Cheatgrass: Fire and Forage on the Range

Chapter 6. Seral Continuum: Intermediate Step

The exact nature of the step from Russian thistle to the next level in the succession of invasive exotic plants is not known. John Weaver and Frederic Clements stressed that the stages in succession are largely determined by two factors: (1) how each previous stage or plant assemblage modifies the ecosystem, and (2) by the stand renewal process—the means by which the previous plant community was destroyed.1 Russian thistle finds a home on bare soil created by severe mechanical disturbance such as road grading and livestock movements along trails or around salt blocks. Road shoulders that are graded every year will never proceed beyond Russian thistle dominance. If the Russian thistle plants mature without additional disturbance, on the other hand, succession will proceed in subsequent seasons. When Russian thistle plants mature, break loose at the soil surface, and tumble off the site, they leave behind a seedbed significantly changed both physically and biochemically . The seedbed is still largely bare, but the stubs of the Russian thistle plants provide microtopography that captures subaerially deposited dust and heavier particles moved across the soil surface by wind action. In the Great Basin, the deposition of fine silt-sized particles on the soil surface and the illuviation of these particles into the soil profile after rain are major parts of the soil-building process. An endless supply of these fine particles is constantly being eroded from the playa surfaces.2 chapter 6 Seral Continuum The Intermediate Step 79 Stand renewal does not have to be an all-or-nothing event. In a firmly established cheatgrass stand, for example, the area around rodent burrows may continue to support Russian thistle. Wildfire in a cheatgrass stand may burn all the litter, especially the material close to sagebrush canopies, where it burns hot enough to kill most of the cheatgrass seeds in the seed bank. When that occurs, the site will return to Russian thistle dominance. Away from woody fuel, cheatgrass seed banks usually survive and cheatgrass dominance is not interrupted. The stand renewal process that induces regression in succession may vary in spatial distribution and intensity on a given site. If the bare-ground seedbeds on Great Basin rangelands largely belong to members of the goosefoot family, the next stage in succession is dominated by three members of the mustard family, Brassicaceae. The most widely distributed and dominant mustard on Great Basin rangelands is tumble mustard , a native of Eurasia (fig. 6.1). Very little is known about the timing and mode of introduction of this species, but tumble mustard was apparently introduced relatively recently in California.3 The oldest California herbarium specimens date from 1910–1918. The second mustard species, tansy mustard, is a native of North America and the rarest of the species in the secondary succession toward cheatgrass dominance. Tansy mustard is highly invasive and may not be native to much of the Great Basin where it is now found. The taxonomy of tansy mustard is very complex, with at least eight recognized subspecies.4 Tumble mustard is the most frequently found species , but in some sites tansy mustard is dominant. Shield cress, the third mustard species, is also a native of Eurasia. Like tumble mustard, shield cress was first recorded in herbariums in California early in the twentieth century. It is much more salt tolerant than tumble or tansy mustard. In the Great Basin, shield cress is most abundant in the upper shadscale zone and the lower portion of big sagebrush communities, especially on pluvial lake plains, where it is often a roadside dominant species (fig. 6.2). All three of the Great Basin’s mustard species have a mucilaginous seed coat,5 a character found in many different plant families. Eight percent of the 233 Great Basin species we surveyed had seed mucilage. Interestingly, most of the species with mucilaginous seeds were colonizing weeds. Seed mucilage occurs in so many species in so many plant families that it probably conveys a variety of benefits. A commonly proposed benefit is as an aid in dispersal. Seeds with copious amounts of mucilage are sticky and may 80 c h e a tg r a s s disperse to new sites by sticking to animal feet and legs, bird feathers, or vehicle tires.6 For species of the temperate desert environments of the Great Basin, effects on moisture relations during germination may be a much more biologically significant aspect of...