Recommendation to remove the large valley oak at the Walter Hayes Elementary School, for safety reasons

Transcription

1 Chuck McDonnell Palo Alto Unified School District, 85 Churchill Ave, Palo Alto, CA March 6, 2012 (Revised from August 23, 2011) Recommendation to remove the large valley oak at the Walter Hayes Elementary School, for safety reasons The following met at Walter Hayes School yesterday to review and discuss this tree one last time in order to come to a group agreement to either save or remove this tree: Chuck McDonnell (PAUSD Facilities Manager, Dave Dockter (City of Palo Alto Planning Department Arborist), Bill Reulman, Jr. and Sr. (Bill s Tree Care & Landscape, PAUSD s main tree service contracting company), Rafael Ruiz (City of Palo Alto Transportation Engineer), Brian Truong (PAUSD Project Manager) and Urban Cummings (Canopy Board Member). After an hour of evaluating and discussing this tree, we all came to the decision that the oak really should be removed it is simply too great a danger and a liability for the District in its present condition and high traffic location. Page 1 of 9

2 Instead of removing the tree we seriously considered the option of Veteran Tree Management, that is, a drastic reduction of the canopy and overall size of the tree, in order to make it safe enough in our opinion, to remain. This would require shortening the scaffold branches of the tree so that they were merely stubs, and not really branches any more. With luck the oak might have enough stored energy to produce some epicormic growth to produce foliage (and thus food for the tree) and keep itself alive for a few to several more years. This drastic pruning would impose great stress on the tree, which would accelerate the rate of the already-present decay within the trunk and at least some of the scaffold branches. If the large oak were removed however, this would open up a relatively large area for new tree planting, without the risk of having recently planted trees damaged when the large valley oak is removed. As I suggested in my previous report, I felt that California black oak, Quercus kelloggii, would be a good tree to plant in the vicinity of the old oak. I originally recommended that one new tree be planted, but Dave Dockter, Chuck McDonnell and I decided that three black oaks planted to grow as a small grove would be a better choice. California black oak is native to California as well as the Palo Alto vicinity, and so it should fare well and will not require irrigation after a two to three year establishment period after planting. This is a deciduous tree species (it loses its leaves in the Fall of the year) and it provides very nice yellow Fall foliage color. Deciduous trees planted in the location of the previous valley oak will allow more sun into the building directly behind in the winter while providing shade during the summer. I have included black oak photos, and also a sketch of where the new trees will be planted, on page 9. The photo on the previous page was taken on August 22, 2011 when I first evaluated the tree and produced my original report dated August 23, In this first report I recommended removal of the tree for safety reasons. My business partner Tim Ellis performed a tomographic scan 1 of the tree on November 16, 2011, not as a diagnostic test but because I was under the impression that the tree was going to be removed. We frequently scan trees that will be removed if we can see them cut apart at the level of our scan, which furthers our knowledge and skill in tree tomography, which is a relatively new diagnostic technique. We have scanned several District tree prior to their removal. The scan of the Walter Hayes oak was performed at no charge to the District and for educational purposes only. The results of this scan are explained on pages 6-8, but it is important to understand that I was confident in recommending removal of this tree without scanning it beforehand. 1 Tomographic scanning (of trees): Tomography is a method of imaging the interior of an object by sending invisible waves (sound, magnetic, x-ray, etc.) through an object. The changes that the waves experience as they pass through the object provide data, which through mathematical algorithms is translated into a two-dimensional color-coded image. Interpretation of the image provides information on the mechanical and sometimes chemical properties of the interior of the object. A common type of tomography used for medical imaging is the MRI scan, which uses magnetic waves. For trees, sonic (sound wave) and electric resistance tomography is used to investigate the interior of tree trunks and large branches. Sonic tomography measures wood density and electric resistance tomography measures hydration and other chemical properties of the wood. Sonic and electric resistance tomography provides more and higher quality information than either method used alone. Page 2 of 9

