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 of ecosystem processes leads to significant desirable plants, alterations in the biomass and in the di- changes in the water and nutrient cycles, radiation balance versity of life forms, accelerated soil degradation and haz- and wind regimes. This is particularly important since ards for human occupancy” (UNEP 1977). About 6 million degraded ecosystems have leaky nutrient cycles compared hectares is irretrievably lost or degraded by desertification to undamaged landscape elements (Allen and Hoekstra each year and about 135 million people are severely af- 1992). Aronson and others (1993a) presented a general fected by desertification (UNEP 1984). In 1975 it was es- model for the restoration and rehabilitation of degraded timated that in the Sudan the desert land had expanded arid and semi-arid ecosystems that included “vital eco- south 90 to 100 km in about 17 years (Tivy 1990). Deser- system attributes,” of which several are landscape scale tification is a dynamic self-accelerating process resulting attributes. from positive feedback mechanisms driving a downward There are several examples of landscape level planning spiral of land degradation (Tivy 1990). Desertification of restoration or rehabilitation activities. Aronson and oth- has two main physical characteristics—vegetative degra- ers (1993b) described an ecologic and economic rehabilita- dation and soil degradation. Restoration of these degraded tion program for landscape scale problems of a degraded sites is restricted by several site-specific obstacles that Espinales landscape in central Chile. Thurow and Juo must be addressed on each site (Table 1). There are also (1991) described an integrated management program for several indicators that may suggest destructive landscape- an agropastoral watershed in Niger and argued that wa- level interactions with the potential to disrupt restoration tersheds are the most appropriate level for manipulating efforts (Table 2). hydrological and geochemical processes. Our understand- Management approaches to the restoration of degraded ing of the functional interactions controlling landscape dy- ecosystems have emphasized artificial revegetation or im- namics is far from complete and our ability to direct those proved grazing management. Although improved grazing processes is less well developed. However, existing theo- management must be part of any long-term management retical, empirical, and practical information provides in- plan, current ecological understanding suggests it is un- sight that suggests the elements of a new paradigm for likely to significantly improve severely degraded ecosys- arid land restoration. tems. Even complete removal of livestock does not insure

27 Table 1—Site-specific obstacles to aridland restoration. Although these obstacles have local origins, they may cause problems on other parts of the landscape or may also occur elsewhere in the landscape.

Deterioration of soil structure (surface crusting, compaction, reduced macroporosity, low aggregate stability, reduced infiltration) Wind or water erosion Reduced soil organic matter Reduced water holding capacity Soil salinity levels elevated beyond natural conditions Nutrient depletion Reduced capacity to retain nutrients Reduced vegetative and litter cover Low functional and species diversity Depleted seed bank diversity Reduced activity and diversity of soil organisms

secondary succession leading toward recovery (Walker and and the water gains velocity and energy, producing rills, others 1981; Whisenant and Wagstaff 1991; Friedel 1991; channels and eventually gullies. Several problems occur Laycock 1991). The traditional successional concept of as soil structure deteriorates, but the more common prob- vegetation returning to a predictable, relatively stable- lems on arid lands are surface crusting, accelerated ero- state following disturbance is not valid in many arid eco- sion, salinization, reduced macroporosity, reduced aggre- systems (Westoby and others 1989; Friedel 1991; Laycock gate stability, and reductions in the diversity and activity 1991). Most arid ecosystems seem to have multiple, alter- of soil organisms. These processes reduce infiltration and native stable-states (Friedel 1991; Laycock 1991). Move- increase water loss from runoff and evaporation. As soil ment between these steady-states (i.e. ecosystem recovery) water reserves are depleted, less vegetation is produced requires significant management inputs (Friedel 1991). and the degradation of soil condition accelerates. Thus, despite the economic problems of rehabilitating de- Soil improvement strategies should be directed toward graded arid ecosystems, some management intervention the eventual goal of retaining and using