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Most ecosystems across the planet have been degraded in some way by human encroachment. Everyone is so focused on the Amazon rainforest, the Great Barrier reef, or hundreds of other beautiful, stunning places that they overlook the many deserts across the world that are also in the same condition. Whether it’s the Mojave, the Sahara, the Gobi, or one of the other lesser known deserts across the world, many of them are in a state of decline. This coupled with the tough, low nutrient conditions in deserts can make the natural comeback of the system slow. As such, many studies and efforts have been made to speed up this process and what is needed. The restoration of desert ecosystems across the globe requires attention to soil nutrients and retention, local flora needs and interactions, and the reestablishment of native fauna.
What caused the degradation of deserts like the Mojave? Like many places human development is the main cause of this issue, farming, cattle, mining and other activities that strip the land of habitat for monetary gain. Of these the most damaging and widespread cause for habitat destruction is the overgrazing of the land by cattle. According to Robert Webb and Steven Stielstra (1979) the cattle consume plants and trample others leaving to more disturbance than the ecosystem can handle. After a century of this it has left these ecosystems barren, with studies showing that this decrease in plant cover leads to a sharp increase in erosion and runoff (Webb & Steven 1979). After all of this damage there is a lot of work to do so it helps to know where to start.
So, let’s start from the ground up, with almost all restoration efforts one of the first things that you need to do is look at the content of the soil. You need to know what can grow where as well as if there are any deficiencies that need to be addressed before you can start planting. As you can imagine desert soils tend to be on the sandier side and low in nutrients. In the Encyclopedia of Soils in the Environment the World Soil Map section by H. Eswaran and P. F. Reich (2005) goes over the two main soil types found in the arid regions of the world, these being Entisols and Aridisols. Entisols are soils that lack any significant formation and are commonly found in heavily eroded areas of the world. Since they are being eroded constantly, they lack the upper, more nutrient rich organic topsoil and are very mineral in nature. Due to this broad classification there are several different sub-categories of Entisols with the desert variety being Psamment. Which is largely consist of the shifting sands that make up the mobile dunes populating desert regions.
In the same section by Eswaran and Reich (2005) Aridisols are explained on the other hand to have layering and some nutrients but lack proper hydration. As such these arid soils are unable to support all but the most adapted plant life, as according to the text there is no point in the year where there is enough soil hydration to allow for 3 months of continuous growth. This also leads to another issue with Aridisols that makes growth difficult for most plants in that they also tend to contain a buildup up salts that then get redistributed across the horizons. Despite the water-soluble nature of these salts the lack of rainfall keeps the soil very alkaline (Eswaran, Reich 2005). Still, despite the soil quality many plants like cacti, legumes, and deep-root trees call this ecosystem home and is where a lot of restoration efforts are focused.
Because of the arid, shifting nature of the soil and slow plant growth that results of it there are many challenges that need to be overcome. One of the biggest issues with desert degradation across the world is the that when the plant life is destroyed or overgrazed it can no longer hold down the sand that is resides on. This then leads to many issues such as the increased frequency and severity of sand and dust storms or the onslaught of desertification. Desertification being the degradation of “arid, semi-arid, and dry sub-humid areas” most often caused by human activities such as the overgrazing of livestock (Li, Y. 2009). Due to this the restoration of a desert often involves coming up with ways to keep soil retention.
In a research paper by Li, X. R., Xiao, H. L., He, M. Z., and Zhang, J. G in Ecological Engineering, 28(2), (2006) they looked at one such method which involved the use of straw laid out in a checkerboard pattern across a degraded desert landscape. The straw acts as makeshift foliage and keeps the retention of the desert soil long enough for the desert plants to grow and fix the soil in place. The process involves the placing down of “Mechanical sand fences” which are created out of bamboo and willow to act as windbreaks. After which wheat straw, which can be bought cheaply in very large quantities, is then placed in the ground enough so that it is partially sticking out in the checkerboard pattern. After which native xerophytic shrubs are the planted in the 1-meter by 1-meter squares. Due to the arid nature of the environment and lack of rainfall these structures can stay functional for up to 5 years, giving the plants long enough time to develop.
Knowing this, the group of researchers led by Li, X. R. (2006) then set out with their own study to test this procedure for themselves. They set up 4 testing sites and used a variety of shrubs and herbs in the checkerboards or “quadrates” for a total of 120 plots. After which they took soil samples, measured the dust deposition along the checkerboards and looked at the bulk density and pH levels over the course of the study. After 6 years the research was concluded and they found that using the straw checkerboards before the native xerophytes where planted led to an increase in the overall increase in species richness, ground cover, dust deposition, and the accumulation of silt and clay in the topsoil. This is shown to be an effective method in the desert restoration of areas that have lost their sand-binding vegetation cover.
