Print PageShrub encroachment may fundamentally change how ecosystems function due to alteration of labile and recalcitrant carbon pools, soil and plant communities, and eco-hydrological interactions. Our understanding of the long-term, residual impacts of encroachment on fundamental ecosystem processes, such as soil respiration (Rs), is hindered by the short time scales over which these processes and their drivers are quantified or related to each other. In this study, we quantified the “memory” or time scale over which past or antecedent soil moisture and temperature conditions impact current Rs. This study was conducted within a shrubland ecosystem at the intermediate stage of encroachment, and measurements of daily integrated Rs, soil moisture, and soil temperature (two depths, surface and subsurface) were made in three primary microsites created by encroachment (under shrubs, near grasses, open space). We fit a simple process model to the Rs data in a hierarchical Bayesian framework to quantify the duration of the memory and the relative importance of moisture and temperature in past weeks for Rs. We also incorporated the effects of “current” soil moisture and vapor pressure deficit because it has been shown that each may impact Rs.
Results/Conclusions
Antecedent moisture and temperature positively affected Rs beneath shrubs but had a negative effect in open and grass microsites. It appears that surface temperature and subsurface moisture were significant for Rs under shrubs, but surface moisture and temperature at both depths were important for Rs in open and grass microsites. In terms of temporal scale, soil moisture and temperature one to four weeks in the past have the most significant effect on Rs in the grass microsite. For open space, the memory extends from one to six weeks in the past. The shrubs had the longest memory, with conditions eight to nine weeks in the past showing the strongest effects on current Rs. Inclusion of memory effects in the Rs model increased the R2 of observed vs. predicted Rs by 12%. Current soil water content positively affected, but VPD negatively affected Rs in grass and shrub microsites. Surface rather than deeper soil moisture was important for Rs in all microsites, and mean rather than maximum daytime VPD was more significant for Rs in the grass and shrub microsites. Our findings show that shrubs can lengthen the memory of Rs, where moisture and temperature conditions relatively far in the past can impact the current status of soil respiration.
