2024 Abstracts

Soil Nutrient Controls on Biomass Productivity Across Lowland Tropical Forests


José A. Medina-Vega1* (medinaja@si.edu), Stuart J. Davies1, Jeffrey Chambers2,3, ForestGEO collaborators


1Forest Global Earth Observatory, Smithsonian Tropical Research Institute, Washington, DC; 2University of California–Berkeley, CA; 3Lawrence Berkeley National Laboratory, Berkeley, CA



The future capacity of tropical forests to sequester carbon remains uncertain, partly due to the diverse nutrient requirements of trees [e.g., nitrogen, potassium, calcium, phosphorus (P), and magnesium] beyond carbon for their growth. Additionally, tropical forests exhibit considerable soil fertility variation, resulting in a heterogeneous spatial distribution of trees with diverse species, sizes, ages, and ecological functions. Consequently, the responses of tree growth and overall biomass accumulation may not be consistent across different forests or even within the same forest. The project addresses a fundamental yet crucial hypothesis aimed at understanding the role of nutrients in regulating forest productivity. Specifically, the team posits that trees situated in exposed canopy positions show a more pronounced response to nutrient limitations compared to their counterparts in shaded areas. To test this hypothesis, researchers assessed the association between aboveground woody biomass productivity and soil nutrients, focusing particularly on soil P, across eight lowland tropical forests. The team found that across all canopy levels, there is no consistent association between mean aboveground biomass accumulation and soil P among and within these eight sites, except one site (Lambir in Borneo). However, across the eight sites, aboveground biomass accumulation in the upper canopy layers tends to increase with increasing soil P. In contrast, the understory layers do not respond to soil P changes. The response of biomass accumulation to nutrient limitations is not uniform among and within forests.

Specifically, aboveground biomass accumulation is P-limited, particularly in exposed canopy positions. This is attributed to the higher productivity of canopy plants with increased access to light, more nutrient requirements for rapid tissue growth, and heightened respiration rates. Conversely, understory plants experiencing limited light availability exhibit reduced growth and nutrient requirements due to a lower carbon supply for tissue development.