PS 15-199 - Variation in seasonal water relations of four gymnosperm tree species in a snowpack-dependent ecosystem

Abstract: The availability of water to plants is a critical factor that varies considerably with geographic and topographic regions. In some areas of the western US, trees rely heavily on spring snowmelt that saturates the soil, and soil water content gradually decreases throughout the trees’ growing season. Two strategies that plants can use to tolerate water stress are 1) limiting water loss through stomatal closure and 2) altering their cellular water relations (through turgor loss point or osmotic potential changes). The extent to which a plant limits water loss can be characterized along a spectrum ranging from extreme isohydry to extreme anisohydry. Isohydric plants conserve minimum water potential under times of stress by closing their stomata, whereas anisohydric plants prioritize the ability to fix carbon and risk their hydraulic safety. The determination of where a plant falls on this iso/aniso-hydric spectrum can be achieved by examining its “hydroscape,” which represents the range of water potentials observed during a dry-down period. Additionally, performing pressure-volume curves can reveal whether plants are altering turgor loss points or osmotic potentials during a dry-down.

This study investigated the variations in hydroscapes and cellular water parameters of four gymnosperm species in a snowpack-dependent ecosystem throughout the growing season. We hypothesized that anisohydric species would adjust their leaf water parameters more than isohydric species during limited soil moisture to maintain photosynthesis and leaf turgor. The results revealed that Juniperus osteosperma (Utah juniper) and Juniperus scopulorum (Rocky Mountain juniper) were the most anisohydric species, with lower measured minimum water potential during low soil water potential. Pseudotsuga menziesii (Douglas fir) and Pinus ponderosa (ponderosa pine) were comparatively more isohydric, implementing strategies that preserved their minimum water potentials throughout the season. All four species displayed seasonal changes in osmotic potential and turgor loss point values, which appeared to be unrelated to their iso/aniso-hydric nature. Water availability and a plant's corresponding water use strategy are critical factors for determining the available landscapes that a plant can occupy. Combining plant water use strategies with plant distribution models can improve their prediction capabilities, which would be particularly useful when trying to predict distribution in response to precipitation alteration due to climate change.