LEAF PHENOLOGY, PHOTOSYNTHESIS, AND THE PERSISTENCE OF SAPLINGS AND SHRUBS IN A MATURE NORTHERN HARDWOOD FOREST

We quantified leaf phenologies of saplings and overstory trees of suga r 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 irradi ance in the shrub layer, and in the leaf CO2 exchange of viburnum, and sugar maple and beech saplings were also measured. Leaf expansion occ urred earlier in the spring and green leaves were retained later in th e autumn in saplings and shrubs than in overstory trees. During the sp ring light phase (before overstory closure), large CO2 gains by all th ree shrub-layer species occurred as a result of a combination of relat ively large leaf area, high photosynthetic capacity, and high irradian ce. Throughout the summer shade phase, photosynthetic capacity at a gi ven irradiance remained relatively constant, but CO2 gain was typicall y limited by low irradiances. Even though irradiance in the shrub laye r 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 ga ins during the spring light phase, and to a lesser extent during the a utumn light phase, may be important to the carbon balance and long-ter m persistence of saplings and shrubs in the usually light-limited shru b layer of a northern hardwood forest. Therefore, for some late-succes sional species, leaf phenology may be an important characteristic that permits their long-term persistence in the shrub layer of mature nort hern hardwood forests.