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 o f 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 cli mate on the rate of decomposition in semiarid communities. A litter-ba g experiment was performed using needle/leaf litter from Pinus pondero sa, Pinus edulis, Juniperus monosperma, Gutierrezia sarothrae, and Bou teloua gracilis. The five litter types are representative of the domin ant local vegetation and offer a range of litter qualities. The bags w ere placed along a gradient, running from Great Basin Desert scrub (19 60 m) through a pinyon-juniper woodland (2100 m) and up into a pondero sa pine forest (2280 m). Samples were collected and analyzed over a pe riod of 2 yr. Decomposition was closely correlated with the relative p roportion of easily decomposed carbon fractions to recalcitrant fracti ons for the first year. Litter from G. sarothrae and B. gracilis conta ined relatively low levels of lignin and high levels of cellulose and carbohydrates, and these litter types exhibited significantly faster r ates of decay than the highly lignified pine and juniper litter. The o rder of the relative rates of decomposition was G. sarothrae much grea ter than B. gracilis > J. monosperma > P. ponderosa = P. edulis. There was no correlation between initial litter nitrogen content and the ra te of decomposition, suggesting that decomposition is limited by carbo n 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 cl imate change may not increase the rate of decomposition in the Southwe st unless accompanied by increases in available moisture.