Background/Question/Methods To predict shifts in species ranges, better understanding of the physiological mechanisms affecting species range limits is needed.  In alpine treeline ecotones, tree growth is thought to be limited primarily by climatic factors.  Evidence often presented for climatic constraints is elevated mobile carbon pools under cold temperatures, suggesting carbon processing problems. However, this approach assumes that passive source-sink carbon relationships dictate tree growth.  Common garden studies have shown differences in non-structural carbohydrate pools between provenances of species, suggesting genetic differences in carbon processing. The objectives of this study were to (a) compare ecophysiological differences between two provenances of limber pine (Pinus flexilis) seedlings planted in common gardens along an elevational gradient, (b) to ascertain whether patterns of non-structural carbohydrates were associated with environmental or genetic factors, and (c) to assess how seedling ecophysiology corresponded to demographic patterns of growth and survival.    

Recently germinated seedlings of limber pine from local high-elevation (HI) and low-elevation (LO) seed sources were transplanted after snowmelt into three common gardens arrayed along an elevation gradient (3100 m—3600 m) at Niwot Ridge, CO.  Seedling survival and growth were monitored weekly.  In both early (June) and late (August) summer, seedling carbon exchange (net photosynthesis, A_net; dark respiration, R_d; carbon balance, A_net:R_d) and water relations (stomatal conductance, g, water use efficiency, WUE, and water potential, ψ) were estimated using gas exchange and a pressure chamber.  Non-structural carbohydrates (NSC) and plant biomass were also determined. 

Results/Conclusions Survival, plant biomass, leaf area, and shoot:root ratios were 12-40% greater in LO seedlings compared to HI seedlings along the elevation gradient.  LO seedlings had 35-77% greater A_net and A_net:R_d, and 25% greater NSC concentrations than HI seedlings at most elevations. g was significantly greater in LO seedlings, but there were no differences in WUE (photosynthesis/transpiration) or ψ between provenances. Similar to previous common garden experiments with conifer trees, LO seedlings had greater survival and growth than HI seedlings, which was attributed to the greater A_net and A_net:R_d in LO seedlings.  Neither plant biomass nor NSC concentrations were correlated with carbon source:sink metrics (A_net:R_d). The greater carbon balance, NSC, and growth in LO seedlings along the elevation gradient strongly indicates differences in genotypic regulation of mobile carbon pools, and not simply passive source-sink allocation patterns.  Therefore, using NSCs to ascertain growth limitations to trees requires greater understanding of active and passive controls of plant carbon balance, NSC pools, and plant growth.