||Drylands (arid and semi-arid lands, such as deserts and many grasslands) make up about 35% of the United States and over 40% of the terrestrial surface globally. Indeed, drylands are our planet’s largest biome. These relatively arid ecosystems also maintain very large stocks of carbon; for example, dryland soils store nearly twice as much soil organic carbon as temperate forest soils. In addition, due to high solar irradiance, drylands have the potential to affect future climate not only via changes to carbon cycling, but also through changes in albedo and energy balance. Nevertheless, research on the potentially shifting contribution of drylands to regional and global carbon and energy budgets has received relatively little scientific attention. This lack of attention represents an important deficit in our understanding and ability to forecast the effects of global change: climate models predict rapidly rising temperatures for the already hot and moisture-limited dryland regions, and such changes could dramatically affect these extensive landscapes. For example, a recent study showed that biological soil crusts (the soil surface community of mosses, lichens and cyanobacteria, which form a critical component of dryland ecosystems) can respond to climatic change with rapid mortality events. In spite of the importance of such eventualities, what they could mean for carbon cycling and storage, for coupled nutrient cycling, and for energy balance across the widespread dryland biome remains almost wholly unknown. Our lack of understanding of the direction, magnitude, and mechanisms behind these projected changes – as well as the absence of a theoretical framework for incorporating biocrusts into global models – greatly constrain our ability to appropriately represent drylands in modeling efforts. At the same time, the main message from the limited existing data is that dryland ecosystems have the potential to respond dramatically to climatic change, and that such changes could, in turn, affect climate at the global scale.
This project will make great strides in advancing our understanding of the role drylands play in global climate, both now and into the future. To elucidate the mechanisms behind and the consequences of climate-induced change to dryland communities, we have planned a multi-disciplinary yet integrated approach that combines in situ field manipulations; cross-ecosystem comparisons; the creation of ‘libraries’ of dryland vascular plant, biocrust, and soil spectral data; and a small-scale modeling effort (to be followed in later years by a larger-scale modeling effort). In particular, we propose to test the following four hypotheses: Hypothesis 1) Climate-induced changes to the composition and cover of vascular plant and biocrust communities will have dramatic effects on carbon and nutrient cycling;
Hypothesis 2) Biogeochemical drivers of and responses to these changes in communities will be inconsistent across different dryland ecosystems; Hypothesis 3; Changes to vascular plant and biocrust communities in drylands will have significant effects on energy balance; and, Hypothesis 4) Introducing a predictive representation of biocrust structure and function into the Community Land Model (CLM4.5), including physical and biological biocrust properties, will improve predictions of water, energy, and carbon fluxes between dryland systems and the atmosphere.