388 Nowak et al.: Effect of Plot and Sample Size on Timing and Precision lation estimate (Table 2). Average time per plot increased from approximately 62 min (SE 7.4) for a one-twenty-fourth acre plot (0.017 ha) to 106 min (SE 14.0) for a one-sixth acre (0.067 ha) plot. Number of permissions (lots) also increased from an average 1.9 (SE0.1) to 3.1 (SE0.2), and number of trees measured per plot increased from 2.6 (SE 1.1) to 6.5 (SE 1.5). All three plot sizes produced total population estimates with a sampling error within 1 SE of the estimated population total of 251,000 trees, but as plot size increased, the total estimate moved closer to the 251,000 estimate and SE decreased (Table 2). The trend of the overall estimate decreasing with plot size (Table 2) suggests that the sample size was not large enough for the two smaller plot sizes. The effect of increasing the number of plots for the smaller plots sizes such that the total sample area remains the same among all plots sizes remains to be investigated. A similar pattern occurred when accessing tree cover from digital maps using plot sizes that ranged from one-twenty-fourth acre (0.017 ha) to one-fourth acre (0.1 ha), including an FIA cluster plot. Number of permissions increased and percent of plot in parcel with plot center decreased as plot size increased (Table 3). The one-fourth acre (0.1 ha) plot produced the closest esti- mate of actual tree cover value and had the lowest SE and rela- tive SE. The one-twenty-fourth acre (0.017 ha) plot produced the estimate farthest from the actual tree cover value, although it was still within 1 SE from the true mean and had the highest SE and relative SE. The FIA cluster design, which is being used in the urban forest health monitoring program (Cumming et al. 2008), produced estimates of tree cover with a slightly higher SE and relative SE than a one-sixth acre (0.067 ha) single plot design. The FIA plot design also required nearly double the permissions of the one-sixth acre (0.067 ha) plot design (Table 3). Average time to set up a residential plot (e.g., gain permission and establish plot center) was 15.6 min (SE1.9); average time needed to estimate cover types was 12.8 min per plot (SE 1.1). Thus, the average fixed time per plot was approximately 30 min. The average time to record all measurements on one tree was 12.2 min (SE 0.9). Effect of Sample Size on Total Population Estimate Precision The relative standard error (RSE) of total number of trees drops significantly with the first 50 to 100 one-tenth acre (0.04 ha) plots established, from 54.1% RSE with 10 plots to 17.1% RSE at 100 plots. After approximately 100 plots, the RSE continues to drop, but a reduced rate per additional plot (Figure 1). The av- erage RSE for 200 plots is 12.1% (Table 1). DISCUSSION The key to assessing urban forests is to determine the optimal number of plots and plot size needed to gain the desired preci- Time (min) Plot size (ac) 1/24 (0.017 ha) 1/10 (0.04 ha) 1/6 (0.067 ha) zStandard error. yActual estimated number based on 100 one-tenth acre (0.04 ha) plots is 251,000 trees (SE 35,000). xRelative standard error. ©2008 International Society of Arboriculture Mean 61.8 84.1 106.1 SEz 7.4 9.9 14.0 No. of lots Mean 1.9 2.8 3.1 SEz 0.1 0.2 0.2 Mean 2.6 4.6 6.5 sion of an estimate at minimal cost. Unfortunately, there is not much information in the literature on costs of urban field plots and structural variability across the urban forest. A general rule of sampling is increasing the plot size and number of plots tends to increase precision, but at increased cost. Data presented in this article begin to reveal the increases in precision and time costs associated with different sample designs for sampling trees in urban areas. Assuming an average tree density of 204 trees per acre of urban tree cover (504 trees/ha cover) (Dwyer et al. 2000) and a national average tree cover of 27.1% (Nowak et al. 2001), the average time to set up and measure a one-tenth acre (0.04 ha) urban plot in the United States would be approximately 95 min (five plots per 8 hr day). However, this estimate does not include travel time. The longer the distance between plots and the slower the traffic, the fewer the number of plots that can be measured per day. This estimate also includes plot permissions; however, plots on several land uses often do not require permission and access setup time could be reduced. Also, the fewer the trees per plot or fewer variables measured, the more plots can be mea- sured per day. A reasonable estimate of average number of one- tenth acre (0.04 ha) plots per day for a field crew of two people would be approximately three plots per day for a full suite of tree and plot measurements in a midsized city. Number of plots per day will vary by the amount of tree cover in a region because when tree cover increases, the amount of time measuring trees increases. In desert regions, urban tree cover averages 9.3% (Nowak et al. 2001) and average plot setup and measurement time would be approximately 51 min. In grass- lands (urban tree cover averages 17.8%), average plot time would be approximately 72 min. In forested areas (urban tree cover averages 34.4%), average plot time would be approxi- mately 113 min. Again, these estimates do not include travel time or office time needed to establish plot locations and maps. The standard UFORE model sampling approach establishes approximately 200 one-tenth acre (0.04 ha) circular plots in ran- domized grid or stratified random sample. The selection of 200 plots was based on an estimated amount of plots that could be surveyed by field crew of two people during a summer season (14 weeks), given an average data collection rate of three plots per day. In some cities with high tree cover and/or traffic vol- umes, data collection will take longer than 14 weeks. In addition to data collection time, there are also costs associated with es- tablishing the locations of the plots, transportation, equipment, data entry or data transfer, and data analysis and reporting costs. The use of 200 one-tenth acre (0.04 ha) plots produces a reasonable population estimate if a 12% RSE is acceptable to the user. Depending on the desired precision, a smaller sample size may provide adequate estimates of the urban forest population. Table 2. Average time, number of lots accessed, trees per plot, and total population estimate from 26 residential plots measured in Syracuse, New York, U.S. using different plot sizes. No. trees per plot SEz 1.1 1.3 1.5 Range 0–27 0–33 0–34 Estimatey 429,998 316,968 267,922 No. of residential trees SEz 178,366 90,708 61,220 RSEx 41.5 28.6 22.8
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