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HomeMy WebLinkAboutFinal- McConaghay Arborist Rpt. 12-21 1343 Bishop Street, San Luis Obispo, CA 93401 Tel: 805 234 8760 Email: rodney@heritagetreeconsulting.com December 16, 2021 To: Richard McConaghay – 234 Broad Street, San Luis Obispo, CA 93405 From: Rodney Thurman – Heritage Tree Arboricultural Consulting Re: Tree Health and Risk Assessment Report, 234 Broad Street, San Luis Obispo Mr. McConaghay, The following report is in response to your request for a tree health and risk assessment report regarding one (1) California Sycamore (Platanus racemosa) growing in the backyard of your property located at 234 Broad Street. Assignment: • Perform health and structural integrity inspection of 1 tree. • Perform pruning assessment. • Prepare written arborist report addressing tree health, structural integrity and pruning as well as residual risk of tree failure for a period of one year from the date of this report. Introduction: My inspection was conducted October 5, 2021. There was 1 mature California Sycamore growing along a seasonal creek on the northern side of your back yard at 234 Broad Street. Out of concern for your safety and in an effort to protect your home, you had 2 large *scaffold branches reduced to mitigate possible failures. The City of San Luis Obispo has asserted that the pruning was excessive and has compromised the health and longevity of the tree. You have hired Heritage Tree Arboricultural Consulting to perform a tree health and pruning assessment as well as a tree risk assessment to determine if your actions were appropriate and warranted. Additionally, you have asked me to assess what residual risk of failure remains. Summary: Based on structural defects found in scaffold branch unions, the reduction pruning performed on this tree was appropriate. Based on its current structure and health, the tree can remain in the landscape. It will be important to maintain the canopy in a reduced form in order to keep tree risk levels Low. Yearly health and risk inspections should be completed to ensure the tree is still viable and structurally sound enough to be retained. *See Appendix B - Glossary Terms 2 Observations Tree Observations The tree, at the time of my inspection, was approximately 25’ tall. Its diameter at standard height (DSH) was 49”. DSH is measured at 4.5’ above ground. The new growth on the tree was healthy and vigorous with no visible signs of insects or disease. There were numerous regrowth shoots which were in response to the recent loss of canopy. The exterior of the trunk was sound with no outward signs of decay. See Appendix A – Photos 2,5,6,8&9. I was able to access the tree with a ladder so I could inspect the union point of the branch scaffolds and trunk. At the union, there was a decay cavity with an opening 2’8” wide. The decay area inside the tree was much greater than the size of the opening. I was able to probe the cavity vertically with a pole to a depth of 8’ 7”. See Appendix A – Photo 7 The roots were intact and healthy with no visible defects or recent disturbances. The area around the tree was covered in non-permeable flagstone set in concrete. You indicated to me that the pavers had been set before you purchased the home 20 years ago. See Appendix A – Photos 5&6. You had the tree pruned in the summer of 2021 to reduce end-weight on large scaffold branches. Comparing photos from Google street view to the current height of the tree, I estimate the reduction was approximately 75%. See photos below. See Appendix A – Photos 1 and 2 for expanded view. Dec 2020 Google image of tree Oct 2021- Tree post trim with regrowth Target Observations When assessing tree risk, it is important to identify potential targets a tree or part of tree could strike if it were to fail. Without a target, there is no risk to assess. Targets may include, people, buildings, cars, fences, utility lines or any other property of value that could be injured or damaged. 3 In the case of this tree, the major targets were the home and associated out-buildings, people and electrical/communication lines. all targets were within striking distance of the tree before it was trimmed. The home, and utility lines had and occupancy rate of 100% since they are stationary. People living in the house had an occupancy rate of at least 75%. See Appendix A – Photos 1,3,4&6 which illustrate targets in proximity to the tree. Structural Integrity Assessment (post tree reduction) When I assessed the tree for structural integrity, I began my inspection at the roots and moved up the trunk to the scaffold branches and regrowth canopy. • Roots The roots although surrounded by stone pavers set in concrete, looked healthy and intact. I did not observe any decay or structural damage that gave me concern that the roots were compromised or were in danger of uprooting under normal conditions. See Appendix A – Photo 5. • Lower Trunk The exterior of the trunk had no signs of outward decay. When I sounded the trunk with a rubber mallet, I did not hear any echoing returns that would indicate a thinning outer shell. See Appendix A – Photos 5 & 6. • Upper Trunk at Scaffold Branch Union At the transition of the trunk to the two main scaffold branches, I found an open decay area in the trunk between the scaffold attachments. The opening was 2’8” at its widest. The actual decayed area opened up into a much larger cavity inside the tree. See Appendix A – Photo 7. When I probed the decay area vertically with a metal rod, I was able to insert it 8’7” deep. The decay area narrowed as I probed deeper into the tree, however the density of the internal wood fibers in the area were still soft. Although it may seem contradictory that I did not hear any hollow areas when I sounded the lower trunk, the reality is that the trunk has a 49” diameter and the outer wood can still be sound while inner wood is rotten. The main concern with the decay cavity, was the more extensive rot at the scaffold branch attachments. The weight of the scaffold branches before reduction pruning would have been in the range of 5 tons each. I make this assertion based on the estimation that a cubic foot of live, healthy wood from a hardwood tree is approximately 50 pounds. The scaffold branches were approximately 30” diameter at their widest. 