Oxygen Isotopes in Tree Cellulose Reveal Ecohydrological Relationships in the Colorado Rocky Mountains

Abstract:

Oxygen isotopes in xylem water or cellulose can be useful as an indicator of the moisture sources for trees in modern and paleo contexts. In the snow-dominated high elevations of the Rocky Mountains, the oxygen isotope ratio in the cellulose of conifer trees may provide valuable information about the seasonal origins of tree water use, which can be key to understanding how snowpack or summer rains have varied through time and how this has affected tree growth. In this pilot study, annual and sub-annual tree-ring cellulose isotopic measurements from Picea engelmannii were compared with monthly meteoric oxygen isotope measurements, precipitation amount (P) and temperature (T) data to investigate the climate-proxy response and to refine the use of wood cellulose oxygen isotopes as a hydroclimate proxy. Preliminary results indicate that annual-ring cellulose generally coincides with year-to-year variation of volume-weighted meteoric (p-weight) oxygen isotopes between the years 2007-2016. The relationship among cellulose oxygen isotopes, p-weight oxygen isotopes, and temperature is more pronounced for the annual growing season average (May-Sep). Sub-annual tree-ring increments indicate inter-annual differences between cellulose oxygen isotope values and the timing of p-weight oxygen isotopes, which may indicated tree-level discrimination between cool-season and growing season source water depending on environmental conditions for that year. The statistical relationship between sub-annual cellulose oxygen isotopes and the climate variables (P and T) suggests that T is an important control on tree-ring growth in May through August. May and July P corresponds to the early portion of the growth ring, while August P corresponds to late season growth. Our results indicate that there is a connection between seasonal climate and oxygen isotope values derived from tree-ring cellulose in high-elevation, snow-dominated systems. An improved understanding of this relationship will assist in proxy-refinement and contribute to robust future reconstructions of hydroclimate.