Oak Tree Effects on Soil and Herbaceous Vegetation in Savannas and Pastures in Wisconsin

Abstract

To study tree/nontree interactions, soil characteristics, aboveground biomass and plant composition were compared in areas under and adjacent to canopies of open grown bur oaks (Quercus macrocarpa) and northern pin oak x black oak hybrids (Q. ellipsoidalis x velutina) in two savannas and two pastures in S-central Wisconsin. One savanna and one pasture were paired on loam soils, the other sites were on sandy soils. In general, soil moisture was higher below tree canopies than outside canopies during a drying trend and was similar between areas after a period of rain. Soil organic matter and potassium (K) decreased with increasing distance from tree boles on all sites, while phosphorus (P) showed a similar trend only on the pastures. Pastures had greater soil organic matter and P than savannas of similar soil texture, while the loam soil sites had higher soil pH, organic matter, K, Ca and Mg than sandy sites of similar disturbance history. Areas below canopies received 67% of the ambient rainfall and 27-48% of the ambient photosynthetically active radiation and had 2.3-5.2 C lower soil temperatures compared to areas outside the canopies. Although soil moisture, nutrient and organic matter levels seemed more favorable for plant growth under the canopies, aboveground biomass was lower below canopies compared to open areas at the two savannas, while biomass was equal between canopy and open areas on the one pasture that was measured. This indicates that other factors, such as light, were more important in determining plant biomass. For all sites, plant composition under tree canopies differed from that outside the canopies. Savanna plant species were predominantly perennial and native while pasture species were mostly perennial and exotic. Grazing, fire and previous introductions of prairie species may have been major factors in determining the prevalent species (>5% cover) for a given site. Poa spp. were more prevalent below canopies compared to open areas on all sites, despite their different disturbance histories.