HOW DO LONG-TERM CHANGES IN PRECIPITATION SEASONALITY AFFECT DRYLAND CARBON DYNAMICS? EVIDENCE FROM A 21-YEAR MANIPULATIVE CLIMATE-CHANGE EXPERIMENT

Abstract:

Semi-arid shrublands of the Intermountain West have been identified as potential carbon (C) sinks, contingent on precipitation amount and timing. To date, there is a dearth of long-term, manipulative studies that have quantified the effects of altered precipitation amount and seasonality on aboveground C storage and turnover. We measured aboveground C pools and fluxes at leaf, soil, and ecosystem scales during the 2014 growing season, at a 21-year ecohydrological experiment site in Idaho, USA (a cold desert ecosystem). At this experiment, winter- and summer-only irrigations (200 mm for each treatment) have been applied to plots to supplement ambient precipitation, since 1993. We hypothesized that increases in winter precipitation would stimulate net aboveground C uptake and storage relative to ambient and increased summertime precipitation regimes. Our hypothesis was generally supported. Midday net ecosystem exchange in winter-irrigated and summer-irrigated plots was nearly 4X greater than in control plots, although the seasonal timing of peak fluxes was different between these treatments. Similarly, cumulative-seasonal net ecosystem exchange was about 4X greater in irrigated plots relative to control plots, although the seasonal patterns of net C uptake differed. We observed no differences in leaf-level photosynthesis among treatments. Midday and evening soil respiration were often 3X greater in summer-irrigated plots compared to control plots throughout much of the growing season. Soil respiration in winter-irrigated plots was only slightly greater than in control plots. Aboveground biomass in winter-irrigated plots was 2X that in control plots, and biomass in summer-irrigated plots was only modestly different from control plots. Most of the long-term increases in biomass were due to increased shrub cover and size, but not shrub density (# individuals/m²). Our findings suggest that expected increases in wintertime precipitation in western North America could result in increased aboveground C uptake and storage in shrubland communities in cold deserts ecosystems. Soil respiration appears to be the component of aboveground C flux most responsive to changes in precipitation seasonality. Environmental factors regulating soil respiration might be key to net C balance in semi-arid ecosystems.