Carbon and water fluxes from ponderosa pine forests disturbed by wildfires and thinning

Background/Question/Methods: We address impacts on carbon, water, and energy balances of southwestern ponderosa pine forests from a) intense fires that have increased because of fuel accumulation and climate warming, and b) thinning used to reduce fuels and fire intensity.  We measured impacts of thinning using experimentally induced thinning with one year of pre-treatment data followed by two years of post-treatment data paired with data from an unthinned/unburned control stand.  We measured impacts of intense fire by comparing the control stand to a stand exposed to intense fire a decade before our measurements. We measured exchanges of CO₂, water, and energy at all sites continuously for three years (2006-8) using the eddy covariance approach.
Results/Conclusions: 1) Intense fire that converted forest to early successional grassland reduced total ecosystem carbon stock more than thinning used to reduce fire intensity. 2) Both disturbances converted forest from a net annual CO₂ sink to a source, with stronger reduction by intense fire. 3) Reduced gross primary production after disturbance was a stronger driver of shifts from carbon sinks to sources than changes in total ecosystem respiration. 4) Thinning ameliorated constraints on CO₂ net ecosystem exchange caused by drought. 5) Both disturbances increased importance of soil CO₂ efflux to total ecosystem respiration.  6) The fire-induced vegetation change from forest to grassland altered site energy balance and microclimate, whereas thinning did not. 7) Both disturbances reduced forest evapotranspiration, with a larger effect of fire-induced vegetation change.