Abstract. The accuracy and validity of urban forest resource data that was collected by trained volunteers was established, using an actual case study in Brookline Massachusetts. Results indicate that the data collected by trained volunteers is valid, and its accuracy compares favorably to levels found among a control group of Certified Arborists. Indirect benefits associated with this type of volunteer effort include the development of a more informed urban forest constituency, increased environmental awareness, an increased political voice, and an improved quality of life for urban residents. The cost of utilizing community volunteers to conduct urban forest inventories is competitive with similar programs conducted by professional Arborists.
 
 
 
 

Volunteer initiatives in urban forestry, primarily addressing the implementation of tree planting programs within communities, have been outlined and the benefits of these programs described in the literature (1,5). The development of volunteer initiatives in establishing effective urban forestry programs and the assisting of municipal tree care programs have also been discussed by Skeira and Evans (9,4). Berry (2) proposes that "successful urban forest programs are those that break down the boundaries between concepts and roles that traditionally have created a dualism of culture and nature." They work because they create ways for citizens to play an important role in shaping their environment. Probart (8) proposes that, "effective volunteerism provides the community with unique opportunities that are generally unavailable to them," enabling communities to identify and fulfill their environmental goals, including urban forest resource management.

Successful strategies for utilizing volunteers to perform urban forest resource inventories are detailed in the literature (3,4,7,8,6). Buchanan suggests that restricted funding is a primary reason that professionals are not always used for tree inventory data collection and proposes alternative methods including interns, volunteers and high school youth (3). Empowerment of citizens to partake in the betterment of their own community is one of the primary benefits of utilizing volunteers to assist in an urban forest resource inventory as outlined by Lipkis (5).

The costs associated with using a professional to gather urban forest resource data can be calculated based on studies by Buchanan (3). He estimates that in 1991 "experienced urban forestry technicians can routinely collect data on 200 to 400 trees per day depending on the amount and complexity of information collected on each tree." In this case it would cost approximately $1.00 to $2.00 per tree to collect the data, based on an average of $50.00 per hour. While the use of volunteers to assist in gathering data on urban forest resources has been described, the benefits are more difficult to quantify since the costs of recruitment, training, mobilization, and supervision must be factored into a cost/benefit analysis. These costs will vary for each community that utilizes volunteers, making cost/benefit analyses more difficult. Additionally, the benefits of empowerment and community involvement are difficult to accurately assess, but must be considered. The evolution of a group of involved citizens as urban forest advocates and a focused political voice must also be considered when discussing the benefits of volunteer efforts.

While the literature suggests that volunteers can be used to complete urban forest resource inventories, there is a lack of information that describes the reliability of the data that is collected by the volunteers. Strategies for utilizing volunteers in tree care programs, including inventories, have been examined, but there is little or incomplete information concerning the validity and accuracy of the data collected by the volunteers. The purpose of this paper is to provide an accurate assessment of the accuracy of using community volunteers to complete an urban forest resource inventory, to examine the costs associated with implementing this type of inventory program, and to outline the political and organizational impacts that are likely to occur in the development of this type of program.

Materials and Methods

Volunteer Data Acquisition. A street tree inventory was conducted in Brookline, Massachusetts by 97 community volunteers during the Spring of 1994. Brookline was the site of one of the earliest street tree inventories in the country in the late 1800’s and has an active urban forestry program in the community with a strong citizen advocacy. The inventory program was coordinated by the University of Massachusetts Department of Forestry and Wildlife who assembled a diverse team of cooperators including the Town of Brookline, the Arnold Arboretum of Harvard University, the Brookline Greenspace Alliance and Boston Edison. This project was funded by the USDA Forest Service, and the Massachusetts Department of Environmental Management as an urban forestry grant. The project team included professional urban forestry personnel as well as educators, municipal officials, conservation personnel, citizen advocates and representatives of local utilities.

Approximately 11,250 trees found along 104 miles of roads were inventoried. Inventory teams documented the species, condition, DBH, management needs, root zone cover, percentage of impervious material over the root zone, presence of cavities, dead wood, and overhead utilities. The specific variables that were collected for each tree were determined based on results of the 1993 Survey of Massachusetts Urban Forest Management Programs conducted by Ryan and Bloniarz. This survey also reviewed inventories from communities across the country. The inventory information was entered into a Hunter Husky 16 portable computerized data recorder provided to each team. The trees location was also plotted on a base map that was provided by Boston Edison, the electric service provider to Brookline. The base map included streets, building footprints, and property boundaries. The spatial location of each tree enabled its position to be entered into a Geographic Information System (GIS) by the University of Massachusetts’ Resource Mapping Unit, providing the basis for an operational street tree management program based on GIS technology. Use of the computerized data recorders allowed for immediate use of the data and simplified incorporation into the GIS database, reducing the chance of errors that could occur during data entry and transfer. The data acquisition took place on three weekends during June 1994.

