Elevation affects soil respiration in a northern Idaho mixed-conifer forest

In temperate forest ecosystems, soil surface CO₂ efflux (R_soil) is estimated to represent from 50% to 80% of total annual ecosystem respiration. Forest stands at higher elevations generally show lower annual productivity, and this may result in lower soil respiration rates due to reduction of carbon substrate quantity for both root and soil heterotrophic respiration. At a mixed-conifer forest in northern Idaho, summertime soil respiration was measured monthly at nine plots contained in three elevation classes, varying from 854 m to 1521 m (above sea level). Mean plot-level R_soil varied from 1.1 µmol C m^-2s^-1 to 5.9 µmol C m^-2s^-1. At the subplot level, "CO₂ chimneys" (measurement locations showing 2 to 3 times higher R_soil than the plot-level mean R_soil) persisted throughout the growing season, causing high variation at the plot level. One-way ANOVA revealed some significant differences in mean R_soil by elevation class. High elevation plots had significantly lower R_soil than low elevation plots in July and September, and lower R_soil than middle elevation plots in August and September. No differences occurred among elevation classes in June or October. Temperature at 5 cm below the forest floor at each measurement location significantly described R_soil at each plot using least-squares regression, but there was no trend in slopes by elevation class. We conclude that factors other than soil temperature and moisture control soil surface CO₂ efflux in northern Idaho mixed-conifer forests, and some of those factors provide preferred diffusion pathways. We propose further work to explore the relationship between substrate availability, surface conditions, and R_soil.