The vertical distribution and function of tree roots in the soil affect ecosystem water balance by influencing soil water availability, ground water recharge, and the quantity of water transpired into the atmosphere. However, it is not well understood what factors drive the depth of water uptake, or functional rooting depth, and therefore the sources of water that trees are using. In order to address this issue, we studied the role of tree species, position within a catchment, and soil characteristics on functional rooting depth in 9 temperate tree species in the Susquehanna Shale Hills Critical Zone Observatory in Central Pennsylvania. We used natural abundance stable isotopes to identify the source or sources of water in tree tissues, taking advantage of the differences in oxygen and hydrogen isotopic signatures caused by greater evaporative processes in precipitation and shallow soil layers compared with deeper soil layers and ground water. Over two field seasons, small canopy branches were sampled from a total of 57 trees, and analyzed for oxygen-18 and deuterium signatures. Potential sources of water were also sampled including ground water, precipitation, and soil water at various depths. Xylem water isotopic signature ranged from -1.97 to -7.6 per mil for oxygen-18 and -5.7 to -60.3 for deuterium. Isotopic signature varied by species, ranging from a mean oxygen-18 signature of -4.3 ± 0.1 per mil for Acer saccharum, to -7.0 ± 0.1 for Carya tomentosa, and -34.1 ± 1.0 per mil for Acer saccharum to -52.1 ± 0.5 per mil for Quercus velutina for deuterium. It was estimated that the majority of trees were using shallow soil water from the top 30 cm of soil. Individual variation was related to tree species, diameter at breast height, tree height, elevation, and soil order. Our results suggest seasonal trends in the depth of water uptake, as a result of water availability from recent precipitation. The species-level variation, and trends with tree size and with soil characteristics suggest opportunities for modeling the depth of tree water uptake over larger scales.
Gaines, K., Eissenstat, D.M. (2012): Patterns in tree water isotopic signature at the Shale Hills Critical Zone Observatory. AGU Annual Fall Conference Proceedings.
This Paper/Book acknowledges NSF CZO grant support.