Diverging conditions of current and potential future urban forest patches

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, USA. 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 strata 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 heterogenous 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 towards 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.