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.
Robert P. Griffiths
Although forest meadows in the Central Oregon Cascade Mountains make up a relatively small fraction of total area, they contain a large variety of plant species that greatly enrich biodiversity over the landscape (Hickman, 1976). Under present climatic and forest management conditions, many high-dry mountain meadows of Pacific Northwest are being invaded by the surrounding forest providing an opportunity to study changes in soil properties in response to vegetative succession (Franklin et al., 1971: Magee and Antos, 1992; Yakimenko, 1997; Miller and Halpern, 1998).
It is likely that these meadows were originally established and maintained by aboriginal burning (Miller and Halpern, 1998). However, factors responsible for the current invasion are not known with certainty but climate change, fire suppression and termination of sheep grazing may all have played a role (Popenoe et al., 1992; Miller and Halpern, 1998). In a comprehensive study of mountain tree invasion in the Central Oregon Cascade Mountains, Miller and Halpern (1998) noted that in addition to the allegoric factors mentioned above, autogenic factors may also control this process (i.e. local influence of trees on the establishment of seedlings by altering microclimate). In addition to controlling moisture, the pioneer trees could also be altering soil properties resulting in increased seedling establishment.
Vegetation can profoundly influence both the chemistry and biology of soils; altering soils in a way that enhances plant community resiliency to perturbation (Perry et al.1989). To a large degree, this finding explains why forests that have been disturbed by fire, disease, wind-throw, harvesting or other factors typically return to the same vegetative assemblage that was present before the disturbance. Similar mechanisms may explain why invading vegetation alter soils to favor trees rather than the original meadow vegetation.
Comparative studies between grasslands and forests have shown large differences in soil chemistry (Göceoðlu, 1988; Hart et al., 1992; Popenoe et al., 1992; Ross et al., 1996; Yakimenko, 1997); litter decomposition rates (Hunt et al., 1988; Köchy and Wilson, 1997) and food web compositions (Hunt et al., 1987; Ingham at al., 1989). Our study was designed to measure changes in both the chemical and biological characteristics of high-elevation mountain meadows as adjacent forests invade them. This includes an analysis of the transition zone to obtain a rough idea of which soil properties are most rapidly altered in response to the tree invasion.
Within the context maintaining biodiversity and habitat diversity, forest managers are looking techniques to reverse invasion high elevation mountain meadows by surrounding trees (Popenoe et al., 1992). One of the objectives of this study was to provide basic information about biogeochemical transformations associated with tree invasion that could be used to monitor the effectiveness of different treatments.