1. Temperate deciduous forests in the United States are located in the most densely populated states across the northern and mid-Atlantic east coast. Land development and associated human activities result in small forests that are susceptible to anthropogenic influences, such as urbanization and non-native plant invasion. 2. The overall objective of this study was to assess spatial and temporal drivers of forest vegetation structure and diversity in small forests embedded across developed landscapes. We assessed woody plant composition across spatial gradients (i.e., urbanization and non-native plant invasion) and a temporal gradient (i.e., time since canopy closure) across 38 forests along the east coast of the United States in northern Delaware and southeastern Pennsylvania. 3. Surprisingly, we found the invasion gradient was not related to the urban gradient across our forests. Across all forests, the canopy consisted of native species (98% of all trees), whereas the forest understory was most vulnerable to nonnative plant invasion (65% of all woody plant stems). Greater native species richness in forest canopies and understories with increasing urbanization supports the conclusion that urban forests maintain native species and are not inherently degraded ecosystems. Non-native plant invasion has a strong influence on understory plant communities, and the duration of intact forest canopy had a strong negative correlation with non-native plant invasion suggesting intact forests can resist invasion. 4. Synthesis. This is the first study to compare simultaneously the importance of invasion and urbanization in determining plant community composition in forests embedded across developed landscapes, and to discover that younger forests harbor more invasive plants and urban forests maintain native plants. K E Y W O R D S nonmetric multidimensional scaling, non-native plant invasion, temperate deciduous forest, urban forests, urbanization, woody plant community | 2367

Despite their diversity and abundance, the importance of native, forest bee communities to pollination services and inherent biological diversity conservation is often overlooked. We studied forest bee communities in Delaware and Pennsylvania, USA, to better understand how forest bee community structure varies with changing land use and microhabitat quality among small, urban and suburban forest fragments in the mid‐Atlantic United States. Our hypotheses were that (1) microhabitat quality would affect relative abundance of bee taxa, (2) surrounding landscape composition would drive local patch colonization–extinction dynamics, and (3) forest patch size would not affect community structure and composition. We found a lack of spatial autocorrelation among forest patches, indicating the importance of individual fragments in the autonomous generation and/or maintenance of bee communities. Community analyses revealed the importance of both landscape context and microhabitat quality in defining forest bee communities. By partitioning beta diversity into its constituent components, we also found that landscape composition drove changes in the relative abundance of taxa, while both landscape and microhabitat characteristics significantly influenced species turnover. Neither landscape composition nor quality of the microhabitat influenced initial site occupancy or colonization–extinction dynamics. Bisected by highways, suburban neighborhoods, agricultural lands, and urban development, many urban and suburban forest patches in the eastern United States are similar in composition to our study sites. As many native forest bees have limited capacity for long‐range movement between forest patches, these remaining forest fragments are critical to the conservation of unique and speciose forest bee communities.

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The impacts of urbanization, such as urban heat island (UHI) and nutrient loads, can influence tree function through altered physiology and metabolism and stress response, which has implications for urban forest health in cities across the world. Our goal was to compare growth-stimulating and stress-mitigating acclimation patterns of red maple (Acer rubrum) trees in deciduous forests embedded in a small (Newark, DE, US) and a large (Philadelphia, PA, US) city. The study was conducted in a long-term urban forest network on seventy-nine mature red maple trees spanning ten forests across Newark and Philadelphia. We hypothesized that red maples in Philadelphia forests compared to Newark forests will be healthier and more acclimated to warmer temperatures, elevated CO 2 concentrations and reactive nitrogen (N r) deposition, and higher nutrient/heavy metal loads. Therefore, these red maples will have higher foliar pigments, nutrients, and stress-indicating elements, enriched δ 15 N isotopes and increased free polyamines and amino acids to support a growth-stimulating and stress-induced response to urbanization. Our results indicate red maples are potentially growth-stimulated and stress-acclimated in Philadelphia forests experiencing a greater magnitude of urban intensity. Red maples in Philadelphia forests contained higher concentrations of foliar chlorophyll, %N, δ 15 N, and nutrients than those in Newark forests. Similarly, lower foliar magnesium and manganese, and higher foliar zinc, cadmium, lead, and aluminum reflected the difference in soil biogeochemistry in Philadelphia forests. Accumulation patterns of foliar free amino acids, polyamines, phosphorous, and potassium ions in red maples in Philadelphia forests shows a reallocation in cellular metabolism and nutrient uptake pathways responsible for physiological acclimation. Our results suggest the approach used here can serve as a model for investigating 'plant physiology' and the use of urban trees as a biomonitor of the impacts of 'urban pollution' on urban forests. The results suggest that cellular oxidative stress in trees caused by pollutant uptake is mitigated by the accumulation of free amino acids, polyamines, and nutrients in a larger city. Our study provides a framework for determining whether trees respond to complex urban environments through stress memory and/or acclimation.

