2018 ESA Annual Meeting (August 5 -- 10)

PS 3-37 - Shrub (Prosopis velutina) recruitment in a semi-arid grassland: Precipitation-herbivory interactions reveal few constraints

Monday, August 6, 2018
ESA Exhibit Hall, New Orleans Ernest N. Morial Convention Center
William Rutherford1, Steven R. Archer1, Luis Weber-Grullon2 and Osvaldo E. Sala2,3, (1)School of Natural Resources and the Environment, University of Arizona, Tucson, AZ, (2)School of Life Sciences, Arizona State University, Tempe, AZ, (3)School of Sustainability, Arizona State University, Tempe
Background/Question/Methods

Grasslands across the US and globally have experienced marked increases in woody plant cover. Causes are highly debated and include changes in climatic, grazing, and fire regimes. Investigations have focused on these factors in isolation of each other. Here, we examine how climate-herbivore interactions at the critical establishment phase of the shrub life cycle influences shrub encroachment in semi-arid grasslands where plant recruitment is often presumed to be episodic with respect to precipitation (PPT). We ask the following question: “How do PPT, ant/rodent herbivory, and livestock grazing interact to influence velvet mesquite (Prosopis velutina) recruitment?” We test the hypothesis that mesquite recruitment is contingent on the coincidence of high PPT, livestock grazing, and lows in rodent/ant populations.

Automated Rainfall Manipulation Systems (ARMS) manipulated PPT, wherein 10 plots received +65% ambient PPT, 10 received -65% ambient PPT, and 10 received ambient PPT. Half of each plot was clipped to simulate heavy ungulate grazing; the remaining half was unclipped. Exclusion treatments (None, Rodents, Ants, Rodents+Ants) were installed within each PPT x Grazed combination. Mesquite seeds (n=5) were placed within each treatment combination in July 2017. Seedling recruitment and herbivory were recorded daily for the first four days, then weekly through September 2017.

Results/Conclusions

Precipitation and herbivore exclusion significantly (p ≤ 0.0003) impacted velvet mesquite seedling recruitment (F23, 240 = 2.62; p = 0.0001), but simulated ungulate grazing of grasses did not (p = 0.26). Recruitment on drought (-65% ambient) treatments (mean ± SE) was significantly (p < 0.005) lower than that on +65% ambient and ambient treatments (45.4 ± 1.6% vs. 54.4 ± 2.2% vs. 55.6 ± 2.2%, respectively). Herbivory on seedlings was lowest under drought conditions (18.3 ± 1.5%, p < 0.005) and highest under wetter/supplemented conditions (25.9 ± 1.9%). Recruitment declined 67% over the 2.5-month observation period. Highest levels of seedling herbivory occurred during wetter periods (Jul. 2017) compared to the dry period (Aug.-Sept. 2017). Seedling herbivory was highest on plants accessible to both rodents and ants (39.8 ± 0.8%) compared to one or the other (rodent accessible: 28.4 ± 1.3%; ant accessible: 20.3 ± 0.9%). These preliminary data demonstrate that short-term, monsoonal recruitment of mesquite in grasslands was (i) only modestly reduced by drought with insignificant differences between ambient/wet condition (ii) was not influenced by grass defoliation and (iii) was modestly constrained by rodent/ant herbivory. Collectively, results suggest short-term mesquite recruitment can potentially occur under most PPT-livestock grazing-herbivore combinations.