What do you do though if there is not enough nutrients or water to go around, or if the plants you are introducing have issues taking root and getting established? Restoration of desert ecosystems can be a hard undertaking for these reasons. Even in the restoration of more nutrient rich and temperate climates transplanting can lead to some plants failing. This is exasperated in the extreme conditions found in deserts, more so in the degraded ecosystems you are trying to restore. Transplant survivability in these conditions can be as low as 25% or even worse if animal herbivory is in overabundance (Abella et al. 2015). Therefore, this needs to be at the forefront of any restoration effort in the revegetation of the site.
What are ways you can get around this issue though? An experiment by Scott R. Abella, Lindsay P. Chiquoine, Alice C. Newton, and Cheryl H. Vanier for the Journal of Arid Environments v115 (2015) looked at solutions to this. They looked at rooting hormones, nutrient rich slurry, soaking treatments, irrigation, and topsoil replacement between 23 different plant species. The young plants were grown in a nursery where they were monitored before being subjected to the various treatments. They found that the IBA rooting hormone, soaking, and slurry treatments did not increase survivability in or out of the nursery. The topsoil replacement fared a lot better though as it increased the survival of the plants to 56%, up from original 25%. However, irrigation proved to be the winner, increasing survivability of the plants up to a 65% average. This information is valuable in understanding what the dos and don’ts in restoring this type of ecosystem in not only increase plant survivability but also avoiding unnecessary expenses.
Some plants however are a lot pickier in where they can grow, and this can lead to new challenges in trying to recreate these conditions. Research by Bacilio, M., Hernandez, J. P., and Bashan, Y. (2006) from the journal Biology and fertility of Soils, 43(1) looked at the Cardon Cactus, a major plant crucial to restoration in the Sonoran Desert that was largely removed for land development. The cactus is largely found on patches of increased soil fertility and nutrients nicknamed “resource islands”. With these not always being available they looked at the effects of compost and the growth promoting bacterium Azospirillum brasilense to stimulate the growth instead. They found that while the bacterium did very little to increase cacti growth the compost had better results. The compost, added in ratios between 6% and 25%, simulated “resource island” soil that the cactus prefers, and resulted in increased growth. This shows that soil variation such as “resource islands” can be recreated artificially in the context of restoration.
However, what if you wanted to recreate these conditions naturally, as restorationists often do, how would you go about it? Deserts being by nature very nutrient poor often are dependent on biotic interactions of plants and animals, on such being the Legume. In Desert Restoration: The Role of Woody Legumes by Ross A. Virginia (1990) he goes over the important role played by woody legumes such as mequite (Prosopis glandulosa), smoke tree (Psorothamnes spinosus), palo verde (Cercidium floridum) and others. The most important role that they play is that they are key nitrogen fixers in the Sonoran and Chihuahuan Deserts. They are important to increasing soil nutrients and the “resource islands” that are important to the ecosystem (Virginia, R. 1990).
Ross explains that as the trees grow, they develop deep root systems with bacteria that fix Nitrogen increasing soil fertility. During this development the legume trees periodically lose their leaves that then decompose. This decomposition releases all of the nutrients and cations that the tree took up from deeper in the soil and deposits it on the surface creating a rich topsoil. Over time this changes the soils chemical and physical properties creating “resource islands” that the Cardon Cactus and other key species rely on. Despite their importance Ross does explain that getting the trees to propagate and establish can be time consuming and difficult. To properly establish their deep roots system requires a level of stored soil moisture that only happens once or twice a decade naturally. This can be aided by restoration practices such as “deep ripping or 3m deep auguring holes” to improve seeping of water.
So, we have talked about soil types, soil fixation, and soil nutrients, as well as steps needed to introduce native plant species. Of course, all of this development doesn’t just benefit native vegetation, but invasive species as well. The bane of every restoration project is dealing with the invasion of overly aggressive nonnative plants and animals that wreak havoc on what you’ve painstakingly restored. Of course, the normal procedure would be to cut and spray on a regular basis to keep them at bay. While this definitely works it can be time consuming as well as labor intensive. To reduce the impact by the invasive species and the need for spraying, some researchers have been looking into the use of native species to keep them at bay (Abella, et al. 2012).
In a study by Scott Abella, Donovan Craig, Stanley Smith, and Alice Newton (2012) they looked at native plant species and if they could be used to reduce the number of invasive species present in the restored ecosystem. The most common type of invasive species in desert ecosystems are prairie grasses. Many of them propagate after the rainy season and dry up in the summer, leading to fires which are uncommon in this region. This fire then destroys the slow growing native populations while the grasses, which are pyrophytes, simple pop up again in a vicious cycle. Some of these invasive grasses that they looked at controlling where Bromus rubens (red brome), and the exotic Mediterranean grasses Schismus arabicus & barbatus.
