Some Like It Dry: Differences in Water Use Strategies between Historic and Range-Expanding Populations of Juniperus virginiana

Premise of research. Woody plant expansion is a global problem in which trees and/or shrubs replace herbaceous species. Often the woody plants are native species, and the expansion may be attributable either to natural stochasticity or to changes in management of grazing or fire. In other cases, woody plant species are expanding their historic range, and changes in management may be insufficient to explain observed changes in range. One possible mechanism is adaptation of populations at the range edge to the local environment. Methodology. We investigated drought responses of plants sourced from three populations of the widespread, grassland-encroaching Juniperus virginiana and one population of the more drought-resistant Juniperus scopulorum. Seedlings of the four different populations were grown in a common garden in northeast Ohio, part of the native range of J. virginiana. We exposed plants to either an acute or chronic drought and determined water potential at 50% loss of function (P50) for shoots. Pivotal results. The most resistant shoot P50 measured ranged from 2.7 to 4.7 MPa less negative than P50 reported in the literature for woody tissues. Juniperus virginiana plants sourced from the most arid site maintained high stomatal conductance when water was available and retained hydraulic conductance at lower water potentials compared to J. virginiana from wetter locations. Juniperus virginiana from the most arid site exhibited the largest hydraulic safety margin, with shoot P50 as much as 3.25 MPa less negative than minimum shoot water potentials measured. Conclusions. Our data show that estimates of Juniperus spp. drought tolerance based on woody tissues misrepresent actual plant drought response. Juniperus virginiana plants from more arid sites exhibit a more exploitative water use strategy, which could promote expansion by facilitating rapid water uptake in pulse-driven systems, while a decrease in hydraulic conductance in shoots could promote survival by preventing hydraulic failure of more costly woody tissue.