3 The following section of this report is repeated from my August 23, 2011 report: We met on site last week to look at and discuss this tree. Unfortunately the old oak should be removed. There is no doubt in my mind about this. The tree presents a significant hazard because of extensive trunk decay and the targets in the vicinity that will be struck when the tree falls apart the very busy Middlefield Road to the south and the school classroom building to the North. The tree is heavily infected with a wood decay fungus, Ganoderma applanatum. The shelf-like protrusions emerging from the trunk are the conks (fruiting bodies) of this fungus that release spores. I am pointing to one of these conks in the lower left photo. The presence of these conks (and also their number and size) indicates that there is extensive internal decay within the trunk. In two upper photos I have drawn arrows that point to a crack in the bark where the wood below is dead and decayed. The fungus and its decay extend beyond this area, however. Page 3 of 9

4 There is no method to remove the fungus from the tree. The fungus is like a network of cancerous threads permeating the wood in the tree. The conks are reproductive structures like flowers; they are just the tip of the iceberg and an indicator of extensive decay within the tree. Removal of the conks will not affect the fungus within the tree that is causing the decay. In addition to decay in the trunk, there is also a fair amount of unrelated decay from old pruning wounds throughout the canopy including in large scaffold branches. Ganoderma applanatum requires a wound in order to infect. This oak has had a lot of pruning in the past; much of that pruning has been detrimental and has left large wounds that are prone to decay. This improper pruning happened long ago Palo Alto Unified School District takes much better care of their trees now. In fact, some of this pruning may have occurred before Walter Hayes School was even built. Middlefield Road has been in Palo Alto for a long time, and the land on which the tree is located has probably gone through many different owners and uses prior to the school being built. In the photos above, arrows indicate old pruning wounds where large branches were removed. There is visible decay here. It is likely that the wood decay fungus gained entry through these wounds, as there are conks that emerge just below the wounds and continue down the trunk on that side of the trunk. Page 4 of 9

5 Tree Data: Species: Quercus lobata Common Name: valley oak Trunk diameter: 54.7 inches at 4.5 feet above the ground Tree size (height x canopy spread): 68 feet tall (measured with laser hypsometer) by approximately 80 feet wide North to South over Middlefield Road and the school property. Condition 2 : Vigor: 40 Structure: 20 Estimated Risk Rating 3 : 10 (Severe) Preservation Suitability: Poor Notes: The canopy of this tree extends almost all the way across the width of Middlefield Road to the North to about 20 feet over the school building to the South. Canopy density is only about 10% of normal, so the tree is definitely declining from a vigor standpoint in addition to having significant structural problems. I counted 13 Ganoderma conks on the trunk ranging from 1 inch in length and width to over 6 inches wide. There is only 6 inches between the exposed portion of a buttress root and the sidewalk bordering Middlefield Road, so the road was built quite close to the tree trunk! This oak has existed for a long time in its present location. I imagine that a lot has gone on and changed around the tree since it first germinated from an acorn. The age of this tree is probably somewhere between years old. We may be able to provide a more accurate estimate by looking at the stump before it is grinded. In fact, it might be a nice idea to keep a slab of the stump close to the ground as an educational opportunity for the students of Walter Hayes School. Cut a slab about 6 inches thick, cover it lightly with a plastic tarp and keep it in the shade. Horses and buggies through the electric and hybrid cars of today have passed by this tree. The tree has put up with a lot of abuse in the past, starting with a road built within inches of its trunk and probably several phases of development and redevelopment prior to the current school. It has endured and survived. But it is nearing its life s end now, and it is important from a safety and liability perspective that we remove this oak before it falls apart and causes serious injury or property damage. 2 Condition & Preservation Suitability Ratings: Trees are rated on their condition on a scale of zero to 100 with zero being a dead tree and 100 being the most perfect tree imaginable (which rarely exists like a supermodel in human terms). There are two components to tree condition vigor and structure, and they are each rated separately. Averaging the components would not be useful because a very low rating for either component could be a good reason to remove a tree from a site -- even if the other component has a high rating. Numerically speaking, 100 is Excellent (an A academic grade), 80 is Good (B), 60 is Fair (C), 40 is Poor (D) and 20 is unacceptable (F). Condition of the tree is considered relative to the tree species and present or future use of the site to obtain the tree s Preservation Suitability Rating (i.e. Is this tree worth keeping on this site, in this location? ). Preservation suitability ratings are: None, Poor, Fair, Good and Excellent. Fair/Poor and Fair/Good are intermediate ratings. 3 The Risk Rating for the tree is estimated at the time of inspection for the existing site, on a scale of 3 to 12 with 3 equivalent to Low and 12 equivalent to Severe. The Failure Potential, Size of Part and Target Rating are separate sub-factors that are evaluated and rated separately on a scale of 1 (Low) to 4 (High). The tree s risk rating is the sum of the sub-factors. For the subject oak tree the Failure Potential was rated as High (3), Size of Part was rated as >30 inches (4), and Target Rating was Frequent Use (3) for a total of 10 points. Forms and methodology for the rating system are taken from the industry standard text, A Photographic Guide to the Evaluation of Hazard Trees in Urban Areas, 2nd Ed. HortScience. International Society of Arboriculture Page 5 of 9

