A 100% tree inventory using i-Tree Eco protocol: A case study at Auburn University, Alabama

Abstract

The Auburn University campus in Auburn, Alabama, served as a site for a case study evaluating the applicability of i-Tree Eco to complete a 100% tree inventory. The 2009-10 inventory of the managed areas of main campus encompassed 237 ha. Information collected from each tree included diameter at breast height (dbh), tree height, crown width, percent dieback, and a tree condition rating. The complete inventory included 7,345 trees on the main campus with Lagerstroemia spp. (crapemyrtle), Quercus phellos (willow oak), and Pinus taeda (loblolly pine) being the most numerous species. Average dbh and total height of all trees were 16.4 cm and 8.5 m, respectively, with an estimated canopy cover of approximately 16%. Tree condition ratings, recorded twice for each tree, indicated that percent dieback alone is not a sufficient measure of tree condition. Field data were analyzed by the United States Department of Agriculture Forest Service (USDA FS) using i-Tree Eco which provided vital information on ecosystem services. Ecosystem services data estimated by i-Tree Eco for the Auburn main campus (237 ha) and Davis Arboretum (5.5 ha) were separated to provide an evaluation of the differences between an urban managed and a protected forest. The ecosystem services reported included air pollution removal and carbon storage and sequestration. Air pollutants reported were carbon monoxide (CO), ozone (O3), nitrogen dioxide (NO2), particulate matter < 10 microns (PM10), and sulfur dioxide (SO2). Trees in the arboretum had an average dbh of 24.4 cm and basal area of 12.04 m2/ha (std. dev. = 19.4 and 0.12, respectively) as compared to 16.4 cm and 2.24 m2/ha (std. dev. = 19.6 and 0.13, respectively) for the main campus. The managed areas of campus stored 6,652 kg of carbon per ha and sequestered 291 kg/year/ha of carbon. The Davis Arboretum stored 41,975 kg of carbon per ha and sequestered 1,758 kg/year/ha of carbon, 6x the campus amount on a unit area basis. Trees from the main campus removed 2,970 kg/year of air pollution (12.5 kg/year/ha) compared to 560 kg/year for the arboretum (102 kg/year/ha), which was 8x the amount on a unit area basis compared to the main campus. Relative tree condition ratings indicated there was little difference in tree condition between the two areas; however, the larger diameter trees in the arboretum had higher condition ratings than those on the main campus. Models that predict ecosystem services in urban areas are useful tools for urban forest managers and arborists. Tree crown form is an important component of these equations; however, there are few equations that predict ecosystem services available for urban, open-grown trees. Predictive open-grown crown width equations were developed for three native species common in urban forests in the southeastern United States (US). The species used were Quercus lyrata (overcup oak), Quercus nuttallii (Nuttall oak), and Quercus phellos (willow oak). To our knowledge, these are the first predictive open-grown crown width equations developed for these species in the southeastern US. Dbh (independent variable), dbh2 (independent variable), and average crown width (dependent variable) data were used to create the equations which yielded R2 values of 0.96, 0.94, and 0.91 for overcup, Nuttall, and willow oak, respectively. These equations can aid urban landscape and utility planners in predicting crown width at various trunk diameters, reduce field collection time by reducing the need to measure crown width in the field, and with time, be used to validate species specific equations, e.g. leaf biomass, for these and other southeastern urban-planted tree species. Ecosystem services information obtained from the complete inventory of the Auburn main campus and Davis Arboretum provided a dataset used to evaluate the standard plot sampling protocol of i-Tree Eco. Air pollution removal and carbon storage and sequestration values estimated by i-Tree Eco were the ecosystem services factors utilized for this assessment. To achieve an 80% estimate of the total campus value for air pollution removal and carbon storage and sequestration, 622, 870, and 483, 0.04 ha plots, respectively, with at least one tree present would have to be inventoried, as opposed to the standard i-Tree Eco sampling protocol of 200-0.04 ha plots. Based on the proportion of area with and without trees, the Auburn campus would require 20, 30, and 16% of the total area to be inventoried for air pollution removal and carbon storage and sequestration, respectively, to obtain the necessary number of plots with at least one tree present. In this study, i-Tree Eco procedures were an effective and efficient tool, based on having not incurred any major problems, and provided valuable information regarding Auburn University’s and the Davis Arboretum’s urban forest structure and functions. The ecosystem services results demonstrate how important and necessary naturalized and protected areas are in our urban environments and how small areas can have large impacts, because they may contain more trees on a unit area basis, which are typically larger and in better condition due to less disturbance. This study also provided a first step in the evaluation of the i-Tree Eco sampling protocol; however, efforts to test these methods at sites throughout the southeastern US and to evaluate stratified sampling are needed to provide the most accurate evaluation for urban forests. i-Tree Eco has the potential to become the urban forest inventory standard; however, more research is needed not only throughout the southeastern US but also other regions to more completely validate i-Tree Eco.