The legacy of Pleistocene glaciation on soil conditions and productivity of a Rocky Mountain conifer forest

Background/Question/Methods

The interaction between soil conditions and climate variability plays a central role in the functioning of montane conifer forests. Although soil moisture availability to trees is largely dependent on climate, the depth and texture of soil exerts a key secondary influence. I hypothesize that soil texture plays a more prominent role in the growth and photosynthetic function of lodgepole pine during dry rather than wet years, as the water holding capacity related to soil textural conditions will be most divergent during years with water scarcity. I have examined physiological and growth responses of lodgepole pine to interannual variation in precipitation across a chronosequence of Pleistocene glacial tills aged from 700k to 12k years before present (YBP). First, glacial till surfaces were located and needles from different age cohorts (2008-2012) were collected. Bulk carbon d¹³C values were obtained and cross-referenced with a subsample of hollocelluose. Soils from three distinct glacial events (Pre-Bull Lake, Bull Lake, Pinedale) were analyzed for gravimetric water content  

Results/Conclusions

Preliminary data shows that the d¹³C values differ from needles of different aged cohorts (from 2008-2012) on trees growing on different glacial tills. Needle tissue from trees growing on older soils (700k – 300k YBP) with high clay content show substantially less stomatal restriction than do needles from trees growing on younger soils (23k-12k YBP). A sub-sample of cellulose verifies the use of bulk carbon as an appropriate method for linking δ¹³C values from needle cohorts to the respective growing conditions for the year of needle production. Initial data also confirms differences in water holding capacity of the glacial tills, with younger soils having a lower water holding capacity than older soils. To fully understand the effects of drought on stomatal regulation the trees will undergo tree ring isotopic analysis, specifically selecting rings formed in drought vs. non-drought years. The overall goal of this research is to understand how lodgepole pine responds to drought across a mosaic of sites differing in soil textural conditions.