The next 9 blog posts will summarize my reading assignments for the EBIO 3rd semester exam. The exam is scheduled for 3 hours and involves my four committee members asking me questions about anything at all! I was required to put together a reading list covering 4 main topics: biological soil crusts & drylands, microbial ecology, ecosystem services, and community, restoration, and disturbance-succession ecology. Obviously, I actually have 7 topics, which I managed to squeeze into "4". The reading list is a guide for the exam. To help me through this exam preparation process, I will use these blogs to summarize what I am learning over the next 9 weeks.
The first two weeks of my reading are focused on literature of global drylands and biological soil crusts, a familiar domain. Drylands are categorized into four groups depending on their annual precipitation levels and other climatic factors: hyper-arid (<25 mm), arid (25-250 mm), semiarid (250-500 mm), and dry sub-humid. For readers in Wyoming and Colorado, the average annual precipitation of Boulder, CO is 526 mm, or dry sub-humid. In Cody, WY it is 267 mm, so it is semi-arid. The area where I work in Utah receives 219 mm (semi-arid). The driest location in the U.S. is Death Valley, with 60 mm annually, making it arid. The driest location in the world is the Atacama Desert of Chile which receives 15 mm annual precipitation, making it hyper-arid. The Earth’s poles are also very dry. About 45% of the Earth’s terrestrial area is dryland. And about 40% of the world’s population lives in and relies on drylands. Drylands are vulnerable to degradation due to limited water availability. There are a limited number of days each year with enough water availability for plants (and other organisms) to grow and do other life processes, so growth/recovery after disturbance can be quite slow compared to places with more water. One of my favorite ideas from my reading this week was presented in Bowker et al. (2007). For drylands in a degraded state, physical processes rather than biotic processes dominate. I can really feel this idea when I stand out in the field we are trying to restore in Utah. There is so much bare ground and so much erosion occurring that is important for us to overcome physical barriers (erosion) so that biotic processes (vegetation growth) can get going. Desertification is a term used to describe degradation in drylands. The term is broadly used, so estimates of desertification range from 4-74% globally. A definition from Bestelmeyer et al. (2015) is that desertification is an “undesirable state change in drylands” or “persistent and severe reductions in biological productivity due to unsustainable land uses, such as poverty and migration”. My advisor, Nichole Barger, is currently part of the UN Convention to Combat Desertification (UNCCD) due to desertification being a widespread problem that threatens human health and well-being in many places. The UNCCD was established to “assist countries to implement the Convention and address desertification, land degradation and drought”. Their main initiatives include Land Degradation Neutrality, the Great Green Wall, and the Drought Initiative. Our lab’s research does not directly engage with UNCCD efforts, but I think it is important to recognize that the research we do is applicable to more than just the site in Utah where we work. Any progress we make toward restoration of drylands will benefit land restoration efforts around the globe. This is important because human population is expected to continue to grow, land use will increase or shift, and land degradation will continue while climate change is expected to result in altered rainfall, higher temperatures, more severe droughts, and more frequent extreme events in the coming years across global drylands. In other words, this restoration work has value today and in the future in many places around the world. So how do scientists know if a landscape is degraded? This is actually a challenging question because we have to first know what a “healthy” landscape looks like for a certain area. And there is not just one single healthy land state. For instance, one important ecological concept is succession. The community that lives on a landscape will change over time. For example, after fire, the Yellowstone ecosystem will move through a predictable succession of vegetation communities: bare rock; lichens, mosses; small annual plants; perennial grasses; grasses, shrubs, and small shade-tolerant trees; shade-tolerant trees. Lodge-pole pine trees dominate the climax community of that ecosystem. Just as there are multiple versions of “health”, there are also multiple levels of “degradation” for any given piece of land. Because of this complexity, people organize the different ecosystem states and the processes that move ecosystems from one state to another into State and Transition models. These are box and arrow diagrams, specific to a particular landscape, with narratives to describe as much as possible about the land condition. Sometimes it is fairly easy to know what will cause a landscape to shift and how much time it may take (like with succession after fire) because we have seen it before in similar ecosystems. Other times, it is very challenging to know what will cause a shift, what that landscape will look like, and what it may take to push the landscape back to a condition that is preferrable to people. For instance, the unpredictable effects of climate change or intensified land uses may have unknown states and transitions. In restoration ecology we worry about thresholds. These are state transitions that occur quickly and that may have no route back to a prior state. These are extremely challenging to rehabilitate and may result in land abandonment. I like the way Bestelmeyer et al. (2015) describes relationships between rangelands and croplands. In this paper, the authors incorporate human interests and pressures that result in particular state transitions into existing State and Transition Models, arguing that land-use is an important factor to include in these models. In their example, you can start by imagining a cropland, perhaps it is your favorite corn field. If the corn farmer is not very careful, soil erosion can lead to persistent changes in the soil profile (changes in soil texture, soil depth, water-holding capacity, or nutrient availability). These things start to