water where it is required for improvement. falls (Sanders 1990). The only sustainable method of ac- complishing these objectives is to reduce the amount of Soil Improvement—Soil erosion is the most common bare ground by establishing a vegetative cover. Unfortu- and damaging form of soil degradation since it degrades nately, the potential vegetative cover is typically low in the physical, chemical and biological components of the arid ecosystems. Sanders (1990) suggested a strategy to soil. Excessive erosion depletes nutrients, decreases root- improve degraded soils that has both immediate and long- ing volume, reduces plant-available water reserves—and term objectives. The immediate objectives are to: (1) pre- most tragically—is irreversible. Wind erosion is a serious vent soil crusts; (2) reduce soil erosion; and (3) retain the problem in arid and semi-arid regions. The United Nations precipitation on site. These immediate objectives are Environment Program (UNEP 1977) estimated that 80% strongly correlated but can be considered separately. They of the 3,700 million ha of rangeland around the world are are critical steps toward longer-term objectives that require affected by wind erosion. Wind erosion has greater impacts more time for development: (1) increase soil organic matter on the fine, nutrient-rich components of soil, such as silt, content; (2) increase water holding capacity; (3) improve soil clay and organic matter, leaving less fertile sand, gravel structure; and (4) restore sustainable nutrient dynamics to and other coarser materials. The rate of wind erosion gen- the soil system. The immediate objectives are met by in- erally depends on soil erodibility, surface roughness, climate, creasing the vegetative cover or increasing mulch and lit- the unsheltered travel distance of wind across a field, and ter cover on the soil surface. This reduces the detrimental vegetative cover. Soil surface crusts reduce permeability effect of raindrop impact on the bare soil surface, which is and accelerate runoff and erosion. On bare soil surfaces, the primary cause of crusting. Providing soil cover reduces running water is not slowed or absorbed by organic matter,

Table 2—Indicators suggesting problematic landscape-level interactions with the potential to disrupt arid land restoration efforts.

Gully cutting (upslope or downslope from restoration site) Excessive soil deposition Altered water table (might be higher, lower, or of reduced quality) Low volume and diversity of seed immigrants Accelerated nutrient losses to adjacent landscape element (fluvial, aeolian, or subsurface processes) Low natural recruitment of plants Increased salinity resulting from accelerated run-on of low quality water Inadequate pollination Excessive animal damage (herbivory or seed predation) Reduced landscape diversity

28 the extremes of soil temperature, reduces evaporation, re- ecosystems bark and wood chip amendments can contrib- duces erosion, increases infiltration of water into the soil, ute to a stable below-ground biota that facilitates a more and increases soil water content. Although arid environ- sustainable above-ground flora (Whitford and others 1989). ments may not have the potential to produce enough veg- Severely depleted soils treated with readily decomposed etation to cover much of the soil surface, the cover and organic materials developed soil biota and processes simi- resulting benefits should be maximized. lar to less damaged soils, but the beneficial effects lasted Soil surface treatments such as pitting, terracing, or only two years (Whitford 1988). Unlike cultivated soils microcatchments reduce runoff and increase infiltration. where nitrogen immobilization by high Carbon/Nitrogen Microcatchments, pits, and contour furrows retain water ratio materials is undesirable, recalcitrant organic mate- and increase infiltration and storage of water. Soil modi- rials may be desirable in arid environments (Whitford and fication procedures have a finite design life determined others 1989). by erosion rate, depth and precipitation events. Previous Decomposition is an essential nutrient cycling process pitting and contour furrow uses on arid rangelands con- (Whitford and others 1989) regulated by water and organic centrated on the establishment of herbaceous species. In matter availability in arid ecosystems (Steinberger and general, those practices greatly improved establishment others 1984). Stable soil decomposition processes require success and productivity for several years. However, they a diverse soil biota (Santos and others 1981; Santos and were temporary and seldom lasted much beyond the de- Whitford 1981). Decomposition potentials of severely dis- sign life of the soil modification and did not expand ben- turbed soils may