Woody encroachment is occurring rapidly and at a global scale, altering the water balance and ecosystem dynamics. One of the dominant woody encroaching species within the Great Plains of the US is Juniperus virginiana L. (eastern redcedar; ERC). There are many proposed explanations, but one proposed driver is high drought tolerance, however, much is still unknown about the mechanisms by which J. virginiana is able to increase its range and survive in previously unfavorable locales.
In this study we use a combination of soil moisture, stem xylem conductivity, and pressure-volume curves to examine differences in stem xylem conductivity, xylem resistance to embolism, shoot water potential, and turgor loss point, among other leaf level physiological metrics to identify tree level variation in a closed canopy ERC forest and an adjacent, recently encroached grassland. We specifically investigated: 1) How does soil and shoot water potential differ by canopy cover, 2) Is there variation in shoot level drought tolerance strategy by canopy cover, and 3) How does xylem conductivity and embolism vary between the open and closed forests?
Results/Conclusions
We found a significant difference in soil moisture, with the closed canopy forest being 35% drier than in the adjacent encroached grasslands. Stem xylem embolism was 8% higher in the closed canopy site. Tree canopy size, height, shoot saturated water content, and relative water content at turgor loss point were significantly correlated (p=0.006, p<0.001, p=0.011, p=0.027, respectively) with water potential in the closed canopy site, which averaged 19% more negative than the adjacent grassland site. There were no significant correlations between water potential and measured physiological traits within the encroached grassland site. Turgor loss point and tree height had significant correlation (p=0.047, p=0.002, respectively) with water potential when both sites were combined.
Our results indicate that J. virginiana individuals located in closed canopy stands experience greater water stress and have higher rates of embolism than individuals in the adjacent (open canopy) grasslands. However, lower shoot turgor loss points indicate a higher resistance to drought stress. These results could have implications on the longevity of closed canopy forests of J. virginiana in semi-arid regions.