About Astragalus brauntonii Parish
Braunton's milkvetch, known scientifically as Astragalus brauntonii Parish, is a large perennial herb that grows from a woody caudex and can reach up to 1.5 meters (4.9 feet) tall. Its thick hollow stems are covered in coarse white hairs. Leaves grow up to 16 centimeters (6.3 inches) long and are composed of many pairs of oval-shaped leaflets. The inflorescence is a dense spike holding up to 60 bright lilac flowers. Each pealike flower is roughly one centimeter long, with a reflexed hood. After withering, the flowers turn brown but stay attached to the plant instead of falling off. This plant is pollinated by native Megachile bees and native bumble bees of the genus Bombus. Its fruit is a small, bent legume pod. Astragalus brauntonii grows in coastal prairie grasslands, coastal sage scrub, and chaparral plant communities. It is often found in disturbed areas, particularly those with carbonate soils. There are 16 known remaining populations in California, located in the southwestern Transverse Ranges (eastern Santa Monica Mountains, east end of the Simi Hills, south base of the San Gabriel Mountains) and northern Peninsular Ranges (northwest side of the Santa Ana Mountains), within Los Angeles, Orange, and Ventura Counties. A disjunct population (variety lativexillum) grows in the coastal hills between Tijuana and Rosarito, in extreme northwest Baja California, almost 240 kilometers south of the southernmost populations of var. brauntonii. It grows sympatrically with Astragalus tijuanensis. Astragalus brauntonii is an opportunistic species that depends on disturbance, primarily natural wildfire, to reproduce. Unlike other fire-adapted perennials in chaparral, it does not resprout from underground vegetative structures. Instead, it relies on a long-lived dormant seedbank, a trait it shares with another rare perennial legume, Thermopsis macrophylla. The beanlike seeds need scarification from fire or mechanical disturbance to break down their tough seed coats before they can germinate. Seeds persist for years in the soil until fire or disturbance allows sprouting, so populations of the plant often spring up in areas that have recently burned. Alteration of natural fire regimes is a major threat to the species. Historically, natural fires in Southern California occurred at intervals of 50 to 100 years. Despite modern fire suppression, the current fire frequency is substantially higher, at 15 years or less. Wildlands near urban areas face more frequent fires and ignition sources. This converts native woody perennial vegetation to landscapes dominated by invasive non-native annual grasses and forbs. These non-native grasslands change the fire cycle further, as they ignite easily and burn more quickly. Shortened fire cycles reduce the species' seedbank, because mature plants die before they can produce large numbers of seeds. Non-native annuals then outcompete A. brauntonii, reduce its critical habitat, and cause additional alteration of the fire cycle.
