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Leaf phenology in relation to canopy closure in southern Appalachian trees¹

² Department of Botany, University of Wisconsin, Madison, Wisconsin 53706 USA ³ Smithsonian Tropical Research Institute, Center for Tropical Forest Science, Unit 0948, APO AA 34002 USA ⁴ Department of Forest Resources, University of Minnesota, St. Paul, Minnesota 55108 USA

ABSTRACT

Leaf phenology varies markedly across tree species of temperate deciduous forests. Early leafing in spring may increase light capture and carbon gain prior to canopy closure, allowing saplings to survive in understory sites deeply shaded in midsummer. We quantified sapling leaf phenology for 18 tree species and seasonal variation in understory light availability at three sites along a ridge–slope–cove landform gradient in the Great Smoky Mountains National Park. Early leafing species (e.g., Aesculus flava, Carpinus caroliniana) broke bud an average of 24 d before late leafers (e.g., Magnolia fraseri, Nyssa sylvatica). Canopy closure occurred 14–18 d earlier and summer understory light was on average 63–74% lower on intermediate and mesic sites than on the xeric site. Early leafing species intercepted 45–80% of their growing season photon flux before canopy closure vs. 8–15% for late leafers. However, earlier leafing increased exposure to freezing temperatures by 5.5% per week near the mean time of bud break. Early leafing is strongly correlated with midsummer shade, risk of freezing temperatures, and distribution on mesic sites across a "main spectrum" of 15 deciduous species. Differences in leaf phenology and resultant impacts on spring carbon gain may help determine tree shade tolerance and distribution in southern Appalachian forests.

Key Words: bud break • canopy closure • early leafing • environmental gradient • freezing • leaf flush • light gains • photon flux density • shade tolerance

Received for publication 20 March 2008. Accepted for publication 29 August 2008.

FOOTNOTES

¹ The authors thank the administrators of the Great Smoky Mountains National Park, especially K. Langdon, for logistical support in carrying out this study. They are also grateful to C. Parker of USGS for technical assistance in the design and construction of electrical fences to deter black bears from destroying data loggers, to the State Office of Climate of North Carolina for providing access to long-term temperature records from Asheville, and to J. Dalling, G. Gilbert, and I. Parker for comments on early versions of this manuscript. This research was made possible by a generous grant from the Andrew W. Mellon Foundation and by logistical support provided by the University of Wisconsin-Madison. The authors thank K. Elliot for her expertise in drafting the figures.

⁵ Author for correspondence (e-mail: prioria{at}yahoo.com); Smithsonian Tropical Research Institute, Center for Tropical Forest Science Unit 0948, APO AA 34002 USA