CONTROL OF LITTER DECOMPOSITION IN A SUBALPINE MEADOW–SAGEBRUSH STEPPE ECOTONE UNDER CLIMATE CHANGE

Abstract

Litter decomposition is an important component of the global carbon and nitrogen cycles. Because climate exerts strong controls over rates of litter decomposition, climate change may alter both cycles. Climate change can influence litter decomposition rates directly, or indirectly through changes in litter quality. The relative importance of climate and litter quality in controlling rates of decomposition varies across ecosystem types. Thus, ecosystem responses to climate change are not readily predictable. This study examines in situ litter decomposition rates of native plant litter of different growth forms (grass, forb, and shrub) in two microclimates (xeric and mesic). In a Colorado subalpine meadow-sagebrush steppe ecotone, a climate-warming treatment was used to determine the influence of litter quality, microclimate, and warming on the rates of decomposition. Three one-year litter bag experiments were performed using senescent leaf litter of the three growth forms from a xeric microclimate (shrub, Artemisia tridentata; grass, Festuca thurberi; forbs, Delphinium nuttallianum, Erigeron speciosus) and a mesic microclimate (shrub, Pentaphylloides floribunda; grass, Festuca thurberi; forbs, Erythronium grandiflo- rum, Ligusticum porteri). A reciprocal transplant litter experiment was performed in the third year to determine the direct effect of warming on litter quality and subsequent litter mass loss rates. Evidence suggests that decomposition was limited by moisture in the xeric zone and by temperature in the mesic zone. Decomposition rates were strongly correlated with the initial lignin:N ratio of the litter. The forbs had a much lower initial lignin:N ratio and, therefore, decomposed at a much higher rate than did the grasses or shrubs. In a changing climate, initial microclimate and changes in litter quality of the bulk litter produced as a result of shifts in species composition may be more important in determining rates of litter decomposition than the direct effect of warming.