PARENT SESSION
Oral Session #17: Plant Ecology: Water Relations.
Presiding: W. Pockman
Monday, August 5. 1:00 PM to 3:45 PM. Coconino Meeting Room, TCC.

Competition between trees and grasses for water overwhelms effects of hydraulic lift in African savannas.

LUDWIG, FULCO^*,1, DAWSON, TODD², DE KROON, HANS³, ¹ Wageningen University, Wageningen, The Netherlands² University of California, Berkeley, Berekeley, California³ Nijmegen University, Nijmegen, The Netherlands

ABSTRACT- In a semi-arid savanna ecosystem in East Africa we conducted a field experiment to study the effects of hydraulic lift on tree-grass interactions. Under large Acacia tortilis trees, which were known to show hydraulic lift, we experimentally tested whether trees facilitate grass production through hydraulic lift or compete with grasses for belowground resources. Trenching around trees to prevent tree-grass interaction lead to an increased water content in the topsoil. This higher soil moisture availability in trenched plots resulted in an increased grass biomass production relative to the un-trenched control plots indicating that trees and grasses were competing for water in the topsoil. Stable isotope analysis of plant and source waters in the un-trenched control plots showed that grasses which compete with trees use a greater proportion of deep water compared with grasses in trenched plots. This indicates that either grasses use hydraulically lifted water provided by Acacia trees or grasses take up deep soil water directly by growing deeper roots when competition with trees occurs. We conclude that the positive effects of hydraulic lift for neighboring species is limited in this savanna system and that facilitative effects of hydraulic lift may usually be overwhelmed by water competition in (semi-) arid areas.

KEY WORDS: Hydraulic Lift, Tree-grass interactions, Competition, Facilitation