No abstract

A current, widespread example of vegetation change is the invasion of grassland by woody plants. This is associated with an increase in soil heterogeneity, and it has been argued that woody plants both cause and benefit from high heterogeneity. We know of no experimental demonstrations of differences between grasses and woody plants in their effects on heterogeneity. Here we compare heterogeneity between mixed-grass prairie and aspen forest, and we report the results of a soil transplant experiment that tested for differences between these vegetation types in their effects on soil resource heterogeneity. We measured the heterogeneity of resources and plant mass along 10 transects in both prairie and aspen forest in spring and summer. Light and available nitrogen (N; sum of ammonium and nitrate) were significantly more variable in forest than prairie, as were root and understory shoot mass. The variability of soil moisture and topography did not differ between prairie and forest. In our experiment, N and water in cores of prairie soil moved to forest attained the relatively high variability of forest soils. Further, forest soils moved to prairie attained the relatively low variability of prairie soils. In summary, both the biomass heterogeneity measurements and the soil transplant experiment suggested that plant uptake contributed to greater heterogeneity in forests.

No abstract

No abstract

Few studies have compared scales of heterogeneity among plant communities. We predicted that differences in the sizes of dominant species should allow us to detect small-scale (<256 cm) heterogeneity in mixed-grass prairie but not in adjacent aspen forest. We examined light penetration, soil moisture, available N, elevation, species composition, and plant mass at 10 locations in prairie and forest in both spring and summer. Variables were measured in 1-cm2 plots arranged in pairs separated by 0-256 cm. Several variables in prairie (elevation, litter mass, light penetration, and species composition) showed significant evidence for scale within the range examined. In contrast, only one variable in forest (light penetration in summer) showed evidence for scale in the same range. The scale of heterogeneity in prairie was consistent with the scale of two possible causes, species composition and elevation variability due to northern pocket gopher (Thomomys talpoides Richardson) activity, both of which varied significantly in prairie but not in forest. Whereas some aboveground factors (light and litter mass) varied within the range examined, belowground factors (water and N) did not, suggesting that the scale of heterogeneity differs between above- and below-ground factors. In total, the results suggest that differences in the scale of heterogeneity between prairie and forest reflect the relative sizes of the dominant plants.Key words: aspen, forest, heterogeneity, light, nitrogen, prairie, scale, soil, water.