Restoration and canopy-type effects on soil respiration in a ponderosa pine-bunchgrass ecosystem

In ponderosa pine (Pinus ponderosa Douglas ex P. Lawson & Lawson)-bunchgrass ecosystems of the western USA, fire exclusion by Euro-American settlers facilitated pine invasion of grassy openings, increased forest floor detritus, and shifted the disturbance regime toward stand-replacing fires, motivating ecological restoration through thinning and prescribed burning. We used in situ soil respiration over a 2-yr period to assess belowground responses to pine invasion and restoration in a ponderosa pine-bunchgrass ecosystem near Flagstaff, AZ. Replicated restoration treatments were: (i) partial restoration - thinning to presettlement conditions; (ii) complete restoration - removing trees and forest floor material to presettlement conditions, native grass litter addition, and prescribed burning; and (iii) control. Within treatments, we sampled beneath different canopy types to assess the effects of pine invasion into grassy openings on soil respiration. Growing season soil respiration was greater in the complete restoration (346±24 g CO₂-C m^-2) and control (350±8 g CO₂-C m^-2) than the partial restoration (301±5 g CO₂-C m^-2) in 1995. In 1996, the complete (364±17 g CO₂-C m^-2) and partial (328±7 g CO₂-C m^-2) restoration treatments had greater growing season respiration rates than the control (302±13 g CO₂-C m^-2). Results suggest that restoration effects on soil respiration depend on interannual soil water patterns and may not significantly alter regional C cycles. Soil respiration from grassy openings was 15% greater than from soil beneath presettlement or postsettlement pines in 1995 and 1996. A lack of active management will decrease belowground catabolism if pines continue to expand at the expense of grassy openings.