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.
This week my reading included two papers on drylands/desertification and three papers on biocrusts. The Maestre et al. (2016) paper was a fantastic review and a good example of why my professors have been encouraging me to read lots of reviews at this stage. This paper discusses 4 important global change drivers in drylands (climate change, grazing, land cover change (woody encroachment), and nitrogen deposition) to explain how drylands may change in the future. This paper also offers an alternative perspective on dryland classification. The Aridity Index is a better way to classify drylands because it takes into account both mean annual precipitation (as discussed in blog #1) and mean potential evapotranspiration. Drylands have an index less than 0.65. Delving into the four drivers of change, let's first think about climate change. There are four expected climatic changes for drylands: higher temps (~+4C), shifts in seasonal rainfall, greater frequency of extreme events like drought or heatwaves, and increased aridity. Research has shown that with drought, there are alterations in plant forms, interactions, and cover. Increasing temperatures result in declines in multifunctionality. Increasing aridity is associated with reduced soil organic matter and total nitrogen. Increasing aridity can imbalance soil nutrient stoichiometry. Drylands are home to different below-ground microbial communities than other climates with Actinobacteria (bacteria) and Ascomycota (fungi) dominating the communities. We do not know much about how these communities respond to increasing aridity (or at least this was not discussed in this review). The second driver of change in drylands is nitrogen deposition. My biggest question after reading this section is where nitrogen is predominantly coming from in dryland systems? Regardless, the main idea is that increased nitrogen results in reduced native plant diversity, changes in plant species composition, soil acidification, increased aluminum toxicity, and altered nitrogen cycling. The third driver is intensified livestock grazing which is associated with less soil carbon, less soil stability, less nutrient cycling, and less water infiltration. The final driver is woody encroachment and the authors did their own analysis of how plant traits and aridity result in different ecosystem function. There is an entire section of the paper about the relative importance of these 4 drivers and two applications of this information (for desertification and restoration). My favorite ideas in the Maestre et al. paper are in thinking about why cyanobacteria are interesting and warrant further study. They are highly motile and have the ability to be the first colonizers after disturbance, they are powerful soil stabilizers, and we know that cool-adapted species are replaced by warm-adapted organisms as you move across climatic gradients. For these reasons, the authors suggest that cyanobacteria could be indicators of global change, something you might measure in addition to all the other plant and microbial and soil measurements to tell you that the environment may be shifting (or that restoration may be needed). The perspectives presented in Maestre et al. (2016) were understood in 1990 when Schlesinger published Biological Feedbacks in Global Desertification. This paper also tries to integrate climate change, grazing, and land cover change into a single story for explaining their observations of woody encroachment and desertification at the Jornada Experimental Range in New Mexico. Their idea was that any increase in the heterogeneity of resources across a dryland landscape may result in positive feedbacks that drive desertification. The general story goes like this: as you increase grazing intensity on a landscape, there is a decline in grass cover and reduced competitive potential of grasses compared to other vegetation. Trampling by the grazers compacts the soil, which results in lower infiltration rates, greater runoff, and more erosion (nutrients leach away). Shrubs maintain soil moisture compared to non-vegetated surfaces and so effective infiltration occurs mostly under the shrubs. Nutrient cycling is confined to under shrubs as well. This creates "islands of fertility" or perhaps just remnant fertile homogeneous soil areas. Feedback in this system has to do with water; transpiration is reduced in shrublands compared to grasslands, so temperatures actually increase with woody encroachment through lower humidity and precipitation. These hot dry soils have slower nitrogen accumulation which feeds back to promote shrubs whose growth is less tightly tied to nitrogen turnover in the soil. I am not sure that this narrative continues to hold value or that it is broadly applicable to many systems. I'd have to read a review article (like Maestre) to understand if these ideas are still held. But this paper has a lot of value in reminding me of the importance of biogeochemistry... I dug into nitrogen