After hundreds of thousands of acres of aspen in the West perished during the 2000s, William Anderegg, a Princeton University forest and climate researcher, set out to test tree physiologist Nate McDowell's hypothesis that drought killed trees in one of two ways: thirst or starvation. Anderegg found that aspen primarily died of thirst, but it wasn't an "instantaneous failure." Mortality peaked six years after the drought did, suggesting that, even when precipitation improved, the trees couldn't repair drought-damaged plumbing systems and slowly died. He also found that the dead stands were unlikely to regrow. After fires kill aboveground growth, surviving aspen roots usually send up new shoots. However, after the drought, root systems also died.
Aspen are water-loving trees, drinking primarily from the topsoil, where moisture is controlled by the timing of snowmelt and the heat and dryness of summer air. Earlier snowmelt, and hotter, drier summers are both expected to become more common, and further reduce that water supply. "The climate," Anderegg says, "is leaving aspen behind."
In the past, cold temperatures limited the growth of the boreal forests that cover Northern latitudes. Climate change, it was thought, might increase growth in these forests. But that's not turning out to be the case in Western Canada. A 2011 study found that in forest plots unaffected by beetle outbreaks, wildfire or logging, background mortality rates have increased by about 5 percent a year since 1963. The pattern was not counterbalanced, as it was in Eastern Canada, by new growth or faster growth in surviving trees. The increase in mortality seems tied to an increase in water stress due to declines in precipitation and increases in summer temperature – a drought double-whammy.
Boreal forests are globally important because they are gigantic carbon sinks, absorbing more carbon dioxide than they emit. Disturbingly, the 2011 study suggested that if the mortality trends continue, Western Canada's boreal forest could become a net source of carbon, emitting more over time from its decaying wood than it can absorb.
The growth rates of coast redwoods and giant sequoias have surged since the 1970s, according to preliminary results of ongoing research. But this seemingly good news should be taken with a lump of salt. As temperatures continue to rise and water availability decreases, the trees could reach a threshold beyond which growth declines.
California's ancient giants are resilient in ways many trees aren't. They have massive food reserves, are resistant to insects and fungi, and have thick bark that doesn't burn easily. The trees can even re-sprout crowns after losing all limbs and foliage to wildfire. "It's going to be really hard to outright kill these things," says Anthony Ambrose, a redwood physiologist at the University of California-Berkeley. But if they become stressed by drought in the future, they may become vulnerable to pests and disease they currently tolerate. "That's one of those big unknowns."
White spruce, one of the most common species in Western North America's boreal forests, are responding unevenly to warmer winters and summers. In interior Alaska, white spruce reacted positively to warmer temperatures until the mid-20th century. Then, as temperatures kept going up, tree growth rates started to decline, sparking concern that the trees might start to die. The exact mechanisms that are harming white spruce are still uncertain, but temperature-induced water stress is a prime suspect.
Where the forest meets the tundra farther north, some white spruce have shown the same pattern, but others are growing faster, according to a high-profile 2011 study. Interestingly, temperature no longer seems to be the primary factor controlling tree growth here, as it was before 1950. "There is now a new sheriff in town regulating annual growth," physiologist Nate McDowell and climatologist Park Williams wrote in an analysis of the 2011 study. That sheriff's identity remains unknown, however, highlighting scientific uncertainty about the mechanisms that control growth, survival and death in the boreal forest.