Consequences of Cool-Season Drought-Induced Plant Mortality to Chihuahuan Desert Grassland Ecosystem and Soil Respiration Dynamics

Variation of water use efficiency across seasons and years: Different role of herbaceous plants in desert ecosystem

Desert ecosystems often structured in two distinct layers of woody and herbaceous plants. Changes in community composition alter the fractional coverage by bare soil, woody and herbaceous plants, with potential effects on water and carbon fluxes. We used eddy covariance measurements and chamber method in two similar shrub-dominated desert communities (Tamarix community and Haloxylon community) to assess inter- and intra-annual variations of ecosystem water use efficiency (EWUE) (where we distinguished whole ecosystem EWUE as EWUE_E, and EWUE of shrub and herbaceous layers as EWUE_Shrub and EWUE_Herb) in central Asia. In the Tamarix community, 11 years of carbon and water fluxes showed that years with larger herbaceous cover (referred to as shrub-herb years) had significant higher EWUE_E than years with lower herbaceous cover (referred to as shrub years), with the values of 1.07 ± 0.11 vs. 0.68 ± 0.03 g C/kg H₂O. There was a significant positive correlation between EWUE_E and the maximum herbaceous plants cover. In the Haloxylon community, chamber measurements during a shrub year demonstrated that the shrub layer contributed most to the gross ecosystem productivity (GEP) and evapotranspiration (ET) of the system, with the herbaceous layer contributing around 30% at the beginning of the growing season, and decreasing to nearly zero during the middle and at the end of the growing season. The shrub layer EWUE_Shrub was significant higher than that in the herbaceous layer (EWUE_Herb) throughout the growing season (1.82 ± 0.11 vs. 1.06 ± 0.32 g C/kg H₂O). EWUE_Shrub was positively correlated with EWUE_E, but there was no relationship between EWUE_Herb and EWUE_E in a shrub year. This study shows that the variability of the herbaceous layer across seasons and years in these desert ecosystems is crucial for predicting water and carbon cycling under ongoing and projected climatic change scenarios in shrub-dominated desert ecosystems.

Productivity responses of desert vegetation to precipitation patterns across a rainfall gradient

The influences of previous-year precipitation and episodic rainfall events on dryland plants and communities are poorly quantified in the temperate desert region of Northwest China. To evaluate the thresholds and lags in the response of aboveground net primary productivity (ANPP) to variability in rainfall pulses and seasonal precipitation along the precipitation-productivity gradient in three desert ecosystems with different precipitation regimes, we collected precipitation data from 2000 to 2012 in Shandan (SD), Linze (LZ) and Jiuquan (JQ) in northwestern China. Further, we extracted the corresponding MODIS Normalized Difference Vegetation Index (NDVI, a proxy for ANPP) datasets at 250 m spatial resolution. We then evaluated different desert ecosystems responses using statistical analysis, and a threshold-delay model (TDM). TDM is an integrative framework for analysis of plant growth, precipitation thresholds, and plant functional type strategies that capture the nonlinear nature of plant responses to rainfall pulses. Our results showed that: (1) the growing season NDVIINT (INT stands for time-integrated) was largely correlated with the warm season (spring/summer) at our mildly-arid desert ecosystem (SD). The arid ecosystem (LZ) exhibited a different response, and the growing season NDVIINT depended highly on the previous year's fall/winter precipitation and ANPP. At the extremely arid site (JQ), the variability of growing season NDVIINT was equally correlated with the cool- and warm-season precipitation; (2) some parameters of threshold-delay differed among the three sites: while the response of NDVI to rainfall pulses began at about 5 mm for all the sites, the maximum thresholds in SD, LZ, and JQ were about 55, 35 and 30 mm respectively, increasing with an increase in mean annual precipitation. By and large, more previous year's fall/winter precipitation, and large rainfall events, significantly enhanced the growth of desert vegetation, and desert ecosystems should be much more adaptive under likely future scenarios of increasing fall/winter precipitation and large rainfall events. These results highlight the inherent complexity in predicting how desert ecosystems will respond to future fluctuations in precipitation.

When vegetation change alters ecosystem water availability

The combined effects of vegetation and climate change on biosphere-atmosphere water vapor (H2 O) and carbon dioxide (CO2 ) exchanges are expected to vary depending, in part, on how biotic activity is controlled by and alters water availability. This is particularly important when a change in ecosystem composition alters the fractional covers of bare soil, grass, and woody plants so as to influence the accessibility of shallower vs. deeper soil water pools. To study this, we compared 5 years of eddy covariance measurements of H2 O and CO2 fluxes over a riparian grassland, shrubland, and woodland. In comparison with the surrounding upland region, groundwater access at the riparian sites increased net carbon uptake (NEP) and evapotranspiration (ET), which were sustained over more of the year. Among the sites, the grassland used less of the stable groundwater resource, and increasing woody plant density decoupled NEP and ET from incident precipitation (P), resulting in greater exchange rates that were less variable year to year. Despite similar gross patterns, how groundwater accessibility affected NEP was more complex than ET. The grassland had higher respiration (Reco ) costs. Thus, while it had similar ET and gross carbon uptake (GEP) to the shrubland, grassland NEP was substantially less. Also, grassland carbon fluxes were more variable due to occasional flooding at the site, which both stimulated and inhibited NEP depending upon phenology. Woodland NEP was large, but surprisingly similar to the less mature, sparse shrubland, even while having much greater GEP. Woodland Reco was greater than the shrubland and responded strongly and positively to P, which resulted in a surprising negative NEP response to P. This is likely due to the large accumulation of carbon aboveground and in the surface soil. These long-term observations support the strong role that water accessibility can play when determining the consequences of ecosystem vegetation change.