Landscape Dynamics and Arid Land Restoration Steven G

Landscape Dynamics and Arid Land Restoration Steven G. Whisenant Abstract—Restoration strategies that initiate autogenic Most rangeland improvement recommendations begin succession—by using rather than by combating natural processes— with the premise that activities (investments) should focus have great potential for arid ecosystems. Damaged ecological pro- on sites with the greatest potential for a positive economic cesses must be restored to restoration sites. Landscape dynamics return. That is sound advice from a financial investment can be directed toward restoration objectives with strategies that: viewpoint. However, the failure to consider landscape inter- (1) reduce or eliminate the causes of degradation; (2) address soil actions may create unanticipated problems. For example, degradation and initiate soil improving processes; (3) establish in arid regions, depositional areas at the base of hills are vegetation that addresses microsite availability, soil improvement, commonly selected for restoration efforts because of their and nutrient cycling problems; and (4) arrange landscape compo- inherently better soil, nutrient and water relations. The nents to reduce detrimental landscape interactions while increasing best restoration effort on those sites may fail due to prob- synergies among landscape components. Landscape configuration lems on other parts of the landscape. Accelerated sheet can be designed to: (1) encourage synergies among landscape com- erosion on hill slopes can lead to channel deposition that ponents; (2) reduce nutrient losses to adjacent landscape compo- steepens the slope gradient. This initiates channel en- nents; (3) facilitate natural seed dispersal mechanisms; (4) attract trenchment that creates steep channel banks susceptible beneficial animals; and (5) reduce detrimental animal activities. to mass failure or slumping. This leads to lateral erosion of the stream channel against an adjacent hill slope and further steepens the hill slope gradient and removes the concave portion of the valley bottom. This increases sur- Artificial revegetation of arid ecosystems is expensive, face erosion rates while reducing the opportunity for sedi- risky, and the benefits are often short-lived. Current ment storage at the bottom of the hill slope. Other land- approaches to ecosystem rehabilitation are extensions of scape scale problems (such as those involving nutrient traditional agronomic technologies developed under more cycling, geomorphology, hydrology, herbivory, granivory, hospitable climates. These agronomic approaches produce propagule transport) are less obvious, but can be just as linear rows of uniformly spaced plants rather than natu- disruptive. rally occurring vegetative patterns. Ecological restoration Landscapes are an assemblage of different vegetative ele- is an alternative approach that attempts to minimize man- ments that may have patches or corridors of other vegeta- agement intervention (and expense) by stimulating natural tion types embedded in a matrix of a distinct vegetation successional processes to develop stable structural and type. Unique landscape combinations are formed from inter- functional dynamics. actions of geomorphology, hydrology, colonization patterns, Restoration efforts have traditionally been designed and and local disturbances (Forman and Godron 1986). The implemented for specific sites—with the boundaries deter- landscape matrix is the primary vegetation type surround- mined by fences or ownership patterns. These restoration ing patches of other vegetation types. The distribution— efforts focused on site specific attributes and objectives with- not the movement—of energy, materials, and species in out considering interactions with the surrounding land- relation to the sizes, shapes, numbers, kinds, and configu- scape. Since all parts of a landscape are functionally linked, rations of landscape elements makes up the ‘structure’ of this site specific focus contributed to several problems. The that landscape (Forman and Godron 1986). Landscape failure to view restoration sites as integral components of a function—or dynamics—is the interaction among the land- larger, highly interconnected landscape has often produced scape elements that involves the flow of energy, materials, inherently unstable “restored” landscapes. The processes water, and species among the elements. and products of unstable landscape components can disrupt The concepts of landscape restoration ecology can be ap- the stability of the other parts, resulting in widespread fail- plied to all ecosystems, but this discussion is focused on ure throughout the landscape. We have the potential to im- large arid ecosystems that cannot be completely restored by prove restoration success by incorporating landscape pro- artificial methods. Western North America is an excellent cesses essential in the establishment and maintenance of example, since it contains millions of hectares that require ecological systems. restoration or rehabilitation, but the need far exceeds our ability to provide it. This situation is common, perhaps the rule rather than the exception in arid and semi-arid ecosystems. Our success in rehabilitating these systems has not In: Roundy, Bruce A.; McArthur, E. Durant; Haley, Jennifer S.; Mann, been good, but even if we had the capability to restore them, David K., comps. 