Effects of three global climate change factors on soil water and sap flow of Larrea tridentata in the Mojave Desert
AuthorHolmes, Christopher Dale
AdvisorNowak, Robert S
Environmental and Natural Resource Sciences
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<bold>ABSTRACT</bold>Water availability is the primary control of plant productivity and composition in arid plant communities. Effects of simulated global changes on soil water and sap flow of <italic>Larrea tridentata</italic> in a natural Mojave Desert ecosystem were measured over five years and two years respectively. Global change treatments were summer irrigations that imitated a northward shift of summer monsoons associated with global warming, nitrogen additions that imitated increased nitrogen deposition from anthropogenic sources, and biological crust disturbance that imitated disturbance from increased land use. Averaged over all data, irrigated plots had 40% and 24% more soil water than non-irrigated plots at 0.2 m and 0.4 m depths, respectively. Soil water from irrigation treatments consistently reached 0.6 m depth but only persisted in irrigated plots until October or November and hence is unlikely to benefit deeper rooted C3 vegetation that is active during late winter and spring. At no time during the study did soil water infiltrate beyond the rooting zone, suggesting that deep percolation of soil water is very rare for this ecosystem. A significant year-long effect of intact biological soil crusts was not found. However within 2-4 days following summer irrigation, plots with intact biological soil crusts were wetter than plots with disturbed biological soil crusts to 0.6 m depths. No consistent effects of nitrogen deposition on soil water content were found. <italic>L. tridentata</italic> is one of the most abundant and widespread perennial plants in deserts of southwestern North America. Averaged over all treatments, <italic>L. tridentata</italic> exposed to summer irrigation treatments have significantly greater sap flow than plants in non-irrigated plots. This irrigation effect lasts approximately seven weeks after the last irrigation when a transition is made and plants in non-irrigated plots begin to have significantly greater sap flow until the irrigations of the following year. Not only is<italic> L. tridentata</italic> able to respond to an increase in soil water during the hot summer months, but evidence is also presented that suggests<italic> L. tridentata</italic> has an additional physiological response to the increase in soil water within a two week period. <italic>L. tridentata</italic> plants exposed to the highest nitrogen addition consistently had the greatest rates of sap flow. The effect of the biological soil crust treatment on sap flow of <italic>L. tridentata</italic> was variable and dependant on interactions with the irrigation and nitrogen treatments. These results demonstrate that predicted impacts of global changes have the potential to have important effects on the Mojave Desert by affecting the primary production of <italic>L. tridentata</italic> and the ecohydrology of the ecosystem.