Juniperus spp. are successfully encroaching grasslands and savannas of the southern Great Plains, and changing the composition and dynamics of these ecosystems. One well known case is the encroachment of eastern redcedar (Juniperus virginiana) into the oak-dominated Cross Timbers forests of Oklahoma that is changing this ecoregion into a mixed juniper-oak forest. The overall objective of this study was to understand the inter- and intraspecific interactions between eastern redcedar and post oak (Quercus stellata) for oak-redcedar mixed forests in the Cross Timbers of north-central Oklahoma. Our specific objective was to understand how xylem water potential (Ψ), net photosynthetic rate (Pn), and water use efficiency (WUE) of eastern redcedars and post oaks differ among pure oak, pure eastern redcedar, and mixed oak-eastern redcedar stands. A total of 24 trees (7 trees of each species in the pure stands, and 5 trees of each species in the mix stand) of different sizes were measured for plant Ψ with a pressure chamber and net photosynthetic rate using a LICOR-6400 (LICOR Inc., Lincoln, NE). Pn, midday water potential (Ψm), and soil water volumetric content were measured biweekly during the 2017 growing season and monthly during winter, while predawn water potential (Ψp) was measured monthly.
Results/Conclusions
When soil moisture was low, we found that oaks growing with eastern redcedars had significantly less negative Ψp compared with oaks growing in pure stands. The Pn of oaks declined during summer with high temperatures but was always greater than that of eastern redcedars which maintained a relatively constant rate throughout the year. The Pn, WUE, Ψp, and Ψm of eastern redcedar in the mixed stand were not significantly different from those in the pure eastern redcedar stand. When oaks and eastern redcedars grew together in the mixed stand, Pn and WUE of oaks were significantly greater than those of eastern redcedars, and Ψp and Ψm of oaks were less negative than those of eastern redcedars when soil moisture was low. Our results suggest that eastern redcedars influence Ψ of oaks mixed stands, but the physiological performance of eastern redcedar did not differ in pure vs mixed stands. Moreover, eastern redcedar exhibits anisohydric behavior with large decline in Ψ to sustain Pn during dry periods. These traits may allow eastern redcedar to successfully encroach into the forest midstory under prolonged water-limited conditions in this sub-humid region.