constrain plant productivity, which limits recovery of soil quality. Year after year, these problems build up, resulting in the farmer noticing that her ears of corn are not as delicious or maybe her annual yield is slowly declining. After a while, the farmer decides that the cost of producing corn in this field is not worth the investment and so the cropland is abandoned and converted to rangeland. Erosion continues. You can imagine that the land may eventually become unsuitable for even rangeland use with limited forage available. At this point, it may be extremely challenging to rehabilitate the landscape to any of its prior states. Knowledge of the states and transitions for landscapes is important, as described above, if you want to be able to ensure sustainable land use. One paper I read described how biocrusts have been missing from dryland state and transition models. This is a major problem because as the authors see it, biological soil crusts cover a huge area in drylands and have very important ecosystem functions. A loss in biocrusts, can trigger a state transition across a threshold. So from a restoration perspective (preventing or reducing an undesireable state transition), Bowker et al. (2007) outline three key barriers to overcome in biocrust rehabilitation from less challenging to more challenging: propagule limitation, resource deficiencies, and active soil erosion. Bowker et al. also give us a definition of ecological restoration as defined by the Society for Ecological Restoration, “assisted recovery of a degraded, damaged, or destroyed ecosystem and attempts to return an ecosystem to its historic trajectory”. In restoration there are many different benchmarks that one may want to consider as targets for the restoration like composition, structure, pattern, heterogeneity, function, dynamics, and resilience. For example, when I delve into the microbial ecology of biocrust rehabilitation with my current research project using 16S rRNA gene sequencing, I will be able to understand composition (which microbes are present) better than we could with visual assessments. However, I have to combine samples across a plot and then mix them all together, so I lose information about structure, heterogeneity, and function by taking a sequencing approach. For those things, we would need to use other monitoring tools. And of course, we need that baseline or reference condition to compare our results to. Bowker et al. provides an overarching framework of 18 different dryland ecosystem variables that one could measure which are vital to the ecosystem: perennial species richness, annual species richness, total plant cover, aboveground phytomass, beta diversity, life form spectrum, keystone species, microbial biomass, soil biota diversity, biomass productivity, soil organic matter, maximum available soil water reserves, coefficient of rainfall efficiency, rain use efficiency, length of water availability, nitrogen use efficiency, microsymbiont effectiveness, cycling indices. These are not specific to biocrusts, but you could address at least 11 of these variables with a biocrust project. These could be useful as people plan ways to assess the outcomes of their rehabilitation efforts whether you are comparing the degraded site to a reference or just to itself over time. In Eldridge et al. (2020), the authors wanted to know the overall impact of biological soil crusts on water. They used a meta-analysis approach where you gather all of the papers that have ever been published on biocrusts and water quality or water delivery. They selected 7 water variables that had enough coverage in the literature and then they combined all of that information together. In this type of analysis, you can compare results across studies using a technique called a response ratio. In this case, they wanted to know how higher biocrust cover would affect each variable. So, for water infiltration, for example, you take the value that was measured for water infiltration at high biocrust cover and divide it by the value measured for water infiltration at low biocrust cover. You take the log of that value to get the response ratio. With this work, Eldridge et al. determined how biocrusts generally influence water and how that varies in different regions, with different soil textures, with different biocrust types, with different scales, and with various disturbances. The final paper this week was Antoninka et al. (2020) which is an example of biocrust rehabilitation research that directly addresses the 3 barriers described by Bower et al. in 2007. This project used a complex combination of different restoration techniques straw borders, three different soil stabilization tackifiers, surface roughening, inoculation, shading, and watering. In the end, they determined that active erosion was not important at their site, so any treatment they tried that would have improved soil erosion, did not have a huge impact on biocrust recovery. Instead, as shown in other projects, shading and inoculation worked the best. They also showed that soil texture matters for biocrust recovery (this is related to water availability). Like my own biocrust recovery project, the recovery of biocrusts using special treatments was in some cases slower than control plots. The site might have recovered on its own without human aid. References Antoninka, A. et al. 2020. Addressing barriers to improve biocrust colonization and establishment in dryland restoration. Restoration Ecology 28, S150-S159. https://doi.org/10.1111/rec.13052 Bestelmeyer, B.T. et al. 2015. Desertification, land use, and the transformation of global drylands. Front. Ecol. Environ. 13(1), 28-36. https://doi.org/10.1890/140162 Biological Soil Crusts: An Organizing Principle in Drylands. Weber, B., Budel, B., Belnap, J. 2016. Bowker, M.A. 2007. Biological soil crust rehabilitation in theory and practice: An underexploited opportunity. Restoration Ecology 15(1), 13-23. https://doi.org/10.1111/j.1526-100X.2006.00185.x Eldridge, D.J. et al. 2020. The pervasive and multifaceted influence of biocrusts on water in the world’s drylands. Global Change Biology, 6003-6014. https://doi.org/10.1111/gcb.15232
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AuthorSierra is a graduate student in the Barger Lab at CU Boulder studying microbial ecology for dryland restoration. Archives
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