Influence of Climate, Construction Disturbance, and Site Factors on Tree Resistance, Resilience, and Performance in Urban Forests

Abstract

As urban areas increase in physical size, distribution, and population, increased importance will be placed on the benefits provided by trees in the urban forest (Dwyer, et al., 2000). Appropriate tree species selection based on environmental criteria is critical in order to achieve the larger and longer lived trees (Chacalo, et al., 1994) needed in urban landscapes. An analysis of urban planting sites can be difficult and costly when dealing with the highly variable nature of the urban environment. Site analysis approaches that model tree growth using easily obtainable site variables, such as soil surface, planting space width, and regional climatic factors are crucial for developing species specific site selection criteria designed to maximize tree size and longevity. Tree longevity is a function of species, adjacent land use, and tree health (Nowak, et al., 2004). The long-term performance of urban trees is a complex issue involving many factors from species to site conditions to climate factors and construction activities. Yet our current understanding of appropriate tree selection to maximize benefits over the long-term is limited. There is a gap in the literature on the long-term influence on tree growth and performance following infrastructure construction activities, and the effect of urban site characteristics. While climate in the urban environment is well documented over the life-span of the majority of living urban street and park trees (~28 years on average (Roman & Scatena, 2011)), the impacts to tree growth related to climate in combination with urban environmental factors is less well understood. Larger trees have been shown to provide greater benefits (McPherson E. G., 2003; Scott & Betters, 2000), yet our understanding of what constitutes a “larger” urban tree in terms of specific benefits has not been well defined. The research in this dissertation examined the influence of climate, construction, and physical environment on the growth of urban trees primarily through the use of tree ring analysis for trees in the cities of Minneapolis and Saint Paul, Minnesota. Chapter 1 examines the influence climate over the past 30 years on the growth of municipally-managed street and park trees. Chapter 2 investigates the effect of sidewalk construction on growth of street trees that survived the initial construction. Finally, chapter 3 evaluates four different approaches to quantifying tree performance in urban environments. Minneapolis and Saint Paul municipal forestry departments provided inventory data for their managed park and street trees to allow for selection of genera, species, and individual trees planted in both cities. Acer platanoides, Celtis occidentalis, Gleditsia triacanthos and Tilia spp. were selected as tree genera and species commonly planted in Minnesota and elsewhere in the Midwest of the United States. The results presented in Chapter 1 compare growth of trees in parks to trees growing along city streets using annual basal area increment determined from tree cores. Additionally, the chapter examined the influence of precipitation and mean monthly temperature growth over the period of 1982 to 2013 and identified the monthly climate variables that significantly influenced growth. Differences in growth among species and between sites (i.e. parks and along streets) were found as well. The period between 1982 and 2013 also contained two drought events and the resistance and resilience of species was quantified with significant differences occurring in response to the two droughts that were linked to tree age and tree size. In chapter 2, the effect of sidewalk construction on tree growth was quantified in terms of mean basal area increment (BAI), resistance, resilience, and recovery. As in Chapter 1, differences in growth response were detected among the four species investigated. Resistance and resilience showed statistically significant differences among species. To further quantify the growth impact of construction, time to recovery was also examined as the number of years post-construction needed for a species to regain previous levels of basal area growth. The interaction of boulevard or planting space-width on tree recovery was analyzed and found to have significant impact on growth recovery for all species. Finally, to investigate the influence of urban site characteristics on tree growth and performance, four separate measures of tree growth were analyzed: canopy projection area (CPA), diameter at breast height (DBH), growth rate as a ratio, and a tree performance index (TPI). The TPI was created in an attempt to include DBH, CPA, height, and growth rate into a single metric defining tree performance in urban environments. Species performance differed based the metric analyzed. The amount of pervious surface area under the canopy had a consistent positive influence on all species regardless of the performance metric investigated. Other factors influencing growth included: damage to tree trunk, the presence of stem girdling roots, nearness of neighboring trees, and tree age (all of which varied in significance, direction, and magnitude of effect on based on the performance metric analyzed). This chapter discusses the complexity in assessing performance and highlights the importance of identifying the primary objective as part of the tree selection process.