Forest practitioners worry about the inevitable impacts of anthropogenic climate change on trees and other forest denizens. Pests and disease are other ongoing concerns, as are invasive plants. The direct consequences of climate change are difficult enough to project, but ensuing effects on interactions between species are the greater wildcard. It is double jeopardy when climate warming favours exotic shrubs over native species, especially trees, as reported in this issue of New Phytologist by Polgar et al. (pp. 106–115). By comparing modern observations of leaf-out phenology in Concord,Massachusetts (USA),with recordskeptby theeighteenth century philosopher and naturalist, Henry David Thoreau, the authors show that woody plants, and particularly the invasive ones, are now breaking bud and flushing leaves some 18 d earlier than in Thoreau’s time. This is partly a response to global warming but has more to do, so far, with the heat island effect in the Boston area.
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