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HomeMy WebLinkAboutAgenda Packet - BC - 1995.02.02NOTICE BURLINGAME BEAUTIFICATION COMMISSION FEBRUARY 2, 1995 5:30 P.M. CITY HALL CONFERENCE ROOM "A" A G E N D A I. ROLL CALL II. MINUTES OF JANUARY 5, 1995 III. CORRESPONDENCE IV. OLD BUSINESS A. REQUEST FOR REMOVAL OF MAGNOLIA @ 1516 LOS MONTES B. ARBOR DAY - MARCH 7, 1995/FRANKLIN SCHOOL V. NEW BUSINESS A. P.G.&E. LINE CLEARANCE STUDY VI. REPORTS A. Park Director B. Chairman C. Commissioners VII. FROM THE FLOOR 4 --"%k a� �'�-� �v .�-�, G ��ii�- � ���� �� S. �,/� ��.= .��-G--emu. (��-�._- /�"- °��. � Cam° - D�--c,c_..- r.�t� ..�Z�� ^,/z �J� �iC�-��-rt.� , L�-�e�' . � � � �� ✓h �� �,,�� vim-- � /� cz� �Yl.� Gt - 4' � �z �G �� i EU� i �PHYTOSPHERE RESEARCH 1027 DAVIS STREET, VACAVILLE, CA 95687-5495 (707) 452-8735 Memorandum Date: January 19, 1994 To: Rich Quadri Director of Parks City of Burlingame 850 Burlingame Ave, CA 94010 From: Ted Swiecki and Elizabeth Bernhardt Subject: PG&E Line Clearance Efficiency Study PG&E is in the process of switching from their previous practice of topping trees under utility lines to using natural pruning to maintain clearances. We are undertaking a scientific study for PG&E which will allow us to compare the economics of natural pruning and topping for maintaining clearance around electric distribution lines. We have selected London Plane as the tree species for study. We are trying to conduct the study in a relatively small geographic area to minimize the effect of variability due to climate. We originally planned to conduct the study in the San Jose area, but difficulty in finding sufficient numbers of London Plane trees under power lines has forced us to expand the study area north. The study will use trees in San Jose, Mountain View, Menlo Park, Redwood City, San Carlos, and San Mateo. We have also located 47 potential pairs of London Planes in Burlingame and we would like to include some of these pairs in the study. We anticipate we would use at most about 20 pairs from Burlingame. However, we will not plan to include any of these trees in the study until we receive approval from you. We have enclosed a summary of the study, as well as an overview which gives the rational behind the study and the chosen study design. Please do not hesitate to call if you have any other questions about the study. As I mentioned on the phone, this will be a six -year study, and it is critical to the study that each pair remains in the study for its six -year duration. Thank you for your consideration. We hope to hear from you soon. 1• PHYTOSPHERE RESEARCH 1027 DAVIS STREET, VACAVILLE, CA 95687-5495 (707) 452-8735 �-' January 19, 1995 PG&E LINE CLEARANCE EFFICIENCY STUDY Arboricultural researchers and practitioners have advocated the use of natural pruning as a superior alternative to topping for maintaining utility line clearance. However, there is currently very little data to support the widely -held belief that using natural pruning for line clearance will reduce overall pruning costs, improve tree health, and reduce service interruptions caused by branch failures. PG&E has contracted with Phytosphere Research to conduct a scientific study that compares natural pruning and topping for maintaining line clearance. The study will allow the researchers to compare the cost efficiency of utility clearance pruning by the two methods and examine how the two pruning methods affect tree condition. The investigators will determine whether the repruning interval differs for trees pruned by the two different methods. They will also determine whether the two pruning methods have different effects on tree health or the incidence of branch failures over the six -year duration of the study. Data will be collected on time and materials costs associated with the two different pruning methods. These data will be combined with information on repruning intervals in order to construct economic models which compare the costs of natural pruning with those of topping over varying time horizons. Matched pairs of trees will be used as experimental units to help control for the effects of factors that could affect the outcomes under study. The matched pairs of trees in the study will be randomly selected from within the study area. Pruning operations for each tree will be timed, and trees will be observed periodically to assess regrowth toward the conductors. Trees will be repruned once they encroach into a six-foot clearance envelope around the conductors, and observations will continue until the next repruning is required, or the year 2000, whichever comes first. London plane (P/atanus acerifo/ia) was selected as the tree species for study. It is a large- statured, relatively fast-growing and long-lived species. It is one of the most common urban trees in PG&E's service area and one of the most frequently -planted street trees in California. Of the potential prior pruning histories, only previously -topped trees will be included in the study. This is the most common pruning history in PG&E's service area. Only trees directly under power lines will be used in the study because top -trimmed trees are more common than side trimmed trees in urban areas. There is also some evidence that branch regrowth may be faster for top -trimmed trees than for side -trimmed trees. Horticultural consultant Deborah Ellis (408-378-5564) will be responsible for field evaluations at the northern California study location. Questions about the study can be also be addressed to the lead researchers Ted Swiecki and Elizabeth Bernhardt of Phytosphere Research (707-452-8735) or to Ken DiVittorio (510-866-5839) of PG&E. PG&E Line Clearance Efficiency Study Overview January 19, 1995 BACKGROUND Pruning cuts made at the point of branch origin or at laterals large enough to assume the terminal role are referred to as thinning cuts. Pruning cuts made at other points along branches generally result in the formation of branch stubs, and are referred to as heading cuts (Harris 1992). In pruning for utility line clearance, the pruning method which uses thinning cuts to remove interfering branches and redirect growth away from conductors is referred to as natural, directional, Lateral, or drop -crotch pruning. The use of heading cuts to obtain clearances and shape a tree is called topping, heading, roundover pruning, tipping, or stub -cutting. In this document, we use the terms natural pruning and topping, respectively, to refer to the two general pruning methods described above. Various arboricultural researchers and practitioners have advocated that natural pruning be used whenever possible when pruning for utility line clearance. It is known that in many species, topping results in the production of numerous fast- growing, weakly attached branches near the ends of the cut stubs. These can quickly grow into the conductors and are prone to breakage, increasing the chances of tree -related service interruptions. The branch stubs left after topping are also more likely to be invaded by decay fungi and wood -boring insects than are branches with properly -made thinning cuts. Wood decay originating from these stubs may adversely affect the structural stability and longevity of topped trees. Based on these observations, it has been assumed that the use of natural pruning for utility line clearance will result in increased pruning intervals, greater service reliability, and improved tree health compared to topping. However, our review of the literature turned up only two published studies which compare the use of natural pruning with topping for utility line clearance (Goodfellow and others 1987, Johnstone 1988), but neither study was able to draw strong conclusions. PG&E is required by law to maintain clearance between its high voltage power lines and tree branches. PG&E has recently instituted several changes in its distribution line clearance program which are intended to improve efficiency, cost- effectiveness, and service reliability. These modifications include: PHYTOSPH ERE RESEARCH 1 PG&E Line Clearance Efficiency Study Overview January 19, 1995 • a new administrative structure, • a change from time and materials contracts to unit cost contracts, • a change from grid -based to circuit -based pruning, • a new inspection and scheduling system, • a change from topping to natural pruning methods. Since a number of significant changes are being instituted simultaneously, tracking future cost reductions and efficiency gains will not provide information about the effects of the individual program changes. Although it has been assumed that the change to natural pruning will provide long term benefits, there are currently no data that clearly indicate what impact natural pruning has on utility line clearance costs. The proposed study will provide data to compare the economics of using topping versus natural pruning for distribution line clearance. STUDY OBJECTIVES AND APPROACH The overall purpose of this study is to compare two alternative methods of pruning to maintain distribution line clearance, namely topping and natural pruning. More specifically, the proposed study has been designed to accomplish two major objectives: Study objective 1. Determine whether trees that have been pruned for line clearance using the natural pruning method have a different repruning interval than trees that have been pruned using the topping method. Study objective 2. Determine whether long-term costs associated with the use of natural pruning for line clearance differ from costs associated with the use of topping. A third objective will also be addressed in this study: Study objective 3. Determine whether the two pruning methods have different effects on tree health or the incidence of branch failure over the short term. PHYTOSPHERE RESEARCH 2 PG&E Line Clearance Efficiency Study Overview January 19, 1995 EXPERIMENTAL DESIGN London plane (Platanus acerifolia) was