Evolution in the smallest valves (stomata) guides even the biggest trees

The regal stature of the redwood sister-giants, Sequoia and Sequoiadendron, makes them internationally renowned icons of size and longevity. Yet an examination of the evolutionary history of the sequoiid clade (which includes the dawn redwood— Metasequoia) quickly dispels the impression that these trees are an immutable part of the landscape. Fossil data show that these species are relicts of a lineage that once extended North to the Arctic Circle and as far south as Australia ( Hill and Brodribb 1999, Ahuja and Neale 2002), and like many other basal lineages of the Cupressaceae, the three extant sequoiid genera have been subject to systematic extinction that has left only monotypic representatives alive today. Similar to other basal Cupressaceae, Sequoia and Sequoiadendron are confined to humid forest or fog belts, a habitat that appears to be very vulnerable to climate warming and associated drought ( Pounds et al. 1999). This is an important context because an extreme drought event is currently occurring across the entire range of both species, raising pertinent questions about how effectively these species are able to manage water stress. Of particular concern is whether this current drought represents a threat to the survival of trees. Answering such a question demands quantitative knowledge of what constitutes a damaging level of water stress in these species. A study by Ambrose et al. (2015) in this issue of Tree Physiology provides some answers to this critical question, while at the same time providing new insights into the surprisingly different ways these sister-species respond to water stress.