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Carbon gain in young seedlings of Abies lasiocarpa and Picea engelmannii at alpine treeline.
Johnson, Daniel*,1, Germino, Matthew *,2, Smith, William*,1, 1 Wake Forest University, Winston-Salem, NC, USA2 Idaho State University, Pocatello, ID, USA
ABSTRACT- Most research concerning the occurrence and stability of alpine timberlines has focused on broad correlations with annual temperature regimes, without addressing specific mechanisms. Timberline migration to higher altitude is dependent upon new seedling establishment away from the forest edge into the timberline ecotone. Reductions in carbon gain in these establishing seedlings have been shown to decrease survival. The purpose of this study was to compare photosynthetic carbon gain and mortality in seedlings of two codominant tree species of the subalpine (Abies lasiocarpa and Picea engelmannii), across a representative altitudinal gradient from the subalpine forest to the upper treeline (2965m-3198m). Differences in environmental factors (temperature, light, and water) that might be responsible for differences in carbon gain were also examined. Mean daily photosynthesis in seedlings of both Abies lasiocarpa and Picea engelmannii was reduced (19 and 29 % respectively) at the higher-elevation site. Minimum nighttime temperatures below 2°C were more frequent at higher elevation and were correlated with reduced maximum photosynthesis the following day. Reduced water status was correlated with reduced carbon gain equally at both sites early in the season before snowmelt, as well as late in the season when soils began to dry. Reductions in photosynthesis at the high-elevation site do not appear to be related to water status because similar values of xylem pressure potentials were observed between sites. Sunlight regimes were highly variable at both sites due to structural shading, although this variation was substantially reduced on cloudy days (44% of all days measured). Moreover, cloudy days resulted in greater integrated daily carbon gain, especially at lower elevation, based on a photosynthesis model that compared potential (simulated) versus actual (measured) daily carbon gain. Photosynthetic responses to temperature, sunlight and water indicated that sunlight and nighttime temperature were the major limiting factors.
Key words: Picea engelmannii, Subalpine, Abies lasiocarpa , Treeline