Water relations and drought response of Pinus strobiformis

Abstract

Southwestern white pine (Pinus strobiformis Engelm.) faces dual threats of climate change shifting its environmental niche and mortality due to a nonnative, invasive fungal pathogen. To inform efforts to sustain this species, we established experimental field trials in three common gardens along an elevational gradient with drought treatments to assess trait responses in southwestern white pine. We measured predawn and midday water potential on 44 maternal families from 10 populations at each garden. We used regression between predawn and midday water potentials to estimate hydroscape area, an index of stomatal regulation of transpiration. We measured leaf carbon isotope ratio and estimated carbon isotope discrimination and leaf mass per area to understand the effects of gardens and treatments on stomatal aperture and leaf structure. Water stress caused by experimental drought and temperature decreased leaf carbon isotope discrimination and leaf mass per area, indicating formation of thin leaves with low stomatal conductance in response to heat and drought. The hydroscape area of southwestern white pine suggests tight control of transpiration via stomatal closure, similar to other isohydric pines. Families with greater stomatal closure (inferred from carbon isotope ratio) at the warm, dry garden had higher survival than other families, suggesting an important role of isohydry in acclimation of southwestern white pine to expected habitat drying and warming.