Examining the response of lodgepole pine transpiration to snow melt and summer rainfall in subalpine Colorado, USA

Background/Question/Methods

The hydrologic cycle plays a central role in regulating ecosystem structure and function.  Investigation of ecohydrologic processes is needed to improve our understanding of ecosystem changes that could result from climate change and mountain pine beetle mortality in Colorado’s subalpine forests.  Here, we present data from a subalpine hillslope dominated by lodgepole pine (Pinus contorta) at the Niwot Ridge LTER site on the Colorado Front Range that aims to improve the process-level understanding of the source and fate of water between subsurface storage and forest uptake.  This study utilizes storm event-based sampling during the 2011 growing season to investigate a paradox between water sources and rooting depth in lodgepole pine. Findings from Niwot Ridge have shown that lodgepole pine, typically believed to be a shallowly rooted species, appears to be strongly dependent on water from snowmelt for the entire growing season suggesting that trees were accessing water from deeper in the soil than summer (monsoon) rain typically infiltrates.  The relationship between precipitation event size and depth of infiltration on a seasonal and event basis, the effective rooting depth of lodgepole pine, and hysteretic responses of transpiration to soil moisture over a growing season are examined using measurements of tree processes (sap flux and leaf water potential) and hydrological parameters (precipitation, soil moisture), as well as, stable water isotope compositions (δ²H and δ¹⁸O) of xylem water, mobile and immobile soil water, snow, rain, and stream water.

Results/Conclusions

Snow melt averaged -157.7 ‰ and -20.9 ‰ for δ²H and δ¹⁸O respectively; whereas, summer rain averaged -60.8 ‰ and -8.6 ‰.  Initial results from xylem water samples indicate transpiration source water was isotopicly similar to snow melt and deep soil moisture. However, transpiration appears to respond to rain events. Lodgepole pine sap flux increased by 15-30% within 24 hours of monsoon events and decreased over 72 hours or until subsequent rainfall. Furthermore, soil moisture in deep layers (60 and 70 cm) responded to large summer rain events of 0.7 mm and greater. Understanding the mediation of hydrologic processes by trees like lodgepole pine will improve modeling of hydrological and ecological processes and knowledge of forest susceptibility to climate change and other disturbance impacts.