© 1998 Heron Publishing—Victoria, Canada
Leaf phenology, photosynthesis, and the persistence of saplings and shrubs in a mature northern hardwood forest
David S. Gill (1, 2), Jeffrey S. Amthor (1, 3) and F. Herbert Bormann (1)
1. School of Forestry and Environmental Studies, Yale University, New Haven, CT 06511, USA / 2. Department of Biological Science, California State University, Fullerton, CA 92834, USA / 3. Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA / Received October 24, 1995
Summary
We quantified leaf phenologies of saplings and overstory trees of sugar maple (Acer saccharum Marsh.) and American beech (Fagus grandifolia Ehrh.), and the shrub hobblebush viburnum (Viburnum alnifolium Marsh.) in a 72-year-old northern hardwood forest. Seasonal changes in irradiance in the shrub layer, and in the leaf CO2 exchange of viburnum, and sugar maple and beech saplings were also measured. Leaf expansion occurred earlier in the spring
and green leaves were retained later in the autumn in saplings and shrubs than in overstory trees. During the spring light
phase (before overstory closure), large CO2 gains by all three shrub-layer species occurred as a result of a combination of relatively large leaf area, high photosynthetic
capacity, and high irradiance. Throughout the summer shade phase, photosynthetic capacity at a given irradiance remained relatively
constant, but CO2 gain was typically limited by low irradiances. Even though irradiance in the shrub layer increased during the autumn light
phase as the overstory opened, CO2 gains were modest compared to springtime values because of declining leaf area and photosynthetic capacity in all three species.
The CO2 gains during the spring light phase, and to a lesser extent during the autumn light phase, may be important to the carbon
balance and long-term persistence of saplings and shrubs in the usually light-limited shrub layer of a northern hardwood forest.
Therefore, for some late-successional species, leaf phenology may be an important characteristic that permits their long-term
persistence in the shrub layer of mature northern hardwood forests.
Keywords:
Acer saccharum, American beech, carbon gain, Fagus grandifolia, hobblebush viburnum, Hubbard Brook, shade tolerance, sugar maple, Viburnum alnifolium.
