Document: JAM-3-35-36

C₃ woody plant invasion in a C₄ grassland: Are grasses and shrubs functionally equivalent?

MCCARRON, J.K.* and A.K.KNAPP

Kansas State Univeristy, Manhattan, KS, USA ¹

Abstract:
Anthropogenic disturbances and changes in land management have enabled many woody species to expand into grasslands. In C₄ dominated tallgrass prairies, invasion of C₃ shrubs have increased with the suppression of fire. Research on the mechanisms responsible for this expansion of woody species and its consequences relies on the assumptions that the C₃ shrubs are functionally equivalent and distinct from C₄ grasses. Specifically, C₄ grasses are thought to have greater water use efficiency and higher photosynthetic rates than the C₃ shrubs during the hot dry summers. In addition, grasses are thought to better compete for soil water in shallow soil layers and can take advantage of smaller precipitation amounts, while shrubs, rely more on deeper soil water and are less susceptible to short-term droughts. To evaluate these assumptions in tallgrass prairies, three C₃ shrub species, Cornus drummondii, Prunus americana, and Rhus glabra, and the dominant C₄ grass, Andropogon gerardii, were selected for intensive study on Konza Prairie. Andropogon gerardii had higher photosynthetic potential (17.8 mol m ^-2s^-1) than either C. drummondii or P. americana, but not R. glabra (11.1, 12.9, and 17.8 mol m ^-2s^-1, respectively). Rhus glabra had greater water use efficiency than either C. drummondii or P. americana, however, all were lower than A. gerardii (1.36, 0.95, 0.93, and 1.99 mol CO₂ / mol H₂O, respectively). Photosynthetic rates for A. gerardii and P. americana had greater ties to shallow soil moisture than C. drummondii or R. glabra. Our results indicate that these three shrubs could not be considered functional equivalents, nor were they, as a group, distinct from A. gerardii.

Keywords: Woody plant invasion, shrubs,Cornus drummondii, Prunus americana, Rhus glabra, Andropogon gerardii

This abstract is being presented at: 10:15 AM in session:
Oral Session #19: Grassland Restoration.