Carbon Cycling in Ponderosa Pine Forests

Human activities are increasing carbon dioxide (CO₂) concentrations in the atmosphere with unknown environmental consequences. Fires are a major source of CO₂ to the atmosphere and fire management in southwestern ponderosa pine (Pinus ponderosa Laws.) forests may be an important component of the U.S. carbon (C) budget. Historically, these forests experienced frequent surface fires that constrained C accumulation, but fire exclusion since the late 1800’s has temporarily increased C storage in tree biomass and the forest floor. Forests with anomalously high fuel loads will eventually burn in stand-replacing wildfires, or experience fuel reduction treatments such as thinning and prescribed burning. The main objective of our project is to quantify the effects of fire suppression, wildfire, and fire management strategies on long-term C storage in ponderosa pine forests of the southwestern U.S. We are tackling this problem with both short and long-term approaches.

In one study funded by the USDA Carbon Cycle Science program we are monitoring CO₂ within and above the forest canopy to infer net ecosystem C exchange with the atmosphere (eddy covariance towers). We have towers in unburned, wildfire, and restored (by mechanical thinning and prescribed fire) forest. Our role is to provide land-based measurements of tree growth and soil CO₂ flux to compare with eddy covariance measurements.

A second study funded by the USDA NRI program will measure long-term (decades to centuries) C accumulation in live aboveground tree biomass, live coarse root biomass, and forest floor detritus of ponderosa pine forests in northern Arizona following: 1) fire suppression, 2) stand-replacing wildfire, 3) fuel reduction via thinning, and 4) fuel reduction via prescribed burning. Long-term thinning and burning field experiments and previously collected dendrochronological records from this region provide a unique opportunity to estimate long-term C accumulation in all four management scenarios. We will also conduct field work to test mechanisms that underlie differences in C storage among these fire management scenarios.