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Photosynthesis during an Episodic Drought in Abies lasiocarpa and Picea engelmannii across an Alpine Treeline

Abstract Effects of abiotic factors on daily and annual photosynthetic carbon gain were evaluated in Abies lasiocarpa and Picea engelmannii at three sites across an alpine treeline ecotone in the Medicine Bow Mountains of southeastern Wyoming (U.S.A.). In addition, the year 2001–2002 was characterized as an episodic drought (seventh driest year since 1895), including a winter that generated only 45% of normal snowpack. Both species had approximately 50% lower xylem water potentials and photosynthesis compared to previous studies for the same species and locale. In A. lasiocarpa, estimated total photosynthesis for the measurement period (Atot) was greatest (28.7 mol m−2) at the mid-ecotone site (∼3198 m), followed by the alpine site (24.6 mol m−2) at ∼3286 m, and then the forest site (19.4 mol m−2) below timberline (∼2965 m). Similar results occurred in P. engelmannii (17.6, 23.4, and 25.3 mol m−2, respectively). These differences appeared to be most influenced by stomatal rather than non-stomatal effects based on comparisons of photosynthesis, leaf conductance, and internal CO2 concentrations through summer. Although photosynthesis over the summer appeared limited primarily by annual water limitation, the two higher elevation sites had significantly greater values that were associated with microclimatic differences in sunlight incidence and, possibly, temperature.