1995. Proceedings: wildland shrub and arid land resto- we would never have the money to apply that technology to ration symposium; 1993 October 19-21; Las Vegas, NV. Gen. Tech. Rep. all the areas that need it. Restoration strategies that ini- INT-GTR-315. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station. tiate autogenic succession—by using rather than by combat- Steven G. Whisenant is Associate Professor of Rangeland Ecology and ing natural processes—are most appropriate for extensively Management, Texas A&M University, College Station, TX 77843-2126. 26 managed arid ecosystems. The objectives of this paper are Processes to Increase Resource not to outline a comprehensive restoration program—but Availability to introduce concepts that contribute toward landscape- level planning of restoration efforts on arid lands. Landscape considerations are incorporated into arid land restoration efforts with strategies that: (1) reduce or eliminate the causes of degradation; (2) address soil degradation Directing Landscape Dynamics and initiate soil improving processes; (3) establish vegetation that addresses microsite availability, soil improvement, The restoration of degraded arid lands has several limi- and nutrient cycling problems; and (4) arrange landscape tations: (1) resource (water, nutrients, soil organic matter, components to reduce detrimental landscape interactions propagules) levels are uniformly low; (2) harsh microenvi- while increasing synergies among landscape components. ronmental conditions limit seedling recruitment; and (3) an- Remove Causes of Degradation—Deforestation and imals have a greater potential to disrupt restoration efforts abusive grazing practices reduce soil organic matter, litter, in arid systems. Since plant establishment and growth in vegetation and infiltration. The reduced perennial plant arid lands is limited by available water, successful resto- cover associated with degradation results in less organic ration strategies increase water availability and/or reduce matter being produced and added to the soil. As soil organic evaporation and transpiration. Water availability is in- matter is reduced, aggregate stability is reduced and the creased with strategies that harvest water, increase infil- soil is more easily crusted by raindrop impact. Raindrops tration and increase water retention. Evapo-transpiration falling on exposed soil surfaces with low aggregate stabil- can be reduced with strategies that lower soil and leaf tem- ity detach fine soil particles from the soil surface. These peratures (shade) and increase litter accumulations on the fine particles fill soil pores and create soil surface crusts soil surface. Herbivores and granivores may have large with a continuous surface sealing. Soil surface crusts (soil impacts on the vegetation of arid landscapes. They affect sealing) are “thin layers of compacted soil with greatly re- the vegetation directly by consuming the vegetation and duced hydraulic conductivity, capable of decreasing the seeds and indirectly by altering the fire regime. Animals infiltration of soil surfaces subjected to rainfall” (Bohl and and the arrangement of landscape components also influ- Roth 1993). After drying, surface crusts seal the soil sur- ence the movement of seed across landscapes. face, reducing infiltration and aeration. Deforestation, over- The application of landscape considerations to arid land grazing and cultivation degrade the vegetation and initiate restoration problems might focus on capturing flows of the process of desertification. scarce resources across the landscape or on reducing frag- Desertification is a common result of degradation. De- mentation and reintegrating fragmented landscapes. sertification is the spread of desert-like conditions (Lal Tongway (1991) suggested a landscape approach that iden- and others 1989), or the “…impoverishment of arid, semi- tifies processes controlling the flows of limiting resources arid and sub-humid ecosystems by the impact of man’s into and through landscapes. Hobbs (1993) argued that activities. This process leads to reduced productivity of fragmentation

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