Global change, including anthropogenic nitrogen deposition, is upon us. These changes have and will continue to affect plants, including Pacific Northwest native conifers. Understanding the response of these native conifers to global change will help managers plan for future forest management. Urban areas already experience higher temperatures, more CO2, and more nitrogen compared to rural areas. The effects of urban areas on trees can therefore be used to predict the effect of global change. We use Thuja plicata trees to address three hypotheses: That 1) trees experience higher temperatures and nitrogen levels at urban edges, 2) trees experience decreased reproduction at urban edges relative to rural edges and urban centers, due to higher temperatures causing low soil moisture, and 3) trees experience increased growth at urban edges. At 5 forested urban parks and 3 rural parks, we located study sites near and far from the edges. Each study site was centered around a focal Thuja plicata tree. Data collected from sites include hourly temperature, soil NO3-, seedling counts (for reproduction), and an increment core (for tree growth). The data were analyzed to determine the effect of temperature, nitrogen, and urban edge on growth and reproduction.
We found that temperatures were higher at edge locations compared to center locations, and that temperatures were especially high at urban edges, but we found no difference between overall rural and urban temperatures. Soil nitrogen was higher in urban areas, but not significantly different between edge and center locations. Tree growth was greater in urban areas than rural areas, with no edge effects detected in either. We did not observe any difference between urban and rural areas in Thuja plicata reproduction alone. We did observe a difference between all conifer reproduction at edges vs. centers; however, the direction of that difference was opposite in urban vs. rural parks. In urban parks, reproduction at edges was lower than at centers. Overall, seedling counts were significantly lower than in other, less disturbed forest locations (e.g. a National Park). We suspect that factors in addition to temperature and nitrogen are having significant impacts on seedlings. For example, herbivory by introduced gastropods is thought to play a significant role in urban forests.