Directing spatial patterns of recruitment during an experimental urban woodland reclamation

Studies of biological invasions indicate that natural recruitment of new sp ecies can occur as a "nucleation" phenomenon, in which scattered colonizati on. fuci spread and coalesce. Ecological reclamation of damaged lands might make use of this potential for enhanced natural dispersal, by inoculating sites with multiple small plantings to attract animal dispersers and other mutualists from nearby remnants of natural habitat. We conducted an experim ental test of this proposition. On a 6-ha section of an abandoned municipal landfill in the New Jersey Mead owlands, we installed 16 clusters of 21 trees and shrubs in an array of fen ced plots. Clusters contained seven native species known to: (1) attract bi rd dispersers to introduce propagules from remnants of off-site habitat; (2 ) contribute propagules by virtue of high reproductive output and clonal gr owth; and (3) accelerate woodland succession on open, degraded habitats. Av erage plant size was varied, with half the plots receiving larger tries and shrubs, to test whether woody plant size would enhance any attractive func tion. An additional eight empty plots were studied to estimate background r ates of recruitment and to test for a fencing effect. Site preparation incl uded the addition of 90 cm of fresh substrate, including organic matter, an d a cover crop of annual grasses. Recruitment of woody plants inside and su rrounding the experimental plots was examined fur five years, and results w ere compared on the basis of treatment and recruitment mode (avian, wind, o r clonal dispersal). Woody plant recruitment into experimental plots was rapid and substantial, primarily via dispersal from natural sources. Plots with larger plants attr acted significantly more recruits at the outset. but this difference dimini shed over time. Fall seed rain samples yielded a mean estimate of 426 seeds /m(2) within plots. However size distributions of recruiting woody species increasingly shifted toward larger individuals each year. Experimental mani pulations that opened seed beds for woody plant recruitment had short-lived effects. indicating a narrow window of opportunity for establishment. Spre ad of the planted species themselves was generally weak, although clonal gr owth contributed substantially to spread on the margins of plots. Most recruitment outside experimental plots was from external sources. A st rong proximity component was found fur bird-dispersed recruits, which were highly clustered near planted plots, with the highest densities near source populations on the site margin. Wind-dispersed trees and shrubs, by contra st, were nor associated with planted plots and were concentrated near one c orner of the site. Discounting plot interiors, total recruitment density fo r the site after 5 yr was similar to 800 woody stems/ha, 36% via avian disp ersal. 10% via clonal spread, and the remainder via wind-borne propagules. New recruits represented 26 woody plant species, all but four from external sources, and only five common species contributed more than a few recruits . We conclude that techniques for manipulating natural seed dispersal hold promise for ecological restoration, provided that background populations ar e available to supply colonists.