Influence of coniferous tree invasion on forest meadow soil properties on Bunch Grass Ridge and Deer Creek near the Andrews Experimental Forest

Measurements were made along transects running from mountain meadows, into transition zones where trees were invading meadows and then into mature forest to determine if the invasion of high central Oregon Cascade Mountain meadows by the surrounding forest altered soil properties. Prior studies shown that meadow soil chemical and biological characteristic change when they are invaded by surrounding trees. For instance meadow litter has been shown to be enriched in nitrogen when compared with tree litter. In this study, differences in nitrogen pools and cycling were observed supporting the view that nitrogen is more available in meadow soils than in forests and that these differences change rapidly when tree invade mountain meadows. As trees invade meadows, ß-glucosidase activity is also rapidly reduced suggesting that qualitative changes are taking place in microbial populations as microorganisms adjust to changes in litter quality. High correlations between litter depth and most variables suggest that meadow litter may control other aspects of biogeochemical cycling; a relationship not observed in the transition zone or the mature forest. With one exception, the values observed in the transition zone were intermediate between those in meadows and those in forest soils. In most cases, the values found in this zone were closer to those found in mature forests than in meadow soils suggesting that when trees invade meadows, soil properties are rapidly shifted toward those found in forests. These rapid changes may alter soils so that they are more likely to support trees than grass. This may partially explain why, areas where trees had been cut after they became established as small islands within the meadow are rapidly recolonized by trees rather than grass.