Canopy trees in an urban landscape – viable forests or long-lived gardens?

Abstract

Seedling recruitment shapes tree communities, including those found in altered landscapes such as urban forests. However, little data exists on local- and broad-scale tree seed immigration and recruitment in these communities. Interspecific competition and seed predation are two major causes of recruitment failure for plants. In North American urban forests, these pressures may be exacerbated by an altered disturbance regime and prevalent invasive species combined with dense populations of rodents and browsers. Preliminary investigation in an urban forest on the eastern shore of Lake Erie indicated very low long-term tree seedling recruitment in the mature canopy stands. Our competing hypotheses were that seed establishment (habitat suitability) and seed limitations (seed availability) explained the tree recruitment failure. We tested seed establishment using field experiments (burning and vertebrate herbivore exclosures) and seed limitation by introducing native tree seeds. Moreover, we tested also seed limitation by examining local and regional seed input using seed traps. We found that seedling recruitment increased significantly with experimental reductions in predators and competitors, suggesting strong biotic establishment limitations in the urban forest. In addition, seed rain correlated significantly with immediately proximate parent plants, but no species arrived beyond what occurs within 50 m of the experimental plots. Essentially, then, the existing canopy species are not replacing themselves and extant seeds are not immigrating to replace them. At the patch scale, habitat quality, particularly seed predation and browsing, as well as competition from nonnative understory shrubs, constrained native tree recruitment in the urban forest. At the landscape scale, the evidence of poor long-term seed recruitment and the lack of long-distance seed input also suggest low native tree seed availability. The tree recruitment failure suggests that, in the absence of active management, this urban forest may eventually convert to an invasive-species dominated urban shrubland.