Carbon balance of conifer seedlings at high elevations: Comparison of a treeline and non-treeline species

How variations in plant carbon balance may explain the climatic range limits of species, specifically conifers at alpine-treeline, is currently in debate.  Environmentally induced changes in growth, carbohydrates, and/or CO₂ exchange have been hypothesized to limit tree growth at high elevations.  Relationships among these factors have been examined along elevation gradients for saplings and adults, but not for seedlings during their critical establishment phase.  We compared 1) growth and carbon allocation, 2) diurnally integrated-CO₂ exchange, and 3) nonstructural carbohydrate concentrations (NSC), in germinant seedlings of Pseudotsuga menziesii (non-treeline species) and Abies lasiocarpa (treeline species) in the Tetons of Wyoming.  Seedlings were outplanted at 2450 m (high forest) and 3000 m (near treeline) elevation at sites with similar canopy coverage and day lengths.  We sampled every 2-3 weeks over the 75 days that the 3000 m site was snow-free.  Overall, elevation effects on CO₂ exchange and growth were similar between species.  At the needle level, shoot respiration rates decreased at the higher elevation, but photosynthetic rates did not, likely due to greater acclimation to lower temperatures near treeline.  At the whole-plant level, seedlings at the higher elevation had less shoot growth, specifically less needle area per individual, resulting in less total moles of carbon flux (both uptake and efflux) over the study period.  NSC concentrations were greater at the higher elevation at times, especially in P.menziesii early in the growing season. Also during early development, A.lasiocarpa had less NSC than P.menziesii and developed more needle tissue.  Early differences in the developmental coordination of NSC, needle area, and CO₂ exchange lead to differences in seedling carbon balance.  Specifically, the treeline species’ early carbon allocation to needles lead to a positive-feedback on carbon acquisition, and thus yielded more whole-plant carbon gain and growth than the non-treeline species at these high elevations.