Large Trees Decline in Yosemite; Climate Change is a Likely Culprit
Scientists from the United States Geological Survey and the University of Washington compared records of tree density and diameter from the period of 1932-1936 against records collected between 1988-1999. The results were startling: since the 1930s there has been a 24 percent decline in large-diamter trees. This could have very serious long-term impacts. Large-diameter trees are critical habitat for mosses, spotted owls, and orchids, among other species. These trees generally produce more seeds and are more resistant to fires, making regeneration after a fire and forest growth much slower.
The study did not specifically look at the causes of this decline, but climate change is a likely suspect. According to USGS scientist Jan van Wagtendonk, the warming trend has increased the length of the dry season as well as reduced the winter snowpack. This snowpack provides a good portion of the water that is necessary for the growing season. The prolonged, dry summers don't help the situation and can make the trees more prone to pathogens and insects.