31-year tree-ring record of growth and photosynthetic gas exchange in lodgepole pine across a glacial till chronosequence

Often overlooked, but critically important for predicting responses of forested ecosystems to climate change, is the role of soil physical and hydraulic properties that exert strong controls on available soil moisture for tree growth. In the central Rocky Mountains where precipitation input and primary productivity are concentrated in high elevation catchments, Pleistocene-aged glacial tills of varying age and state of weathering create a patchwork of soils with highly contrasting soil texture and hydraulic properties. I expect soil physical properties related to water retention, coupled with variable climate conditions, will impact tree physiological growth and gas exchange parameters by exposing trees to different amounts of accessible soil moisture. I measured increment growth and carbon isotope composition (delta13C) of tree rings over a 31-year record (1983-2013) for two populations of lodgepole pine (Pinus contorta) on each of three different aged glacial till surfaces in the Medicine Bow Mountains of Wyoming, USA. Till deposits were from the pre-Bull Lake ("pBL"; 500k + ypb), Bull Lake ("BL"; 125k -- 250k, ybp), and Pinedale ("PD"; 12k -- 25k, ybp) glaciation events. Soils developed on pBL till had significantly higher clay content compared to soils developed on the BL or PD tills. Responses of tree-ring width to environmental changes were site specific. Inter-annual variation in tree-ring widths were positively correlated with annual maximum snow water equivalent (SWEmax) at the PD sites only. Carbon isotope discrimination (Delta13C) determined from bulk-wood ring tissue, and the calculated ratio of internal to ambient CO2 concentration (Ci/Ca) of leaves exporting sugars to wood growth, were highest and corresponding values of the ratio of photosynthesis to stomatal conductance (A/g) were lowest for trees growing on soils developed in young PD tills over the entire 31-year record. A/g for trees growing on PD and pBL sites increased significantly over the 31-year record, corresponding to the 53ppm increase in atmospheric CO2 concentration over the 31-yr period, while A/g of trees growing on BL tills showed no significant change over this time period. A relationship between A/g and growing season temperature (Tavg; Apr. -- Sept.) was only observed for trees growing on soils developed on young PD tills; A/g increased with higher Tavg on these young tills. Taken together my data show that sensitivity of growth and photosynthetic gas exchange in lodgepole pine to variation in precipitation is strongly dependent on geomorphic setting established by past glaciation within high-elevation catchments of the Rocky Mountains, and the expected increase in A/g with rising atmospheric CO2 concentration varied in magnitude across different soil types.