PARENT SESSION
Poster Session 1: Photosynthesis and Water Relations
Monday, August 8, 5:00 PM - 6:30 PM, Exhibit Hall 220 A-E, Level 2, Palais des congrès de Montréal

Experimental warming and altered rainfall distribution in southern oak savanna: plant-soil water relations.

Volder, Astrid^*,1, Briske, David¹, Tjoelker, Mark¹, ¹ Texas A&M University, College Station, TX

ABSTRACT- The structure and function of savanna ecosystems may be mediated, in part, by grass-tree competition for soil water. In southern oak savanna in Central Texas, Quercus stellata (post oak) is the dominant deciduous tree, Schizachyrium scoparium (little bluestem) is the dominant grass (C₄), and Juniperus virginiana (juniper) is a woody evergreen invader. Climate warming and changes in seasonal distribution of rainfall have the potential to modify soil water availability and alter competitive interactions between grass and trees. In replicated rainfall exclusion shelters, we manipulated rainfall events to both intensify summer drought and augment cool season rainfall compared to the long-term mean. We planted monocultures of the three species and two grass-tree combinations in each shelter. Plots were subjected to a warming and control treatment. This factorial design enabled us to investigate effects of rainfall distribution and warming both independently and in combination. Rainfall redistribution significantly decreased soil water by 31 % during the dry summer phase and caused more negative pre-dawn and mid-day water potentials for all three species. Mid-day water potentials were most negative for oak and least negative for little bluestem and this species order was reversed for pre-dawn water potentials. Summer mid-day water potential data suggest that juniper experienced competition for water from little bluestem, while little bluestem was benefiting from the presence of juniper. The presence of grass increased pre-dawn water stress during intensified summer drought for both woody plants, yet the presence of woody plants did not affect pre-dawn water potential of the grass. There is a strong interactive effect between global change drivers and species combination on plant-soil water relations that will mediate savanna responses to rainfall redistribution and climate warming.

Key words: climate change, oak savanna, rainfall redistribution, water potential