Volunteers were recruited from the community using a variety of methods including the media, mailings to target environmental organizations, and presentation in public forums. The Brookline Greenspace Alliance, and umbrella organization for community based environmental and open space groups in Brookline assisted in the recruitment effort, and provided a number of volunteers for the inventory. The Brookline Tree Planting Committee and Conservation Commission assisted in the logistical organization of survey teams and assigned specific survey locations for each team. Over 95% of the volunteers were Brookline residents and ranged from 13 to over 70 years in age.

Each volunteer completed a training program consisting of 12 contact hours with instructors from the University of Massachusetts Department of Forestry and Wildlife Management and the Arnold Arboretum. The training program was conducted in two parts. The first segment consisted of a four hour reception, orientation and informal discussion of the overall purpose and focus of the inventory project. The second training session was primarily instructional and was conducted at the Arnold Arboretum of Harvard University. It included classroom and practical field instruction. The volunteers were trained on the specific steps needed to complete the urban street tree inventory and received training on a series of items ranging from tree identification and condition assessment to map reading skills and data entry procedures. All of the volunteers were required to attend the two training sessions in order to ensure that all survey personnel had received a standard level of training.

The goal of the training curriculum was to introduce all aspects of the survey process to the volunteers and to develop skills in tree identification, maintenance requirements, hazard assessment, and assessing the overall condition of the tree. It was critical that the training program develop a uniform technique for gathering information on the trees in order to ensure consistency in data acquisition by the volunteers. The training curriculum was developed specifically for the Brookline inventory and contained information on situations that were likely to occur along the Town’s streets. A sample survey of the street tree population, made prior to the curriculum development, enabled the tree identification segment of the training to focus primarily on the trees that were expected to be found on the streets in Brookline, eliminating the need to teach volunteers about trees they would not find while completing their survey. Additionally, the training focused on examples of streetscape situations that were found along Brookline’s roads and used slides and graphics that contained landmarks that could be identified by community residents.

A detailed questionnaire was completed by each volunteer, prior to the training, in order to determine their specific experience and strengths that would be beneficial to a survey team. Survey teams were comprised of three volunteers each, and were organized with at least one person experienced in tree identification assigned to each team. The use of three member teams allowed for the survey process to move more smoothly since each member was assigned a specific task that could be completed while the other members worked on their assigned role. Since several tasks could be completed simultaneously, such as DBH measurement, recording the tree’s location on the map, and data entry, the use of three volunteers reduced the amount of work required of each volunteer thereby expediting the survey of each tree. Using three team members also allowed for the formation of a consensus, if questions arose with any aspect of the effort. Although the time necessary to map and record the required data on each tree varied, the time necessary at each tree averaged just over 2 minutes. Response to the three team approach was very favorable among the volunteers and resulted in a camaraderie and cooperative effort that was cited by many volunteers as the highlight of the survey experience. Pilot testing, in which teams of two survey personnel were used, proved to be slower, more cumbersome, and less rewarding to the participants. Any tree that a team considered to be a problem or that they had a serious question about was flagged and field inspected by a Certified Arborist.

The survey teams were each given maps of the particular road segments that they were to expected to survey and were allowed to proceed at their own pace and direction. These maps were also used to record the location of each tree as it was surveyed. A unique identification number was assigned to each tree and was recorded on the map for each tree. The same number was automatically generated by the data recorder for each tree and was saved along with the permanent number marked on the survey maps. This ensured that the location data for each tree was attributed to the correct tree as entered into the datarecorder. The identification number enabled the trees to be accurately added to the GIS database and serve as a permanent record for inventory purposes. Additionally, in one sample area, permanent tags containing the unique identification numbers were attached to the trees in order to test durability, susceptibility to vandalism, and usefulness in locating a specific tree more easily.

