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Abstract Trees in residential environments are affected by a unique combination of environmental and anthropogenic factors, including occasional insect outbreaks that are increasing in frequency and severity due to climate change. We studied loblolly pine trees infested by bark beetles in a residential backyard in a southeastern US city. We investigated the responses of tree and stand‐level transpiration to environmental factors (solar radiation, atmospheric vapor pressure deficit, and soil moisture), severe weather events (strong winds and heavy storms), bark beetle infestation, and human actions (insecticide treatments and tree removals). We used constant heat dissipation probes to make continuous sap flux measurements (J₀) in tree boles. Over 22 months of the study,J₀of trees with confirmed infestation decreased from ~90 to ~60 g cm⁻² day⁻¹andJ₀of the rest of the trees increased from ~60 to ~80 g cm⁻² day⁻¹. One infested tree died, as itsJ₀steadily declined from 110 g cm⁻² day⁻¹to zero over the course of 2 months, followed by a loss of foliage and visible signs of severe infestation 6 months later.J₀was sensitive to variations in incoming solar radiation and atmospheric vapor pressure deficit. In most trees,J₀linearly responded to soil water content during drought periods. Yet despite complex dynamics ofJ₀variations, plot‐level transpiration at the end of the study was the same as at the beginning due to compensatory increases in tree transpiration rates. This study highlights the intrinsic interplay of environmental, biotic, and anthropogenic factors in residential environments where human actions may directly mediate ecosystem responses to climate. 

ABSTRACT. Urbanisation has led to increasing homogenization of plant communities across cities. However, it is unclear whether these patterns extend to cosmopolitan plant species at the genetic level. We examined genome‐wide genetic patterns in six widespread plant species (three Poaceae and three Asteraceae) across five cities in the USA (Boston, Baltimore, Minneapolis‐St. Paul, Phoenix, and Los Angeles) using reduced‐representation sequencing. We assessed genetic structure, differentiation, and patterns of isolation by distance (IBD) and environment (IBE) to determine if species were genetically homogeneous or differentiated by city, percentage of impervious surface, or both. Most species exhibited limited population structure overall, withPoa annua(annual bluegrass),Taraxacum officinale(dandelion), andCynodon dactylon(Bermuda grass) showing no significant genetic differentiation among cities, a pattern consistent with high gene flow mediated by human activity. Notable exceptions included city‐level differences inErigeron canadensis(horseweed) andLactuca serriola(prickly lettuce), especially in Phoenix. We also observed low genetic diversity inDigitaria sanguinalis(crabgrass) from Phoenix, suggesting recent founder effects or selection via environmental filtering.Erigeron canadensis,the only native species studied, displayed stronger differentiation by city, along with significant isolation by temperature and distance. Among all species, we found no evidence for population structure by impervious surface. Our findings indicate that widespread population genetic structure patterns of cosmopolitan plants are likely to depend more on species attributes (e.g., self‐compatibility) and human‐mediated dispersal than on urbanisation per se. 

Urban forests provide ecosystem services important for regulating climate, conserving biodiversity, and maintaining human well‐being. However, these forests vary in composition and physiological traits due to their unique biophysical and social contexts. This variation complicates assessing the functions and services of different urban forests. To compare the characteristics of the urban forest, we sampled the species composition and two externally sourced traits (drought tolerance and water‐use capacity) of tree and shrub species in residential yards, unmanaged areas, and natural reference ecosystems within six cities across the contiguous US. As compared to natural and unmanaged forests, residential yards had markedly higher tree and shrub species richness, were composed primarily of introduced species, and had more species with low drought tolerance. The divergence between natural and human‐managed areas was most dramatic in arid climates. Our findings suggest that the answer to the question of “what is an urban forest” strongly depends on where you look within and between cities. 

Many of the world’s major cities have implemented tree planting programs based on assumed environmental and social benefits of urban forests. Recent studies have increasingly tested these assumptions and provide empirical evidence for the contributions of tree planting programs, as well as their feasibility and limits, for solving or mitigating urban environmental and social issues. We propose that current evidence supports local cooling, stormwater absorption, and health benefits of urban trees for local residents. However, the potential for urban trees to appreciably mitigate greenhouse gas emissions and air pollution over a wide array of sites and environmental conditions is limited. Consequently, urban trees appear to be more promising for climate and pollution adaptation strategies than mitigation strategies. In large part, this is due to space constraints limiting the extent of urban tree canopies relative to the current magnitude of emissions. The most promising environmental and health impacts of urban trees are those that can be realized with well-stewarded tree planting and localized design interventions at site to municipal scales. Tree planting at these scales has documented benefits on local climate and health, which can be maximized through targeted site design followed by monitoring, adaptive management, and studies of long-term eco-evolutionary dynamics.