摘要: The Upper Sonoran Desert and Desert Grasslands covers over 10 million acres in southern Arizona. The environment is erratic with daily temperatures reaching over 100 degrees F in the summer and occasional freezing in the winter. P... 展开 The Upper Sonoran Desert and Desert Grasslands covers over 10 million acres in southern Arizona. The environment is erratic with daily temperatures reaching over 100 degrees F in the summer and occasional freezing in the winter. Precipitation ranges from ten to about sixteen inches annually, depending on elevation. Winter and summer rainy seasons are usually well defined, and prolonged droughts are common. The harshness of this climate poses special problems for native plants and they have had to develop various strategies to survive. Woody plants are xerophytic, having various means of protection against loss of water by excessive transpiration. Strategies that work in some years fail in others, although significant shifts in species composition usually occurred over long periods. These strategies also affect the success of various herbicide treatments. Several species have few leaves and rely on photosynthesis in stems; thus, foliar applied herbicides have not proven effective. 收起