Urban Tree Residues: Results of the First National Inventory

Project Approach

A mail and telephone survey was performed of arboriculture and urban forest industries to determine the quantity and characteristics of urban tree and landscape residues. The arboriculture and urban forest industry is comprised of the following groups, or generators of residue: Commercial Tree Care Firms; Municipal/County Park and Recreation Departments; Municipal Tree Care Divisions; County Tree Care Divisions; Electric Utility Power Line Maintenance; Landscape Maintenance / Landscaper / Nursery firms; and, Excavator / Land Clearance firms.

The methodology used in this effort involved identifying representative populations for each of the generator groups, obtaining mailing lists from multiple organizations, preparing and administering the survey form, and collecting and analyzing the data. Because the survey effort was based on random sampling, it was important to establish statistical significance for each of the various categories of generators and residue forms. Statistical significance provides assurance to data users that the information is reliable and can support policy decision-making.

The analytical undertaking was a two-part effort. The first phase focused on analyzing and reporting the survey data. The second phase was a scale-up effort that provided estimates of national values for each of the generator and residue types for each region identified based upon the survey-obtained statistics. Each of these two efforts are reported separately below.

To facilitate the analysis and use of the survey data, it was useful to group the country into specific geographic subdivisions. Data were obtained on a state-by-state basis and were subsequently aggregated to the same geographic regions utilized by the U.S. Department of Energy’s Regional Biomass Energy Programs. These established programs have been collecting information on regionally important biomass resources since the early 1980s, and a considerable knowledge base currently exists that facilitates comparative analysis.

The mail survey requested information regarding the quantity and characteristics (e.g., chips, logs, brush, etc.) of the annual tree and landscape residue generated by organizations. Other survey questions were related to methods and costs of residue disposal. Finally, questions regarding the influence of natural disasters such as hurricanes and ice storms were also included on the survey form. With regard to natural disasters, respondents were asked to provide information for a 10-year period rather than a single year.

National Survey Results

The tabulation of survey results by generator group for the nation are illustrated in Table 1. For all generator categories, 3,878 organizations were identified and mailed at least one survey document. Also, for all generator categories, 1,710 (44 percent) surveys were returned. The number of organizations that were either “out-of-business” (186) or “declined to answer” (11) was 197 or approximately six percent of the total. Thus, there were 1,513 useful surveys. Of the total useful surveys, 181 organizations (12 percent) reported that they did not generate any residues. In all, there were 1,331 useful surveys that provided information about residue generation.

Table 2 summarizes the survey results of residue generation by region for all generators. Note that all of the information reported in Table 2 is actual survey results and not extrapolations or projections. For the United States, the generators reported slightly over 17.7 million cubic yards per year of residue produced. As shown in Table 2, most tree and landscape residue is generated in the form of chips. Over 11 million cubic yards of chips, or 67 percent of total residue, are produced every year. Unchipped tops and brush follow at 1.3 million cubic yards per year or nine percent of the total. Unchipped mixed wood, followed by grass clippings and fall leaf collection, are next with over 968,000, 406,000 and 395,000 cubic yards produced, respectively, each year. The least amount of urban tree and landscape residue comes in the form of whole stumps and unchipped logs. According to survey responses, the greatest volume of residues are generated in the Southeastern region, representing 43 percent of the national total, followed by the Northeast region.

The types of natural disasters that produce urban tree and landscape residue are presented in Table 3. Strong winds and storms constitute the dominant factor in contributing to tree residues, accounting for 61 percent of the reported natural disaster types. Hurricanes and tornadoes constitute separate categories and, when combined with strong winds, these three categories represent approximately 78 percent of the total natural disaster types. The volume of residue associated with natural disasters mirrors the frequency distribution except for the category referred to as “freeze.” For freezing conditions, considerable volume of residue is produced albeit on an infrequent basis.

Table 4 presents information on methods used to dispose of urban tree and landscape residue in the United States. As indicated, a large quantity (42 percent) is given away. Seventeen percent of the residue is landfilled, while 12 percent or two million cubic yards per year is sold. The highest percentage of that which is sold is used for mulch or sold as firewood.

