Water-limited ecohydrology and carbon sink-source dynamics of desert grasslands and shrublands during dry years, Chihuahuan Desert, USA

Background/Question/Methods

State change of grassland to shrubland is a concern for the viability of arid ecosystems worldwide. Differences in grassland and shrubland functioning result in altered landscape-scale responses and carbon uptake patterns when ecosystem state change occurs, and these differences may be exacerbated in future climate scenarios. In this research we characterized the hydrology and ecology of native C₄ mixed blue grama (Bouteloua gracilis) and black grama (B. eriopoda) grassland and C₃ creosote (Larrea tridentata) shrubland in the northern Chihuahuan Desert, USA, during a drier than average period from 2007-2011. Our primary goal was to determine the patterns and causes of carbon sink-source dynamics at grassland and shrubland sites using aboveground and belowground measurements of net primary productivity and flux tower measurements of carbon exchange.

Results/Conclusions

Grassland and shrubland carbon sink-source patterns from 2007-2011 differed from each other at daily, seasonal and annual timescales and were influenced by a number of biotic and abiotic variables. Grasslands were a carbon source in all years except for 2010, while shrublands were a carbon sink in all years. This difference is driven by high grassland surface respiration at daily, annual and seasonal timescales, which may be a legacy effect of belowground stored carbon. Differences in sink strength are also facilitated by a significantly longer growing season in shrublands (P < 0.05) than in grasslands. Shrubland carbon fluxes are less sensitive to fluctuations in soil moisture and surface energy exchange during the growing season than grasses, although C₄ grasses have a higher ecosystem water-use efficiency. If changing patterns of winter and monsoon precipitation result in greater seasonal variance in water availability in the future, this may highly favor shrublands over grasslands in the Chihuahuan Desert and other similar ecoregions.