Before 1900, low-intensity forest fires occurred regularly in the Sierra Nevada, but because of the potentially harmful effects of larger fires to timber supply wildlife and humans,nearly all fires have been actively extinguished in the California mountain range this century. A UC Davis researcher recently analyzed data covering more than 7,000 square miles in the central and northern Sierra to see how fire suppression and logging may have affected tree sizes, densities and species composition.To do this, Jim Bouldin, a graduate student in plant biology,compared data collected in the 1930s with data collected in the 1990s, both covering the same geographic area. He found two- to fourfold increases in densities of trees with diameters less than 24 inches and accompanying density decreases in trees larger than 36 inches. He also found that the largest increases in small trees were in the more shade-tolerant, less fire-tolerant species such as incense cedar and white fir, while the decreases in larger trees were most obvious in the more economically valuable pines such as ponderosa and sugar. Before fire suppression began, the smaller firs and cedars would have been killed by light surface fires while the older trees with thick bark would tend to survive. Bouldin is exploring in more detail what the relative importance of logging, fire suppression and natural mortality may have in causing the present pattern, since, for example,an increase in density of smaller trees could have been caused by either logging or fire suppression. Bouldin presented his findings at the Ecological Society of America meetings in August.