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Ecological Section

Givnish, TJ [1], Lopez, Omar R. [3], Montgomery, Rebecca A. [2], Lopez, Krista Farris [3].

Leaf phenology and hydraulic conductance as determinants of shade tolerance in temperate forest trees.

Early leafing and prolonged photosynthetic activity under an open canopy can increase shade tolerance by providing a “spring subsidy” that allows saplings to persist in microsites that are densely shaded in midsummer. However, such plants also require narrow, safe but inefficient xylem to avoid cavitation due to late frosts. Consequently, late leafers with wide, highly efficient but unsafe xylem may gain an advantage in sites that are well-lit in midsummer and have high evaporative demand. To test these ideas, we measured light regimes, static and dynamic photosynthetic light responses, hydraulic conductivities, and the timing of leaf flush and drop in 17 common tree species in Great Smoky Mountains National Park. Photon flux density varied seasonally, from nearly 100% full sunlight during early spring to 3-13% full sunlight in midsummer on mesic to xeric sites, respectively. Amax (μmol CO2 g-1 s-1) for early leafers (e.g., Aesculus lutea, Halesia monticola) was more than 150% higher than that for late leafers (e.g., Liriodendron tulipifera, Nyssa sylvatica). Across species, spring Amax was negatively correlated with time of sapling bud break (r2 = 0.32*) and leaf longevity (r2 = 0.25*). Instantaneous light compensation points varied from 1 to 15 μmol m-2 s

1 - University of Wisconsin-Madison, Botany, 430 Lincoln Drive, Madison, Wisconsin, 53706-1381, USA
2 - University of Minnesota, Forest Resources, 1530 Cleveland Avenue N, Green Hall 115, St. Paul, Minnesota, 55108, USA
3 - University of Wisconsin-Madison, Botany, 180 Sutton Road, Bryson City, North Carolina, 28713, USA

shade tolerance
leaf phenology
forest ecology
whole-plant compensation point.

Presentation Type: Paper
Session: 29-11
Location: Wasatch (Cliff Lodge)
Date: Tuesday, August 3rd, 2004
Time: 10:45 AM
Abstract ID:520

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