CANCELLED - Differing controls of perennial grass mortality over consecutive years in a Sonoran Desert savanna – implications for rangeland restoration efforts.

Background/Question/Methods

Perennial grass seedling transplantation is a feature of many rangeland restoration efforts, but transplant survival is contingent upon numerous of ecological factors that may differ as transplants mature. In the establishment phase, transplants may be particularly sensitive to abiotic variables (e.g. soil texture, climate) and their influence soil moisture. As transplants establish roots in deeper soil where moisture is less ephemeral, survival may become contingent upon resource competition. To test this hypothesis, we transplanted greenhouse-grown Digitaria californica into experimental plots beneath five rainout shelters in a Sonoran Desert rangeland. Shelters contained temperature/relative humidity sensors, and plots were instrumented with soil moisture sensors at 10 cm and 25 cm depth; soil samples were collected for texture analysis at these same depths upon moisture sensor installation. Transplants were planted in the fall of 2019 and plots were subsequently irrigated with rainwater on a schedule that reflected the 1970-2020 average precipitation for the site. Near the end of the summer 2020, spring 2021, and summer 2021 growing seasons, the height and basal diameter of all surviving transplants were recorded, and plot-wide vegetation cover was estimated via the point frame method; soil samples were also collected to quantify nitrogen pools at these time points.

Results/Conclusions

In summer 2020, grass transplant survival was higher in shelters that had higher silt and clay content. Soils in these shelters also had greater cumulative days of ‘wet’ soil ( > 0.05 m3/m3) at both 10 cm and 25 cm depths, suggesting that transplants benefitted from improved soil moisture conditions associated with finer-textured soils. Analyses about the influence of microclimatic differences (e.g. vapor pressure deficit) related to rainout shelter geographic orientation are on-going. In spring 2021, inorganic soil nitrogen species were depleted in all rainout shelters. Preliminary results suggest that grass transplant mortality in summer 2021 was higher in plots with greater living vegetation cover, suggesting density-dependent interspecific competition for diminishing nitrogen pools. Forthcoming analyses will examine the role of intraspecific competition by comparing transplant survival to the size and abundance of conspecifics in the plots. These analyses will aid in our understanding of how to increase the likelihood of successful rangeland restoration strategies.