selected as the tree species for study. It is a large-statured, relatively fast-growing and long-lived species. It is one of the most common urban trees in PG&E's service area and one of the most frequently -planted street trees in California. Only previously -topped trees will be included in the study. This is the most common pruning history in PG&E's service area. Only trees directly under power lines will be used in the study because top -trimmed trees are more common than side trimmed trees in urban areas. There is also some evidence that branch regrowth may be faster for top -trimmed trees than for side -trimmed trees. Both regrowth following pruning and the amount of time required to prune a tree can be affected by a wide range of factors. To reduce the number of confounding factors and the overall level of variation in the data, we have selected a paired experimental design. In our paired design, the basic experimental units are pairs of matched trees. Trees in each pair will be matched, to the degree possible, for factors that are likely to affect shoot growth in response to the pruning treatments. Pairing will also be used to minimize variation associated with factors that could affect the amount of time required to prune trees. In each pair of trees, one tree will be randomly assigned to each of the two pruning methods, topping and natural pruning. For the initial pruning, both trees in a pair will be pruned on the same day by the same crew. An auditor will be present on site to record tree and site data and the time associated with various pruning operations. Experimental pruning standards Topping should follow standard practices, using heading cuts with no special attempts to direct tree growth away from the conductors. Natural pruning should follow guidelines put forth in ANSI A300 (ANSI 1993), Shigo (1990), and Harris (1992). PHYTOSPH ERE RESEARCH 3 PG&E Line Clearance Efficiency Study Overview January 19, 1995 Standard Illustrations 1. Pruning cuts should be placed just beyond the branch collar or the branch bark Shigo (1990) ridge. ANSI A300 Harris 1992 2. Damage to the bark around pruning cuts should be minimized through the use of Shigo (1990) multiple cuts when removing large branches. ANSI A300 Harris 1992 3. Cuts should be made back to the trunk or to branches that are at least one third Shigo (1990) the diameter of the part which is being removed. 4. Direct branch growth away from the conductors through the removal of medium- Shigo (1990) sized branches, minimizing the pruning of small branches in the outer crown. Harris 1992 5. Strategic selection of pruning cut locations should be made to minimize total Shigo (1990) number of pruning cuts needed. 6. An entire branch should normally be removed if more than 50% of its live foliage is removed. 7. Avoid removing more than one-third of the live foliage from the entire tree in a single pruning. For the topping method, pruning should result in a clearance of about 10 feet from the conductors or about 4 feet from the clearance envelope. For natural pruning, the clearance distances will vary with tree form and branch pattern, but minimum branch -conductor clearances must be greater than 6 feet. EVALUATION Trees in the experiment will be observed at intervals to determine whether branches have grown near the 6 foot clearance envelope around the conductors. When repruning is required, each tree will be repruned using the same method as in the initial experimental pruning. Unless regrowth rates for a pair of trees are identical, it is unlikely that the two trees in a pair will be pruned on the same date for the second experimental pruning. Following the second experimental pruning, periodic observations will be continued until the tree requires a third pruning. It is not anticipated that observations would continue beyond the time that the third experimental pruning is required. Experimental trees will be returned to the normal pruning cycle after the final set of data is collected, so there will be only two experimental prunings of each tree. The study will run for 6 years. It is anticipated that 2 pruning cycles will occur in this time for most of the trees in the study. PHYTOSPH ERE RESEARCH 4 PG&E Line Clearance Efficiency Study Overview January 19, 1995 M LITERATURE CITED American National Standards Institute. 1993. Standard practices for trees, shrubs, and other woody plant maintenance - ANSI A300. Amherst, NH: National Arborist Association, Inc. Goodfellow, J., Blumreich, B., and Nowacki, G. 1987. Tree growth response to line clearance pruning. J. Arboriculture 13:196-200. Harris, R. W. 1992. Arboriculture: Integrated management of landscape trees, shrubs, and vines. Second edition. Englewood Cliffs, NJ: Prentice -Hall. Johnstone, R. A. 1988. Economics of utility lateral trimming. J. Arboriculture 14:74-77. Shigo, A. L. 1990. Pruning trees near electric utility lines. Durham, NH: Shigo and Trees, Associates. PHYTOSPH ERE RESEARCH 5