The Society for Ecological Restoration (SER) defines ecological restoration as "the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed" (SER 2004). Similar to other ecosystems, the general principles of desert restoration include:

• Prioritizing areas for restoration, using criteria such as the degree of degradation of the ecosystem (e.g., lost plant cover, soil erosion, or missing indigenous biota), or the need to maintain water quality or soil productivity for human resource use.
• Determining reference conditions, representing models of what are considered natural, healthy conditions.
• Setting objectives or targets for restoration, often based on reference conditions and the degree to which current conditions differ from reference conditions, as modified by what is desirable or feasible from an environmental management standpoint. For instance, restoration goals can be modified to favor higher proportions of certain grass species in the restored ecosystem that are desirable for meeting the grazing needs of livestock kept by humans.
• Grounded in an understanding of ecological knowledge specific to the ecosystem undergoing restoration and often drawing from fields such as engineering, design treatments to accomplish restoration objectives. Ideally any ongoing sources of degradation (e.g., dust erosion reducing soil integrity), which may have resulted in the need for restoration in the first place, are reduced to the extent possible as part of restoration.
• Conducting monitoring and research to evaluate if restoration goals were met and to inform implementation of future projects.

A common misconception about ecological restoration is that it seeks to exactly replicate past conditions. While in rare cases projects may seek to do this, often it is not desirable or even feasible to replicate past ecosystems. For example, if a species has gone extinct from an ecosystem and no genetic material remains, that species will not be able to be reestablished. Climates also have changed and are changing, such that replicas of past ecosystems from different climatic time periods are unlikely to be able to persist in the same locations in a different climate.

The essence of restoration is reestablishing the evolutionary trajectories of ecosystems that were disrupted, such as reestablishing the cyclic pattern in deserts of perennial plant species facilitating the establishment of others by modifying the environment in small patches of soil (Padilla & Pugnaire 2006). This approach explicitly recognizes that the evolutionary trajectories of ecosystems may change with changing climates, enabling restoration practices to be tailored to reestablishing trajectories that ecosystems would have had (if not disrupted) in new climatic conditions. Often restoration can and does result in reestablishing desert ecosystems that are more productive, resilient, and able to supply higher quality resources (e.g., water) and benefits (e.g., air quality, recreation opportunities) to humans.

Locations and Unique Environments of Deserts

Deserts are typically defined as regions where evaporation and transpiration (water release by plant tissues) exceeds precipitation, which is generally <25 cm/year (Noy-Meir 1973). Deserts occupy 20–35% of Earth's land surface depending on mapping criteria. Deserts primarily occur at 30° latitudes where dry tropical air descends, in the interiors of continents far from ocean moisture, and in rain shadows adjacent to mountains (Figure 1).

While deserts are best known for being hot and dry, the extreme variation (daily, seasonally, and among deserts) in climate is a key feature of deserts (Crawford & Gosz 1982). For example, hot temperatures exceeding 50°C occur, whereas extremely cold temperatures of -20°C have been recorded in Asia's Gobi Desert. These extreme environments have led to numerous adaptations of desert organisms, such as plants that have moisture-conserving leaves or that reside as seeds in the soil for long time periods until climate is favorable for growth (Evenari et al. 1982).

Disturbances and Natural Recovery

Many types of human-caused disturbances have occurred in desert ecosystems (Burke 2001). Failed agricultural attempts, clearing land, road building, off-road vehicle use, water diversion, grazing by non-native herbivores, and fires fueled by non-native plants are some of the many disturbance types of deserts (Figure 2). Depending on their type and severity, these disturbances reduce the abundance of indigenous species, alter soil properties, diminish ecosystem services for humans, and create hazards to humans such as blowing dust from erosion of devegetated soils. These disturbances also can alter a common feature of deserts — a spatial pattern of fertile islands (nutrient-enriched, shaded areas below the canopies of perennial plants) alternating with openings between perennial plants (Crawford & Gosz 1982). Fertile islands are key areas for recruitment of plants and biological activity.

Natural recovery from disturbance in deserts is typically slow. For example, the average time for the reestablishment of perennial plant cover following a variety of disturbances in North America's Mojave and Sonoran Deserts was 76 years, and even partial recovery of species composition required over two centuries (Abella 2010). After severe disturbances, or in the case of desertification, recovery through natural processes following disturbance is not necessarily possible, creating a need for restoration.