Urban forested natural areas are an important component of the forest and tree canopy in northeastern United States urban areas. Although similar to native forests in surrounding regions in structure, composition, and function, these natural areas are threatened by multiple, co-occurring biological and climate stressors that are exacerbated by the urban environment. Furthermore, forests in cities often lack application of formal silvicultural approaches reliant upon evidence-based applied ecological sciences. These include both urban- and climate-adapted silvicultural techniques to increase the resilience and sustainability of native forests in cities. With this in mind, we convened a group of urban forest practitioners and researchers from along a latitudinal gradient in the northeastern United States to participate in a workshop focused on co-developing long-term, replicated ecological studies that will underlie the basis for potential silvicultural applications to urban forests. In this article we review the process and outcomes of the workshop, including an assessment of forest vulnerability, and adaptive capacity across the region, as well as shared management goals and objectives. We discuss the social and ecological challenges of managing urban oak-dominated mixed hardwood forests relative to non-urban forests and identify potential examples of urban- and climate-adapted silviculture strategies created by practitioners and researchers. In doing so, we highlight the challenges and need for basic and long-term applied ecological research relevant to silvicultural applications in cities.

Environmental conditions, such as temperature, carbon dioxide, and nutrient availability, are altered by urban conditions at regional scales with potential for impact on tree leaf structure. Our goal was to compare leaf morphological characteristics driven by physiological acclimation in red maple (Acer rubrum L.) trees in deciduous forests embedded in a small (Newark, DE) and a large (Philadelphia, PA) city. The study was conducted in six urban forests on eighteen mature red maple trees in a long-term urban forest network. We hypothesized that red maples in Philadelphia forests compared to Newark forests will have a thicker upper epidermal layer, spongy palisade and mesophyll layer, longer and wider stomates, and lower stomate density. Additionally, we hypothesized that red maples in Philadelphia forests compared to Newark forests will have lower leaf water content and specific leaf area, and greater leaf thickness, fresh leaf weight, dry leaf weight, and leaf dry matter content. Our results for stomate length and stomate width supported our predictions; red maple leaves had longer and wider stomates in Philadelphia forests than in Newark forests. The increased stomate size in red maple trees suggests potential altered gas exchange behavior and mutual abiotic stress mitigation responses in red maple to greater urbanization impacts in Philadelphia forests. This supports previous findings of possible physiological and biochemical acclimation of red maple trees to urban conditions. Furthermore, the findings from this study suggest red maple trees may be a good biomonitor of regional scale impacts in urban environments.Forests and trees have the potential to improve environment quality in urban areas through blockage, uptake, and immobilization of pollutants [12,13]. Plant leaves have the ability to trap, sequester, and compartmentalize pollutants [14,15]. Urban forest trees may also provide important benefits through pollution regulation of urban conditions. However, this is debatable due to biogenic volatile organic compound (BVOC) production by urban trees [16 ], and for some pollutants, like particulates, the aerodynamic effect of tree canopies can influence particulate residence time, recirculation, and concentration in urban environments [17,18]. Alternatively, urban air pollutants can affect tree growth and function; tropospheric ozone has been reported to induce visible injuries, de-regulate stomatal conductance, and reduce photosynthesis and leaf area [19,20]. These complex interactions with urban tree canopies and atmospheric pollutants may stress trees, leading to shorter lifespan even with enhanced growth rates [21]. Thus, the ability of urban trees to mitigate urban conditions and improve environment quality and sustainability of cities may be dampened by their response to altered urban environmental conditions [17,22,23].The morphological and anatomical traits in leaves can serve as a bioindicator of plant response to altered environmental conditions, specifically air [15,24] and soil [2,25,26] polluti...

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