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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.

The Wind River old-growth forest, in the southern Cascade Range of Washington State, is a cool (average annual temperature, 8.7°C), moist (average annual precipitation, 2223 mm), 500-year-old Douglas-fir 4 -western hemlock forest of moderate to low productivity at 371-m elevation on a less than 10% slope. There is a seasonal snowpack (November-March), and rain-on-snow and freezing-rain events are common in winter. Local geology is characterized by volcanic rocks and deposits of Micocene/Oligocene Micocene-Oligocene 5 (mixed) Micocene and Quaternary age, as well as intrusive rocks of Miocene age. Soils are medial, mesic, Entic Vitrands 6 that are deep (2-3 m), well drained, loams and silt loams, generally stone free, and derived from volcanic tephra. The vegetation is transitional, between the Western Hemlock Zone and the Pacific Silver Fir Zone, and the understory is dominated by vine maple, salal, and Oregon grape. Stand structural parameters have been measured on a 4-ha plot. There are eight species of conifers, with a stand density of 427 trees ha )1 and basal area of 82.9 m 2 ha )1 . Dominant conifers include Douglas-fir (35 trees ha )1 ), western hemlock (224 trees ha )1 ), Pacific yew (86 trees ha )1 ), western red cedar 7 (30 trees ha )1 ), and Pacific silver fir (47 trees ha )1 ). The average height of Douglas-fir is 52.0 m (tallest tree, 64.6 m), whereas western hemlock averages 19.0 m (tallest tree, 55.7 m). The regional disturbance regime is dominated by high-severity to moderateseverity fire, from which this forest is thought to have originated. There is no evidence that fire has occurred in the forest after establishment. Primary agents of stand disturbance, which act at the individual to small groups of trees scale, are wind, snow loads, and drought, in combination and interacting with root-rot and butt-rot fungi, heart-rot fungi, dwarf mistletoe, and bark beetles. The forest composition is slowly shifting from dominance by Douglas-fir, a shade-intolerant species, to western hemlock, western red cedar, Pacific yew, and Pacific silver fir, all shade-tolerant species. The Wind River old-growth forest fits the regional definition of Douglas-fir ''old growth'' on western hemlock sites.

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