Influence of Growth from on Ecophysiological Responses to Variable Sunlight in Subalpine Plants

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Abstract
Periods of variable sunlight due to intermittent cloudcover are common in the subalpine zone of the central Rocky Mountains. To evaluate the ecophysiological significance of daily variations in sunlight, responses in net photosynthesis, stomatal conductance to water vapor, and water use efficiency (CO2 uptake per unit H2O lost) were measured in 12 subalpine species including trees, shrubs, and herbs. Measurements were made during intervals of alternating full sun and shade throughout a growth season that included daily and seasonal periods of water stress. In general, photosynthesis responded rapidly in all growth forms to changes in sunlight due to clouds, while stomatal conductance responded more slowly or not at all. Large reductions in photosynthesis were measured during clouds periods for all species, while stomatal conductance was considerably less responsive to changes in sunlight in woody species. The greatest and most variable reductions in stomatal conductance during shade (<10 to a 68% reduction of full sun levels) occurred in herbaceous species. Carbon gain in the woody species was enhanced because stomatal conductance remained relatively high during both sun and shade periods. Water use efficiency in most species was generally lower during shade periods compared to full sun periods. However, woody plants maintained higher xylem pressure potentials that were much nearer shade values compared to herbs. The greater decrease in stomatal conductance and photosynthesis in herbs during shade resulted in improved water use efficiency, although carbon gain was reduced during both shade and the subsequent full—sun period. These apparent growth strategies during non—steady—state response periods appear opposite to generalizations made previously for woody vs. herbaceous species under steady—state conditions.