PARENT SESSION
Oral Session # 35: Biogeochemistry III: Arctic, Alpine, and Tundra Systems.
Presiding: A Hartley
Tuesday, August 5. 1:30 PM to 5:00 PM, SITCC Meeting Room 203.

Carbon uptake limitations in Abies lasiocarpa and Picea engelmannii across an alpine treeline ecotone.

Brodersen, Craig^*,1, Germino, Matthew², Smith, William¹, ¹ Wake Forest University, Winston-Salem, NC² Idaho State University, Pocatello, ID

ABSTRACT- The debate over the mechanistic factors that limit the altitudinal limit of forest trees to specific elevations, and no higher, has been a source of controversy for over a century. Environmental effects on both photosynthetic carbon gain and respiratory-driven growth processes have been used to evaluate limitations at the alpine treeline. It was hypothesized here that microsite factors associated with ecological facilitation are more important for determing the potential for photosynthetic carbon gain than altitudinal differences. Throughout the summer of 2002 adult trees of the codominant Abies lasiocarpa (subalpine fir) and Picea engelmanii (Engelmann spruce) were selected for gas exchange measurements (Li-COR 6200) of net photosynthesis, as well as microlimate, at three different altitudes that spanned the width of the alpine treeline ecotone (3,154 m; 3,205 m; 3,345 m). High elevation alone did not appear to be a limiting photosynthetic activity, as the two highest sites studied showed the highest daily values of carbon gain (0.7, 1.2, 1.9 mol m^-2s^-1, respectively), as well as integrated annual values (0.96, 2.20, and 1.79 mol m^-2s^-1, respectively). Additional data also support the idea that microsite sky and wind exposure (both day and night), and associated biophysical factors, may be more definitive in predicting carbon gain at treeline than the effect of altitude. Microsite differences in sky and wind exposure may be responsible for higher carbon gain at the middle elevation site compared to either the forest or upper ecotone sites. Adaptive trade-offs between too much and too little sun exposure, cold nighttime sky exposure, and wind effects reflect a complex interaction between needle temperatures, low temperature photoinhibition of photosynthesis, and water relations related to snow burial and deposition patterns.

Key words: Alpine, Abies lasiocarpa, Treeline, Picea engelmannii