Carbon Cycling in Forests

The response of harvested forests to climate change

A field-based climate change simulation experiment was established in 2008 in Penn State's Stone Valley Forest to identify the responses of soils and tree species to predicted increases in temperature and precipitation.  Research plots were heated by ~2 degrees C with suspended infrared lamps and irrigated with +20% of the average long-term precipitation in a forested area that was harvested prior to the experiment.  In the plots we are monitoring: tree seedling growth, phenology, and ecophysiology; early successional community composition and phenology; and soil temperature, moisture, nutrient fluxes, nutrient pools, and soil biotic community responses.  With these measurements we will recognize both how forest composition may change with predicted changes in climate and the mechanisms that drive the responses. 

Results show that warming increased soil N mineralization within the first year post harvest.  We also observed that single factor treatments increase soil CO₂ fluxes, however the two climate change factors combined (warming + irrigation) decrease soil CO₂ fluxes relative to ambient levels. Possible mechanisms for these differences include depletion of soil labile C, change in quality or quantity of plant inputs, or change in the soil microbial community.  We are assessing the functional dynamics of the soil microbial community through extracellular enzyme assays, community-level physiological response profiles, and selective substrate induced respiration methods.   

The depth distribution of CO₂ in soils

 The flux of CO₂ from soils to the atmosphere is one of the largest components of the global C cycles.  Yet, on the ground, this flux varies tremendously across forested landscapes.  The source of this variation and the implications for scaling up plot-level soil respiration measurements are active areas of research. We collaborate at the Shale Hills-Susquehanna Critical Zone Observatory in measurements of soil CO₂ concentrations and fluxes.  On concave and planar slopes we have been monitoring the depth distribution and surface CO₂ fluxes on transects from the ridge top to the toe slope. Simultaneously, we monitor soil temperature and water content.  These data, collected mainly by undergraduate researchers, will increase our understanding of how soil development and soil moisture variability control the accumulation of CO₂ in soil pore spaces and the flux of CO₂ from soils to the atmosphere. 

Fire and C storage in ponderosa pine forests

Fires are a major source of CO₂ to the atmosphere and fire management in southwestern ponderosa pine 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 trees 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 research is to quantify the effects of fire suppression, wildfire, and fire management strategies on 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 monitored 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 was 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 measured 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.