Diverging conditions of current and potential future urban forest patches

Abstract

Forested natural areas in cities provide a range of social, ecological, economic, and health benefits. Ensuring the delivery of these benefits requires an understanding of current and potential future forest conditions, yet urban forest dynamics are not well-understood. Here, we address this knowledge gap by examining forest structure and composition in 126 plots distributed across three forest patch sizes (large [95–126 ha], medium [1–19 ha], and small [0.05–0.65 ha] patches) in the city of New Haven, CT. We detected significant shifts in forest structure and composition suggesting a suite of distinct successional trajectories within each patch size. Large patches comprised 95% native tree species in the canopy (>5 cm dbh), sapling (1–5 cm dbh), and seedling (<1 cm dbh) layers, suggesting that these large patches will continue to be native-dominated in the future—a primary objective for urban forest management. Moreover, in these large patches, shifts in the dominant species in each stratum suggest that as large patches move through succession they will transition from moderately shade-tolerant forest types (i.e., oaks) to shade-tolerant types (i.e., American beech)—a similar successional trajectory to surrounding second-growth forests in the region. Medium patches were the most heterogeneous patch size sampled. Despite this heterogeneity, they generally resembled large patches in the canopy and sapling layers but diverged in the seedling layer. In medium patches, the invasive Norway maple replaced American beech as primary seedling species suggesting a future shift toward non-native-dominated forest in the future. Small patches were the most compositionally and structurally distinct patch size sampled. Only 36% of the canopy trees and saplings in small patches were native species; however, this increased to 71% native in the seedling layer. Additionally, the most important seedling species included those that are bird-dispersed highlighting the potentially valuable role that these small, non-native, forest fragments play as wildlife corridors. Collectively, our study demonstrates that multiple forest types, dynamics, and conditions can be found within a single city and that forest patch size offers a helpful framework to begin to parse out these differences.