from 18% of tree basal area in 10-year-old forests to 26% in old-growth forests. Initially, N fixers increase in relative abundance as well, to about 20% of trees, although they decline to 10% of trees in old-growth forests. N fixers are significantly larger than non-fixers, on average, particularly in older forests. Relative growth rates of N fixers are 2.5 fold higher than non-fixers in the youngest forests, although this disparity disappears by 25 years. In the youngest forests, N fixers recruit into the 5 cm size class at a similar rate as non-fixers, but N fixer recruitment decreases rapidly through succession. N fixers survive better than non-fixers, particularly in young forests, where they are 15 times less likely to die. According to our individual-based model, high survival is the dominant driver of N fixers’ increase in basal area, whereas low recruitment is the dominant driver of decreasing abundance and the larger average size of N fixers. Combined with other results, our data suggest that N fixers function differently in tropical forests compared with temperate forests, and that attempts to understand these differences should focus on differences in survival and recruitment.