Plants adjust nutrient acquisition strategies to nutrient availability over tropical forest succession

Michelle Wong, Cary Institute of Ecosystem Studies, Millbrook, NY, Nina Wurzburger, Odum School of Ecology, University of Georgia, Athens, GA, Jefferson Hall, Smithsonian Tropical Research Institute, Balboa, Panama, S. Joseph Wright and Kristin Saltonstall, Smithsonian Tropical Research Institute, Panama, Lars Hedin, Ecology and Evolutionary Biology, Princeton University, Princeton, NJ, Michiel van Breugel, Center for Tropical Forest Sciences, Smithsonian Tropical Research Institute, Balboa, Panama, Sarah A. Batterman, School of Geography, University of Leeds, Leeds, United Kingdom

Background/Question/Methods
Tropical forests contain high functional biodiversity and sequester large amounts of carbon in primary and secondary forests. Increasing evidence suggests that the low nutrient availability of highly-weathered tropical soils will limit the tropical carbon sink. However, it is unclear to what extent trees can overcome nutrient limitation through the use of nutrient acquisition strategies, such as the production of root phosphatase enzymes and investment in symbioses with arbuscular mycorrhizal fungi. These strategies come at a carbon cost, incentivizing trees to reduce investment when they are not limited by nutrients and stimulate investment when they are. To identify nutrient limitation and to test how trees adjust investment in these strategies, we used a large-scale ecosystem nitrogen and phosphorus fertilization experiment across three stages of tropical secondary succession and a mature forest in Panama. We hypothesized that trees regulate these strategies as nutrient limitation shifts with forest succession, with higher investment in mycorrhizae in the rapidly-growing, nitrogen-limited young forests and increasing phosphatase activity as forests mature and shift towards phosphorus limitation. We also hypothesized that nutrient addition would either relieve limitation and suppress tree investment in the strategy associated with that nutrient, or stimulate investment in strategies for acquiring other nutrients.
Results/Conclusions
We found that trees regulate nutrient acquisition strategies over forest succession and in response to nutrient availability. First, mycorrhizal colonization was highest in younger forests and phosphatase activity increased across secondary forest succession. Second, relative to the mature forests, nutrient addition changed mycorrhizal colonization less so in the younger forests, suggesting stronger, and more consistent, investment in nutrient acquisition in rapidly-growing young forests. In the mature forests, however, nitrogen alone and an interaction of nitrogen and phosphorus decreased colonization, suggesting a shift in strategies as nutrient limitation was alleviated. Phosphatase activity responded dynamically to nutrient addition across all forest age classes. In the youngest two forests, nitrogen addition alone increased phosphatase activity, but not in the older forests. In contrast, phosphorus addition reduced phosphatase activity in the older forests, but not in the youngest forests. These results are consistent with the theory that, over secondary succession, tropical forests shift from nitrogen to phosphorus limitation. Our findings support the idea that nutrient acquisition strategies are dynamic on short and long timescales, and play a key role in alleviating nutrient limitation and facilitating the tropical carbon sink.