Christopher Doughty, Andrew Abraham, Tomos Prys-Jones, Tom Kolb

A better understanding of carbon use efficiency and carbon allocation during disturbance is critical to improve simulations of the global carbon cycle and understanding future climate impacts. Forest thinning of high stem density, high elevation dry western US forests is becoming more common to reduce severe fire danger but there are uncertainties about how forest thinning may impact forest carbon use efficiency, carbon allocation and energy flow through the food chain. In three, quarter ha stands with similar soils, elevation and climate along a forest thinning chronosequence near Flagstaff (AZ), we measured total net primary production (NPP of wood, fine root, and leaves), total autotrophic respiration (Ra of wood, rhizosphere, and canopy respiration), gross primary production (GPP = NPP+Ra) and large mammal herbivory (with camera traps and dung counts) over a ~2-year period. We found strong seasonality in all carbon cycling variables and herbivory, peaking during the warm, wet monsoon period. Carbon was produced more efficiently in the thinned stands, with carbon use efficiency (CUE = NPP/GPP) of ~0.50, versus the un-thinned stand with CUE of 0.34. GPP was similar in the two thinned stands ~3.5 Mg C ha-1year-1 but was about 30% greater in the un-thinned stands (5.0 Mg C ha-1 year-1). Finally, the thinned stand had higher understory NPP, large herbivore consumption and had triple the total energy going into primary consumers despite reduced total GPP. Overall, the thinning, and the return to a more natural pre-fire suppression forest structure, increased the carbon use efficiency and energy flow from primary producers to primary consumers.