Effect of Plot and Sample Size on Timing and Precision of Urban Forest Assessments Methods Effect of Plot Size on Data Collection Time and Total Population Estimate Precision

Abstract

Accurate field data can be used to assess ecosystem services from trees and to improve urban forest management, yet little is known about the optimization of field data collection in the urban environment. Various field and Geographic Information System (GIS) tests were performed to help understand how time costs and precision of tree population estimates change with varying plot and sample sizes in urban areas using random sampling approaches. Using one-tenth acre (0.04 ha) plots, it is estimated that, on average, approximately three plots per day can be measured with plot data collected on several variables for all trees greater than 1 in (2.54 cm) in diameter along with general plot, ground cover, and shrub data. A field crew of two people can gather approximately 200 one-tenth acre (0.04 ha) plots during a 14 week summer field season depending on city traffic, city area, and tree cover conditions. These 200 plots typically yield approximately a 12% relative standard error on the total number of trees. Measuring the urban forest structure (i.e., species composi-tion, number of trees, tree sizes and locations, tree health) can give managers and planners a basis with which to develop and evaluate programs for managing urban trees and forests throughout a city. In addition, long-term monitoring of urban forest structure can provide essential data related to rates and factors of change affecting population totals, tree mortality, tree planting and natural regeneration, tree health, and species changes. An accurate quantification of urban forest structure is also needed to assess the various ecosystem services and values pro-vided by the urban forest. Urban vegetation, particularly trees, provides numerous benefits that can improve environmental quality and human health in and around urban areas. These ben-efits include improvements in air and water quality, building energy conservation, cooler air temperatures, reductions in ul-traviolet radiation, and many other environmental and social benefits (Nowak and Dwyer 2007). By having accurate infor-mation on urban forest structure, managers can understand what the current urban forest provides in terms of various en-vironmental benefits and also alter the structure of the urban forest (e.g., tree plantings, species and site selections, and tree maintenance and removals) to enhance these benefits in the fu-ture. One of the best ways to assess the entire urban forest is through sampling procedures. However, varying sample and plot sizes affect total cost (time) of data collection and the precision of the urban forest estimate. The purpose of this ar-ticle is to illustrate, based on field data collection tests, how plot and sample size of randomly located circular plots in urban areas can affect data collection time, number of permissions needed to access plots, and precision of tree cover and total tree population estimates. These types of data have been lack-ing related to urban forest sampling and can be useful in devel-oping sampling schemes to help provide desired precision of estimates and understand the costs associated with obtaining that precision.