Conifer invasion of forest meadows transforms soil characteristics in the Pacific Northwest

In many areas, chemical and biological characteristics of soils change when trees colonize meadows. To determine if the invasion of high meadows by forests in the central Cascade Mountains of Oregon altered soil properties, we measured soil properties along transects from mountain meadows through transition zones, where trees were becoming established, into mature forest. The differences observed in this study support the view that N is more available in meadow soils than in forest soils and that N pools and cycling change markedly when trees invade mountain meadows. β-Glucosidase activity in the transition zone soil was close to that in the forest soil and much lower than that in the meadow soil, suggesting qualitative changes in microbial populations as microorganisms adjusted to changes in litter quality. High correlations between litter depth and most variables in meadow soil, which were not observed in transition zone soil or mature forest soil, suggest that litter may control other aspects of biogeochemical cycling in meadows. With the exception of laboratory respiration, the values observed in the transition zone soil lay between those in meadow soil and those in forest soil; in most cases, they were closer to those in forest soil. This suggests that soil properties shift rapidly toward those found in forests as trees invade meadows. These rapid changes may alter soils so that they are more likely to support trees than grass.