Astragalus tyghensis: Actual vs. predicted population sizes
A 1990 study (Kaye et al., 1990) documented the abundance of Astragalus tyghensis on public and private lands in Oregon. Long-term monitoring for Astragalus tyghensis (Tygh Valley milkvetch) was conducted annually between 1991
A 1990 study (Kaye et al., 1990) documented the abundance of Astragalus tyghensis on public and private lands in Oregon. Long-term monitoring for Astragalus tyghensis (Tygh Valley milkvetch) was conducted annually between 1991 and 2000. This work lead to descriptions of the species’ life-history, its long-term trends at five sites, and the development of computer models of population behavior based on demographic processes. Despite the improvement in our knowledge of the species’ distribution, it remains rare and restricted to a relatively small area of Wasco County, Oregon. Analysis of aerial photographs shows that the habitat of the species has declined markedly since the area was settled and land was converted to range and cultivation (Kaye et al., 1990). The studies described here help assess the predictive power of population models for this species, as well as the health and long-term trends of individual populations. Until 2006, no follow up monitoring of the species was conducted. In this report, we detail the results of monitoring Astragalus tyghensis (Tygh Valley milkvetch) on Bureau of Land Management (BLM) and state land in 2006 – 2008 and compare these data to predictions from the population models created using the data from 1991–2000.
The goals of this cooperative project were to:
1. Re-sample permanent monitoring plots at five sites (four BLM and one State of Oregon) to determine current population status.
2. Compare current population size and structure to projections of computer models developed using population data collected form 1990–2000.
- In 2006 – 2008 sites 10, 25 and 41 appear to be relatively stable while sites 4 and 13 appear to be in decline.
As in previous years, much of the variability between sites and years appears to be driven by the abundance of seedlings.
The structure of A. tyghenssis populations was generally skewed toward smaller plants in all years and at all sites.
There were differences among sites in reproductive plant diameter, length of longest stem, inflorescence number, and number of seeds per plant during each year of the study. As in the previous two years, the largest plants (both diameter and length of longest stem) were found at site 13. The number of inflorescences per plant has declined since 2006, from an average of 12 in 2006, to 4 in 2007, and 1.6 in 2008.