Ecology of Rusby's Milkvetch (Astragalus rusbyi), a Rare Endemic of Northern Arizona Ponderosa Pine Forests

Abstract

Rusby’s milkvetch (Astragalus rusbyi Greene) is endemic to basaltic soils northwest and west of Flagstaff, Arizona. Recent interest in this species is due in part to its addition to the U.S. Forest Service Region 3 sensitive species list in 1999 and its occurrence in ecological restoration projects and proposed fuels reduction projects that involve tree thinning and prescribed burning. Some of its habitat has been subjected to large wildfires over the last few decades, and other areas have undergone ecological restoration treatments, while much of its range in ponderosa pine forest is slated to undergo such treatments in the near future. In a ponderosa pine restoration study area northwest of Flagstaff, A. rusbyi was an indicator species of remnant grass patches and increased following tree thinning and prescribed burning. However, in an area less than 3 km away, there appeared to be no relationship to restoration treatments, trees per ha, pine basal area, or canopy cover, but A. rusbyi did appear to be sensitive to an extreme drought event in 2002 and may have remained dormant in that year, a pattern that has been observed in other rare Astragalus species. A. rusbyi has a foliar nitrogen content of 4.4% and a foliar C:N mass ratio of 11. It is classified as a competitive ruderal species, meaning it is able to compete well with other understory species, but is not very tolerant of stresses, such as deep shade. We currently do not have a thorough understanding of the ecology of this species, or the effects of ecological restoration or fuels reduction treatments. In this paper we will discuss ecology of other members of the genus Astragalus and explore the relationships of A. rusbyi to moisture, vegetation treatments and overstory mortality.