Decoupling litter respiration from whole-soil respiration along an elevation gradient in a Rocky Mountain mixed-conifer forest

Litter respiration (R L ) represents a significant portion of whole-soil respiration (R S ) in forests, yet climatic correlations with R L have seldom been examined. Because R L is reduced at low humidities and R S is reduced at low temperatures, these components may show divergent trends with elevation in western North American forests. Using a litter-removal experiment along a forested 750 m elevation gradient in the Rocky Mountains of northern Idaho, USA, we measured R S on soils from which litter had been removed (R NL ) and, by difference, R L . Mean R L represented 16% (SE = 2%) of mean R S from July through October of 2007 and 2008. R S was highest at warmer times and sites, and was not suppressed by low soil moisture. In contrast, R L was highest at cooler times, when humidity and gravimetric litter water content were highest. R L was highest at mid-elevations, representing neither the warmest nor wettest sites. Sixty-three percent of variability in site R L was explained by both mean annual temperature (MAT) and mean annual relative humidity (MARH), including a positive interaction effect between MAT and MARH. Our results imply that the equilibration of litter with atmospheric humidity is an important control over litter respiration rates.