4 A cross section of a 1’x30” diameter branch is 4.9 cubic feet. With that in mind, each linear foot of the branch would weigh 245 pounds. Given that the scaffolds would have extended approximately 40 feet above the decay area, the weight of a single branch would have weighed as much as 9,800 pounds or 4.9 tons (E.g., 245x40=9,800). With the weight of the scaffold branches in mind and the extensive decay at their attachments, they would have been susceptible to failure at the trunk attachments. Pruning Assessment Considering what we know in terms of structure of the tree at the scaffold branch attachments, the reduction pruning was warranted and appropriate. Although it looks extreme and not what I would recommend for a healthy tree, in this case it was the best way to prevent large, scaffold branch failures. In assessing the Google street view photos provided in Appendix A photos 1, 3 &4, I noted that there were no lateral branches on the lower half of the scaffolds that would have been appropriate to prune to. Typically, pruning cuts should be made at a branch union so the pruning wound will seal over faster. Pruning to branch unions also helps maintain a tree’s natural form. Due the need to substantially reduce the weight of the scaffolds to lessen risk of failure into the home, associated out buildings and utility lines, pruning cuts were made at approximately 20 feet above the ground. Tree Risk Tree risk is determined by identifying the targets e.g. (people, buildings, cars or property) that could be injured or damaged by a tree failure, the occupancy rate, exposure of the target to the tree hazard, the size of part of the tree that may impact the target if it were to fail, the likelihood of impact on the target, and the consequences of failure on the target. The part of the tree most likely to fail and cause the largest impact is used to determine the highest overall risk for the tree. When considering likelihood of failure, the definitions are as follows: • Imminent – Failure has started or most likely to occur in the near future. Even if there is no significant wind or increased load. The imminent category overrides the time frame stated in the scope of work. • Probable – Failure may be expected under normal weather conditions within the specified time frame. • Possible – Failure may be expected in extreme weather conditions, but is unlikely during normal weather conditions during a specified time frame. • Improbable – The tree or part is not likely to fail during normal weather conditions and may not fail in extreme weather conditions within the specified time frame. I did not perform a tree risk assessment on this tree before it was reduced but I can make some assumptions based on photos of the tree before trimming and my structural integrity inspection post trim. I used the tree risk matrices created by the International Society of Arboriculture (ISA) to determine what would have likely been the risk of scaffold branches failing into targets. See explanation of matrix and matrix table on the following page. 5 For Matrix 1, the Likelihood Matrix, the likelihood of failure for a part of the tree must be determined. For this tree, the likelihood of failure before reduction, highlighted in yellow, was selected as “Probable”. The likelihood of impact on a target, i.e., home, people or utilities, also highlighted in yellow, was selected as “High”, therefore the Likelihood of Failure and Impact, highlighted in red, was “Likely”. The result of “Likely” from Matrix 1 was then plugged into Matrix 2, The Risk Rating Matrix. Then the Consequences of Failure were determined. In this case, the consequences of failure, highlighted in yellow, were “Severe”. The result of the two selections in Matrix 2 was “High” which determined the risk of the part of tree being assessed. Mitigating Risk: Once a risk level has been determined for a tree or tree part, a determination whether the tree risk can be reduced through mitigation is needed. Mitigation can be made by using one or more of the following: moving the target, reduction pruning or other pruning prescriptions, installing cabling, bracing or support structures. If the mitigation action(s) outweigh the benefit or do not reduce the risk of failure, removal of the tree may be necessary. To my knowledge, a tree risk evaluation was not performed prior to the tree being reduced. Nevertheless, a determination of perceived risk was made by the tree trimming contractor, and reduction cuts were carried out. It is now my job to determine what residual risk of tree failure remains. In the next section I have included an updated tree risk assessment including risk mitigations and recommendations. Tree Risk Assessment (post tree reduction) Targets • Home at 234 Broad Street and associated buildings • People in home • Electrical and communication utility lines 6 Risk The parts of this tree being considered for risk rating are the remaining sections of the scaffold branches that are attached to the decay cavity in the upper trunk. The following matrices charts have been highlighted to show how risk was determined for the parts. I used Matrix 1 the “Likelihood