RESTORATION AND CANOPY-TYPE EFFECTS ON SOIL RESPIRATION IN A PONDEROSA PINE-BUNCHGRASS ECOSYSTEM

In ponderosa pine (Pinus ponderosa Douglas ex P, Lawson & Lawson)-bunc hgrass ecosystems of the western USA, fire exclusion by Euro-American settlers facilitated pine invasion of grassy openings, increased fores t Door detritus, and shifted the disturbance regime toward stand-repla ting fires, motivating ecological restoration through thinning and pre scribed burning, We used in situ soil respiration over a 2-yr period t o assess belowground responses to pine invasion and restoration in a p onderosa pine-bunchgrass ecosystem near Flagstaff, AZ. Replicated rest oration treatments were: (i) partial restoration - thinning to presett lement conditions; (ii) complete restoration - removing trees and fore st floor material to presettlement conditions, native grass litter add ition, and prescribed burning; and (iii) control. Within treatments, w e sampled beneath different canopy types to assess the effects of pine invasion into grassy openings on soil respiration. Growing season soi l respiration was greater in the complete restoration (346 +/- 24 g CO 2-C m(-2)) and control (350 +/- 8 g CO2-C m(-2)) than the partial rest oration (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)) restoratio n treatments had greater growing season respiration rates than the con trol (302 +/- 13 g CO2-C m(-2)). Results suggest that restoration effe cts on soil respiration depend on interannual soil water patterns and may not significantly alter regional C cycles. Soil respiration from g rassy openings was 15% greater than from soil beneath presettlement or postsettlement pines in 1995 and 1996. A lack of active management wi ll decrease belowground catabolism if pines continue to expand at the expense of grassy openings.