Document: GRE-3-40-41

Uncoupling the relationship between oak trees and patch fertility in Midwestern savannas.

HOUSEMAN, G.R.* and K.L.GROSS

Michigan State University, Hickory Corners MI 49060 USA 1

Abstract:
The soils beneath isolated trees in savanna habitats often have higher fertility than adjacent grassland and this microhabitat heterogeneity may enhance diversity in these communities. Although it is assumed that tree establishment and growth fosters this enhanced fertility, an alternative hypothesis is that tree seedling establishment may be limited to higher fertility microsites in the grassland. If tree seedlings are better able to establish in high quality microsites, then fertility under tree seedlings should be significantly higher than adjacent grassland. I compared soil resource levels under isolated oak trees, seedlings, and adjacent grassland habitats in two savannas in southwestern Michigan. Site A had 63% higher peak biomass and 26% higher soil moisture than site B. On the higher productivity savanna, there was no difference in peak biomass or soil moisture among the three microhabitat types. However, NH4 pools were significantly higher under trees (0.22 ug g-1) than under seedlings (0.09 ug g-1) or in adjacent grassland (0.07 ug g-1) and there was no difference between the seedling or grassland habitats. In the lower productivity site, both available NH4 (0.25 ug g-1) and ANPP (113.66 g m-2) were significantly higher under trees than under seedlings or in grasslands which did not differ from each other. Soil moisture was higher under trees (1.2%) and seedlings (1.2%) than in grassland (0.6%) microhabitats in the lower productivity site. Thus, we found evidence for enhancement of fertility by trees rather than establishment of trees in high quality microsites. The observation that the magnitude of the effects of trees differed between sites of different productivity suggests several testable hypotheses for future studies.

Keywords: Savannas, productivity, soil fertility, soil moisture, soil nutrients

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This abstract is being presented at: 3:30 PM in session:
Poster Session #17: Vegetative Analysis.