Astragalus mulfordiae: Population dynamics and the effect of cattle grazing in the Vale District, BLM

EXECUTIVE SUMMARY
This document summarizes the monitoring of Astragalus mulfordiae (Mulford’s milkvetch) on land managed by the BLM Vale District. In 2018, we revisited permanent plots testing for the effects of herbivory on A. mulfordiae, along with permanent transects looking at long-term population trends and plant community composition in A. mulfordiae habitat.
•In 2018 we observed a decrease in number of A. mulfordiae. We counted a total of 103 plants, 49 in caged plots and 54 in uncaged plots, compared to 242 found in 2017. Only nine new plants were observed. Despite there being over a two-fold increase from 2014 to 2017, the total number of plants remained at about one third of the numbers observed in 2008-2010.
•Thirty-two plots that were originally established in high densities of A. mulfordiae no longer contained the species, almost doubling the number of empty plots found in 2017. No previously-empty plots were re-populated.
•We found that the size (diameter) of A. mulfordiae plants differed by treatment at Snively and South Alkali in 2018, but when averaged across all observed plants, neither treatment nor site was associated with a significant difference in plant size.
•Reproductive effort was variable across sites, with Brown Butte, North Harper North, and Snively having the fewest reproductive plants (three, two, and two, respectively) and South Alkali having the most (21). North Harper South had the highest average number of fruits per reproductive plant (39), but there was no significant difference between sites or treatments.
•Focusing on plots established in 2008, we observed a decrease in total number of plants from 2017 to 2018. Due to seedling die-off, the increase in A. mulfordiae observed in 2017 did not lead to significant population growth in 2018. The long-term decreasing trend in population size exhibited from 2008-2014 across all plots and sites appeared to resume in 2018.
•Along population monitoring transects, the number of A. mulfordiae decreased from 134 to 46 in 2018. Despite a pulse of seedlings in 2017, the total number of plants still lies far below that observed in 2010 (178 plants total), mirroring observations from the herbivory plots.
•In 2018, 65% of plants observed along transects were reproductive, an increase from 2017 (19% reproductive). Seedlings accounted for 28%, similar to levels observed in 2016, prior to the 2017 increase.
•Plant communities at these sites have varied from 2010 to 2018 with a general trend of increasing exotic species cover, primarily due to the presence of Bromus tectorum (cheatgrass). In 2018, however, only Brown Butte and Snively were dominated by exotic species.
•We found little evidence that cattle grazing has had consistent effects on the populations of A. mulfordiae at these sites.
•Continued monitoring and immediate attention may be necessary to mitigate any future losses of this rare species.