Figure 1 shows the average regional and national costs for landfilling tree and landscape residue. Of the 17 percent of respondents who landfill their residue, landfill costs to dispose of the residue are reported highest in the Pacific Northwest ($15.98 per cubic yard) and lowest in the Southeast ($7.65 per cubic yard). Overall, the national average is $9.12 per cubic yard. Using a rough approximation of three cubic yards equals one ton, the disposal costs per ton range from $23 to $48. The national average is approximately $27 per ton.

• Download figure
• Open in new tab
• Download powerpoint

Figure 1.

Survey results, regional landfill disposal costs per cubic yard for tree and landfill residue.

National Residue Generation Estimates

The survey results were used to calculate national estimates for annual production of urban tree and landscape residues. The scale-up methodology followed accepted statistical procedures and is documented to allow for independent calculation of the national estimates.

In Table 5 the estimated values for the national production of urban tree and landscape residues are presented. The annual residue production is estimated to be 200.5 million cubic yards of green residue per year. Commercial tree care firms and Lawn and Garden/Landscapers produce the greatest amount of residue, almost 147 million cubic yards or 72 percent of the national total. Even though the mean annual production from Lawn and Garden Services/Landscapers is low, this sector has the largest population and therefore is a significant contributor.

Also shown in Table 5 are a variety of statistical parameters for each generator category and the nation. The data for the commercial tree care sector are illustrative of the other generators. The overall population, adjusted to remove firms that do not produce residue, is estimated to be 10,414 firms. These firms, on average, produce 7,004 cubic yards per year of all residue types. The bound on the mean production value represents the 95 percent confidence interval for commercial tree care firms. The bound on the overall estimate (also at the 95 percent confidence interval) for total residue production of 72,937,000 cubic yards is ±27,137,000 cubic yards. This represents a margin of error of 37 percent.

Figure 2 illustrates the distribution of urban tree residues for all generators on a state-by-state basis. The top five states in residue production, from highest to lowest, are: California, Florida, Illinois, Texas, and Pennsylvania. Several states in the Midwest, Southeast, and Northeast also have large volumes. As anticipated, large areas of the West do not contribute significantly to residue production.

• Download figure
• Open in new tab
• Download powerpoint

Figure 2.

National estimate of urban tree and landscape residue.

Conclusions

Important baseline information has been developed that provides insight into the volume and characteristics of green residues. Regional trends have been illustrated as well as patterns concerning the form of residues. The data in this report offer policymakers insight into important regional or business sector considerations that have only previously been addressed with poor or anecdotal data.

For instance, the significance of the lawn and garden service sector contribution to the national residue total has previously been underestimated. Another significant contributor is the municipal park and recreation departments. Although it was recognized that considerable segments of municipal budgets are devoted to tree care, no prior studies have identified this sector as having such a prominent role in the generation of urban green residues.

In many ways the diversity of the industry was confirmed in multiple fashion throughout the survey effort. Beginning with the difficulty in identifying a population estimate for each generator category and carrying through to variation between mean residue generation rates, the industry in general and the individual generator categories each exhibited lack of homogeneity.

It was revealing to discover the lack of knowledge by industry personnel on the actual volume and characteristics of the residue that they generate. Ninety-five percent of the data reported on the survey forms was estimated by the respondents. The lack of formal accounting methods for monitoring residues is an important missing parameter for obtaining the residue information. Because of the regulatory concerns and budgetary significance associated with residue disposal, firms should track patterns, characteristics, and quantity of residues.

A disappointing result of this study was the level of accuracy to attribute to key parameters. The survey effort was well conceived and received assistance from many individuals and organizations. The survey response rate was sufficient to suggest that statistical accuracy could be maintained. The difficulty arose in the variation in responses received from the generators combined with considerable population discrepancies. Large data variations rippled through the computational effort and led to undesirable error margins. Efforts to minimize the confidence intervals without disrupting the integrity of the data were unsatisfactory.