The physiognomic shift in ecosystem structure from a grassland to a woodland may alter the sensitivity of CO₂ exchange to variations in growing-season temperatures and precipitation inputs.  One large component of ecosystem flux is the efflux of CO₂ from the soil (soil respiration, R_soil), which is a function of both biotic (vegetation cover type, litter quality, rooting depths) and abiotic (resource availability, temperature) factors.  The relative importance of these drivers has not been fully quantified under these transitional states, but doing so is particularly of interest within the semiarid southwest where temperature and available moisture vary and covary throughout a growing season and vegetative cover change is rampant.  Determining when ecosystems are temperature sensitive and when they are not is vital for predicting future source/sink status of these ecosystems as they experience woody plant encroachment.  Within this study, we used a combination of the traditional soil-collar technique and soil CO₂ sensors to obtain an extensive temporal and spatial estimation of R_soil at each site along with eddy covariance towers to estimate total ecosystem respiration.  Measures of R_soil were made under grasses, shrubs, and in bare spaces so that the individual responses of multiple microhabitats could be analyzed within each site.
Results/Conclusions

Each of these microhabitats differed significantly in terms of the magnitude of peak flux and the duration of activity in response to precipitation, suggesting that individual measurements within an ecosystem are important for scaling studies.  All three microhabitats followed a similar pattern throughout the growing season in that R_soil rates were significantly lower during the pre-monsoon drought.  However, R_soil under mesquites began to increase about DOY 100 (~ 85 days prior to the onset of the monsoon), indicating that the earlier leaf-out of the woody plants was leading to an earlier development of measurable R_soil.  R_soil rates regressed against soil temperature showed a hysteresis in R_soil within each day, with rates being higher in the late afternoon/evening.  The degree of hysteresis varied among the microhabitats, but was consistently greater under the woody plants.  Furthermore, the degree of hysteresis varied throughout the growing season among all microhabitats.  Assuming a simple linear relationship between R_soil and soil temperature resulted in a significant underestimation of ecosystem-scale R_soil­­.  These results suggest that as ecosystems shift in vegetative cover to woody plants and temperatures within the southwest increase, ecosystem respiration rates may increase more rapidly than if the ecosystem were still dominated by grasses.