Background

Cheatgrass (Bromus tectorum).

This invasion has had profound negative consequences for native biotic diversity in the western United States. Many native plants have been directly replaced. This, in turn, has altered soil food webs, decomposition cycles, and soil nutrient availability (Ingham and others, 1989; Belnap and Phillips, in press). Native animal diversity has been concomitantly reduced through alterations in type and timing of food and cover. Wildfire cycles have been accelerated, further reducing native biodiversity. Loss of habitat has resulted in, and continues to perpetuate, the endangerment of many plant and animal species (Monsen, 1994; Rosentreter, 1994). In California alone, 29 species are state-listed as a result of annual grass invasion (CA Native Plant Society, pers. comm.) Consequently, the restoration of ecosystems dominated by annual grasses has been identified by many federal agencies as a top national priority. Despite the widespread and rapidly increasing damage caused by this invasion, little is known about the geologic and ecologic conditions of annual grass colonization and spread. Knowledge about these conditions, at scales of rhizospheric (root zone) nutrient uptake to the landscape, is urgently needed to alert land managers to potential future invasion and ultimately to identify possible ways to halt continued damage to many ecosystems.

USGS geologist stands in an area invaded by cheatgrass surrounded by native grasses.

USGS geologists and ecologists have started work to understand the conditions of cheatgrass invasion into native grasslands in the area of Canyonlands National Park on the central Colorado Plateau and elsewhere in the southwestern U.S. New observations reveal possible landscape-scale links between cheatgrass footholds, on the one hand, and climate, soil texture, soil composition, and geomorphic features, on the other hand. At the same time, ecological experiments have developed new theories about plant-nutrient requirements and uptake mechanisms for native and non-native species in the region. Combined, these approaches would support new appreciation about how interactions among biogeochemical and physical processes might influence ecosystem processes and health, and already they result in new hypotheses about cheatgrass invasion that we will test.

In more detail, the new studies have identified several factors that seem to favor or retard the spread of cheatgrass on the central Colorado Plateau. First, cheatgrass invades soils where the potassium to magnesium ratio is high, suggesting that potassium uptake is limiting (Woodward and others, 1984; McKnight and others, 1990). Second, field studies suggest an essential role for phosphorus availability in promoting cheatgrass growth, tied to spatial and temporal variations in soil properties that influence dynamics of P sorption and desorption reactions (Lajtha and Schlesinger, 1988; DeLucia and others, 1989; Belnap and Harper, 1995; Lajtha and Harrison, 1995; M. Miller and J. Belnap, unpub. data). Third, we have discovered that windblown dust has delivered most nutrients (including Mg, P, and possibly K) to these soils and that these nutrients are elevated in the silt plus clay fraction (Reynolds and others, 1998; M. Reheis and P. Lamothe, unpub. data). Finally, we have observed some young cheatgrass footholds in topographic lows (e.g., swales behind stabilized dunes), which can be expected to preferentially accumulate fine-grained (silty) eolian sediment. Despite these leads, we have no unifying explanation for observed distributions of cheatgrass or for patterns of invasion. Moreover, we lack critical information about landscape-scale geomorphologic influences and about local-scale soil compounds, such as carbonates and iron oxides, that may absorb and release essential nutrients, such as P, depending on soil and climatic conditions.

Problems and Hypotheses

1. What factors and conditions drive vegetation type at the landscape level? Specifically, what is the role of eolian dust in increasing soil fertility to make otherwise low-fertility landscapes susceptible to cheatgrass invasion? (This question applies as well to other noxious weeds, especially annuals.)

2. What factors and conditions drive vegetation type at the local level? Specifically, how do different soil components respond to ambient environmental, geochemical, and rhizospheric conditions that control nutrient availability and uptake?

Objectives

References

Belnap, J. and S. L. Phillips. In press. Soil biota in an ungrazed semi-arid grassland: response to an exotic annual grass (Bromus tectorum) invasion. Ecological Applications.

DeLucia, E. H., W. H. Schlesinger, and W. D. Billings. 1989. Edaphic limitations to growth and photosynthesis in Sierran and Great Basin vegetation. Oecologia 78: 184-190.

Ingham, E. R., D. C. Coleman, and J. C. Moore. 1989. An analysis of food-web structure and function in a shortgrass prairie, a mountain meadow, and a lodgepole pine forest. Biology and Fertility of Soils 8:29-37.

Lajtha, K., and W. H. Schlesinger. 1988. The biogeochemistry of phosphorus cycling and phosphorus availability along a desert soil chronosequence. Ecology 69: 24-39.

Lajtha, K., and A. F. Harrison. 1995. Strategies of phosphorus acquisition and conservation by plant species and communities. Pages 139-147 in H. Tiessen, ed. Phosphorus in the global environment: transfers, cycles, and management. SCOPE 54. Wiley, Chichester, UK.

McKnight, K.B., K.H. McKnight and K.T. Harper. 1990. Cation exchange capacities and mineral element concentrations of macrofungal stipe tissue. Mycologia 82(1): 91-98.

Monsen, S.B. 1994. Selection of plants for fire suppression on semiarid sites, in Proceedings, Ecology and Management of Annual Rangelands, S.B. Monsen and S.G. Kitchen, editors. USDA Forest Service, General Technical Report, INT-GTR-313, p. 363-373.

Reynolds, R.L., J. Belnap, M. Reheis, and N. Mazza, 1998, Eolian dust on the Colorado PlateauÑmagnetic and geochemical evidence from sediment in potholes and biologic soil crust, in Busacca, A., (ed.), Dust aerosols, loess soils, and global change: Washington State Univ., College of Agriculture and Home Economics Misc. Publ. No. MISC0190, p. 231-234.

Rosentreter, R. 1994. Displacement of rare plants by exotic grasses. In: Proceedings, Ecology and Management of Annual Rangelands, S.B. Monsen and S.G. Kitchen, editors. USDA-USFS, INT-GTR-313, p. 170-175.

Ryan, J., D. Curtin, and M. A. Cheema. 1985a. Significance of iron oxides and calcium carbonate particle size in phosphate sorption by calcareous soils. Soil Science Society of America Journal 49: 74-76.

Woodward, R.A., K.T. Harper, and A.R. Tiedemann. 1984. An ecological consideration of the cation-exchange capacity of roots of some Utah range plants. Plant and Soil 79: 169-180.