not recover for many years (Harris and efits beyond the soil depression. others 1991). Respiration-to-biomass ratios (soil meta- Reducing crusting and erosion while increasing the in- bolic quotient) in German mined soils were not stabilized filtration and retention of water in the soil improves plant 50 years after mining (Insam and Domsch 1988), although establishment and growth. This begins the accumulation soil metabolic quotients were found to decrease with each of soil organic matter which improves physical, chemical increasing successional stage (Insam and Haselwandter and biological characteristics of the soil. As this process con- 1989). This decrease is probably a reflection of K-selected tinues, water and nutrient retention are improved and soil soil organisms beginning to dominate. However, these structure is increased. Plants contribute to soil organic car- studies of soil metabolic quotient relative to rehabilitation bon reserves through decomposition of leaves and stems progress were conducted in mesic environments and have falling on the soil surface and through exudates from roots not been examined in arid ecosystems. This possible rela- and decomposition of dead root material. These processes of tionship between the metabolic quotient and succession soil development increase vegetation production and accel- suggests the potential to influence the speed, direction, erate the rate of soil development. The processes of vegeta- and stability of arid land restoration by manipulating the tion and soil development are mutually dependent. Ulti- microbial community. mately, the nutrient dynamics of the system is stabilized. Vegetation Strategies—Vegetation can be used to Soil biological properties are degraded through the ef- mediate harsh microenvironmental conditions; capture fects of reduced organic matter, reduced biological activ- wind- and water-borne soil, nutrients, and organic matter; ity, reduced diversity of soil flora and fauna, and unfavor- improve soil conditions; increase soil nitrogen; and create able changes in biological processes (Lal and others 1989). structural diversity to attract birds that transport seed. This reduced biotic activity adversely affects nutrient cy- Woody plants capture wind-blown organic materials, soil cling, soil physical properties and makes soils less hospi- particles, nutrients (Virginia 1986) and microorganisms table for plant growth. Organic matter quality may be (Allen 1988). Shrubs also improve microenvironmental another important factor in the recovery, persistence and conditions by moderating wind and temperature patterns stability of the soil biota. The reduced soil biotic diversity (Allen and MacMahon 1985; Vetaas 1992). Not only do and activity typical of degraded soils (Fresquez and others shrubs improve soil and microenvironmental conditions, 1987; Mott and Zuberer 1991), reduce enzymatic capabil- they may reduce nutrient and water losses from disturbed ity of the soil microflora and thus hinders nutrient cycling landscapes. Perennial, nitrogen-fixing legumes are be- and organic decomposition. Although bacterial numbers lieved to be essential components of many arid and semi- tend to recover rapidly following disturbance, the species arid ecosystems (Jenkins and others 1987; Jarrell and balance is altered in favor of ruderal species capable of Virginia 1990) as well as in alternative steady-state sys- rapid growth on readily available substrates. These rud- tems (Knoop and Walker 1985). Woody legumes can ben- eral species correspond to r-selected organisms (Andrews efit disturbed arid landscapes with low water, nitrogen, and Harris 1986) and often dominate following distur- and phosphorus levels (Bethlenfalvay and Dakessian 1984) bance, but are less abundant under stable, climax condi- because of their ability to develop symbiotic associations tions. In contrast, autochthonous microbes metabolize with both rhizobial bacteria and mycorrhizal fungi (Herrera difficult-to-degrade organic matter (OM), have slow growth and others 1993). Keystone species are species believed rates, high affinities for growth limiting substrates and essential to ecosystem structure and function (Westman high starvation survival abilities (Andrews and Harris 1990) and their inclusion may facilitate the restoration of 1986). disturbed ecosystems (Aronson and others 1993a). Woody Recalcitrant organic materials produce low, but con- legumes are often considered keystone species in disturbed tinuous OM sources that persist until perennial root sys- arid and semi-arid ecosystems and should be among the tems begin to supply organic matter (Santos and others first species returned (Aronson and others 1993b) during 1981; Santos and Whitford 1981; Whitford 1988). In arid restoration efforts.

