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Semi-Arid Grassland Ecosystem Functional Collapse After Effects of Five Years of Extreme Drought

Lenners, A.. 2023.

Abstract

A key outcome of climate change is an increase in the frequency and intensity of drought events in many regions of the globe. The largest impacts on ecosystem structure and function are likely to occur in water-limited ecosystems, such as semi-arid grasslands, potentially leading to a collapse of ecosystem function. While short-term studies have been conducted on various grassland ecosystems, the goal of this study is to fill in the gap of the effect multi-year extreme droughts have on the semi-arid shortgrass steppe of the Central US by characterizing the change in structure and function of these ecosystems. The drought was conducted between 2018-2022, and I had conducted various measurements over the summer of 2022 within the USDA-Central Plains Experimental Range (CPER) of Northeastern Colorado. The experimental drought was imposed using four rainfall exclusion shelters, two of which blocked 66% of precipitation from entering and the other two remaining uncovered (control plots). Ten plots in each of the four shelters were measured weekly for soil moisture (%), soil temperature (oC) and soil respiration (CO2 efflux); twice per season for soil nutrient availability; and at the end of the growing season for aboveground (ANPP; stems and leaves) and belowground net primary production (BNPP; roots). The extreme drought resulted in an ~40% reduction in growing season soil moisture and an average 2°C increase in soil surface temperatures. Within the 13 weeks of study, drought led to an ~50% reduction in soil respiration (CO2 efflux). ANPP was drastically reduced (~99%) with extreme drought, while cactus surface area increased 3-fold. The extreme drought treatment also resulted in large reductions in BNPP measured from 0-30 cm (79%); however, root growth was reduced most in the shallowest soil depth (0-10cm) when compared to control plots. Lastly, there was an increase in nitrogen availability (both NH4. and NO3 - ) with extreme drought by the end of the growing season. These results suggest that extreme, multi-year drought can cause an almost complete collapse in ANPP and significantly reduce BNPP particularly in the top 10 cm of the soil profile, which could have important implications for carbon sequestration. It remains unknown what impact the dramatic reduction in ecosystem productivity but accumulation of available nitrogen in the soil will have for recovery of the shortgrass steppe ecosystem post-drought, but it is likely that recovery will be prolonged despite the increase in soil resources.