Matrix” on the form and entered the result into Matrix 2, the “Risk Rating Matrix” to get the overall risk rating. Part – Scaffold Branches In Matrix 1 “The Likelihood Matrix”, I selected the Likelihood of Failure for scaffold branches as Improbable due to the substantial reduction of length. I selected the Likelihood of Impact as Very Low based on the reduced likelihood of failure and the fact that reduced length of scaffold branches would greatly reduce impact to the home and completely eliminate impact on utility lines. The likelihood of impact to people would also be greatly reduced, due to protection provided by the home and the intermittent occupancy of the patio area. The combined Likelihood of Failure and Impact from Matrix 1 was Unlikely. In Matrix 2, the “Risk Rating Matrix”, I entered the result of Unlikely from Matrix 1 into Likelihood of Failure and Impact section of the chart. In the Consequences of Failure section of Matrix 2, I selected Significant, because in the unlikely event that the remaining scaffold(s) were to fail and strike a person, significant harm would be incurred. The resulting risk was Low, which determines the overall risk rating for the tree or part of the tree being assessed. 7 Mitigation and Pruning Recommendations With an overall residual risk rating of Low, it is unlikely that the remaining parts of the scaffold branches will fail within the next year. Further reduction pruning or tree removal is not necessary at this time. It will be important to have follow-up tree risk inspections performed on a yearly basis in order to monitor the decay in the trunk of the tree. Vigorous, resprout branches will need to be managed. The new growth is in response to having a substantial part of the tree removed. The new branches should not be left unchecked, because as they regrow, they will begin to exert weight on the scaffold branches once again. There are two ways to manage the branches. One technique is to remove all resprout growth back to the scaffold branch on an annual basis. This technique is called pollarding. The location of the removed sprout will callous over, and new sprouts will emerge from the same location to create a new canopy the following year. It is important to only remove the sprout and not cut into the wood of the parent branch and cause more damage. Through pollarding the weight being exerted on the scaffold branches will be kept to a minimum. The negative aspect of this technique is that it puts additional stress on the tree because it needs to expend more energy to grow a new canopy each year. The second technique, and the one I prefer, is to do a modified crown restoration. This technique focuses on removing competing resprout branches and keeping approximately 25% of the branches that are strongly attached to the parent branch. The retained resprout branches should be growing in a direction that will form a structurally sound and aesthetically pleasing canopy. The branches that are chosen to remain will also need to be reduced 50% to 75% to ensure they do not grow too tall or become too heavy. All pruning cuts shall be at a bud or branch union. Reducing the branches will cause additional secondary branches to emerge in subsequent years. These secondary branches will also need to be reduced and thinned. The idea behind this technique is to create a stout, more compact canopy that will not exert undue pressure on the scaffold branches. Conclusion: There was a high risk to targets if the tree was kept at the height it once was. Reduction pruning that was performed, although extensive, was appropriate to reduce the risk of scaffold branch failure. The sound structure and vigor of the roots should provide support and nutrients which will allow the tree to continue to grow and provide the benefits of shade, carbon sequestration and erosion control. If recommended annual maintenance and or restoration pruning is performed on an annual basis, the stress on the scaffold branches should be minimal. Going forward it will be important to have follow-up tree risk and health inspections performed on a yearly basis in order to assess the health and structural integrity of the tree. 8 Respectfully Submitted, Rodney Thurman ISA Board Certified Master Arborist PN-2684BUM ISA Municipal Specialist IA Utility Arborist ISA Tree Risk Assessment Qualification Appendices: photographs, glossary of terms 9 Appendix A - Photographs Photo 1 – Google image of home and tree - December 2020. Note overextended scaffolds. overextended scaffold branches 10 Photo 2 – View of reduced tree – Photo by R. Thurman - October 5, 2021 11 Photo 3 – Close up of leaning scaffold in proximity to the home and utility lines. Google image- December 2020. 12 Photo 4 – View from street in front of 261 Broad. Utility and communications lines were potential targets if scaffold branches were to fail. Google image - December 2020. communication lines high voltage lines 13 Photo 5 – View of trunk with flagstone pavers around base. Photo by R. Thurman – October 5, 2021 14 Photo 6 – View of tree from north. Home is at left and rear of tree, art studio on right at right of tree. Photo by R. Thurman - October 5, 2021 15 Photo 7 – View of decay cavity in trunk between scaffold branches. Opening is 2’8”at widest point. Photo by R. Thurman - October 5, 2021 16 Photo 8 – View of tree and regrowth from east side. Photo by R. Thurman - October 5, 2021 17 Photo 9 – View of tree and regrowth from north side. Photo by R. Thurman - October 5, 2021 18 Photo 10 – Close up view of regrowth sprouts. Photo by R. Thurman - October 5, 2021 Appendix B - Glossary Terms • Scaffold Branches - Permanent or structural branches that form the scaffold architecture or structure of a tree.