29 Recent studies also suggest the benefits of restoration These spatial patterns suggest functional characteristics strategies using species producing low-quality litter in useful in restoring degraded arid ecosystems. degraded arid ecosystems. Plants with slow growth and Fertile islands are variously viewed as natural triumphs plants producing low-quality litter are uniquely adapted of concentrating biological mechanisms over dispersing to low-nutrient ecosystems (Aerts and van der Peijl 1993). physical forces (Garner and Steinberger 1989), a symptom Nutrient-poor ecosystems are dominated by species with of degradation (Schlesinger and others 1990), or a tool for a low potential growth rate and nutrient-rich ecosystems rehabilitation (Allen 1988). These alternative interpreta- are dominated by species with a high potential growth rate tions are—at least partially—a difference of perspective. (Grime and Hunt 1975; Poorter and Remkes 1990; Poorter Compared to a pristine desert grassland, with a relatively and others 1990). In nutrient-poor environments, nutrient high and uniform distribution of resources (water, N, OM, conserving species develop a higher equilibrium biomass and other soil resources), the conversion to a Prosopis sand- than species with higher nutrient loss rates, although it dune landscape, with its clustered resource distribution may take 3 to 5 years to reach that higher equilibrium involves degradation (Schlesinger and others 1990). In (Aerts and van der Peijl 1993). Nutrient conserving species contrast, clustered resource distribution on rehabilitated produce slowly decomposing litter. Thus, in degraded arid arid mine sites are an improvement over the homogeneous, ecosystems, it is likely that species producing recalcitrant but uniformly low resource levels on mined sites with little litter have greater potential to produce a sustainable sys- vegetation. Severely degraded ecosystems typically have tem. These restoration sites are also less attractive to large uniformly low resource levels and high soil erosion rates. herbivores from other parts of the landscape. Reducing resource loss from severely degraded landscapes is a positive development, even if it partially redistributes Landscape Design—There are significant benefits that resources within the landscape. However, there is much can result from specific landscape arrangements. We can evidence that landscapes capture additional resources dur- arrange landscapes to: (1) encourage synergies among land- ing fertile island development (Garcia-Moya and McKell scape components; (2) reduce nutrient losses to other land- 1970; Allen 1988; West 1989) and that long-term stability scape components; (3) assist natural seed dispersal mecha- and productivity of disturbed arid landscapes may require nisms; (4) attract beneficial animals such as pollinators and the development of fertile islands (Allen 1988). Reducing seed vectors; and (5) reduce detrimental activities of large resource loss and capturing or producing additional re- herbivores and seed predators. sources is an essential component of arid landscape resto- Restoration efforts have the largely unrealized potential ration that contributes to long-term sustainability. to work with underlying landscape processes rather than Garner and Steinberger (1989) hypothesized that biologi- against them by developing strategies that incorporate and cal transport mechanisms concentrate nitrogen (and prob- direct natural processes. Disturbances, such as cultivation ably other resources) while physical mechanisms primarily or abusive grazing practices, homogenize arid landscapes disperse nitrogen. However, physical mechanisms also con- by uniformly reducing water availability, soil nutrients and centrate resources in certain situations. Depressions in the organic matter. This spatial resource leveling produces soil accumulate water, soil, nutrients, organic matter and landscapes where limiting resources are uniformly below propagules. Thus, on barren sites, the concentrating effects the establishment threshold for desirable plants. Under of physical mechanisms (captured flows of water, nutrients, these circumstances, physical or biological features that organic materials and propagules) may contribute to the concentrate resources may initiate recovery by enabling initiation of autogenic landscape restoration. Thereafter, plants to establish and grow on that part of the landscape. biological mechanisms (alteration of soil, microenvironmen- This initiates a series of soil and microenvironmental im- tal, and nutrient relations by the vegetation—particularly provements that begin to positively influence an increas- shrubs) may dominate. ingly larger percentage of the landscape. As an example, Flows of resources through the landscape are important total productivity in arid and semi-arid ecosystems is be- because no landscape component is completely isolated. lieved to be higher if water is distributed in patches rather The dynamics of processes in individual parts of the land- than uniformly (Noy-Meir 1973). scape are strongly