Threats to xylem hydraulic function of trees under new climate normal' conditions

Climate models predict increases in frequency and intensity of extreme environmental conditions, such as changes to minimum and maximum temperatures, duration of drought periods, intensity of rainfall/snowfall events and wind strength. These local extremes, rather than average climatic conditions, are closely linked to woody plant survival, as trees cope with such events over long lifespans. While the xylem provides trees with structural strength and is considered the most robust part of a tree's structure, it is also the most physiologically vulnerable as tree survival depends on its ability to sustain water supply to the tree crown under variable environmental conditions. Many structural, functional and biological tree properties evolved to protect xylem from loss of transport function because of embolism or to restore xylem transport capacity following embolism formation. How the new climate normal' conditions will affect these evolved strategies is yet to be seen. Our understanding of xylem physiology and current conceptual models describing embolism formation and plant recovery from water stress, however, can provide insight into near-future challenges that woody plants will face. In addition, knowledge of species-specific properties of xylem function may help guide mitigation of climate change impacts on woody plants in natural and agricultural tree communities. Many structural and functional tree properties evolved to protect xylem from embolism and loss of transport function or to restore xylem transport capacity following embolism formation. Challenges to transport systems occur during events of extreme environmental conditions and these are predicted to increase in frequency and intensity under the new climate normal' conditions. We review here the current state of knowledge describing principles of embolism formation and plant recovery from water stress to provide insight into future challenges that woody plants will face and guidance to research directions aimed at mitigation of climate change impacts on woody plants in natural and agricultural tree communities.