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

EXECUTIVE SUMMARY
This document summarizes monitoring of Astragalus mulfordiae (Mulford’s milkvetch) on land managed by the BLM Vale District. In 2017, we monitored plots testing for effects of herbivory on A. mulfordiae in permanent plots along with transects monitoring long-term population trends and plant community composition.

• In 2017 we observed an increase in number of A. mulfordiae from 2016. We counted 242 plants total, with 78 in caged plots and 164 in uncaged plots. 124 new plants were observed in 2017. Despite there being over a two-fold increase since 2014, total number of plants still remains less than numbers observed in 2008-2010.
• In 2017, 18 plots that were once established due to high densities of A. mulfordiae no longer contained the species. However, 8 plots that were found empty last year were found to have new A. mulfordiae individuals in 2017.
• We found that size of A. mulfordiae differed significantly by site and by treatment (caged or uncaged) for certain sites in 2017. Plants at Snively, South Alkali, and Brown Butte were the largest while those at North Harper North and Double Mountain were the smallest.
• Reproductive effort was variable across sites, with Double Mountain and North Harper North having the fewest reproductive plants (2 and 5, respectively) and South Alkali having the most reproductive plants (38). North Harper South and Brown Butte had the highest average number of fruits per reproductive plant (120 and 74, respectively), which was mainly due to just a few large plants that produced many fruits.
• In plots established in 2008, we observed an increase in total number of plants from 2016 to 2017. It is promising to observe an increasing trend over the last three years after the decline in A. mulfordiae that had been exhibited from 2008-2014 across all plots and sites.
• In population monitoring transects, number of A. mulfordiae increased from 77 to 134 in 2017. Despite this increase, total number of plants is still less than that observed in 2010 (178 plants total).
• In 2017, 19% of plants observed along the transects were reproductive, which was a decrease from that seen in 2016 (40%). In 2017, 45% were seedlings, which was an increase from 30% observed in 2016.
• The plant communities at these sites have varied from 2010 to 2017. Exotic species cover (as percent of total cover), primarily composed of Bromus tectorum (cheatgrass), has increased from 2010 to 2017 at all sites except North Harper North and South Alkali. In 2017, all sites were exotic-dominated except for North Harper North.
• We found little evidence that cattle grazing has had consistent effects on the populations of A. mulfordiae at these sites. While stocking rates have remained similar on sites, populations have varied greatly over the years. The one site with a higher stocking rate, South Alkali, is grazed during the winter, so impacts on the plants are likely minimal.
• Continued monitoring and immediate attention may be necessary to mitigate any future losses of this rare species.