Potential landscape and nutrient constraints on tropical forest biogeochemical resilience

Abstract:

Tropical forests are a dominant contributor to the terrestrial carbon sink. A critical question is whether these forests will proportionally increase their C sequestration capacity to rising atmospheric CO₂ concentrations occurring worldwide, especially as extreme climatic events are predicted to be more frequent. Tropical forest resilience to rapid shifts in climate, such as intense drought, floods, or hurricanes, depends on the availability and cycling of key elements needed for forest growth and health. Nutrients (carbon (C), nitrogen (N), and phosphorous (P)), cycle at different rates, and in the case of P, availability is constrained by weathering of parent material. Thus, a major challenge in tropical forest biogeochemistry under climate change is accounting for the temporal scale of material flux. The time scale at which landforms change can be on the order of millions of years to within a season. Likewise, biotic biogeochemical time scales range from hundreds of years, such as forest succession, to seconds, as is the case with trace gas fluxes. We present a strategy to capture the nutrient status (key stocks and fluxes) of differently managed forested watersheds using a suite of stable isotopes, radionuclides, and cosmogenic nuclides. Stable isotopes are proven tools to track carbon, nitrogen, and phosphorous (through the ¹⁸O isotopic signature) as they flow through an ecosystem as well as capture physiological responses to management and climate change. Inventories of fallout radionuclides (FRNs) and terrestrial in situ-produced cosmogenic nuclides (TCNs) from soil profiles and stream sediment allow direct quantification of material fluxes at or near the Earth’s surface. Applying FRNs with varying mean lifetimes (e.g., ⁷Be, ¹³⁷Cs, and ¹⁰Be) can quantify fluxes over timescales varying from single storm events to thousands of years, while measurements of TCNs in stream sediment (e.g., ¹⁰Be) are capable of quantifying spatially averaged material fluxes. Coupling these measurements in watersheds with different disturbance and management histories will allow us to test hypotheses that require these spatial-temporal considerations.