Eastern redcedar (Juniperus virginiana, redcedar), a native evergreen conifer, has encroached and established in the midstory of the post oak (Quercus stellata) dominated Cross-timber forests. The impact of this species composition change on ecosystem-level water use remains unknown.
The overall objective of this study was to understand the change of ecosystem-level transpiration after redcedar establishment in the oak forest in the Cross-timbers in the south-central Great Plains, USA. Three stands representative of oak, redcedar, and oak and redcedar mixed forests were selected at the Cross-timber Experiment Range, Stillwater, Oklahoma. Sapflow sensors were installed in a subset of trees in each stand representing the distribution of diameter at breast height (DBH). Sap flow of each selected tree was continuously monitored using three customized FLOW32A-K systems (Dynamax Inc., Houston, TX, USA) for a full year from May 2017 to May 2018. Sap flows were first calculated using equation developed by Granier (1985), then adjusted by the equation reported by Caterina et al. (2014) for redcedar and the equation developed by Sun et al. (2012) for post oak. Daily water use was calculated by summing the hourly water use of individual trees, which was then upscale to stand level based on the total DBH.
Results/Conclusions
The daily water use of post oak and eastern redcedar increased linearly with the tree size. During the growing season, the average daily water use of post oak trees was higher than that of eastern redcedar trees. On the mixed site, eastern redcedar used more water per sapwood area than post oak. Eastern redcedar transpired all year, while the transpiration of post oaks started in the early May and ended in early November for our study period. On the stand level, the annual water use of redcedar and oak mixed forest was higher than the oak forest of similar total DBH. The increase of redcedar in oak forest or replacement of oak by similar redcedar will increase stand level transpiration, potentially reducing water available for streams or recharge to the groundwater.