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Byers, Anya B ¹ ; Harpold, Adrian A ² ; Gochis, David J ³ ; Ewers, Brent E ⁴ ;Brooks, Paul D ⁵ ; Barnard, Holly R ⁶

¹ University of Colorado
² University of Arizona
³ National Center for Atmospheric Research
⁴ University of Wyoming
⁵ University of Arizona
⁶ University of Colorado

Better understanding of hydrologic processes in soil are needed to improve 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 subalpine plots dominated by lodgepole pine (Pinus contorta) at the Niwot Ridge Long Term Ecological Research (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 event-based sampling during the 2011 growing season to investigate a paradox between water sources and rooting depth in lodgepole pine. Previous research at Niwot Ridge has shown that lodgepole pine, typically believed to be a shallowly rooted species, appear to be strongly dependent on water from snowmelt for the entire growing season (Hu et al., 2010). These results suggest that lodgepole pine access water from deeper in the soil than summer (monsoon) rain typically penetrates. Research in a Mediterranean climate (dry summers; rainy winters) has shown that water uptake by trees is also mediated by separation of soil pore water into tightly bound and mobile pools (Brooks et al., 2009). Event-based water isotope sampling aims to determine whether “two water worlds” is a viable explanation for this paradox of water source and rooting depth in a continental subalpine forest as well.

The relationship between tree sap flux density and soil moisture states at varying depths over time, as well as the effective rooting depth and water source for lodgepole pine, were examined using measurements of tree processes (sap flux and water stress), hydrological parameters (precipitation, soil moisture), and stable water isotope composition (δD and δ18O) of xylem water, mobile and immobile soil water, snow, precipitation, and stream water. Data shows that volumetric soil moisture at 10-70 cm depths responds to summer rain events of >1 mm, and that lodgepole sap flux increases within 48 hours of rain events >3mm. Water isotope signatures for rain and shallow soil moisture (10-20 cm), however, are distinctly different from tree, deep soil moisture (40-70 cm), and snow measurements. Though early season isotope results are consistent with previous studies, sap flux and soil moisture response to rain events presents an ongoing paradox that additional isotope analysis of subsurface waters may resolve.

Brooks, J.R., Barnard, H.R. & Coulombe, R.J.J., 2009. Ecohydrologic separation of water between trees and streams in a Mediterranean climate. Nature Geoscience, (December), p. 1-5. Available at: http://www.nature.com/ngeo/journal/v3/n2/abs/ngeo722.html

Hu, J. et al., 2010. Longer growing seasons lead to less carbon sequestration by a subalpine forest. Global Change Biology, 16(2), p. 771-783. Available at: http://blackwell-synergy.com/ doi/abs/10.1111/j.1365-2486.2009.01967.x.