Natural regeneration in urban forests is limited by early‐establishment dynamics: implications for management

Abstract

Urban forested natural areas are valuable ecological and social resources, but long‐term sustainability of these habitats is challenged by environmental and social factors associated with urban ecosystems. Advances in city‐scale assessments of urban forests have increased the resolution of forest community types and conditions, allowing for improved understanding of ecological function, such as natural regeneration, in these urban habitats. By applying metrics of tree regeneration that are commonly used for the management of rural forests, urban ecologists can test the potential for traditional forest management strategies within our cities. In this study, we compare urban and rural oak–hickory forest composition and structure and the capacity for natural regeneration in the New York metropolitan area. Specifically, we use two metrics of advance regeneration that describe the abundance of seedlings and saplings at different size classes to test whether this management for natural regeneration is a viable option. We found differences in recruitment dynamics between urban and rural forests that have implications for the sustainability of these forests and new management strategies. First, after controlling for forest community type, species composition in urban and rural sites was significantly different across multiple strata and within the seed bank. Species‐specific capacity for natural regeneration was different in urban and rural sites, signaling the possibility of divergent successional trajectories. Second, while differences in species composition exist, both urban and rural sites were dominated by native species across all forest strata except for urban seed banks. Third, despite finding significantly lower average annual seedling abundance in urban (1.9 seedlings/m2) compared to rural (7.1 seedlings/m2) sites, we observed greater density of saplings in urban forests, and no significant difference in stocking index between sites. These findings suggest that early‐establishment barriers to recruitment are greater in urban forest sites. However, once established, seedling transition into advance regeneration stages may not be different between site types, and advance regeneration may, in some cases, be more viable in urban forested natural areas. These results highlight functional differences between urban and rural forest recruitment dynamics that may impact on the future community composition of oak–hickory forests in the two landscape settings.

Advancing management of urban forested natural areas: toward an urban silviculture?

“…These techniques have made it possible to create ecological community type maps and modified dichotomous keys that include both regionally common and novel forest types, as well as information on stand condition and a city-wide inventory of all forested natural areas (eg Edinger et al 2016;Forgione et al 2016). This work supports stand assessments oriented toward designing management strategies, such as stocking indices and potential for advance regeneration (eg Piana et al 2021).…”

“…Silviculturists rely on a base of knowledge informed by long-term scientific studies to inform prescriptions and management decisions (Oliver and Larson 1996;Ashton and Kelty 2018). Similar understanding of urban forest stand dynamics (such as the potential for natural regeneration or the response to management prescriptions) is emerging but remains limited (eg Piana et al 2021;Witt et al 2020). In cities, species-specific responses and the ecological thresholds to the multitude of socioecological stressors need to be determined (ie urban forest stand dynamics).…”

“…Recent research suggests that urban conditions may impact plant populations throughout their life history, from seed production to dispersal to adult establishment, potentially shifting plant community dynamics and trajectories (Piana et al 2019). These dynamic changes can result in a greater abundance of nonnative seedlings and dissimilarity between canopy and seedling layer composition and potential shifts in forest community trajectories as compared to nearby rural forest equivalents (eg Pregitzer et al 2019a;Piana et al 2021). Notably, the context of urban forested natural areas may vary, from a forest located in a dense urban core to a woodlot located at the urban-wildland boundary, and therefore the intensity and co-occurrence of urban stressors may also differ.…”

Climate adaptation actions for urban forests and human health

“…364 ► Restore or create conditions that allow tree seedlings to thrive by removing nonnative species in the shrub layer and canopy trees while controlling herbivory. 365,366 ► Implement silvicultural practices to reduce competition around large, long-lived trees to maintain tree health, such as thinning dense stands from below or using prescribed fire in fire-adapted ecosystems.…”

Climate Adaptive Silviculture for the City: Practitioners and Researchers Co-create a Framework for Studying Urban Oak-Dominated Mixed Hardwood Forests

“…Specifically, Urban Forested Natural Areas (UFNA's) are of particular importance for cities in forested biomes. The socio-ecological contexts of these urban greenspace types have only recently been defined (e.g., Pregitzer et al, 2019a;Johnson et al, 2020;Piana et al, 2021a) and can be similar to non-urban native forests in composition, structure, and function (e.g., Pregitzer et al, 2019b;Piana et al, 2021b).…”