Abstract: Tropical forests have an exceptional capacity to sequester carbon, support biodiversity, and cycle nutrients. Regenerating tropical forests are one of the largest carbon sinks in the world: ample soil nitrogen in these forests allows high rates of primary production and rapid regeneration. Nitrogen-fixing trees are major nitrogen inputs in tropical forest systems and are relatively abundant in these biomes. Climate change is predicted to increase the abundance of nitrogen-fixing trees in much of the temperate forests of North America but the effects of climate change on these trees in tropical forests, especially regenerating tropical forests, is largely unknown. To investigate the effects of warming on tropical forest nitrogen cycling, we analyzed annual seedling census data from a warming field experiment in Puerto Rico, comparing seedlings in field plots warmed 4°C above ambient temperatures to seedlings in control plots. The site of the census, which began in 2015, also sustained heavy damage from hurricane Maria in 2017, so the analysis may also illuminate the nitrogen cycle response to warming in regenerating forests. We hypothesize that forest dynamics after the hurricane will differ between warming and control plots: nitrogen-fixers may be more easily recruited in warming plots, but have higher growth rates in control plots.
Preliminary analyses of the long-term seedling dataset provide initial insights into how nitrogen-fixers respond to warming in tropical forests. The abundance of nitrogen fixers increased in control plots and decreased in warming plots before the hurricane but showed an increase in warming plots and a decrease in control plots after. Recruitment of nitrogen fixers decreased in control plots and increased in warming plots after the hurricane. Growth rates of nitrogen fixers in control plots was lower in than in warming plots before the hurricane. However, this relationship reversed after the hurricane: nitrogen fixer growth rate in control plots increased drastically, while growth rate in control plots decreased in contrast. This is important because the switch in abundance and growth rate may indicate a phase of regeneration requires heavy nitrogen inputs in ambient but not warming conditions. To further elucidate the relationships between these patterns, additional data from recent censuses will be integrated and analyzed. Analyses can also be compared to census data that measures soil nitrogen available to plants. This sheds light on the ways and mechanisms by which warming affects nitrogen cycling in regenerating tropical forests.
