Document: BRE-3-35-8

Physiological ecology of juvenile oaks: Implications for forest expansion in grasslands.

DANNER, B.T.* and A.K.KNAPP

Kansas State University, Manhattan, KS 66506 USA. ¹

Abstract:
Worldwide, many grassland ecosystems have been invaded by woody vegetation with a variety of causal factors implicated (e.g., fire suppression, grazing management). Tallgrass prairie is no exception, where gallery forests dominated by Quercus macrocarpa Michx. and Q. muhlenbergii Engelm. have expanded into adjacent grasslands. To assess the degree of competitive interactions between trees and dominant grasses in prairie sites and along the prairie-forest ecotone, we conducted a field experiment in which prairie grasses were removed adjacent to oak juveniles. We predicted that the ecotone environment would facilitate oak success relative to the prairie. Both oak species reponded positively to aboveground removal of grasses in prairie with a 20% increase in photosynthesis. This response was not found along the ecotone. Photosynthesis in ecotone oaks was 33% lower compared to prairie juveniles. Stomatal conductance did not respond to removal of grasses for either species in prairie and ecotone. Greenhouse experiments indicated that seedlings of both species can extend their roots to soil depths greater than 80 cm in 10 weeks. This deep root system may explain the higher predawn water potentials in the oaks compared to the dominant C₄ grasses. Overall, grass removal did not affect oak water relations. We concluded that the grasses negatively impacted carbon gain in oak juveniles in the prairie, and that overstory trees along the ecotone did not facilitate carbon gain or water relations in oak juveniles.

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This abstract is being presented at: 8:30 AM in session:
Oral Session #19: Grassland Restoration.