Abstract
On the dry, western edge of the eastern deciduous forest of the USA (Cross Timbers), the drought-tolerant, evergreen eastern redcedar (Juniperus virginiana) is encroaching into post oak- (Quercus stellata) dominated woodlands. The overall goal of this study was to examine whether the drought tolerance strategies of eastern redcedar provide it a competitive advantage over post oak and whether this is a key attribute facilitating its successful establishment in the Cross Timbers. Specifically, we assessed xylem water potential and leaf gas exchange of these two species growing in single-species stands and in a mixed-species stand. We found that both species exhibit a similar degree of isohydry and close their stomates to the same extent in response to declining xylem water potentials. Both species had similar relative reductions in gas exchange in response to drought, despite differences in xylem anatomy. However, post oak had leaf-level gas exchange rates approximately 5× greater than eastern redcedar during periods of high moisture availability. Therefore, it did not appear that eastern redcedar encroachment into an oak-dominated forest is facilitated by growing season differences in carbon gain, although evergreen eastern redcedar can conduct gas exchange year-round when conditions are favorable while post oak is deciduous. We found that volumetric soil water content (0–45 cm) was lower in the pure eastern redcedar stand than the mixed-species or pure post oak stand which may indicate that eastern redcedar may experience favorable soil moisture conditions when encroaching into open oak woodlands. Moreover, water potentials in eastern redcedar tended to be more negative in pure stands compared to the mixed stand. Our results suggest the two species may be using water from different depths, reducing competition. Overall, our findings indicate that eastern redcedar encroachment into formerly oak-dominated Cross Timbers forests likely will continue under moderate drought, in the absence of fire, with consequences for water budgets, carbon cycling, grazing forage, wildlife habitat, and wildfire risk.
Highlights
Fire exclusion has led to encroachment of trees and shrubs into grassland, savanna, woodland, and forest ecosystems around the world (e.g., Scholes and Archer, 1997; Staver et al, 2011; Schriver et al, 2018)
We evaluated if eastern redcedar possesses physiological characteristics that enable its encroachment in a post oak dominated forest within the Cross Timbers region
Our study found that both post oak and eastern redcedar respond to water potential decrease by closing stomata
Summary
Fire exclusion has led to encroachment of trees and shrubs into grassland, savanna, woodland, and forest ecosystems around the world (e.g., Scholes and Archer, 1997; Staver et al, 2011; Schriver et al, 2018). Throughout the eastern USA, fire-intolerant, mesic tree species have proliferated into oak (Quercus spp.) woodlands and forests (Nowacki and Abrams, 2008). This alters fuel conditions and understory microenvironment to reduce the likelihood of future fire, i.e., mesophication (Nowacki and Abrams, 2008; Kreye et al, 2018). In contrast to the encroachment of mesic hardwoods into oak forests further east, the addition of highly flammable, evergreen eastern redcedar into the midstory of Cross Timbers increases fuel loading and risk of wildfire (Hoff et al, 2018b). As opposed to the increase of mesic hardwoods further east, the encroachment of the drought-hardy eastern redcedar into the Cross Timbers is likely associated with relatively low annual precipitation (average of 660 mm) across Cross Timbers region (Dyksterhuis, 1948; Anderson et al, 2007)
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