COS 5-4 - Soil nutrients drive tropical forest carbon sequestration

Background/Question/Methods

Identifying the factors that regulate carbon sequestration in tropical forests is important for predicting future tropical forest carbon sink and climate change pathways. Theory suggests that soil nutrients (especially nitrogen and phosphorus) can constrain tropical forest carbon sequestration, with the largest sinks per-unit-area likely residing in regenerating forests. However, it remains unclear whether and how soil nutrients limit the sink into tropical forests over the course of secondary succession. To address this knowledge gap, we established a large, long-term ecosystem nutrient manipulation experiment across a tropical forest successional gradient in Panama, including stands aged 0, 10, and 30 years old and a mature forest. Specifically, we asked whether and how soil nutrients limit biomass accumulation and regulate forest dynamics (growth, recruitment, and mortality) over the course of tropical vegetation succession. We identified and measured diameters of trees >1cm diameter for up to 21 years in 76 experimental plots (total area 8.56 ha). We then scaled diameter to aboveground biomass and analysed the responses of aboveground biomass and its dynamics to nutrient addition.

Results/Conclusions

We found that the strength and type of nutrient limitation shift across forest succession. We observed consistent responses in aboveground biomass accumulation and forest dynamics – growth, recruitment and mortality – to the addition of the limiting nutrient over succession. The 0-year-old forest had strong nitrogen and some phosphorus limitation, as reflected in accelerated net biomass accumulation due to increased tree recruitment and growth in response to nitrogen and phosphorus addition. The 10 and 30-year-old forests showed evidence of nitrogen and phosphorus limitations, respectively. In both cases, the addition of the limiting nutrient accelerated net aboveground biomass accumulation by increasing forest growth and slightly decreasing mortality. The mature forest showed some evidence of nitrogen and phosphorus co-limitation on growth and recruitment, but there was no effect of nutrient addition on net aboveground biomass change. In summary, we found a shift from strong nitrogen to weak phosphorus to little evidence of nutrient limitation as forests developed from early to mid-successional to mature forests. Our experiment is the first to demonstrate how nutrient limitation changes over tropical forest succession, and provides insight for policymakers on how carbon sequestration rates in tropical secondary forests could be improved.