cycling once again and remembered the pathways of nitrogen fixation, denitrification, and ammonia volatilization. Microbes control these processes in the soil! For the three biocrust papers, one was specific to the CO Plateau (Zelikova et al.), one provided a broad overview of different metrics you could use to measure the "health" of a biocrust (Mallen-Cooper et al.), and one was a fantastic comparison of climate manipulations and physical disturbance on biocrust communities over long time periods (Ferrenberg et al). The key pieces of information from Zelikova's experiment are that after 2 years of warming biocrusts (+2C), there were no changes to a wide variety of metrics. But after increasing the frequency of summer precipitation events for two years, there were significant moss die-offs. For me, this paper is important because it reminds me that pigment concentrations that we frequently measure vary seasonally as do the percent organic matter and the total nitrogen of the soil. It also help set the stage for what people in my field predict or already know about dryland ecosystem response to climate change...the common questions that are being asked and hypotheses. The paper about functional metrics for biocrusts was important to read because it organizes 22 functional indicators into 5 broad groups. As I develop biocrust studies, I can draw one or more metric from each category to have a more well-rounded analysis of biocrust recovery. The 5 categories are erosion resistance, nutrient accumulation, productivity, energy balance, hydrology. I think there are 2 important concepts from the paper. The first is that most of the measurements we can do are destructive. You have to sample and remove biocrust in order to see how it is doing. The authors argue for a global trait database so that as a field we move away from destructive methods. The second key idea is that there are multiple ways to estimate the reference condition or the potential function for biocrusts. You don't always need an intact reference to compare to, you can also use expert opinion, spatial methods, and literature reviews. This is important because most of the landscapes we work in are altered (100+ years of grazing pressure and now climatic change), therefore, reference sites are hard to come by. For now, we do have the Canyonlands National Park nearby with excellent mature, intact biocrust communities that we regularly rely on as reference material. The final biocrust paper was really cool. In this one, they applied climate manipulations to biocrust communities for 10 years (warming and watering). In just one year, they observed changes for watering treatments. It took 6 years for them to see changes in the warming treatments. Similar to Zelikova et al. , mosses decline, lichen are variable, and cyanobacteria fill the gaps where other functional groups decline. When they compared these findings to 15 years of biocrust trampling data (a physical disturbance instead of climatic), they saw that trampling was much worse for lichens. In theory, these transitions to earlier successional states (cyanobacteria instead of lichens or mosses) should be linked to functional shifts as well since people have observed differences between early and late succession biocrust communities in their ability to fix C and N, their vulnerability to leaching nutrients, their erosiveness, dust production, and water infiltration rates. Early successional biocrusts are known to have lower soil stabilities, less fertility, and less carbon storage. The authors seemed hopeful that because community changes due to climate are relatively slow (6 years or more) there may be potential for long term adaptation and species shifts. References Biological Soil Crusts: An Organizing Principle in Drylands. Weber, B., Budel, B., Belnap, J. 2016. Ferrenberg, S. et al. 2015. Climate change and physical disturbance cause similar community shifts in biological soil crusts. PNAS 112(39), 12116-12121. https://doi.org/10.1073/pnas.1509150112 Maestre, F. et al. 2016. Structure and functioning of dryland ecosystems in a changing world. Annu. Rev. Ecol. Evol. Syst. 47, 215-237. https://doi.org/10.1146/annurev-ecolsys-121415-032311 Mallen-Cooper, M. et al. 2020. A practical guide to measuring functional indicators and traits in biocrusts. Restoration Ecology 28, S56-S66. https://doi.org/10.1111/rec.12974 Reed, S.C. et al. 2019. Biocrust science and global change. New Phytologist 223, 1047-1051. https://doi.org/10.1111/nph.15992 Schlesinger, W.H. et al. 1990. Biological Feedbacks in Global Desertification. Science 247, 1043- 1048. DOI 10.1126/science.247.4946.1043 Zelikova, T.J. et al. 2012. Warming and increased precipitation frequency on the Colorado Plateau: implications for biological soil crusts and soil processes. Plant Soil 355, 265-282. DOI 10.1007/s11104-011-1097-z
0 Comments
Leave a Reply. |
AuthorSierra is a graduate student in the Barger Lab at CU Boulder studying microbial ecology for dryland restoration. Archives
August 2023
Categories |