The study itself (2012) focused on sampling methods for the native plants to find which species effective in invasive reduction. They looked at several species of various growth and aggressiveness, from early grasses and flowers to late shrubs. They found that the planting of early seasonal flowers communities, especially the species Sphaeralcea ambigua also known as desert globemallow, where the best at competing with the invasives. The other species did not do as well though and ended up losing ground. They also evaluated nitrogen levels and how that affected the spread of the invasives finding that the increase in one led to a significant increase in the other (Abella, S. R., et al 2012). There are two things that can be taken from this. One, that when looking to restore an ecosystem it can be beneficial to find more natural solutions or aids to issues. Secondly, that it touches on the potential damaging effects of the nutrient introduction that was touched on earlier.
Keeping with the same theme, the restoration of desert ecosystems can sometimes have a unique perk. As deserts are very inhospitable and extreme environments sometimes the best thing you can do to drive out invasive and exotic species is through restoring the habitat. The degradation and changing of the environment from its initial state can be what allowed the non-native species to thrive and become invasive in the first place. At Ash Meadows National Wildlife Refuge, which resides in an oasis region of the Mojave Desert, they were having issues with invasive fish. Chemical and physical proved to have limited success and so they looked to other solutions like this one (Scoppettone, et al. 2005).
The study was carried out by Gary Scoppettone, Peter Rissler, Chad Gourley, and Cynthia Martinez (2005) and looked at a section of the reserve that over time had become a cold-water marsh. This occurred because in the past large levees and concrete ditches where installed to divert water away from the thermal springs for agriculture. This change in the environment then allowed invasive Gambusia affinis (mosquitofish) and Poecilia latipinna (sailfin molly) to overwhelm the local fish populations. After diverting the thermal spring water back into the marsh increasing the water temperature to an 18 – 32°C range and altering the flow rate to 30cm/s there was noticeable changes. They found that after restoration native fish like the Ash Meadows speckled dace and pupfish recolonized significantly faster and where able to compete the non-natives.
Other animals are important to desert ecosystems as well and should also be a focus on in restoration. In the deserts of the Southwest rodents are a key part of the ecosystem, Michael A. Pattern (1997), a researcher, looked at the reestablishment of rodents in restored shrubland. The site was a previous construction site in Palm Springs, California and had been stripped completely bare of vegetation. As a restoration goal, the main focus was to restore the landscape to the previous 15% shrub cover indicated by the bordering natural patches. Michael however, believed that the vegetation wasn’t a good indicator of fauna health. That even though the 15% shrub cover goal was eventually met, the vegetation compared to bordering undeveloped scrubland was still wildly different with little impact on the native rodent communities. He surmised that this because of non-vegetative factors like water availability and lack of ant competition. This shows the importance of other ecosystem factors in restoration as simple plant cover goals doesn’t give the whole picture.
Once in the ecosystem they can then play important roles in the food chain and the ecosystem interactions that are important to a desert as touched on previously. Going back to the importance of soil health and fertility native animals also pay a role in that as well. In the Chihuahuan Desert, small mammal burrowing, and disturbances help control erosion and runoff. Similar studies, like the one by Alex James, David Eldridge, and Brydie Hill (2009) that looked at the role of burrowing animals in shrublands like the site before. The specific shrubland they looked at was in the desert land of South Australia where they looked at 4 species of vertebrates: greater bilby, European rabbit, burrowing bettong, and Gould’s sand goanna and their effects on the environment. They found that the foraging pits of these animals contained significantly higher levels of carbon and nitrogen with the rabbits and goannas contributing 55% more mineralized nitrogen. This nutrient addition is greatly important in effecting seedling viability and establishment in the shrubland.
Finally, while we are on the subject of animals lets go back to where we started, overgrazing. With proper regulation though it can be an integral part of the ecosystem. A study about the interaction between grazing and arid desert ecosystems was testing in northwest China. Researchers Zhi-Yu Zhou, Feng-Rui Li, Shan-Ke Chen, Hong-Rong Zhang, and Guangdi Li looked at the nitrogen and carbon changed in the soil of desert shrubland after 26 years of monitored grazing. They had found that there was a significant increase in “vegetation cover, plant diversity, productivity, and higher increase in soil C and N levels”. Showing that the effects of controlled grazing and disturbance leads to greater habitat restoration in ecosystem.
Like all ecosystem’s deserts are complex and rely on a lot of organism interactions in order to be healthy. The restoration of them will be a large undertaking and will require a lot of knowhow about the topics gone over in this paper. As the soil lacks nutrients and is highly erodible steps will need to be taken in order to deal with this. Once the soil has been fixed in place and restored with nutrients, either through artificial or natural means then other issues can be addresses. Both plants and animals are dependent on each other and needs to be considered. Invasive species will need to be dealt with, and the site will need to be monitored afterwards to see how things are progressing, as this is a slow growing ecosystem this will take a few years. In the end though all the hard work will pay off in this ecosystem as will the restoration of others to come.
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