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.