Stomatal closure point (SCP), or the water potential at which stomata close to minimize water loss and avoid desiccation, is generally understood to be an intrinsic structural trait for a given plant species and constant across timescales in a given environment. Our understanding of SCP variation has implications for climate projections, as stomata provide the critical link between plant and atmosphere. To verify that species SCP is in fact constant, we evaluated water availability effects on SCP variation in two co-occurring conifers — the relatively drought-sensitive pinon pine (Pinus edulis) and the relatively drought tolerant one-seed juniper (Juniperus monosperma)—across a range of temporal scales. To assess water availability effects on short-term SCP variation, we generated stomatal dehydration response curves for branches of mature, field-grown individuals both immediately after harvesting and after overnight rehydration. To assess water availability effects on SCP variation at intermediate timescales, we monitored mid-day leaf water potential (Ψmd) of mature pinon over six months of regular watering in a greenhouse. We then inferred water availability effects on long-term variation in SCP through a literature survey of leaf water potential and stomatal conductance of mature pinon and juniper at field sites across the species ranges.
Results/Conclusions
Though juniper SCP did not change with overnight rehydration, the branches were collected during the wet season, and the measured SCP was ~1.5 MPa more positive than known dry-season SCP measured at a nearby site. These results suggest that juniper, traditionally considered “anisohydric” with cavitation resistant xylem and less conservative SCP, demonstrates more conservative SCP with additional water availability at seasonal timescales. “Isohydric” pinon pine, on the other hand, became less conservative at short to intermediate timescales, showing ~1 MPa more negative SCP with branch rehydration and allowing Ψmd to drop ~1.5 MPa with regular watering. These results provide evidence that SCP can change in response to water availability at short to intermediate timescales, though it may do so in different directions depending on the species. Our findings suggest that species stomatal response is more plastic than previously understood and, if manifested across the species range, this plasticity may have implications for how we anticipate range shifts under climate change, providing a mechanism for acclimation to altered precipitation regimes.