Restoring Diversity: Factors Influencing Revegetation Efforts in the Mojave Desert

AWARD OF EXCELLENCE

Research

Joshua Tree National Park and Mojave Desert Land Trust, USA | Marinna Wagner, Student ASLA | Faculty Advisors: Susan J. Mulley, ASLA
Cal Poly Pomona

“

Probably the most rigorous research project in this category. . . It had a good balance of secondary resources and primary research. . .

”

- 2018 Awards Jury

PROJECT CREDITS

  • Neil Frakes, Vegetation Branch Chief, Joshua Tree National Park
  • Joshua Tree National Park, Archaeology and Vegetation Departments
  • Mojave Desert Land Trust, Executive and Stewardship Teams
  • Tom Kayne, Technical Advisor
  • Tom Rottman, Technical Advisor
  • William Wagner, Technical Advisor
  • Madena Asbell, Director of Plant Conservation Programs, Mojave Desert Land Trust
  • Scott Kleinrock, MLA, Chino Basin Water District - Thesis Defense Committee
  • Gerald Taylor, MLA, Department of Landscape Architecture Cal Poly Pomona - Thesis Defense Committee
  • James Ochmanek, Consultant
  • Peter Fahnestock, NRCS California
  • Charlie Rossow, Morongo Basin Historical Society
  • Julianne Wagner, Consultant

Citations

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PROJECT STATEMENT

This research examined the capacity for restoration projects to ameliorate disturbed sites in the desert in the context of unprecedented biodiversity loss and potential extinction of species. The study investigated what factors influence revegetation efforts in the Mojave Desert through meta-analysis and field surveys. The goals were to determine influential existing conditions, biophysical, and anthropogenic factors, assess the effectiveness of revegetation techniques, and comprehend vegetation succession following the revegetation of disturbed arid sites.

The Morongo Basin and Joshua Tree National Park was especially limited in published research. It is currently experiencing an increase in negative environmental impacts attributed to increased urban development, recreation, and global warming. This study shows that revegetation is possible, yet factors and strategies are variable and site-specific. The results and analysis are valuable for landscape architects practicing adjacent to the urban-wildland interface in arid environments. In addition, this research suggests the importance of understanding soils and the vegetation successional patterns and the need for increased monitoring of restoration efforts in arid regions.

PROJECT NARRATIVE

Without increasing the intensity and frequency of conservation efforts, a significant loss of biodiversity is highly likely to occur [8, 14]. Deserts, in particular, are endangered by human activity because they are very slow to recover [6, 44, 46]. The Mojave Desert, the smallest and hottest desert in North America, is threatened by increasing urbanization and global warming [4, 45, 46]. The Morongo Basin and Joshua Tree National Park, which are situated near the transition zone between the Mojave and Sonoran Deserts, have been heavily impacted by grazing, recreation, suburban development, and altered fire cycles [4, 9, 10]. The Joshua Tree National Park has never been more crowded than in recent years and environmental damages caused by recreational activity have increased [15]. The multidisciplinary role of landscape architecture is uniquely suited to addressing the issues of disturbed recreational landscapes, such as National Park systems, which are facing increased visitation by humans. This research highlights the need for landscape architects to obtain an expansive ecological understanding of the variables that factor into the health of a certain environmental community. Advanced knowledge of results in a limited risk of negatively impacting sensitive ecosystems including deserts.

Methods

Studies showed that research of the Mojave Desert was limited and focused on certain subregions. The south-central Mojave Desert, which comprises the Morongo Basin and Joshua Tree National Park, was especially limited in published research. Meta-analysis was used to determine the important variables, attributes, and indicators used to assess revegetation in the Mojave Desert ecoregion. Ecological field methods, which included the belt-transect method and soil testing, were used to monitor eight locations in the Morongo Basin and Joshua Tree National Park to determine key factors and the ways in which these sites were recovering. Data sets between reference and revegetation sites were analyzed to determine trends in existing conditions, soils, vegetation, and biological organisms. In addition, biodiversity attributes such as cover, density, and species richness were identified as important indicators of environmental conditions and long-term ecosystem sustainability [35, 36].

