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132 | CITY OF PLEASANTON URBAN FOREST MASTER PLAN <br />STATUS OF THE URBAN FOREST PUBLIC AND PRIVATE TREES <br />2.6 DSH Distribution <br />The most common and least invasive method to approximate <br />the age of a living tree is to measure the trunk diameter <br />at 4.5 feet above the ground (diameter at standard height <br />[DSH]). Since trees vary in size and growth patterns, using <br />DSH to determine age can only be considered an estimate. <br />General age recommendations suggest an urban forest have <br />a distribution of immature trees (40%) to replace failing or <br />aging ones, young (30%) and middle-aged (20%) trees to <br />provide the bulk of economic and environmental benefits, <br />and relatively fewer mature trees (10%) that have most of <br />their life behind them but provided significant environmental <br />benefits for many years (Morgenroth et al. 2020; Richards <br />1983). <br />Table 2-9 shows the DSH distribution of all trees in the <br />2024 City inventory compared to the recommended DSH <br />distributions. The age classes of the City’s trees are not <br />substantially different from the recommended distributions <br />discussed above. There is a lower than recommended <br />percentage (12%) of middle-aged trees, but the City’s <br />population of immature (28%) and young (51%) trees are <br />anticipated to adequately replace mature trees as they <br />reach the end of their life. The current distribution, with a <br />higher proportion of young and immature trees, suggests <br />that the City is well-positioned to sustain its urban canopy <br />over time. However, the low percentage of middle-aged <br />trees could lead to a temporary gap in ecosystem services. <br />This gap occurs as mature trees decline and are removed <br />before younger trees can fully replace their canopy and <br />environmental benefits. Middle-aged trees are crucial as <br />successors to mature trees, ensuring a steady transition and <br />continuity in providing benefits like air quality improvement <br />and carbon sequestration. <br />Recommendations: <br /> ƒDevelop a detailed tree planting succession plan that <br />identifies areas with a high concentration of mature trees <br />and schedules the planting of younger trees nearby. This <br />ensures that as mature trees decline, there are already <br />younger trees in place to take over. <br /> ƒDevelop long-term planting plans aimed at maintaining the <br />age diversity recommendations shown in Table 2-9.