Global warming will […]  lengthen the growing seasons and intensify water cycling—that is, the overall amount of precipitation will increase—in many regions. This combination will result in higher plant productivity, a trend that was already evident throughout most of the United States during the latter half of the twentieth century. But what the long-term effect of such changes will be is not clear. Will the additional productivity be promptly negated by higher rates of respiration in a warmer world? Will most of its increment be stored in long-lived tissues, such as trunks and major roots, or tissues with rapid turnover, such as foliage and fine roots? And, most fundamentally, will global warming eventually convert forests from carbon sinks to carbon sources?