Variation in ecophysiological properties among conifers at an ecotonal boundary: comparison of establishing seedlings and established adults at timberline

Question: Environmental limitations on carbon acquisition and use can impact successful establishment and restrict a species range, such as for trees at timberline. How do ecophysiological properties associated with carbon uptake and allocation change along an elevation gradient for adult compared to seedling conifers in a timberline ecotone? Location: Teton Range in the Rocky Mountains, Wyoming, USA Methods: Photochemical efficiency (F(v)/F(m)), specific leaf area (SLA) and foliar nonstructural carbohydrates (NSC) were compared along an elevation gradient (2200-3050 m) among two age classes (seedling and adult) and species (Abies lasiocarpa and Pseudotsuga menziesii) at timberline during mid-summer. Results: F(v)/F(m) values were relatively high in both seedlings and adults across the elevation gradient, with the exception of a low F(v)/F(m) for seedlings in the site having the lowest soil temperatures. SLA was surprisingly constant within each age class and species across the timberline ecotone. Foliar NSCs did not increase or decrease consistently with elevation in either age class. Nonetheless, NSCs were highly variable among sites, but only in seedlings and not in adults. Conclusions: Elevation effects on these indicators of the efficiency of interception and use of sunlight in the timberline ecotone were minimal during the optimum period of the growing season. However, establishing seedlings had a tendency to exhibit greater responses to the timberline environment, particularly in their allocation of photosynthate to NSC, which may be a constraint to tree establishment at high elevations.