influenced by factors acting in other parts Recognition that arid ecosystems often have clustered dis- of the landscape. Tongway (1991) studied two Australian tributions of plants, nutrients, organic matter, and water sites—one dominated by fluvial processes and the other by lead to the term ‘fertile islands’ (Charley and West 1977; aeolian processes. At each location he identified source and West and Klemmedson 1978; Garner and Steinberger 1989; sink areas and measured nutrient pool sizes and rate pro- Schlesinger and others 1990). The sparse resources typi- cesses (microbial respiration rates). Sink areas were found cal of many degraded arid ecosystems often occur in a clus- to contain significantly higher nutrient pools and higher tered spatial arrangement. Soil depths, soil texture, or- rate processes than the source areas. This led to the recom- ganic matter, nutrient concentrations, irradiance patterns, mendation that restoration strategies link soil-vegetation- wind speed, wind direction, and water storage differ greatly landscape associations to the dynamic processes controlling on a scale of a few meters. Microenvironmental parameters the flow of limiting resources. Tongway (1991) suggested a and soil characteristics vary around individual shrubs, re- restoration strategy based on understanding how limiting sulting in microbial and plant organizational patterns inter- resources ought to be distributed in the landscape and then acting on a scale of a few centimeters (Allen and MacMahon promoting processes leading in that direction. 1985). This spatial variability affects seedling establishment Human-dominated landscapes leak materials such as and plant growth patterns that continue to modify micro- nutrients (Allen and Hoekstra 1992). Cultivated fields and environmental and soil characteristics of the landscape. severely disturbed natural ecosystems retain little of their

30 annual nutrient input. Nutrient losses from disturbed land- The shape and boundary form between two different scape elements occur through aeolian, fluvial, and subsurface landscape elements can affect rates of vegetation recruit- hydrologic processes. As an example, watershed fragments ment (Hardt and Forman 1989). Invasion rates of trees in Poland were identified where the groundwater flow from and shrubs from woodland patches into grassland patches agricultural fields passed under a shelterbelt or small for- was greater where grassland patches projected into wood- est (Bartoszewicz and Ryszkowski 1989). Although aeolian lands (concave border) compared to where the woodland and fluvial processes are more obvious and probably more border was straight or convex (Hardt and Forman 1989). widespread in arid ecosystems, nutrient losses through sub- This suggests the possibility of directing natural succes- surface water flows also occur. Shelterbelts and remnants sion by manipulating the boundary form on the scale of of natural vegetation within a cultivated landscape control tens of meters. The size of an area and distance from seed the nutrient fluxes and increase the nutrient holding capac- sources are important considerations that we typically do ity of the entire landscape. Disturbed landscape elements not consider. with depleted nutrient pools may become a sink that accel- Landscape Configuration—Landscape configuration erates nutrient losses from adjacent, undisturbed landscape is a key factor in succession (Risser 1992). Successional elements. Careful landscape design contributes to the re- changes are driven—in part—by differential species avail- tention of nutrients, water, and other materials. The flows ability. The arrangement of landscape components partially of water, energy, nutrients, propagules, soil and organic defines their role as propagule donors or receptors. With matter that flow into, within, and out of landscape elements artificial seeding activities we can manipulate the spatial can be manipulated to help achieve restoration objectives, configuration of propagule donor sites. By concentrating since landscapes rich in ecotones are believed to lose fewer resources on many donor sites distributed over the entire nutrients (Ryszkowski 1992). landscape, we provide a continuing source of propagules. In Deep-rooted shrubs or trees on parts of a landscape may arid ecosystems—where seedling establishment is episodic— regulate the hydrology and nutrient retention capacity this increases the odds of having seeds in the right place at for the larger landscape (Ryszkowski 1989; 1992; Burel the right time. The size of the problem and the shortage of and others 1993; Hobbs 1993). Disruption of that function available resources for arid land restoration suggest the by removing the trees or shrubs can have catastrophic value of a strategy that constructs landscapes with propa- consequences for other parts of the landscape. Agronomic gule donor patches. These donor sites will continue to re- and degraded landscapes lose nutrients through subsur- lease propagules into the adjacent “untreated” areas. The face flows of water and nutrients. Shelterbelts