Associate Scientist Cary Institute of Ecosystem Studies Millbrook, New York, United States
Background/Question/Methods
The future of the tropical carbon sink is highly uncertain because it may be constrained by nutrients. However, our understanding of how nutrients limit tropical forest remains unresolved. Here I first will review the history of understanding of nutrient limitation in tropical forests and evaluate recent evidence. Second, I use a large-scale ecosystem manipulation experiment in Panama to test how the strength and type of nutrient limitation on net biomass accumulation shifts over tropical forest secondary succession. Begun in 2015, the experiment tracks all trees >1 cm diameter at breast height (DBH) in 76 0.1 ha plots spanning forests aged 0, 10 or 30 years post disturbance and a mature forest. Plots have been treated with either nitrogen, phosphorus or nitrogen and phosphorus fertilizer for the past 7 years, or left as an unfertilized controls, with 4-5 replicate plots per forest age and treatment. I evaluate how nutrients affect net aboveground biomass.
Results/Conclusions
Our understanding of how nutrients limit tropical forest productivity has changed over time. Early work from a few sites using theory, field observations and experiments suggest that tropical forests are phosphorus limited given soil age and time since large-scale disturbance. More recently, however, experiments across forests indicate that tropical forests may not be limited by phosphorus everywhere, and that there may be more heterogeneity in limitation type. Using our fertilization experiment, we found that nutrient limitation shifted over the course of succession such that tropical forests were initially limited by nitrogen, became limited by phosphorus midway through succession and ultimately showed few signs of nutrient limitation in mature forests. The findings point to three conclusions: first, that nutrient limitation in tropical forests is much more heterogeneous than previously considered, both in terms of the type and strength of nutrient limitation. Second, that other factors such as soil type, species composition and species diversity should be evaluated when determining what contribute to nutrient limitation type. And, third, while general principles are emerging for how nutrient limitation changes over succession in tropical forests, we need more widespread, coordinated studies of nutrient limitation to accurately predict the future of the tropical carbon sink.
