Seed Dispersal and Colonization in a Disturbed Northern Hardwood Forest

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Abstract
Seed dispersal and seedling establishment of three dominant tree species were studied on disturbed and undisturbed sites for 3 years (1984-1986) at the Hubbard Brook Experimental Forest in New Hampshire, USA. In the undisturbed northern hardwood forest, year-to-year variation in seed production was smaller for a large-seeded species (Fagus grandifolia Ehrh.) than for species with small (Betula alleghaniensis Brit.) or medium-sized seeds (Acer saccharum Marsh.), and production of seeds by the latter two species differed by several orders of magnitude between years. Recruitment of seedlings into the forest understory correlated with differences in seed input. Seed production of B. alleghaniensis, F. grandifolia and A. saccharum in the undisturbed forest in 1984-1985 (a good seed year) was 9297, 59, and 264 seeds m-2, respectively. Recruitment the following growing season was 39.8, 2.2, and 18.7 seedlings m-2, respectively. The pattern of revegetation by seedlings on a disturbance 205 m wide and 21.9 ha in area was strongly influenced by dispersal of seeds. Seed input and seedling establishment declined exponentially with distance from the disturbance edge for A. saccharum, with no appreciable colonization occurring beyond 15 m from the forest edge even during a good seed year. Seeds of F. grandifolia rarely were dispersed into the disturbed area, and this was reflected in very low seedling recruitment. Dispersal of B. alleghaniensis seeds declined exponentially from the edge and seedling establishment was highest near the edge (47 m-2), decreasing to 10 m-2 at 25 m from the forest edge, and remaining relatively constant at 5 seedlings m-2 in central parts of the disturbance. Seedling recruitment for all three species was restricted to sites where seeds were deposited the preceding year, suggesting that few seeds of those species persisted in the seed bank more than 1 year. As a result, colonization by seedlings and subsequent species composition of the forest opening were greatly influenced by the seed crops immediately preceding and following the disturbance.