As Earth’s climate warms, the shrubs of alder, birch, and willow that grow on arctic tundras are becoming taller and more numerous. Although the trend yields demonstrably more food for reindeer, its other ecological implications, good and bad, are less clear. Now, a large collaboration led by Mark Olson of the National Autonomous University of Mexico has identified a potential threat: Taller plants are more vulnerable to droughts, freezes, and other climatic shocks.
As their maximum height increases, plants in regions that become warmer and wetter develop wider vessels to transport water from their roots to their leaves. Such an adaptation is shaped by the physics of capillary hydraulics. Wider vessels can harvest more water more efficiently. But the wider a vessel, the more vulnerable it is to developing flow-blocking gaps of vapor during periods of drought or freezing. If the gaps persist, the supply of liquid water to the leaves is interrupted, and all or part of the plant dies. Plants in regions that become warmer and more prone to droughts and freezes can therefore be expected to adapt by retaining their original vessel diameters.
To investigate the two adaptations, Olson and his team examined 1535 samples from 537 species in 19 regions, whose mean annual rainfall ranges from 10 cm to 400 cm and whose mean annual temperature ranges from 0 °C to 35 °C. They also grew plants under controlled conditions. By correlating changes in vessel diameter, stem length, and other parameters, they concluded that plants everywhere respond to a warmer climate by growing taller. If the local climate also becomes more variable, those taller plants will be at greater risk of damage, death, and displacement by shorter plants. (M. E. Olson et al., Proc. Natl. Acad. Sci. USA, 2018, doi:10.1073/pnas.1721728115.)