Estimating Reference Conditions

A variety of approaches and techniques can be used for estimating reference conditions in deserts (White & Walker 1997). In some cases, reference conditions could be areas that are relatively undisturbed and located adjacent to a disturbed area that will undergo restoration. When suitable contemporary references do not exist, techniques that estimate past conditions can be employed to reconstruct what the pre-disturbance ecosystem may have been like. Sources of this type of reference information include but are not limited to: historical documents such as journals, old photographs, oral descriptions from long-time residents, land-use records, packrat middens and soil phytoliths (both techniques using plant fossils to reconstruct past vegetation conditions), and climate records.

Desert Restoration Techniques

Once reference conditions are estimated and project goals are established, restorationists design and implement techniques intended to accomplish project objectives. Major desert restoration techniques include: planting and seeding, managing water, manipulating soil properties, and providing cover. Controlling non-native species often also is part of restoration and subsequent maintenance management in the restored ecosystem (D'Antonio & Meyerson 2002). Major desert restoration techniques are summarized in the following sections, together with the special case of riparian restoration within deserts.

Growing plants in nurseries or greenhouses before planting in the wild is typically defined as outplanting. Salvaging plants prior to disturbance (for re-planting after disturbance or at different locations) or moving plants from other locations to a restoration site is called transplanting. Outplants are often grown in greenhouses for ≥ 1 year to enable the plants to develop root systems before planting (Bean et al. 2004). To become established at restoration sites, many plants require initial supplemental watering and protection from herbivores (Figure 3). Achieving a survival of ≥ 50% in desert planting projects is typically considered good (Abella & Newton 2009). Plant establishment on some disturbed soils can benefit by inoculation with mycorrhizae, fungi that form relationships with plant roots and can assist plants in uptake of water and nutrients (Allen 1989).

Seeds are commonly broadcast by hand for small restoration areas or by planes for larger areas. Sometimes seeds are coated as pellets or sown with a protective mulch to conserve moisture and reduce predation (Brown et al. 1979) of the seeds by seed-eating insects and animals (Figure 4). There are some examples of successful seedings for desert restoration (e.g., Abella et al. 2009), but also many examples of complete failures. Using high-quality seed and having seeding coincide with favorable conditions for plant establishment are keys for possible success.

Water can be managed at restoration sites through irrigation and methods such as contouring land to create water catchments (Bainbridge 2007). The utility of water management techniques, such as for increasing plant survival, can be assessed in the context of costs and benefits. For example, if irrigation increases plant survival by only 10%, it might be more economical to simply plant 10% more plants and not irrigate.

Soil health can be restored by additives, contouring, stabilizing, and other techniques to promote favorable soil properties for desired ecosystem development (Bowker 2007). If through soil analyses and plant growth trials certain nutrients are determined as limiting, adding fertilizers or organic matter (which also increases soil water holding capacity) can improve soil conditions. Caution is needed when adding nutrients, however, as non-native species often thrive in nutrient-enriched areas, and many deserts near urban areas are enriched in nitrogen due to deposition from air pollution. In these cases, adding carbon to soils can stimulate uptake of nutrients by soil microbes (which often are limited by carbon as their energy source), making the nutrients less available to plants and potentially providing competitive advantages to native species.In soils contaminated by heavy metals commonly near mining districts, adding organic matter to soils and using species tolerant of pollution can be important.

Revegetating exposed soils can help reduce erosion and stabilize soils, but other materials also can stabilize soils and/or provide cover. "Planting" dead plant material is termed vertical mulching. This technique is used for visual restoration (such as blending closed roads into the surrounding landscape), providing shade structures to assist plant recruitment, protecting seeds, and other purposes (Figure 5). Other mulches such as wood chips and synthetic materials also can be used. Protected areas can be further created by arranging rocks or other structures (Evenari et al. 1982).

Riparian areas occur along watercourses flowing through drier environments and, similar to springs or oases, contain biota that differs from drier areas within the desert. Some of the same techniques discussed above are applicable to restoration in these moister areas. An array of other techniques including streambank stabilization, restoring natural flooding regimes (such as for maintaining processes of sand bar formation within streams), and recontouring of land to reestablish flow patterns of springs also can be needed (Landers 1997).

Challenges and Outlook

Restoring desert ecosystems is challenged by extreme climates, dry soils, seed predation, herbivory, and generally slow rates of plant colonization and growth. Despite these difficulties, at least partial restoration of desert ecosystems is possible. There are examples where restoration techniques such as outplanting initiated ecosystem recovery and accomplished project objectives (Abella & Newton 2009). Failed projects have illustrated that there is little room for error when implementing desert restoration techniques, underscoring the importance of using good practices (e.g., planting good-quality stock at appropriate times of the year). Future research in desert restoration may help improve restoration techniques, provide an understanding of under which conditions different techniques work best, and identify situations where restoration is most feasible and has the greatest probability of success.