'Rooting out the difference: A quantitative test of the two-layer hypothesis in a sub-tropical African savanna,' May 18

Friday, May 18, 10 a.m.
ConocoPhillips Integrated Science Building, Room 105A

Michael Mazzacavallo, graduate student with Biological Sciences at UAA, will defend his thesis on Friday, May 18. Titled "Rooting out the difference: A quantitative test of the two-layer hypothesis in a sub-tropical African savanna," Mazzacavallo's thesis will cover the following topics...

Knowledge of the location, timing and extent of water use by plants is believed to be critical to understanding both plant community development and hydrologic budgets, especially in semi-arid systems. However, the difficulties of belowground research have largely prevented direct measurements of water use by plants in diverse, wildland systems.

Here we combine a depth-controlled tracer technique with micrometeorological measurements and Penman-Monteith-type energy budget models to estimate the amount of water removed from specific soil depths by trees and grasses through a growing season in a stable, sub-tropical savanna on shallow clay soils in Letaba, Kruger National Park, South Africa. Soil moisture content was well predicted by precipitation, modeled evapotranspiration estimates and measurements of deep soil infiltration (R2 = 0.89). Consistent with the two-layer hypothesis, grasses obtained most of their water from the shallowest soils, specifically in the top 25 cm. Trees showed relatively consistent uptake across the top 70 cm. However, because grasses transpired 2.5 times more water than trees, grasses removed similar or greater amounts of soil water from all soil depths than trees.

Results provide the quantitative estimates of water use needed to understand tree and grass coexistence in savannas and how savanna plant growth and water availability is likely to respond to climate change.