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35 <br />30 <br />25 <br />20 <br />L <br />QGJ 15 <br />10 <br />5 <br />0 <br />lr 4• <br />DBH Class (in) <br />Figure 3. Percent of tree population by diameter class (DBH -stem diameter at 4.5 feet) <br />Urban forests are composed of a mix of native and exotic tree species. Thus, urban forests often have a tree diversity <br />that is higher than surrounding native landscapes. Increased tree diversity can minimize the overall impact or <br />destruction by a species-specific insect or disease, but it can also pose a risk to native plants if some of the exotic <br />species are invasive plants that can potentially out -compete and displace native species. In City of Pleasanton, about <br />43 percent of the trees are species native to North America, while 29 percent are native to California. Species exotic <br />to North America make up 57 percent of the population. Most exotic tree species have an origin from Asia (27 <br />percent of the species). <br />50 <br />40 <br />a@ 30 <br />as 20 <br />IL <br />10 <br />0 <br />ire 13 _"3 ` [ fir€ �; 'D X <br />CP <br />a ` D <br />Origin <br />Figure 4. Percent of live tree population by area of native origin, City of Pleasanton <br />Page 5 <br />67 of 219 <br />