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 2016, we monitored plots testing for effects of herbivory on A. mulfordiae in permanent plots along with transects looking at long-term population trends and plant community composition.
• In 2016 we observed similar densities of A. mulfordiae to 2015. We counted 164 plants total, with 65 in caged plots and 99 in uncaged plots. 62 new plants were observed in 2016. Despite a 50.5% increase that occurred from 2014 to 2015, total number of plants still remains much less than numbers observed in 2008-2010.
• In 2016, 24 plots that were once established due to high densities of A. mulfordiae no longer contained the species. This was similar to the empty plots observed in 2012. In 2014, we removed plots that had been empty since 2012.
• We found that size of A. mulfordiae differed significantly by site, and by treatment (caged or uncaged) for certain sites in 2016. Similarly to 2015, plants at Brown Butte, North Harper North, and Snively were the largest while those at Double Mountain and North Harper South were the smallest.
• Reproductive effort was variable across sites, with Double Mountain and North Harper North having the fewest reproductive plants (3 at each site) and South Alkali having the most reproductive plants (36). Brown Butte and Snively had the highest average number of fruits per reproductive plant (186.2 and 118.0, respectively), which was mainly due to just a few large plants that produced many fruits. All of the plants at Snively were reproductive.
• In plots established in 2008, we observed an increase in total number of plants from 2015 to 2016. Although the increase in number of plants was slight this year, it is promising to observe an increasing trend over the last two 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 53 to 77 in 2016. Despite this increase, total number of plants is still much less than that observed in 2012 (121 plants total).
• In 2016 40% of plants in the transects were reproductive, which was a slight decline from that seen in 2015 (53%). In 2016 30% were seedlings, which was an increase from 19% observed in 2015.
• The plant communities at these sites have varied greatly from 2010 to 2016. Exotic species cover (as percent of total cover), primarily composed of Bromus tectorum (cheatgrass), has increased from 2010 to 2016 at all sites but South Alkali. While in 2010 all of the sites were native dominated, in 2015 all sites were dominated by exotic species. In 2016, Double Mountain, North Harper North, and South Alkali were native dominated, and 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. 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. Cattle grazing may have more impacts on A. mulfordiae in other populations with higher stocking rates and grazing during the growth period.
• Continued monitoring and immediate attention may be necessary to mitigate any future losses of this rare species.