or patches spread of a species is believed to be regulated by the dynam- of woody vegetation can effectively limit subsurface water ics of small scattered stands rather than by expansion of migrations of nutrients (Ryszkowski 1989). larger stands (Moody and Mack 1988). Landscape-scale Vegetation—particularly woody plants—can improve restoration designs based on this principle might initiate microenvironmental conditions, capture flows of scarce long-term successional sequences. The stands containing resources, initiate soil development, and capture propagules. species with wind-dispersed propagules might be designed Soil nitrogen and OM content are often greater under shrubs with consideration of the prevailing wind direction during than in adjacent interspaces. These increases have been the season of dispersal. Ultimately, the success of this strat- attributed to nutrient mining by the roots and shrub litter- egy in establishing plants on untreated areas also depends fall (Charley and West 1975; West and Klemmedson 1978; on land management practices and site specific factors. West 1989; Garner and Steinberger 1989). Higher soil OM Restoration efforts might be enhanced by strategies increases infiltration and water availability (Sprecht 1981) that favor certain groups of animals and discourage others which contribute to greater mycorrhizal fungi and phospho- (Archer and Pyke 1991). Where animals are important dis- rus availability (Allen and MacMahon 1985). These posi- persal agents restoration plans should include provisions tive interactions increase herbaceous productivity under for suitable cover and food (Archer and Pyke 1991). The shrubs (Barth and Klemmedson 1978; Garcia-Moya and seed dispersal of bird-dispersed species into open fields was McKell 1970) and can be used to promote autogenic land- increased by an order of magnitude when natural or artifi- scape restoration. cial perching structures were available (McDonnell and Fragmentation of natural systems can cause significant Stiles 1983). This suggests the potential to insure a continu- changes in the water and nutrient cycles, radiation balance, ing seed rain of those species by establishing woody plants and wind regimes of the landscape (Hobbs 1993). For exam- as perching structures in certain landscape components. ple, in a semi-arid portion of western Australia, removal of The development of a comprehensive restoration plan perennial vegetation reduced evaporation and altered soil that incorporates landscape-level dynamics requires sev- water flows, such that peak runoffs increased and water eral considerations (Fig. 1). Immediate and long-term soil tables rose, bringing stored salts to the surface. This frag- improvement objectives are only achieved with vegetation mentation of the native landscape into remnant native restoration strategies that address soil problems. Sustain- patches and cultivated fields severely disrupted landscape able vegetation strategies rely on landscape-level dynamics and ecosystem processes. This not only degraded the agri- that contribute to ecosystem maintenance and develop- cultural potential but it reduced the restoration potential ment. Soil, vegetation, and landscape-level strategies must (Hobbs 1993). External influences on potential restoration be fully integrated and developed to maximize beneficial sites are believed more important than internal processes interactions. and remnant vegetation management must be carried out in the context of the overall landscape (Hobbs 1993).

31 Figure 1—Planning process for incorporating landscape-level considerations into arid land restoration efforts. Planning is presented as a linear process, but implementing restoration efforts requires the simultaneous consideration of several factors. Potential restoration strategies are initially considered and subsequently reevaluated, re- vised, and incorporated into the overall landscape restoration plan. For example, although initial soil treatments may not involve vegetation (such as pitting or mulching), long-term soil improvement is determined by vegetation at the local and/or landscape level.

32 References Grime, J. P.; Hunt, R. 1975. Relative growth-rate: its range and adaptive significance in a local flora. Journal of Aerts, R.; van der Peijl, M. J. 1993. A simple model to Ecology 63: 393-422. explain the dominance of low-productive perennials in Hardt, R. A.; Forman, R. T. T. 1989. Boundary form effects nutrient-poor habitats. OIKOS 66: 144-147. on woody colonization of reclaimed surface mines. Ecol- Andrews, J. H.; Harris, R. F. 1986. r- and K-selection and ogy 70: 1252-1260. microbial ecology. Adv. Microbial Ecology 9: 99-148. Harris, J.; Bentham, H.; Birch, P. 1991. Soil microbial com- Allen, M. F. 1988. Belowground structure: a key to recon- munity provides index to progress, direction, and resto- structing a productive arid ecosystem. p. 113-135 In: ration. Restoration and Manage. Notes 9: 133-135. Allen, E. B. ed. The Reconstruction of Disturbed Arid Herrera, M. A.; Salamanca, C. P.; Barea, J. M. 1993. In- Lands. An Ecological Approach. Boulder, CO: Westview oculation of woody legumes with selected arbuscular Press. mycorrhizal fungi and rhizobia to recover desertified Allen, M. F.; MacMahon, J. A. 1985. Impact of disturbance Mediterranean ecosystems. Applied Environmental on cold desert fungi: Comparative microscale dispersion Microbiology 59: 129-133. patterns. Pedobiologia 28: 215-224. Hobbs, R. J. 1993. Effects of landscape fragmentation on Allen, T. F.; Hoekstra, T. W. 1992. Toward a Unified Ecol- ecosystem processes in the western Australian wheat- ogy. New York, NY: Columbia University Press. 