Forest patch size predicts seed bank composition in urban areas

Questions: As urban areas expand around the world, understanding how to
restore and maintain forests within the urban environment becomes
increasingly important. Given that a comprehensive understanding of
regeneration dynamics is critical to designating appropriate management
interventions we ask the following: how does regeneration, vis-à-vis the
buried seed bank, vary in heterogenous urban forests? And, can forest
patch size be used to predict regeneration and consequent management
interventions? Location: Vacant lots and public parks throughout New
Haven, CT, USA. Methods: We sampled buried seed banks in 131 plots
distributed across three forest patch sizes ranging from large intact
parks (95-126 ha), to small parks (1-19 ha), and vacant lots (.05-.65 ha).
We collected soil samples from the surface mineral soil and stratified
them over sand in a greenhouse over a period of 5 months to record
germination. Results: By examining seed bank floristics in a range of
forest patch sizes we found that species composition and dominance of
specific functional groups shifted with patch size representing a spectrum
of urbanization within just one city. Seed bank floristics in large parks
more closely resembled results from seed bank studies in rural forests
with over 85% native germinants on average. In contrast, vacant lots were
dominated by non-native germinants and more ruderal species indicative of
earlier successional stages. Seed banks in small parks were variable and
in some cases were more similar to large parks or vacant lots.
Conclusions: Our findings suggest that large parks in urban areas may be
largely self-sustaining whereas smaller parks may require more intensive
management for site rehabilitation, especially in early states of
succession. Furthermore, our results confirm the highly heterogeneous
nature of urban forest patches and call for more systematic sampling of
urban areas to capture this variation and improve management prescriptions
and outcomes.