Relative importance of microhabitat, plant form and photosynthetic physiology to carbon gain in two alpine herbs

1. The effects of microhabitat and plant form on sunlight interception, lea f temperatures, frost occurrence and photosynthesis were evaluated for Calt ha leptosepala DC and Erythronium grandiflorum Pursh. Both plants are peren nials that commonly emerge from alpine snowbanks where the combination of c ool temperatures and strong sunlight is among the most extreme for vascular plants. 2. Caltha leptosepala occurred in microsites where colder air accumulates, and has larger, less inclined and more densely clustered leaves compared to E. grandiflorum (which has two steeply inclined leaves). 3. These differences in microsite and plant form led to leaf temperatures b elow 0 degreesC on 70% of nights during the summer growth season in C. lept osepala, compared to only 38% in E. grandiflorum. Leaves of C. leptosepala warmed more slowly on mornings following frosts compared to E, grandiflorum , due to less aerodynamic coupling between leaf and air temperature, and al so a 45% smaller ratio of sunlit to total leaf area due to mutual shading a mong leaves. 4. As a result, night frost did not affect subsequent CO2 assimilation (A) in E. grandiflorum, while frostless nights and warmer mornings led to 35% g reater A in C. leptosepala in the early morning. 5. There were no appreciable differences in the temperature and light respo nse of photosynthesis between the two species. The apparent quantum yield o f A declined only approximate to8% in both species following frost and expo sure to strong sunlight, indicating little adjustment of photosynthetic phy siology. 6. Greater daily carbon gain probably occurs for E. grandiflorum because of its plant form and microclimate, rather than differences in photosynthetic physiology.