PARENT SESSION
Poster Session #45: Gas Exchange.
Friday, August 10, 2001. Presentation from 10:30 AM to 12:00 PM. Exhibition Hall

Gallery forest expansion in tallgrass prairie: mechanisms facilitating oak success.

DANNER, BRETT¹, KNAPP, ALAN¹, ¹

ABSTRACT- Gallery forests, dominated by oaks, have expanded significantly into tallgrass prairie in the past century. Because grasslands are characterized by high temperatures and evaporative demand compared to forests, we hypothesized that belowground, oaks would minimize competition for water with the grasses and that aboveground, the prairie-forest ecotone may facilitate oak establishment by reducing microclimatic extremes. Greenhouse studies indicated that seedlings of both Quercus macrocarpa and Q. muhlenbergii were capable of extending their roots to soil depths of 70cm within 10 weeks. In a drought year (precipitation 44% below long-term average), oak seedling carbon gain was greater under shade conditions, similar to the lower PFD of the prairie-forest ecotone, compared to high light seedlings. However, in oak juveniles, photosynthetic rates were reduced by ca. 20% in ecotone vs. prairie sites. In both seedlings and juveniles, carbon and water relations were unaffected by the removal of the dominant C₄ grasses. We concluded that oak seedling root growth is well adapted to rapidly reach deep soil moisture, avoiding belowground competition with the grasses. Moreover, although low PFD levels characteristic of the prairie-forest ecotone may facilitate carbon gain in oak seedlings, carbon gain in oak juveniles was significantly greater in prairie sites. Since 92% of oak seedlings survived a drought year in prairie sites, facilitation by the prairie-forest ecotone does not appear to be a requirement for gallery forest expansion in tallgrass prairie.

KEY WORDS: gallery forest, forest expansion, Quercus, facilitation