Germination and early establishment of dryland grasses and shrubs on intact and wind-eroded soils under greenhouse conditions

Aims Grassland-to-shrubland transition is a common form of land degradation in drylands worldwide. It is often attributed to changes in disturbance regimes, particularly overgrazing. A myriad of direct and indirect effects (e.g., accelerated soil erosion) of grazing may favor shrubs over grasses, but their relative importance is unclear. We tested the hypothesis that topsoil “winnowing” by wind erosion would differentially affect grass and shrub seedling establishment to promote shrub recruitment over that of grass. Methods We monitored germination and seedling growth of contrasting perennial grass (Bouteloua eriopoda, Sporobolus airoides, and Aristida purpurea) and shrub (Prosopis glandulosa, Atriplex canescens, and Larrea tridentata) functional groups on field-collected non-winnowed and winnowed soils under well-watered greenhouse conditions. Results Non-winnowed soils were finer-textured and had higher nutrient contents than winnowed soils, but based on desorption curves, winnowed soils had more plant-available moisture. Contrary to expectations, seed germination and seedling growth on winnowed and non-winnowed soils were comparable within a given species. The N2-fixing deciduous shrub P. glandulosa was first to emerge and complete germination, and had the greatest biomass accumulation of all species. Conclusions Germination and early seedling growth of grasses and shrubs on winnowed soils were not adversely nor differentially affected comparing with that observed on non-winnowed soils under well-watered greenhouse conditions. Early germination and rapid growth may give P. glandulosa a competitive advantage over grasses and other shrub species at the establishment stage in grazed grasslands. Field establishment experiments are needed to confirm our findings in these controlled environment trials.

Remediation Research in the Jornada Basin: Past and Future

Land degradation in most of the Chihuahuan Desert is characterized by a shift from grass- to shrub-dominated plant communities (Ballín Cortés 1987; Grover and Musick 1990; Fredrickson et al. 1998; see also chapter 10). This shift is associated with increased soil resource redistribution and spatial variability at the plant-interspace scale (Schlesinger et al. 1990; see also chapter 6). Earlier descriptions focused more specifically on the loss of plant species, such as black grama (Bouteloua eriopoda), which were palatable to livestock (Nelson 1934). In 1958, it was estimated that one section (3.2 km2) of black grama grassland could support 18 animal units yearlong, while a similar area dominated by mesquite (Prosopis glandulosa) dunes could support just three animal units (Jornada Experimental Range Staff 1958; see also chapter 13). It was recognized that overgrazing facilitated the increase of less palatable species, including shrubs. Consequently, the objectives of the first organized rangeland research in the Southwest were to identify proper techniques to restore grasslands that had been overgrazed (Jardine and Hurtt 1917; Havstad 1996). Today, we recognize the importance of multiple, interacting factors in addition to overgrazing, and research is more broadly focused on the recovery of ecosystem functions necessary to support multiple ecosystem services. This chapter details this extensive history of research to identify and develop technologies to revegetate, restore, reclaim, rehabilitate, or more generally remediate degraded rangelands. The Society for Ecological Restoration considers that “an ecosystem has recovered when it contains sufficient biotic and abiotic resources to continue its development without assistance or subsidy. It will demonstrate resilience to normal ranges of environmental stress and disturbance. It will interact with contiguous ecosystems in terms of biotic and abiotic flows and cultural interactions” (Society for Ecological Restoration Science and Policy Working Group 2002). Although restoration of perennial grasslands is often cited as the ultimate objective of management intervention in the Southwest, we recognize that in many if not most cases complete restoration of a preexisting plant and animal community is impossible, even if we had perfect knowledge of all of the elements they contained. We also recognize that many of the historic management interventions discussed herein had more limited objectives.

Plant Communities in the Jornada Basin: The Dynamic Landscape

Plant communities of the Jornada Basin are characteristic of the northern Chihuahuan Desert both in structure and dynamics. Although a number of plant communities can be differentiated, five major vegetation types are often distinguished that differ in plant species cover and composition, as well as other factors, such as animal populations, soil properties, and elevation. These five types are black grama (Bouteloua eriopoda) grasslands, playa grasslands, tarbush (Flourensia cernua) shrublands, creosotebush (Larrea tridentata) shrublands, and mesquite (Prosopis grandulosa) shrublands. Similar to many other parts of the Chihuahuan Desert, these plant communities have experienced major shifts in vegetation composition over the past 50–150 years (York and Dick-Peddie 1969). The most dramatic changes in vegetation and associated ecosystem processes have occurred as a result of a shift in life form due to woody plant encroachment into perennial grasslands (Grover and Musick 1990; Bahre and Shelton 1993). This encroachment of shrubs has occurred in many arid and semiarid regions of the world, including the Western United States, northern Mexico, southern Africa, South America, New Zealand, and Australia (McPherson 1997; Scholes and Archer 1997). A number of drivers have been implicated in these grass–shrub dynamics, including various combinations of livestock grazing, small animal activity, drought, changes in fire regime, and changes in climate (Humphrey 1958; Archer 1989; Allred 1996; Reynolds et al. 1997; Van Auken 2000). The causes of shrub invasion are quite variable and often poorly understood, although the consequences consistently lead to the process of desertification (Schlesinger et al. 1990). This chapter describes the characteristics of each vegetation type and the documented changes in each type at the Jornada Basin. We then discuss the key drivers influencing these dynamics. Vegetation in the Chihuahuan Desert region has been classified as desert-grassland transition (Shreve 1917), desert savanna (Shantz and Zon 1924), desert plains grasslands (Clements 1920), desert shrub grassland (Darrow 1944), and shrubsteppe (Kuchler 1964). Desert grassland is often used as a general descriptive name for the area (McClaran 1995), although landscapes at the Jornada and throughout the northern Chihuahuan Desert often consist of a mosaic of desert grasslands, Chihuahuan Desert shrublands, and plains-mesa sand scrub (Dick-Peddie 1993).

Growth and Root NO3- and PO43- Uptake Capacity of Three Desert Species in Response to Atmospheric CO2 Enrichment

In a phytotron experiment, we examined growth and rates of NO-3 and PO3-4 uptake in seedlings of two desert C3 shrubs (Larrea tridentata and Prosopis glandulosa) and a desert C4 perennial grass (Bouteloua eriopoda) grown under CO2 partial pressures of 35 or 70 Pa. Plants were grown in soil but uptake studies were conducted on roots of intact seedlings placed in nutrient solutions containing both NO-3 and PO3-4. Elevated CO2 increased total biomass by 69 and 55% in Larrea and Prosopis seedlings and by 25% in Bouteloua. NO-3 and PO3-4 uptake rates were more than doubled in Bouteloua at high compared to ambient CO2. In contrast, CO2 enrichment inhibited root NO-3 uptake capacity in Larrea by about 55% without a significant effect on PO3-4 absorption rate; rates of NO-3 and PO3-4 and uptake in Prosopis were insensitive to CO2 treatment. Elevated CO2 enhanced the proportion of biomass allocated to the fine roots in Bouteloua but markedly reduced this fraction in Larrea and Prosopis. Foliar N concentration of both shrubs decreased in response to elevated CO2, but was unaffected in Bouteloua. We suggest that compensatory changes in root size and activity are critical in determining interspecies variation in plant nutrient relations under high CO2.