It was determined by Town of Brookline officials that only trees found from the back of sidewalk in toward the centerline of the road would be recorded, since these are the trees that are managed by the town’s Forestry Division. In areas that contained no sidewalks, trees that were growing within 10 feet of the curb or edge of pavement were recorded. This determination was unique to Brookline, and must be made prior to any survey process by the municipality. Trees growing in traffic islands and small parks less than 1/4 acre in size were also surveyed.

Accuracy Assessment. In order to determine the accuracy of the data that was collected by the volunteer teams, it was necessary to develop a method to examine their work. A random sample of 473 trees was generated and these were re-examined by a two person team of Massachusetts Certified Arborists (MCA/2), experienced in urban forest resource inventories. These trees were examined immediately following the volunteer effort in order to accurately assess the tree’s condition before environmental changes could occur. The same criteria variables that were used by the volunteers to assess the trees were used by the professionals. Once the data on the trees was collected it was compared with the data collected by the volunteers through the use of frequency tables or agreement matrixes, showing the number of times the two Certified Arborists (MCA/2) agreed with the volunteers. This would allow for a determination to be made as to the relative accuracy of data collected by volunteers as compared to practicing professionals.

It was also important to examine how well the two Certified Arborists compared to other professionals by establishing a level of agreement for collected data. This was done by setting up a separate study in which a series of trees were examined by the two Certified Arborists and also by ten other Certified Arborists (MCA/10). In this manner, the observations made by the two Certified Arborists (MCA/2) could be tested against observations made by other professionals in this Control Group (MCA/10). By determining how often they all agreed on specific variables it would be possible to ensure that the sample data that they collected in Brookline was representative of that of typical Certified Arborists, so that it could be compared with the data collected by the volunteers. It would also validate their use as experts to complete the random sample survey. Agreement levels between the two Certified Arborists (MCA/2) and the Control Group (MCA/10) would be compared to agreement levels between the volunteers and the two Certified Arborists (MCA/2) enabling a determination to be made as to the accuracy and validity of using community volunteers to complete urban forest resource inventories.

Results and Discussion

Project Costs. The total number of volunteer hours donated for the project was 3484, of which 3124 were provided by Brookline residents. Volunteer staff from the University of Massachusetts, the Arnold Arboretum, the Brookline Greenspace Alliance, and the Town of Brookline provided 360 hours of donated services to facilitate the inventory project. Based on a conservative rate of $10.00 per hour for volunteer time, $34,840.00 in donated services was provided. This amount can be translated into direct costs saved by the town for completion of the inventory. Paid staff provided 120 hours of labor, primarily in logistical support and coordination for the project. Assuming a rate of $50 per hour, paid staffing totaled $6,000.00 for the project.

Each volunteer averaged 20.2 hours of field work, plus an additional 12 hours of orientation and training, bringing the average total donation to 32.2 hours. Each survey team covered an average of 2.08 miles of roadway per day, collecting data on 225 trees along the way. This resulted in 11,250 trees being inventoried over a six-day period.

The total project budget was $49,340.00, including all labor and equipment, and is shown in Table 1. Costs for the inventory data acquisition included the rental of 8 computerized data recorders, printing, photocopying, and miscellaneous costs. An additional 8 data recorders were loaned to the project by the Massachusetts Department of Environmental Management. Since nearly $38,000.00 of this budget was donated services, the actual total cost to the Town of Brookline was $11,500.00.

Table 1. The total budget includes $37,840.00 in volunteer in-kind services, resulting in an actual cost to the Town of $11,500.00.
 

		Total Amount
Labor or Item		of Material or Service
Community Volunteers @ $10./hr.*		$31,240.00
Volunteer Support Personnel*		$3,600.00
Paid Staff		$6,000.00
Data recorder rental		$3,000.00
Donated data recorders*		$3,000.00
Printing & Photocopying		$1,500.00
Misc. Materials		$1,000.00
Total Project Budget		$49,340.00
*Volunteer Services		-$37,840.00
Total Cost to Town		$11,500.00

Based on the actual out of pocket expense disbursement of $11,500.00 by the Town, the cost per tree surveyed in the inventory was $1.02. This is very competitive with pricing of inventories that would be completed by professional arboriculture firms. Post-inventory evaluation of the entire project has identified several areas that could be streamlined in order to reduce the amount of overhead and support costs, thereby enabling future inventories to be performed at a lower cost. This would include soliciting donations for photocopy and printing costs, relying on skilled volunteers for some support services, and reduction of mailing costs.

