Factors influencing tree colonization in fragmented forests: an experimental study of introduced seeds and seedlings

Plant colonization is essential to maintain diversity in fragmented ecosystems, but there is little experimental information on this process. We used experimental seed and seedling transplantation to evaluate the ability of the seed of tree species to become established in Southern Ontario forest fragments not containing the species. Carya cordiformis (Wang.) K. Koch, Betula alleghaniensis Britton, Fagus grandifolia Ehrh. and Juglans nigra L. were introduced into the interiors of (1) woodlots not containing members of their species (uncolonized woodlots), (2) woodlots containing conspecifics (colonized woodlots), and (3) well-illuminated sites at the edges of the uncolonized woodlots. With these data, we evaluated the probabilities of seedling colonization by the species and tested the predictions that survival rates are higher in uncolonized woodlots due to escape from host-specific predators and pathogens (Escape hypothesis), and colonization is promoted by seed arrival in well-illuminated sites at forest edges (Edge hypothesis). Both predictions were supported for a subset of the species, indicating two factors that may facilitate colonization. However, the influence of these factors occurred mainly after the second year of seedling establishment and overall survivorship was low due to high rates of mortality among seeds, germinating and first year seedlings. Probabilities of fourth year seedling establishment in uncolonized woodlots were <1% of incoming seed for three of the four species. Successful colonization of these species will depend on relatively large seed inputs to fragments. A previous study indicated that dispersal to and establishment in fragments >150-175 m from a seed source occurs at a rate of less than I seedling/ha over a 4-11-year period. Therefore, management intervention such as seedling transplantation may be necessary to maintain eastern deciduous tree species in fragmented forests. (C) 2003 Elsevier B.V. All rights reserved.