The effects of topography on forest soil characteristics in the Oregon Cascade Mountains (USA): Implications for the effects of climate change on soil properties

Forest soil measurements were made at over 180 sites distributed throughout the H.J. Andrews Experimental Forest (HJA) in the Oregon Cascade Mountains. The influences of both elevation and aspect on soil variables were measured in the early (1998) and late summer (1994). Increased elevation significantly increased soil moisture, mean annual precipitation, soil organic matter, labile C and mineralizable N, microbial activities, extractable ammonium, and denitrification potentials. In contrast, bulk density, pH and soil temperature (1998 only) were significantly lower at the higher elevations. Relative to labile C, mineralizable N was preferentially sequestered at higher elevations. Aspect significantly affected annual mean temperature and precipitation, soil moisture and temperature, soil organic matter, mineralizable N, extractable ammonium, denitrification, and microbial activities. There were no significant higher statistical interactions between elevation and aspect on climatic or soil factors. Soil organic matter (SOM) accumulation at higher elevations is likely driven by a reduction in decomposition rates rather that an increase in primary productivity, however, SOM accumulation on north facing slopes is probably due to both a decrease in decomposition and an increase in primary production. Models of climate change effects on temperate forest soils based on elevational studies may not apply to aspect gradients since plant productivity may not respond to temperature–moisture gradients in the same way across all topographical features.