Temporal Dynamics of Tree Source Water in Sky Island Ecosystems with Ephemeral Snow Pack: A Case Study using Pseudotsuga Meziesii (Douglas Fir)

Abstract

Throughout the world, water is the limiting resource for both natural and human development. A primary source of water throughout the world comes from seasonal snowpack and snowmelt which largely occurs in the mountains. Our results show that the precipitation that falls in the Santa Catalina Mountains has seasonal isotopic variations, with summer monsoon being more enriched than that of winter precipitation. The isotopic signature of soil moisture has a seasonal trend determined by a combination of seasonal precipitation and dynamic soil moisture flow, this also results in a slight seasonal variation of the isotopic signature of groundwater. The Douglas Fir (Pseudotsuga meziesii) utilizes water from the shallow soils (0 – 20 cm), deep soils (30 – 100 cm), and groundwater. The main source of water for the Douglas Fir throughout most of the year is the shallow soils, but during the dry pre-winter season the source of water switches to groundwater due to a drying soil profile. Hydraulic redistribution during the dry pre-winter season likely causes wetting in the shallow soils. If climate change predictions are correct a shift in precipitation could alter the source water of the Douglas Fir such that they may become increasingly water stressed. If Douglas Fir is unable to survive under this increased water stress, it will have important implications for water resources in the region.