Wildfires have always been a feature of the western landscape, despite intense efforts of the past century to suppress them. When multiple fires impact the same rare plant populations in the span of seven years, they become a growing concern to plant conservationists. The Milepost 10, Colockum Tarps, Spartan, and Colockum fires have burned part or all of the entire extent of Whited’s milk-vetch’s (Astragalus sinuatus) habitat near Wenatchee.
A population of Whited’s milk-vetch’s on Bureau of Land Management (BLM) land was impacted by three of these recent fires. The site was historically grazed, but after it was designated as an Area of Critical Environmental Concern (ACEC) in 1985, the site was fenced off to protect the Whited’s milk-vetch population. Since then, studies have been conducted at the site to learn more about this species and the site has been managed to protect it. In 2013, a year prior to the Colockum Tarps fire, the Washington Rare Plant Care and Conservation program (Rare Care) and BLM established 11 permanent plots in the ACEC to monitor the milk-vetch population.
The milk-vetch occurs in areas with deeper soils that also support big sagebrush, and a band of sagebrush occurred across the top of the slope at the site. In some places the sagebrush was dense, which often occurs at sites where the cover of perennial grasses and forbs are reduced by grazing. The Colockum Tarps fire destroyed most of the big sagebrush, and what remained afterwards was subsequently wiped out by the 2017 Spartan fire. As a result, cheatgrass (Bromus tectorum) and tumble mustard (Sisymbrium altissimum), along with several other annual non-native grasses, have greatly increased. Both cheatgrass and tumble mustard germinate early, utilizing water that is a scarce resource in arid environments.
Whited’s milk-vetch likely evolved to tolerate fire in the ecosystem, and the monitoring data in the past six years do not show evidence of a decline after recent fires. However, seedling establishment appears to be episodic, which is typical in arid environments. It is unknown how well new plants will establish in the altered habitat and whether the removal of big sagebrush and competition with non-native species will reduce the overall vigor of plants. Very little is known about what role big sagebrush plays in facilitating Whited’s milk-vetch. A recent study in Oregon showed that big sagebrush improved the reproductive success of certain perennials under drought conditions. Furthermore, research conducted by Julie Combs, as a graduate student of Sarah Reichard, showed that seedling recruitment and establishment is substantially reduced where cheatgrass is present. Therefore, land managers are concerned about the long-term viability of this population and the entire species across its range.
Rare Care is assisting BLM with developing a site management plan that will help guide restoration and management of the ACEC. An important question in developing this plan is how to restore the vegetation community to make it more resilient to wildfire. Another key question for restoration will be deciding what the target vegetation community should be in light of the increased fire frequency, the predicted warmer temperatures, and increased drought stress expected in the coming decades. Reducing the cover of cheatgrass will be key and grass specific herbicides can be effective, but we need to understand what affect they have on Whited’s milk-vetch before they are used in areas it occupies. Increasing perennial grasses and forbs has also been shown to reduce cheatgrass cover and we will develop a plan to increase native vegetation at the site.
Restoration activities have already commenced, and future planting activities are scheduled for the upcoming years. These activities will provide an opportunity for our Rare Care community to contribute to the restoration and conservation of this rare plant and to engage with the local community to foster an appreciation for the wonderful diversity of plants found in the Wenatchee area.