Effects of drought exposure on belowground productivity in a desert grassland
Alterations precipitation as a result of climate change affects plant growth and productivity. Chronic drought, in particular, can have detrimental consequences on above- and belowground productivity. Belowground production is critical for carbon storage, soil quality, and other ecosystems services, yet little is known about how belowground production responds to chronic growing season drought. To address this deficiency, we investigated the impact of drought and changes in precipitation seasonality on belowground production in two desert grassland communities in central New Mexico: shortgrass steppe dominated by blue grama (Bouteloua gracilis) and Chihuahuan Desert grassland dominated by black grama (B. eriopoda). Measurements were conducted in the Extreme Drought in Grassland Experiment (EDGE), which includes two rainfall treatments (66% rainfall reduction annually during the summer monsoon, and a six-week delay in monsoon rainfall seasonality with no change in ambient precipitation), plus ambient plots. In July 2014, at the start of the monsoon season, root ingrowth bags (5 cm diameter by 15 cm deep) were buried at the edge of a clump of dominant grass in five replicates of each treatment at each site (total=150). We used analysis of variance to determine if differences occurred in belowground root production among the treatments at each site.
A total of 207.1 mm and 144.3 mm of precipitation fell at the blue and black grama sites, respectively, during the monsoon season. At the blue grama-dominated site, we found that root production was highest in ambient plots (80.0 + 32.6 g m-2) and lowest in the drought treatment (27.0 + 20.0 g m-2) with intermediate values for the delayed monsoon season (52.0 + 13.7 g m-2). We also found similar patterns at the Chihuahuan Desert grassland. Root production was highest in ambient plots (77.4 + 38.7 g m-2) and lowest in the drought treatment (4.6 + 3.6 g m-2) with intermediate values for the delayed monsoon season (60.1 + 13.2 g m-2). Although these trends are consistent with treatment effects, these results were not significant because of high sample variability. We discuss these results in the context of patterns of aboveground biomass and seasonal rainfall. Overall, our results demonstrate that chronic drought generally reduces belowground production and that only partial recovery occurred in the delayed monsoon treatment.