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Community ecology, climate change and ecohydrology in desert grassland and shrubland

Petrie, M. D.. 2014. University of New Mexico°, Ph.D


This dissertation explores the climate, ecology and hydrology of Chihuahuan Desert ecosystems in the context of global climate change. In coming decades, the southwestern United States is projected to experience greater temperature-driven aridity, possible small decreases in annual precipitation, and a later onset of summer monsoon rainfall. These changes may have profound consequences for ecological systems in the Chihuahuan Desert, which are intensely water-limited and respond to even small pulses of moisture availability. The first chapter (Chapter 2) compares change in the properties of monsoon season precipitation from 1910-2010 in the northern Chihuahuan Desert region to local variability in monsoon precipitation from 2001-2012 at the Desert's northern ecological boundary. From 1910-2010, monsoon precipitation events in the Chihuahuan Desert increased in frequency and decreased in magnitude, and the longest wet and dry periods increased in length. These changes may be masked locally due to high annual variation in total seasonal precipitation, however, which suggests a limited capability for small, average change in the properties of precipitation events alone to influence ecosystem processes in a changing climate. Chapter 3 explores differences in carbon exchanges between established grassland and shrubland communities at the Chihuahuan Desert's northern ecological boundary. During slightly dry years, grassland was highly sensitive to moisture limitation and was a net carbon source, while shrubland was a net carbon sink. These results support the conclusion that ecosystem carbon sequestration in the Chihuahuan Desert will increase if grassland to shrubland state transitions occur. Chapter 4 explores the importance of small rainfall events for grassland nutrient availability and aboveground net primary productivity during dry and wet monsoon seasons. In a warming environment, the residence time of small events as soil moisture will decrease, and results from this chapter show that the experimental removal of only small events during the summer monsoon in desert grassland can promote vegetation senescence and reduce soil nutrient availability, and thus inhibit grassland recovery during subsequent wet monsoons. As a whole, this dissertation research characterizes variation in monsoon season precipitation from local to regional spatial scales and from annual to inter-decadal timescales, explores the potential for greater ecosystem carbon sequestration as a consequence of grassland to shrubland state transitions, and evaluates the ecological role of very small rainfall events during the summer monsoon. While the scope of these topics is broad, this research addresses key questions regarding climate inputs and ecological processes in Chihuahuan Desert ecosystems, and also sharpens scientific understanding of how climate change may be enhanced or moderated by the sensitivity of ecological processes in aridlands to precipitation and temperature.