384 p. belt. Biological Conservation 64: 193-201. Archer, S.; Pyke, D. A. 1991. Plant-animal interactions Insam, H.; Domsch, K. H. 1988. Relationship between soil affecting plant establishment and persistence on reveg- organic carbon and microbial biomass on chronosequences etated rangeland. Journal of Range Management 44: of reclamation sites. Microbial Ecology 15: 177-188. 558-565. Insam, H.; Haselwandter, K. 1989. Metabolic quotient of Aronson, J.; Floret, C.; Le Floc’h, E.; Ovalle, C.; Pontanier, the soil microflora in relationship to plant succession. R. 1993a. Restoration and rehabilitation of degraded Oecologia 79: 174-178. ecosystems in arid and semi-arid lands. I. A view from Jarrell, W. M.; Virginia, R. A. 1990. Soil cation accumula- the South. Restoration Ecology 1: 8-17. tion in a mesquite woodland: sustained production and Aronson, J.; Ovalle, C.; Avendano, J. 1993b. Ecological long-term estimates of water use and nitrogen fixation. and economic rehabilitation of degraded ‘Espinales’ in Journal of Arid Environments 18: 51-56. the subhumid Mediterranean-climate region of central Jenkins, M. B.; Virginia, R. A.; Jarrell, W. M. 1987. Rhizo- Chile. Landscape and Urban Planning 24: 15-21. bial ecology of the woody legume mesquite (Prosopis Bartoszewicz, A.; Ryszkowski, L. 1989. Influence of shelter- glandulosa) in a Sonoran Desert arroyo. Plant and Soil belts and meadows on the chemistry of ground water. 105: 113-120. In: Ryszkowski, L., ed. Dynamics of Agricultural Land- Knoop, W. T.; Walker, B. H. 1985. Interaction of woody and scapes. New York, NY: Springer Verlag. herbaceous vegetation in a southern African savanna. Bethlenfalvay, G. J.; Dakessian, S. 1984. Grazing effect on Journal of Ecology 67: 565-577. mycorrhizal colonization and floristic composition of Lal, R.; Hall, G. F.; Miller, F. P. 1989. Soil degradation: I. the vegetation on a semiarid range in northern Nevada. Basic processes. Land Degradation and Rehabilitation Journal of Range Management 37: 312-316. 1: 51-69. Bohl, H.; Roth, H. 1993. A simple method to assess the Laycock, W. A. 1991. Stable states and thresholds of range susceptibility of soils to form surface seals under field condition on North American rangelands: A viewpoint. conditions. CATENA 20: 247-256. Journal of Range Management 44: 427-433. Burel, F.; Baudry, J.; Lefeuvre, J. C. 1993. Landscape struc- McDonnell, M. J.; Stiles, E. W. 1983. The structural com- ture and the control of water runoff. pp 41-47, In: Bunce, plexity of old field vegetation and the recruitment of R. G. H.; Ryszkowski, L.; Paoletti, M. G. eds. Landscape bird-dispersed plant species. Oecologia 56: 109-116. Ecology and Agroecosystems. Boca Raton, FL: Lewis Moody, M. E.; Mack, R. N. 1988. Controlling the spread of Publishers. plant invasions: The importance of nascent foci. Journal Charley, J. L.; West, N. E. 1975. Plant-induced soil chemi- of Applied Ecology 25: 1009-1021. cal patterns in some shrub-dominated semi-desert eco- Mott, J. B.; Zuberer, D. A. 1991. Natural recovery of micro- systems of Utah. Journal of Ecology 63: 945-964. bial populations in mixed overburden surface-mine spoils Forman, R. T. T.; Godron, M. 1986. Landscape Ecology. of Texas. Arid Soil Research and Rehabilitation 5: 21-34. New York, NY; John Wiley & Sons, Inc. 619 p. Noy-Meir, I. 1973. Desert Ecosystems: Environment and Fresquez, P. R.; Aldon, E. F.; Lindermann, W. C. 1987. producers. Annual Reviews of Ecology and Systematics Enzyme activities in reclaimed coal mine spoils and 4: 25-52. soils. Landscape and Urban Planning 14: 359-364. Poorter, H.; Remkes, C. 1990. Leaf area ratio and net as- Friedel, M. H. 1991. Range condition assessment and the similation rate of 24 wild species differing in relative concept of thresholds: A viewpoint. Journal of Range growth rate. Oecologia 83: 553-559. Management 44: 422-426. Poorter, H.; Remkes, C.; Lambers, H. 1990. Carbon and Garner, W.; Steinberger, Y. 1989. A proposed mechanism nitrogen economy of 24 wild species differing in relative for the formation of ‘fertile islands’ in the desert ecosys- growth rate. Plant Physiology 94: 621-627. tem. Journal of Arid Environments 16: 257-262. Risser, P. G. 1992. Landscape ecology approach to ecosys- Garcia-Moya, E.; McKell, C. M. 1970. Contribution of shrubs tem rehabilitation. pp. 37-46, In: Wali, M. K. ed. Eco- to the nitrogen economy of a desert wash plant commu- system Rehabilitation, vol. 1: policy issues. The Hague, nity. Ecology 51: 81-88. The Netherlands: SPB Academic Publishers.

