A
California has lost half its big trees since the 1930s, according to a study to be published Tuesday and climate change seems to be a major factor（因素）.
The number of trees larger than two feet across has declined by 50 percent on more than 46, 000 square miles of California forests, the new study finds. No area was spared or unaffected, from the foggy northern coast to the Sierra Nevada Mountains to the San Gabriels above Los Angeles. In the Sierra high country, the number of big trees has fallen by more than 55 percent; in parts of southern California the decline was nearly 75 percent.
Many factors contributed to the decline, said Patrick Mclntyre, an ecologist who was the lead author of the study. Woodcutters targeted big trees. Housing development pushed into the woods. Aggressive wildfire control has left California forests crowded with small trees that compete with big trees for resources（资源）.
But in comparing a study of California forests done in the 1920s and 1930s with another one between 2001 and 2010, Mclntyre and his colleagues documented a widespread death of big trees that was evident even in wildlands protected from woodcutting or development.
The loss of big trees was greatest in areas where trees had suffered the greatest water shortage. The researchers figured out water stress with a computer model that calculated how much water trees were getting in comparison with how much they needed, taking into account such things as rainfall, air temperature, dampness of soil, and the timing of snowmelt（融雪）.
Since the 1930s, Mclntyre said, the biggest factors driving up water stress in the state have been rising temperatures, which cause trees to lose more water to the air, and earlier snowmelt, which reduces the water supply available to trees during the dry season.
21．What is the second paragraph mainly about?
A．The seriousness of big-tree loss in California.
B．The increasing variety of California big trees.
C．The distribution of big trees in California forests.
D．The influence of farming on big trees in California.
22．Which of the following is well-intentioned（出于好心的） but may be bad for big trees?
A．Ecological studies of forests. B．Banning woodcutting.
C．Limiting housing development. D．Fire control measures.
23．What is a major cause of the water shortage according to Mclntyre?
A．Not enough snowmelt. B．A longer dry season.
C．A warmer climate. D．Dampness of the air.
