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6.1 C.1 Page 8 Tree Stability Testinq Approximately ten years ago, staff developed a tree pulling test to determine the relative stability of trees growing along streets and in parks. The test was designed to apply a force to the tree from one direction and add to the force with pulsating increases causing the tree canopy to move similar to wind loads during storms. The force applied is not an attempt to pull the tree over or break major limbs from the tree. The force is applied to observe ground movement at the root plate. Staff evaluates the stability of the tree by observing and feeling the root plate movement. If a tree has been root pruned, staff observes the edge of the root plate adjacent to the severed root area for movement. Trees not previously root pruned are observed for any cracks in the soil or root plate movement. A secondary evaluation is made by staff standing on the soil plate and feeling for any movement. The root plate evaluations have provided observational data to make decisions to either remove some trees where movement was detected or retain trees where movement was not detected. There is no measurable data associated with this test process, only observations and staff experience to support the test findings. Trees left standing after this test have a 100% survival rate. Trees have been removed after the test determined instability. There is current research looking at different forces on trees and tree stability being conducted in Europe, Canada, and Australia. The quantitative data is still being evaluated and is specific for each test situation. Staff has contacted three different researchers in order to develop protocols to measure the pull force and any movement quantitatively. This would provide data for making a better decision about a tree with questionable stability, whether a result of root pruning or other factors. The different researchers are measuring both trunk strength and root strength. The measuring devices for the pull forces are similar. However, the measuring devices for the tree movement are different. The strength of the root plate is the primary concern in Redwood City soils, so the need to include any incremental movement at the base of the tree or root plate is key. Instruments which measure stretch or bend along the trunk or limbs do not provide root plate strength data. Staff contacted researchers to determine if a more quantitative measure of root plate stability can be obtained. Advice was received and a root pruned tree scheduled for removal was selected to apply measuring devices to capture data. In December 2005, staff obtained recommended measuring equipment for force pull and directional movement to initiate the testing process. The process combined the previous testing procedure with the new measuring devices. Rope and a vehicle were used for pulling. The measuring device for incremental Page 1 of 2 ~. .--.- .... ._. .__ ..... ___ ._L__.._._' ..... "_,--,,,__,~,~_,.,_,.,,___,_,_','___'_4