DROUGHT’S TOLL ON OUR FORESTS
The region’s forests are suffering. In the past 10 weeks, we’ve had just a couple of hours of rain. Even the pernicious blue periwinkle invading many of the region’s green spaces is showing the strain.
On the east side of Mount Douglas, the ocean spray has crisped. The wet spring provided banner blooming conditions for the native shrub, with bowers of creamy, plumy blossoms bursting forth along pathways throughout the park.
But since the start of August, shrubs growing in open, more exposed areas of the park have had both flowers and leaves shrivel. Many leaves turned from green directly to brown and crunchy — with none of the usual transition through golden fall colours.
The park’s Garry oak trees have started to drop twigs and small branches as well as acorns. The native oaks are better adapted to insufficient water than some other species. This can be seen in the Garry oak’s tough, leathery leaves, which help prevent moisture loss through the tops of the leaves.
But even drought can affect the trees, and when that happens, structural weaknesses in each tree show up.
They start shedding peripheral branches in light winds or under the weight of ripening acorns or scurrying squirrels. When they lose larger branches, the phenomenon is called sudden limb failure or sudden limb drop. Limb failure usually happens in windy or stormy conditions — prolonged drought weakens the trees, allows fungi or insects to attack and further weaken the trees, to the point they can no longer blow in the wind as they could when they were healthy.
This explains the native oak’s typical gnarly silhouette.
During past drought years, such as in 2015, limb failure struck Garry oaks, elm, willow trees and other varieties.
The City of Vancouver recently asked residents to help water young boulevard trees. Young trees, with less developed root systems, are more susceptible to drought, but even mature trees suffer.
As the number and duration of droughts increase around the world, scientists are working to predict how future parched conditions will affect plants, especially trees. Part of that is determining what exactly drought does to a tree to kill it.
The question of ultimate cause of mortality extends to more organisms than trees. For example, the medical profession and scientists have long argued about what the final fatal mechanism is that triggers human death.
Some say we die because our brains’ cells stop getting the oxygen they need to function. Others champion heart failure, which stops circulation of oxygen-laden blood to the brain and other vital organs.
Similar disagreement occurred regarding trees. But a recent synthesis undertaken by a team of 62 U.S. scientists of drought-manipulation studies reveals the precise mechanisms by which prolonged thirst kills trees.
The researchers found that, in some cases, tree death is a result of carbon starvation. This happens when trees close their pores — essentially starving themselves by blocking the entry of carbon dioxide, which is needed for photosynthesis.
Trees use 99 per cent of the water they suck up through their roots to keep open those pores, called stomata, but respond to the drought stress by closing the pores. When that happens, they turn to stored sugars and starches to stay alive, and die if these internal food resources run out before a drought ends.
A second culprit in tree death, however, is hydraulic failure — the inability of a plant to move water from roots to leaves. If a tree loses too much water too quickly when it responds to drought stress, air bubbles form in the tree’s water-transporting vessels in the xylem, or sapwood.
This plugs the vessels — similar to how an embolism blocks a human vein or artery. The bubbles trigger hydraulic failure in the tree by preventing water from moving from the roots to the leaves, which causes the tree to dry out and die.
The scientists found that hydraulic failure always happens when trees die from drought, whereas carbon starvation contributes to tree death only about half the time.
This information allows scientists and forest managers to better predict droughts’ effects on forests in a region as climate change alters regional weather patterns and forest range.