Abstract
Eastern redcedar (Juniperus virginiana L., redcedar) encroachment is transitioning the oak-dominated Cross-Timbers of the southern Great Plain of the USA into mixed-species forests. However, it remains unknown how the re-assemblage of tree species in a semiarid to sub-humid climate affects species-specific water use and competition, and ultimately the ecosystem-level water budget. We selected three sites representative of oak, redcedar, and oak and redcedar mixed stands with a similar total basal area (BA) in a Cross-Timbers forest near Stillwater, Oklahoma. Sap flow 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 over a period of 20 months, encompassing two growing seasons between May 2017 and December 2018. Results showed that the mean sap flow density (Sd) of redcedar was usually higher than post oaks (Quercus stellata Wangenh.). A structural equation model showed a significant correlation between Sd and shallow soil moisture for redcedar but not for post oak. At the stand level, the annual water use of the mixed species stand was greater than the redcedar or oak stand of similar total BA. The transition of oak-dominated Cross-Timbers to redcedar and oak mixed forest will increase stand-level transpiration, potentially reducing the water available for runoff or recharge to groundwater.
Highlights
Woody plant encroachment is transforming the southern Great Plains of the USA, which has significant implications for ecosystem carbon and water dynamics (e.g., [1,2])
We found that post oaks exhibited 74% lower sap flow density (Sd) than redcedar in the mixed stand, wesupport found that exhibited
Our study showed that replacement of post oak by redcedar of a similar basal area increases total transpiration on an annual scale which will alter the hydrological cycle
Summary
Woody plant encroachment is transforming the southern Great Plains of the USA, which has significant implications for ecosystem carbon and water dynamics (e.g., [1,2]). Fire exclusion has increased woody species density and richness and allowed eastern redcedar (Juniperus virginiana L., redcedar), a native, evergreen conifer, to encroach the prairie and infill the oak-dominated areas [5,6,7], which affects a large number of ecosystem services [8,9,10]. Encroachment of redcedar into grasslands increases ecosystem-level water use, resulting in a reduction of runoff and groundwater recharge potential [1,11,12,13]. The effect of redcedar encroachment and infilling into the oak forest on water use and ecosystem-level water budgets remains unknown
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