© 2000 Society for Ecological Restoration 350 Restoration Ecology Vol. 8 No. 4, pp. 350–360 DECEMBER 2000 Barriers to Forest Regeneration in an Abandoned Pasture in Puerto Rico Jess K. Zimmerman 1,2 John B. Pascarella 3,4 T. Mitchell Aide 3 Abstract Sources of forest regeneration (soil seed bank, seed rain) and barriers to seedling establishment were ex- amined in a recently abandoned pasture in eastern Puerto Rico. Few woody species were found in the soil seed bank or in the seed rain. The number of seeds and species in the seed rain and soil seed bank declined with distance from the adjacent secondary forest. Nine species naturally dispersed and colonized plots during the study, with the wind-dispersed tree Tabebuia heterophylla being the predominant colo- nizer (91% of all seedlings). Barriers to seedling estab- lishment were determined using a blocked field ex- periment with eleven woody species representative of a variety of life histories. Each species was planted under the pasture vegetation (control) or in areas where all vegetation was removed (removal). Germi- nation was enhanced for four species in the control treatment, five species were not affected, and two spe- cies did not germinate under either treatment. Sur- vival to 6 months was higher in the removal treatment for two species. Seedling biomass was greater in the removal treatment at 12 months for one species. Seed mass was a good predictor of germination success and final shoot biomass, but not survival. This study dem- onstrates that seeding recently abandoned pastures with a mix of known pioneer species may accelerate the rate of secondary succession, but some species will have to be planted in later successional stages in order to overcome strong barriers to establishment. Key words: germination, pasture, Puerto Rico, regener- ation, seed bank, seed rain, Tabebuia heterophylla , tropical forest. Introduction I dentifying barriers to tree establishment in aban- doned agricultural lands is a useful approach from both applied and basic research perspectives (Nepstad et al. 1990, 1996; Aide & Cavelier 1994). For applied re- search, identifying biotic and abiotic barriers to seed- ling establishment and growth suggests a methodology for restoring forests following the abandonment of agri- culture. Species with easily managed barriers (e.g., dis- persal limitation can be overcome with broadcast sow- ing if there are no other significant barriers) can be used to establish a canopy for species that cannot tolerate the harsh conditions prevalent immediately following aban- donment. An understanding of tree establishment barri- ers can also contribute to knowledge of successional mechanisms (Drury & Nisbet 1973; Connell & Slayter 1977; Horn 1981; Pickett et al. 1987). The potential barriers to tree establishment are nu- merous and have been discussed in many articles de- scribing studies in the temperate zone (e.g., Hill & Can- ham 1989; De Steven 1991 a , b ; Gill & Marks 1991; Reader 1992; Myster & Pickett 1993; Myster 1993). Only more recently have similar questions been addressed in the tropics (Guevara et al. 1986; Purata 1986; Buschbacher et al. 1988; Nepstad et al. 1990, 1996; Aide & Cavelier 1994; Fernandes & Sanford 1995; Holl 1998 a , b , 1999). Most studies identify dispersal, competition from herbaceous vegetation, predation, and poor soil conditions as the most important barriers to tree establishment in a tropi- cal old field. Studies conducted in a tropical setting are contributing to a general understanding of common barriers to tree establishment, leading to the develop- ment of restoration methodology and succession theory in tropical secondary forests as has occurred for tem- perate forests (MacMahon 1987). Dispersal is likely to be a ubiquitous barrier to tree es- tablishment in abandoned agricultural lands. Dispersal around individual trees is limited to a few tens of meters with most fruits and seeds falling under the crown of the parent tree. Many studies report that dispersal into abandoned agricultural lands is similarly limited, result- ing in a significant drop in seed rain and seed banks of woody species as one passes from forest into agricul- tural lands (Nepstad et al. 1990, 1996; Aide & Cavelier 1 Institute for Tropical Ecosystem Studies, University of Puerto Rico, San Juan, Puerto Rico 00936–3682, U.S.A. 2 Address correspondence to J. K. Zimmerman, email jzimmerman@lternet.edu. 3 P.O. Box 23360, Department of Biology, University of Puerto Rico, San Juan, Puerto Rico 00931–3360, U.S.A. 4 Current address: Department of Biology, Valdosta State University, Valdosta, GA 31698, U.S.A.