Demonstration in Ramon LTER site of the compensation hypothesis providing an alternative scenario to desertification

Climate changes bring a new era in which hot dry areas become even hotter and drier. This leads to state shifts and specifically desertification processes. Desertification includes the state shifts from non-desert to deserts and within deserts reduction of primary productivity. Studying shifts in ecosystem states and desertification requires long term ecosystem research and monitoring (LTERM). This can take decades and because temporal variability of precipitation in deserts is very high, it is difficult to distinguish between noise and trends. Space for time substitution is a tool aiding LTER research. Specifically hyper-arid ecosystems (HAE), the driest deserts on earth, are a good tool to study possible scenarios of climate changes. Understanding structure and function of HAE allows us to imagine possible dynamics that can occur in wetter ecosystems. Our new assertion is that if extreme climate change drives arid lands to function under alternate extreme conditions, then arid land ecosystems will function like an HAE as an alternative state, rather than progress to desertification. To support our assertion, we developed a conceptual framework of HAEs that includes a geo-hydrological “abiotic engine” that drives HAE function by soil moisture diversity and plant functional groups. Based on this conceptual framework, we suggest incorporating two new hypotheses in climate change studies to advance our understanding of responses of large-scale, water-limited ecosystems: Hydro-climatic extremes in water-limited ecosystems will reduce the degree of resource conservation by slope ecosystems due to reduction in plant cover and soil. The decreased ecosystem function on the slope will be compensated for by increasing the effect of the abiotic engine on the ephemeral stream, thus enhancing meta-ecosystem functioning in the ephemeral stream. In water-limited ecosystems, climate change toward hydro-climatic extremes will rescale the dominant hydro-ecological processes of pulse–reserve, source–sink, and connectivity along the semiarid, arid, and HA gradients in two ways: Hydro-climatic extremes in water-limited ecosystems will reduce the degree of resource conservation by slope ecosystems due to reduction in plant cover and soil. The decreased ecosystem function on the slope will be compensated for by increasing the effect of the abiotic engine on the ephemeral stream, thus enhancing meta-ecosystem functioning in the ephemeral stream. In water-limited ecosystems, climate change toward hydro-climatic extremes will rescale the dominant hydro-ecological processes of pulse–reserve, source–sink, and connectivity along the semiarid, arid, and HA gradients in two ways: shrinking of both spatial and temporal dimensions; and shrinking in the temporal dimension and expanding in the spatial dimensions. shrinking of both spatial and temporal dimensions; and shrinking in the temporal dimension and expanding in the spatial dimensions. The first rescaling trajectory is related to biodiversity–ecosystem function and the second to the abiotic engine processes. The forecasted increased frequency and intensity of extreme climatic events may strongly affect ecosystem structure and function in the future. It is unclear how ecosystems will function in the long run over a large spatial scale under a new extreme water cycle. This open question calls for a conceptual framework as a fundamental basis for theoretical and experimental exploration of ecosystem function on a large scale driven by an extreme climate envelope. We investigated hyper-arid ecosystems (HAEs) as natural tangible models that already function under an extreme climatic envelope. The results indicate that unique geological settings can retain runoff water from a single significant flash flood that is sufficient to sustain perennials even during drought years. We calculated a water balance for a large rain event and found that 38% of the rain water infiltrated to the main riverbed which supports local trees, 46% exited the watershed to a larger river and only 19% were lost to either evaporation or the bare slopes. We propose a modified pulse–reserve mechanism that provides water to large acacia trees during the hot dry summer in hyperarid areas.