Data Reliability. The data that was collected by the Volunteers was examined to determine how closely it agreed to data obtained by the two Certified Arborists (MCA/2), in order to test the accuracy of the volunteer effort.

The first item reviewed was the level of agreement in tree identification. In this portion of the study, the most commonly occurring trees found along Brookline’s streets were considered. Table 2 shows the most frequently occurring trees found in the 473 tree sample. The list contains trees that occurred more than 10 times in the sample surveyed by the two Certified Arborists (MCA/2) and represent 84 percent of the trees in the sample. Since these trees were the predominant ones found in the study area, they were ones used for the accuracy study. The remaining 16 percent of trees occurred less than 10 times in the sample study, and were not included in the analysis. They represented 33 different types of trees, with most occurring only one or two times.

Table 2. Examination of the level of agreement between Certified Arborists (MCA/2) and Volunteers in tree type identification was based on the most frequently occurring trees found in the sample. These species represent almost 84% of the trees found along Brookline’s streets. The remaining 16% were distributed among 33 different tree types.
 

Tree Name	Number of Cases
Acer platanoides	142
Acer rubrum	40
Fraxinus pennsylvanica	29
Gleditsia triacnathos	28
Platanus x acerifolia	11
Platanus occidentalis	27
Quercus palustris	19
Quercus rubra	41
Tilia cordata	55
Total	392

Frequency tables were calculated for each of the trees found in the sample in order to determine the levels of agreement between the two Certified Arborists (MCA/2) and the Volunteers. The frequency table enables a determination to be made as to the number of times two observations agree, therefore an accuracy level for the volunteers can be established. Table 3 shows a typical frequency table. The table gives the results for identification of Tilia cordata in the sample. In this case the two Certified Arborists identified 55 Tilia cordata, while the volunteers agreed in 41 cases or 75% of the time. Nine of the trees were recorded by the volunteers as Tilia americana, which indicates that they were able to correctly identify the genus in 91% of the cases.

Table 3. Frequency Table showing the number of times that Tilia cordata was identified by the Certified Arborists (MCA/2) and Volunteers. Note that Volunteers and the Certified Arborists (MCA/2) agreed on genus in 50 of 55 cases, or 91% of the time.
 

Response Counts Tilia cordata
							Tree Type
			ap		ar		fp	gt		pc		ta		tc		Total
Certified Arborists (MCA/2)			0		0		0	0		0		0		55		55
Volunteers			1		1		1	1		1		9		41		55

Key:

tc - Tilia cordata

ap - Acer platanoides

ar - Acer rubrum

fp - Fraxinus pennsylvanica
 

gt - Gleditsia triacanthos

pc - Pyrus calleryana

ta - Tilia americana
 
 

The 91% agreement rate on genus identification compares with a 100% agreement amongst Certified Arborists (MCA/10) when tested for the same variable, using a Tilia cordata sample.

Table 4 shows a summary of the agreement percentages for the most frequent tree types, arranged by genus. The agreement levels decreases when identification of both genus and species were calculated, with the lowest levels occurring among Platanus, Fraxinus and Quercus trees. This can be attributed to the similarity of the physical characteristics among the trees found in these genus. Identification of Platanus occidentalis and Platanus x acerifolia, two of the most predominant trees found in Brookline, is difficult for even trained Arborists. Likewise, the varieties of Fraxinus found in the sample leads to confusion, even on the part of Certified Arborists.

Table 4. Percentage of agreement between Certified Arborists (MCA/2) and Volunteers’ tree identification . Levels of agreement as to the genus of sample trees were high between Volunteers and Certified Arborists (MCA/2). Agreement levels decreased when genus and species were examined.
 

Tree Type		Genus		Genus & Species
Acer		95.00%	90.00%
Fraxinus		96.00%	68.00%
Quercus		93.00%	70.00%
Platanus		92.00%	46.00%
Gleditsia		96.00%	96.00%
Tilia		91.00%	73.00%

When considering only the agreement among genus, the levels of agreement are very encouraging. The agreement scores range between 91 and 96%, with only Tilia scoring the lowest at 91% agreement. When examining genus and species, the levels of agreement are lower, and attributed to the similarity in physical characteristics of many of the trees. An example of this is Platanus occidentalis and Platanus x acerifolia, both of which exhibit similar characteristics.

