Date of Award:

5-2016

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Ecology

Committee Chair(s)

Andrew Kulmatiski

Committee

Andrew Kulmatiski

Committee

Karen H. Beard

Committee

Scott Jones

Abstract

Climate change has the potential to cause large-scale changes in plant growth, biodiversity, and biosphere-climate feedbacks. A pervasive aspect of climate change is that as the atmosphere warms, precipitation events are likely to become less frequent but more intense, because warmer air can hold more water. Larger precipitation events can be expected to change plant productivity and community composition, particularly in semiarid ecosystems such as savannas. Savannas are of particular interest because they are spatially expansive at the global scale, they are important to humans for food production, and they are known to be sensitive to changes in soil water availability. Extensive efforts have been made to understand how increases or decreases in total precipitation will affect plant growth, but relatively little is known about how increasing precipitation intensity will affect grass and tree growth in savannas.

Here we use precipitation manipulation shelters in a semiarid savanna system, Kruger National Park, South Africa to examine grass and tree response to changes in precipitation intensity. The shelters collected and stored 50% of ambient precipitation, then redeposited collected water as relatively large precipitation events. Grass and tree growth, and root growth and activity were monitored in treated plots and untreated control plots from 2008-2013.

Small changes in precipitation intensity resulted in large increases in plant available water, particularly at 30-60 cm depths during the first three years of the study. Due to naturally large precipitation events, plant available water was similar between treatment and control plots in the last two years of the study. In the first three years, grass growth decreased and tree growth increased in treatment relative to control plots. Treatment effects disappeared for both grasses and trees in the last two years of the study when treatment effects were small. Our study revealed rapid savanna responses to changes in precipitation intensity.

Our results suggest increased precipitation intensity plays a part in shrub encroachment alongside fire suppression, grazing, and rising atmospheric CO2 concentrations. Increasing precipitation intensity could decrease grass litter buildup, affect fire regimes, and could mean better habitat for browsers, worse habitat for grazers. While small increases in precipitation, like those in this study, are likely to increase plant productivity or aquifer recharge, larger increases may increase runoff and erosion and decrease productivity. Moreover, our data could be used to help develop new crop strains more compatible with climate change in this part of the world.

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