Semi-Arid Grassland Ecosystem Functional Collapse After Effects of Five Years of Extreme Drought
Lenners, A. 2023. Colorado State University
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.
