THE EFFECTS OF LITTER QUALITY AND CLIMATE ON DECOMPOSITION ALONG AN ELEVATIONAL GRADIENT

The process of decomposition is controlled by both biotic and abiotic factors. While it has been widely hypothesized that litter quality and climatic conditions regulate decomposition, the relative importance of these factors appears to vary across biomes. This study examines the decomposition of native plant litter along an elevational gradient in northern Arizona to determine the influence of litter quality and climate on the rate of decomposition in semiarid communities. A litter‐bag experiment was performed usingneedle/leaf litter from Pinus ponderosa, Pinus edulis, Juniperus monosperma, Gutierrezia sarothrae, and Bouteloua gracilis. The five litter types are representative of the dominant local vegetation and offer a range of litter qualities. The bags were placed along a gradient, running from Great Basin Desert scrub (1960 m) through a pinyon–juniper woodland (2100 m) and up into a ponderosa pine forest (2280 m). Samples were collected and analyzed over a period of 2 yr. Decomposition was closely correlated with the relative proportion of easily decomposed carbon fractions to recalcitrant fractions for the first year. Litter from G. sarothrae and B. gracilis contained relatively low levels of lignin and high levels of cellulose and carbohydrates, and these litter types exhibited significantly faster rates of decay than the highly lignified pine and juniper litter. The order of the relative rates of decomposition was G. sarothrae ≫ B. gracilis > J. monosperma > P. ponderosa = P. edulis. There was no correlation between initial litter nitrogen content and the rate of decomposition, suggesting that decomposition is limited by carbon substrates rather than by nutrient content. Decomposition rates were significantly greater at the upper elevation sites, which were colder and wetter. Evidence strongly suggests that decomposition is limited by moisture in these ecosystems. Warmer temperatures resulting from climate change may not increase the rate of decomposition in the Southwest unless accompanied by increases in available moisture.