PARENT SESSION
Oral Session # 56: Plant Ecology IV: Plant - Water Relations II.
Presiding: D Rosenthal
Wednesday, August 6. 1:30 PM to 5:00 PM, SITCC Meeting Room 200.
C3 grasses replace C4 grasses and reduce oak establishment in the prairie-forest border.
Davis, Mark*,1, 1 Macalester College, Saint Paul, MN
ABSTRACT- As a result of changing land-use patterns and introductions of new species, C3 grasses have replaced C4 grasses in many grasslands along the North American prairie-forest border. Since C3 grasses are generally not as well adapted to dry conditions, the replacement of C4 grasses by C3 grasses may alter the competitive dynamics between grasses and woody plant seedlings and hence may affect the establishment success of woody plants. To test this hypothesis, a multi-factorial field experiment was conducted in which water, nitrogen, and grass functional group were manipulated in a study of germination, survival, and growth of oak seedlings. The experimental design also permitted the separate quantification of root and shoot competition by the grasses. The study showed a sharp reduction in oak seedling survival under drought conditions, and to a lesser degree under medium water conditions, for seedlings growing in C3 grasses. Analyses of root and shoot competition showed that the cause for the reduced survival in C3 plots was due primarily to underground processes. These findings confirmed the initial hypothesis that replacement of C4 grasses by C3 grasses would reduce the establishment success of woody plants. However, the explanation for the reduced oak seedling survival in C3 grasses is not clear since soil water levels (measured weekly) did not differ between C3 and C4 plots and other resource levels measured (light and NO3 and NH4) were higher in C3 plots. It is possible that the differential survival patterns could be due to short-term fluctuations soil water levels that were not detected in the weekly soil water measurements. Or, the differential survival might be due to other underground processes, such as herbivory, allelopathy, or mycorrhizal interactions, that might have differed between the C3 and C4 plots. Whatever the cause, the results show that the replacement of C4 grasses by introduced C3 grasses in water-limited environments may alter successional patterns by delaying or impeding the conversion of grass-dominated environments to woody-dominated systems.
Key words: competition, semi-arid environments, woody plant establishment, Quercus