Population dynamics of Mulford’s milkvetch

Although Oregon is well known for its coniferous forests, low valley prairies, and coastline that receive high amounts of rainfall, nearly two thirds of the state exhibits semi-arid habitat conditions.  Oregon’s eastern desert region experiences greater seasonal fluctuation than the western portion of the state and is inhabited by plants and animals that can tolerate extreme differences in weather.  The high desert receives significant amounts of snow in the winter, but quickly dries out in the spring.  The characteristic plant community in eastern Oregon consists of juniper and sagebrush scrublands with bunchgrasses such as needle-and-thread grass (Hesperostipa comata) and Indian rice grass (Achnatherum hymenoides).

Astragalus mulfordiae (Mulford’s milkvetch), listed as a Species of Concern by the US Fish and Wildlife Service, is a member of the legume family (Fabaceae) native to the Snake River Plain of Oregon and Idaho.  In 1995, only 38 populations of Mulford’s milkvetch were known to occur in Oregon, in addition to 34 populations in Idaho.   It was estimated that, in total, there were less than 12,000 living individuals.  Mulford’s milkvetch only occurs in desert shrub communities on sandy substrates, such as lacustrine and alluvial sediments.  The plant relies on environmental cues to initiate regrowth, but it can generally be seen flowering between April and June.  Flowers are yellowish to white in color and measure 6-8 mm in length.  Fruits usually mature in June and July.      

IAE has tracked Mulford’s milkvetch populations in the BLM’s Vale district since 2008, beginning with the establishment of monitoring plots to determine the effects of ungulate and rodent grazing on the populations.  Large animal exclosures and unfenced control plots were established at five different sites around Vale, Oregon.  In the second year of monitoring, transects were established in order to collect data on the plant community associated with Mulford’s milkvetch.   The monitoring protocol used by IAE is consistent with the methods being used to monitor the populations in Idaho, allowing for comparable data sets.  During annual monitoring, data regarding plant life history stage (reproductive, non-reproductive, seedling) and evidence of herbivory and disturbance were documented, as well as quantitative data on plant size and number of inflorescences (flowering stems).  Population trends were difficult to discern after the first few years of monitoring, but continued monitoring may provide more information about the effects of mammal grazing on the size and health of the Mulford’s milkvetch populations.  Currently, there appears to be high turnover of individual plants.  The community sampling transects have revealed ubiquitous invasive and non-native plants that could potentially compete with the native milkvetch.  IAE will continue to monitor these populations to increase understanding of population dynamics for this rare species, and to analyze the potential threats of cattle grazing, rodent activity, encroachment of invasives, and ground disturbance.