Results

The meta-analysis revealed that variable existing conditions, soils, and herbivory had a significant impact on revegetation efforts. Ecological field studies indicated that revegetation was possible and outplanting was effective, yet variable and slow for passive methods including vertical and horizontal mulching. After eleven years, the Indian Cove Borrow Pit, which was revegetated with outplanted shrub species by the National Park, experienced an estimated increase of 4.5 times that of the initial planting. Other sites saw moderate to limited shrub recruitment in both mulch treatments of local decayed plant material. In addition, sites that experienced vehicular impacts and significant soil compaction, resulted in limited recruitment, richness, and density. Ambrosia salsola was dominant across revegetation sites and Hilaria rigida was significantly abundant among reference sites. Existing conditions including high elevation and soil texture with increased silt were correlated with species richness, aerial cover, and density.

Conclusion

Significant factors influencing the recovery of Mojave Desert vegetation include elevation, vegetation community composition, disturbance type, level of compaction, soil type, and soil texture. This study and supporting studies agreed that outplanting of shrub species was more successful than seeding, and that irrigation and mulch techniques were highly variable and warrant further research. Herbivore protection cages are crucial for the survival of seedlings and at least until young plants become hardy enough to withstand herbivore damages [6, 23, 39]. The lack of seeding success was often attributed to the unpredictable and variable precipitation patterns [19, 20, 48] and some studies indirectly attributed failure to granivores [18, 43]. Passive strategies, such as vertical mulching, are promising and inexpensive, however while shrub recruitment is possible, it is slow and unpredictable due to herbivory and inconsistent precipitation patterns. This research contributes to the empirical research for the Mojave Desert, which is sparse in comparison to the wealth of empirical research available for other semi-tropical regions and suggests topics that require further research and experimentation.

Understanding the factors influencing success of revegetation projects and the roles of early succession species can improve restoration success in the future for the Morongo Basin and Joshua Tree National Park. The Mojave Desert region is likely to experience significant disturbances stemming from population expansion, development, resource extraction, and other indirect anthropogenic effects such as nitrogen deposition [4, 5, 21, 46]. It is integral that land managers, ecologist, and landscape architects improve their understanding of revegetation as it relates to the natural distribution of species across the Mojave Desert. Within ecoregion transition zones, increased study of vegetation communities and restoration methods are dually important in the face of impending global warming and resulting northward migrations of species [27, 33] and mortality attributed to the effects of climate change [9, 10, 23].

Next Steps

Revegetation as an ecological restoration strategy is closely aligned with the practice of landscape architecture as it involves replanting vegetation and rebuilding soils on disturbed sites to meet ecological and social goals [17, 31]. Contemporary goals in landscape architecture have been shifting more towards a focus on establishing ecosystems services and it is likely there will be an increasing demand for practitioners to restore ecosystem health and function. In addition, conservation goals have shifted towards focusing on entire ecosystem protection as opposed to single species preservation [8, 11, 14]. These trends will be integral to the way in which landscape architects approach wildland ecosystems that will inevitably experience increased recreational uses in the future. This research suggests there is a need to improve revegetation techniques for severely disturbed arid sites and increase monitoring of past projects to advance success.

Since deserts are slow to recover naturally, environmental disturbances should be minimized and restoration should be promoted [1, 6]. Human's tendency to relate to fauna more than flora often results in minimally funded plant conservation programs [7]. Since deserts are not well studied, there is a lack of funding to properly address the issues regarding anthropogenic disturbance. In conclusion, this research recommends that the best course of action for landscape architects to determine effective methods for mitigation and prevention of environmental degradation is to synthesize data, monitor, observe, and learn from the desert ecosystems that they are striving to restore.