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

Litter respiration (RL) represents a significant portion of whole-soil respiration (RS) in forests, yet climatic correlations with RL have seldom been examined. Because RL is reduced at low humidities and RS 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 RS on soils from which litter had been removed (RNL) and, by difference, RL. Mean RL represented 16% (SE = 2%) of mean RS from July through October of 2007 and 2008. RS was highest at warmer times and sites, and was not suppressed by low soil moisture. In contrast, RL was highest at cooler times, when humidity and gravimetric litter water content were highest. RL was highest at mid-elevations, representing neither the warmest nor wettest sites. Sixty-three percent of variability in site RL 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.