Most research on the occurrence and stability of alpine timberlines has focused on correlations between adult tree growth and mean temperatures rather than on specific mechanisms. Timberline migration to higher altitude is dependent on new seedling establishment in the tree-line ecotone; however, reductions in photosynthetic carbon gain in establishing seedlings have previously been interpreted solely in terms of decreased seedling survival. Our objective was to evaluate the impact of abiotic factors (temperature, light and water) on photosynthetic carbon gain in young seedlings of the two dominant conifer tree species occurring naturally above (tree-line ecotone site, TS) and below (forest site, FS) a Rocky Mountain timberline in southeastern Wyoming, USA. Coincidentally, measurements were made during an unusually dry summer. Mean daily photosynthesis in seedlings of both Abies lasiocarpa (Hook.) Nutt. (subalpine fir) and Picea engelmannii Parry ex Engelm. (Engelmann spruce) was less at TS than at FS (19 and 29%, respectively). Minimum nighttime temperatures below 2 degrees C were more frequent at TS than at FS and were associated with reduced maximum photosynthesis the following day. Low midday water potentials were associated with a reduction in carbon gain at both sites early in the season, prior to snowmelt, as well as late in the season when soils began to dry. However, the lower photosynthetic rates at TS than at FS appeared to be unrelated to seedling water status because seedlings at both sites had similar xylem pressure potentials. Solar irradiance was highly variable at both sites as a result of uneven shading by neighboring trees, although this variation was substantially reduced on cloudy days (44% of all days observed). Compared with sunny days, cloudy days resulted in greater integrated daily carbon gain at both sites (41% increase at TS and 69% increase at FS), based on a simulated photosynthesis model. Photosynthetic responses to temperature, sunlight and water suggest that variable solar irradiance and nighttime temperatures were major abiotic factors limiting photosynthetic carbon acquisition in these young seedlings, especially for seedlings growing in the tree-line ecotone.