By T. Trevor Caughlin, Literature Coordinator
Nitrogen availability limits plant growth in many ecosystems, including some tropical forests. Some tree species have adapted by partnering with nitrogen-fixing bacteria that can convert inert nitrogen gas in the atmosphere into a biologically-available nutrient. The potentially huge amount of nitrogen these tree species can fix (> 100 kg N ha-1 yr-1) points to their importance in forest ecosystems. Indeed, many restoration practitioners recommend planting N fixing trees to accelerate ecosystem development during reforestation. However, the complex interactions between nutrient cycling and tree demography during natural forest regeneration remain poorly understood. A new paper, co-authored by PARTNERS founder Robin Chazdon, disentangles the roles of tree growth, survival and recruitment in driving the abundance of N fixing trees during early succession in Costa Rica.
The study took place in 10 to 41 year old second growth forest plots as well as two old growth forest plots in Sarapiquí province, Costa Rica. These sites are home to long-term demographic studies, where all trees in 1 ha plots in each site are annually mapped and measured. The first 8 to 16 years of these demographic data were used to quantify vital rates (growth, survival, and recruitment) of tree species, including both N fixing trees and non-fixing trees. While comparing vital rates between the two groups of trees is informative, ultimately it is the combination of all three vital rates and the changing environmental variables that affect them that drive the changes in abundance of tree species during succession. To capture tree abundance as an emergent property of vital rates, the authors used a computer simulation to explore the fate of thousands of trees. This computer simulation, an individual-based model, represents demography by randomly determining tree fate at each model time step. The individual-based model enabled the authors to conduct a novel experiment: for each vital rate, they ran simulations where there were no differences between N fixing tree species and non-fixers. The more important a vital rate is for determining N fixer abundance, the greater the expected difference between model predictions and reality.
The authors found that the relative abundance of N fixing trees peaked during the first two decades of forest succession. N fixing trees grew more than twice as fast non-fixing trees during this period, but even more striking were the survival advantages for N fixing trees—15 times higher survival compared to non-fixers. In other words, during early succession N fixing trees almost never died. The individual-based model revealed that this survival advantage is primarily responsible for the higher relative abundance of nitrogen fixing trees during early succession. Why N fixing trees have higher survival remains a mystery. One intriguing hypothesis is that the increased nitrogen available to these trees enables them to produce an arsenal of defensive compounds to ward off leaf herbivores. By linking nitrogen fixation with tree demography, the study provides new insight into secondary succession and illustrates the value of long-term data collection for understanding reforestation.
Citation: Menge, D. N. L., and R. L. Chazdon. In Press. Higher survival drives the success of nitrogen-fixing trees through succession in Costa Rican rainforests. New Phytologist. DOI: 10.1111/nph.13734.
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Photo credit: Pentaclethra macroloba, a N fixing tree that is common in Costa Rican secondary forests. Photo by Joan Simon: https://www.flickr.com/photos/simonjoan/