Other variables were examined to determine the levels of agreement between Certified Arborists (MCA/2) and the Volunteers. These variables included condition assessment, management requirements, and occurrence of cavities or and crotches in the sample trees. Table 5 summarizes the agreement levels between the two Certified Arborists (MCA/2) and Volunteers, and between the two Certified Arborists and the Certified Arborist Control Group (MCA/10), for several different variables that were observed in the sample.

Table 5. Agreement levels showing percentage of agreement between Certified Arborists(MCA/2) and Volunteers and between the Certified Arborists (MCA/2 and MCA/10) when examining individual variables of sample trees.
 

	Volunteers/ Cert. Arborists (MCA/2)	Cert. Arborists (MCA/2) / Cert. Arborists (MCA/10)
Variable	Agreement Level	Agreement Level
Genus	94%	100%
Genus & Species	80%	98%
Condition	83%	89%
Management Need	75%	86%
Weak Crotch	90%	80%
Cavity	92%	93%
Root Zone Cover Material	82%	98%

Table 5 shows that there is a range of agreement levels that are apparent. In most cases, however, there was a clear level of satisfactory agreement, with most agreement levels between 80 and 94%. Table 5 shows that the Certified Arborists (MCA/2 and MCA/10) were not entirely consistent in their levels of agreement, with assessment of Management Needs and identification of Root Zone Cover material representing areas in which there was noticeably disagreement. Assessment agreeement of the tree’s condition by the Volunteers and Certified Arborists (MCA/2) (83%) was nearly the same as that found between the Certified Arborists (MCA/2 and MCA/10) (89%). Assessment of Weak Crotches also showed a 20% disagreement amongst the Certified Arborists (MCA/2 and MCA/10). The other variables were assessed fairly consistently between the Arborists (MCA/2 and MCA/10), with less than 10% disagreement among the remaining variables.

Another noticeable area in which the Certified Arborists (MCA/2) and Volunteers disagree, is in the area of assessing the management needs of the trees. In 25% of the cases, the Certified Arborists (MCA/2) and Volunteers do not agree as to the need to prune or remove the tree. The Volunteers were consistently more conservative in their assessment of the pruning and removal needs of the tree, reporting that trees were in need of pruning more often than did the Certified Arborists (MCA/2). This is probably a result of the fiscal reality that the Certified Arborist faces, realizing that not every street tree can receive the same level of maintenance and pruning as a tree growing at a private residence.

Table 6 shows a complete Frequency Table comparing responses between Certified Arborists (MCA/2) and Volunteers when assessing the management needs of the sampled trees.

Table 6. Frequency Tables comparing the management needs as identified by the Certified Arborists (MCA/2) and Volunteers. The two groups agreed in 75% of the cases, with the Volunteers determining that pruning was needed more often than concluded by the Certified Arborists (MCA/2).
 

Mgt. Need		Mgt.	Need --	Volunteers
Cert. Arborist (MCA/2)	None	Prune	Remove	Consult	Total
None	345	94	4	13	456
Prune	7	5	0	0	12
Remove	0	0	4	0	4
Consult	1	0	0	0	1
	353	99	8	13	473

 

Summary
 

		Management Need
	None	Prune	Remove	Consult	Total
Cert. Arborists (MCA/2)	456	12	4	1	473
Volunteers	353	99	8	13	473

 

As shown in Table 6, the Volunteers recommended pruning in 99 cases while the Certified Arborists (MCA/2) only observed 12 trees that needed trimming. This reinforces the concept of the volunteers being more conservative in their assessment of the tree’s condition. Additionally, the volunteers listed 13 trees that needed Consultation by the Town Arborist, while the Certified Arborists (MCA/2) recommended only 1 tree that was in need of a second professional opinion. Although the two groups agreed in only 75% of the observations, this level of agreement cannot be discounted since review of the trees determined to be in need of pruning by the Volunteers were observed to have some dead wood in the crown, indicating that the Volunteers were very cautious in their determination of management needs.

Condition assessment is a critical component of a tree inventory, providing some of the most useful information to be used in the development of an effective urban forest management system. When examining how often the volunteers and Certified Arborists (MCA/2) agreed on the trees condition, one finds that in 83% of the cases the two groups agreed. Table 7 shows a Frequency Table for condition assessment of the trees. It can be noted that in most cases there is agreement as to condition, with the most noticeable exceptions being that the volunteers are more conservative in their assessment of the condition.

