Future urban forests: The influence of management on outcomes of ecological restoration in New York City

Background/Question/Methods

Many urban forest parcels have been degraded by invasive species and diminution of historic forest biodiversity. This study investigates the long-term fate of efforts to restore ecological integrity to forests in New York City parks. In 1992, the New York City Department of Parks and Recreation’s Natural Resources Group initiated restoration to address widespread invasion by non-native woody plants including multiflora rose (Rosa multiflora), porcelainberry (Ampelopsis brevipedunculata), and oriental bittersweet (Celastrus orbiculatus). Restoration practices were typical of those being used to restore woodlands across the United States: removal of invasive, non-native species by herbicide and mechanical means, followed by planting of desired native tree species. Pre-treatment and initial site conditions following restoration were recorded, and survivorship and growth of planted saplings were monitored for two years.

In 2009, after 17 years, research plots at thirty sites were revisited to test the hypothesis that a more diverse, native-dominated forest structure had been achieved. In 2010, an additional thirty plots that were reported to be similarly degraded in 1992, but which were not restored, were sampled for the same suite of ecological and site characteristics. These plots represent a non-manipulated comparison to the treated sites. Management following initial restoration activities varied between parks, along a spectrum from no revisitation to annual removal of target invasive species.

Results/Conclusions

Many goals of the restoration were achieved, and have persisted over nearly two decades. These include greater recruitment of native tree saplings, higher tree density, and significant differences in ground-layer cover by targeted invasive species. Variation in management was associated with variation in these outcomes, and with the degree to which forests achieved original restoration goals. Restored plots were ecologically distinct from non-restored plots. These data support using proactive removal of invasive woody plants followed by planting of historic tree species as a method to sustain urban biodiversity, and indicate that subsequent maintenance of restored areas improves the success of these efforts.