6 Tomographic Scanning 4 of the tree done on November 16, 2011 We scanned the tree at one level that was within easy reach of the ground and our equipment, and in the vicinity of some fungal conks that were protruding from the trunk. The blue line on the tree below shows the scan level relative to the emergence of multiple large scaffold branches. It is from a failure or pruning wound in this area (arrow) that the decay probably started, and it has been moving down the trunk ever since. 4 For a brief background on tree tomography, please see the Advanced Structural Diagnostics page of my business web site at: Page 6 of 9

7 The scan images (tomograms) Sonic Electric Resistance (4) (3) (4) (3) (3) (2) (4) (1) The sonic tomogram shows relative density. From most to least dense the colors are: black, brown, beige, green, purple, blue and white. The electric resistance tomogram shows electric conductivity/resistance. The colors depicting high to low resistance are: red, orange, yellow, light blue and dark blue. A healthy, sound valley oak will have a mainly blue perimeter with a red interior. I have rotated the scan images so that they match the orientation of the photo below, showing the sonic sensors placed on the tree. Arrows in the scans and the photo point to sonic sensor location #13 (which becomes electric resistance sensor #26 because the number of sonic sensors is doubled for electric resistance scans). Note that sonic sensors #12 and 13 are on either side of a large conk. Annotations in scan images: (1) Dry, dead decayed wood. (2) Probably dead wood but not decayed (or not decayed much) yet. But will decay. (3) Probably early decay (4) OK Page 7 of 9

8 Summary of Scan Images taken in context with the exterior appearance of the tree, and signs and the signs and symptoms of decay that are present: At the level of the scan the tree s structure and load bearing capacity is not compromised much yet. But the area scanned is the lower portion of the total affected (decayed or hollow) area because the decay originates from above. Since the decay originated above, the decay above the scanned area is older and has had longer to develop than the decay at the level of the scan therefore it likely affects a much greater proportion of the interior of the tree. It is at this area; where multiple attachments of large scaffold branches originate that the decay poses the greatest danger. When the decay advances to the point that the branches can no longer be supported, some or all of these large branches will break out of the tree either onto the building or over Middlefield Road. Replanting Photos and Illustrations: Quercus kelloggii, California black oak Left photo: foliage turning to Fall yellow color Right photo: mature black oaks in Yosemite Valley Page 8 of 9

9 (1) (2) (3) Replanting sketch three 36-inch box California black oaks. Oaks #1 and 3 are planted back more toward the classroom building. Oak #2 is planted closer to the street the edge of the trunk is 8 to 10 feet from the school-side of the sidewalk. ************************************************** I hope this information has been helpful to you. Please give me a call or send me an if you have any questions or concerns or need additional assistance. Sincerely, Debbie. ASCA Registered Consulting Arborist #305 ISA Board-Certified Master Arborist WC-457-B ASHS Certified Professional Horticulturist #30022 Page 9 of 9