What Global Warming Means For Old, Tall Trees

Tall, old-growth forests are particularly at risk due to climate change.

CREDIT: SHUTTERSTOCK

As climate change drives up global temperature and increases the frequency of intense droughts, the world’s oldest, biggest trees might be the most vulnerable, according to a recent study published in Nature Climate Change.

Using Darcy’s law — an equation that describes the flow of water through a porous medium — researchers at the Los Alamos National Laboratory looked at what types of vegetation might survive best, and which would most likely die first, due to global warming. The equation takes into account things like plant height and leaf area, among other factors, to measure how effectively water can move through the plant.

The researchers found that the plants that were most likely to survive were short plants with smaller leaf area. Tall trees with large leaf area, on the other hand, were the most likely to die.

“Global warming is global, and all vascular plants obey Darcy’s law,” Nathan McDowell, co-author of the study, told ThinkProgress. “The implications of this is the whole world, anywhere there is plants.”

A crucial component of the equation is vapor pressure deficit — how much moisture the air holds versus how much moisture it can possibly hold. McDowell previously found that even if precipitation trends don’t change, atmospheric warming causes the vapor pressure deficit to go up, which makes conditions feel drier for plants.

“That means that the holes on the leaves of plants — stomata, which allow water out and CO2 in — will lose more water in 50 years than they will today,” McDowell said, “and are losing more water today than they did 50 years ago.”

As atmospheric temperature — and the vapor pressure deficit — increase, plants can do a few things to make up for the loss of water through their stomata, like shedding leaves. But those measures only go so far. At some point, McDowell said, it’s healthier for the ecosystem to replace large trees with plants better adapted to using water in a warmer environment.

“The amount of warming we know is going to happen based on climate change, they can only reduce foliage so much,” McDowell said. “The main outcome is we predict that tall trees in particular should be the most vulnerable.”

Widespread death of tall, old trees would have a large impact on a forest ecosystem — not to mention the effect it would have on worsening climate change even further.

“It’s the big trees that store the most carbon,” McDowell said, noting that the tall trees are also those that do the most photosynthesis, pulling more carbon out of the atmosphere than small trees. Animals also tend to prefer old-growth trees to small, young plants, so a widespread dying of large trees could potentially displace species that depend on old trees for their habitat.

In the short term, McDowell said, changing precipitation trends could mitigate widespread forest mortality. If, due to climate change, an area receives more precipitation than normal, that could make up for the warming-induced dryness and allow large trees to survive.

If that same area then experiences a dry spell, however, the change could be destructive.

“There could be places that do okay for decades, but it always comes back to 100 years from now when a period of low rainfall hits that spot, then those plants are going to be in a place they’ve never been,” McDowell said. “It’s going to be warmer and drier than those plants have ever experienced. It’s pushing plants into a new world that they’ve never experienced, and they just cant get up and walk.”

Though the study didn’t plot a timeline for the tree die-off — it was meant more as a theoretical, big-picture exercise, McDowell said — it did offer some options for forest management in the face of rising temperatures and increasing drought. Forests might benefit from reduction strategies, for instance, to improve the water balance for trees. In a time of drought, fewer trees means less competition for scarce water.

Still, McDowell said, it should be left up to those on the ground to make decisions about how best to adapt their forests to the changing climate, as different regions are expected to see different changes with respect to precipitation or drought. “Local foresters are always going to know best what to do,” McDowell said. “We’re just saying, ‘Here’s something to be thinking about.'”