Light use efficiency of California redwood forest understory plants along a moisture gradient
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We investigated photosynthesis of five plant species growing in the understory at three sites (1,170-, 1,600- and 2,100-mm annual moisture inputs), along the geographical range of coastal California redwood forest, to determine whether greater inputs of rain and fog at northern sites enhance photosynthetic utilization of fluctuating light. Measurements of understory light environment and gas exchange were carried out to determine steady state and dynamic photosynthetic responses to light. Leaf area index ranged from 4.84 at the 2,100-mm site to 5.98 at the 1,170-mm site. Maximum rates of net photosynthesis and stomatal conductance (g) did not vary appreciably within species across sites. Photosynthetic induction after a change from low to high light was significantly greater in plants growing in lower light conditions regardless of site. Photosynthetic induction also increased with the rate of g in diffuse light, prior to the increase to saturating light levels. Post-illumination CO2 assimilation was the largest factor contributing to variation in C gain during simulated lightflecks. The duration of post-illumination photosynthetic activity, total CO2 assimilation per light received, and light use efficiency during simulated lightflecks increased significantly with moisture inputs in four out of five species. Increasing leaf N concentration with increasing moisture inputs in three out of five species, coupled with changes in leaf N isotopic composition with the onset of the summer fog season suggest that natural N deposition increases with rain and fog inputs and contributes to greater utilization of fluctuating light availability in coastal California redwood forests.
KeywordsCommunity diversity Fog Lightfleck Nitrogen isotope Photosynthesis
We enthusiastically acknowledge Stefania Mambelli and Paul Brooks for careful foliar N and isotope analyses; Vanessa Boukili, Anna Simonin, Art Fredeen and Rick Hatcher for faithful assistance in the field; Kevin Simonin, Kevin Tu, and Damon Bradbury for endlessly intriguing conversations on the complex ecosystem processes in California redwood forests; Emily Burns (Limm) for taking an interest understory plants and providing key insights; the California State Park System for access to sites; and the rangers at Prairie Creek State Park for lively discussions over bonfires. The University of California provided logistical support. Funding was provided by a National Science Foundation Postdoctoral Fellowship (BIO-0310103) to L. S. S. and Mellon Foundation and Save the Redwoods League awards to T. E. D.
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