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

This document summarizes monitoring of Astragalus mulfordiae (Mulford’s milkvetch) on land managed by the BLM Vale District.  In 2015, 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 2015 we observed an increase in densities of A. mulfordiae from 2014, which had the lowest densities since 2008.  We counted 164 plants total, with 75 in caged plots and 89 in uncaged plots.  78 new plants were observed in 2015.  Despite a 50.5% increase from 2014, total number of plants still remains much less than numbers observed in 2008-2010.
• In 2015, 23 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, but not by treatment (caged or uncaged) in 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 varied across the sites.  South Alkali had the highest number of fruits per plant while those at North Harper North had the lowest across the sites.  All of the plants at Brown Butte were reproductive.  One plant at South Alkali that had 615 fruits in 2014 had 344 fruits in 2015.
• In plots established in 2008, we observed an increase in total number of plants from 2014 to 2015.  There was a very large increase in plants at North Harper South from 2014-2015 (700% increase), where we observed more plants in 2015 than there were in these plots in 2008 when plots were established.  Although the increases in number of plants were slighter for the other sites, it is promising to observe an increasing trend after the decline that had been exhibited from 2008-2014 across all plots and sites.
• In population monitoring transects, number of Astragalus mulfordiae increased from 42 to 53 in 2015, following a severe decline exhibited in recent years (65% from 2012 to 2014).  One seedling was found in the transect at Double Mountain in 2014, following a year with no plants in 2013.  In 2015, the plant was reproductive.
• In 2015 53% of plants in the transects were reproductive, which was a slight decline from that seen in 2014 (76%).  In 2015 19% were seedlings, which was an increase from 9% observed in 2014.
• The plant communities at these sites have varied greatly from 2010 to 2015. Cover of exotic species, particularly cover of Bromus tectorum (cheatgrass) has continued to increase at all sites, except South Alkali.  While in 2010 all of the sites were native dominated, in 2015 all sites, except North Harper North, were dominated by exotic species.

• We found little evidence that cattle grazing has had consistent effects on the populations of 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.