33 Ryszkowski, L. 1989. Control of energy and matter fluxes United Nations Environment Program. 1984. General in agricultural landscapes. Agriculture, Ecosystems and assessment of progress in the implementation of the Environment 27: 107-118. Plan of Action to Combat Desertification 1978-84. Ryszkowski, L. 1992. Energy and material flows across Governing Council, 12th Session. 16 February 1984. boundaries in agricultural landscapes. pp. 270-284, In: Memeo. Nairobi, Kenya. 23 p. Hansen, A. J.; di Castri, J. eds., Landscape Boundaries: Vetaas, O. R. 1992. Micro-site effects of trees and shrubs Consequences for biotic diversity and ecological flows. in dry savannas. Journal of Vegetation Science 3: New York, NY: Springer-Verlag. 337-344. Sanders, D. W. 1990. New strategies for soil conservation. Virginia, R. A. 1986. Soil development under legume tree Journal of Soil and Water Conservation 45: 511-516. canopies. Forest Ecology and Management 16: 69-79. Santos, P. F.; Whitford, W. G. 1981. Litter decomposition Walker, J.; Thompson, C. H.; Fergus, I. F.; Tunstall, B. R. in the desert. BioScience: 145-146. 1981. Plant succession and soil development in coastal Santos, P. F.; Phillips, J.; Whitford, W. G. 1981. The role sand dunes of subtropical Eastern Australia. 107-131 of mites and nematodes in early stages of buried litter In: D. C. West, H. H. Shuguart, and D. B. Botkin, eds. decomposition in a desert. Ecology 62: 664-669. Forest Succession, Concepts and Application. Springer- Schlesinger, W. H.; Reynolds, J. F.; Cunningham, G. L.; Verlag, New York. Huenneke, L. F.; Jarrell, W. M.; Virginia, R. A.; Whitford, West, N. E. 1989. Spatial pattern-functional interactions W. G. 1990. Biological feedbacks in global desertification. in shrub-dominated plant communities. In: The Biology Science 247: 1043-1048. and Utilization of Shrubs, C. M. McKell (Ed.). Academic Steinberger, Y.; Freckman, D. W.; Parker, L. W.; Whitford, Press, New York, pp. 283-305. W. G. 1984. Effects of simulated rainfall and litter quan- Westman, W. A. 1990. Managing for biodiversity. Bio- tities on desert soil biota; nematodes and microarthro- Science 40: 26-33. pods. Pedobiologia 26: 267-274. Westoby, M.; Walker, B.; Noy-Meir, I. 1989. Opportunistic Thurow, T. L.; Juo, A. S. R. 1991. Integrated management management for rangelands not at equilibrium. Journal of agropastoral watershed landscape: a Niger case study. of Range Management 42: 266-274. Proceedings of the fourth International Rangeland Con- Whisenant, S. G.; Wagstaff, F. W. 1991. Successional tra- gress 4: 765-768. jectories of a grazed salt desert shrubland. Vegetatio Tivy, J. 1990. Agricultural ecology. Longman Scientific and 94: 133-140. Technical. New York. Whitford, W. G. 1988. Decomposition and nutrient cycling Tongway, D. J. 1991. Functional analysis of degraded in disturbed arid ecosystems. p. 136-161 In: The Recon- rangelands as a means of defining appropriate restora- struction of Disturbed Lands. An Ecological Approach. tion techniques. Proceedings of the fourth International (E. B. Allen ed.). Westview Press. Boulder, Colorado. Rangeland Congress 4: 166-168. Whitford, W. G.; Aldon, E. F.; Freckman, D. W.; Steinberger, United Nations Environment Program. 1977. Desertifica- Y.; Parker; L. W. 1989. Effects of organic amendments tion: an Overview. United Nations Conference on De- on soil biota on a degraded rangeland. Journal of Range sertification. August 29 - September 9, 1977, Nairobi, Management 42: 56-60. Kenya.