Table 7. Frequency Table showing condition assessment as identified by the Certified Arborists and volunteers. Agreement between the Certified Arborists and Volunteers occurred in 83% of the cases.
 

Condition Assessment			Volunteers
Cert. Arborists (MCA/2)	Good	Fair	Poor	Dead		Hazard	Total
Good	381	51	8	0		1	441
Fair	12	6	4	0		2	24
Poor	0	2	1	1		0	4
Dead	0	0	0	2		0	2
Hazard	1	0	0	0		1	2
	394	59	13	3		4	473

Summary
 

				Condition
	Good	Fair			Poor		Dead	Hazard				Total
Cert. Arborists (MCA/2)	441		24		4	2			2		473
Volunteers	394		59		13	3			4		473

 

If the ranges of assessment were collapsed into two coarser sets based on observations of Good/Fair and Poor/Dead/Hazard then the level of agreement between the Volunteers and Certified Arborists increases to 96%. This level of agreement is higher than the agreement levels among the Certified Arborist Control Group (MCA/10) and indicates there is a stronger level of agreement between the Volunteers and the Certified Arborists (MCA/2), than is suggested when reviewing the finer condition frequencies illustrated in Table 7.

Comparison of agreement levels between Arborist/Arborist and Arborist/Volunteers shows that for most of the variables assessed the agreement levels are nearly consistent. Identification of cavities, assessment of management needs, and identification of weak crotches had agreement levels that were nearly the same. This indicates that the data collection for these variables, by the volunteers, was nearly as accurate as the professional data acquisition. The validity of the data, as collected by the volunteers, compares to quality of the professional’s assessment of these variables.

Conclusions

Based on the procedures outlined in this study it can be concluded that the use of community volunteers for acquisition of data on trees found in an urban ecosystem can be substantiated and validated. Since many municipalities cannot afford to have professionals carry out a survey of their urban tree populations, community volunteers offer a viable, and less costly alternative. While the total cost of recruitment, training, mobilization, and logistical support for a volunteer effort can be significant, these costs are considerably less than an inventory carried out by professionals and provides a valid assessment of the location, condition, and type of trees found in that unique urban area.

In addition, the use of community volunteers allows for the establishment of a network of proactive constituents who provide a strong political voice that can be used to strengthen urban forest management programs in a community, while empowering the volunteers to play a critical role in the development of a better community. The political strength that a group of community volunteers can provide is difficult to quantify, but it is clear that they provide a voice that represents a strong advocacy for urban forestry issues and includes individuals who have proven their effectiveness through action and involvement. The involvement of the community’s citizens in helping to shape the quality of their neighborhood becomes a primary benefit of volunteer staffing efforts. By utilizing the energy and commitment of trained volunteers, directed by a trained professional Arborist or Urban Forestry professional, momentum for future projects and efforts is established and the framework for organizational development is in place. Volunteers may be called upon to assist in future actions related to greenspace within their community, become involved in other citizen forestry programs, assist in future inventories or become involved in other activities that can improve the quality of their community. The use of community volunteers to assist in gathering data on urban forest resources may serve as the start of a long-term commitment by the residents of a city or town to become involved in urban forest issues and to assist in the development of an effective urban forestry program within their community.
 
 

Acknowledgments. Funding for this project was supported in part from the USDA Forest Service, Northeastern Area and by the Massachusetts Department of Environmental Management.
 
 

Literature Cited

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1. Evans, L. 1993. People empowerment through trees I: ShadeMakers, pp. 135-136. In American Forests (Ed.). 6th National Urban Forest Conference, Minneapolis, Minnesota.
1. Lipkis, A., & Lipkis, K. 1990. The simple act of planting a tree: a Citizen Forester’s guide to healing your neighborhood, your city, and your world. Jeremy P. Tarcher, Inc., Los Angeles, California. 237 pp.
1. Matz, James R. 1993. Organizing Citizen Involvement, pp. 46-48. In American Forests (Ed.). 6th National Urban Forest Conference Proceedings, Minneapolis, Minnesota.
1. Monear, J. 1993. Volunteer pro's and con's: The seven C's of success, pp. 132-134. In American Forests (Ed.). 6th National Urban Forest Conference, Minneapolis, Minnesota.
1. Probart, S. 1993. Effective Volunteerism, pp. 49-50. In American Forests (Ed.). 6th National Urban Forest Conference, Minneapolis, Minnesota.
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