How global warming may make forests shorter and scrubbier

As global warming progresses, look for increasing expanses of majestic forests worldwide to become short and scrubby.

That is the implication of a new study that applies a well-established principle of fluid flow to the inner workings of vegetation.

The analysis doesn't attempt to specify timing or specific locations where such shifts in vegetation would occur. Instead, it uses the principle known as Darcy's law to explore the general types of vegetation most likely and least likely to survive rising temperatures and extreme drought.

Among the most vulnerable types of trees, the study finds, are conifers around the world, particularly the tallest specimens in old-growth forests. Their loss would have "ominous implications" for the natural carbon storage that these forests perform, the study suggests.

Such a shift in vegetation wouldn't be limited to regions with typically dry climates, such as the US Southwest, the study notes. The principle also applies to trees in historically wet or cold forests, according to scientists from the Los Alamos National Laboratory (LANL) and the US Geological Survey (USGS) who conducted the study.

All vascular plants – those with tissue to distribute water and nutrients throughout the plant – obey Darcy's law, notes Nathan McDowell, a forest ecologist at LANL and the lead author of the study, which was published this week by Nature Climate Change.

"Even wet places, when they have their very infrequent dry periods, will be significantly hotter than those trees have ever experienced," says Dr. McDowell – subjecting the forests to stresses that would increase their vulnerability to wildfires, bug infestations, and lack of moisture.

Regions where rainfall is projected to increase with global warming could be spared relatively early forest loss. Yet even these regions will have dry periods, just because of the variable nature of the climate system, McDowell adds. So the combination of drought and added warmth could threaten these forests as well.

The new study grew out of work published online by Nature Climate Change in 2012 that looked at temperature's contribution to tree losses in the Southwest over a 1,000-year period.

It found that temperature's influence on the amount of water that trees release through evapotranspiration was at least as influential in stressing trees as was the amount of rain or snow that the forests received in winter.

As temperatures rise, the atmosphere is able to hold more water vapor. But the gap between what it can hold and what it does hold can be wide. Over land, the atmosphere works to close that gap by drawing moisture from plants and soil. The hotter and drier the atmosphere gets, the more water it draws.

Trees get stressed when the atmosphere draws water from them faster than they can replace it from the soil.

The 2012 study showed that tree growth, or lack thereof, during the past century "was super highly correlated with this evaporative demand" from the atmosphere, says McDowell, who, along with USGS colleague Craig Allen, was part of a 15-member team that conducted the study. As that demand goes up, tree growth goes down.    

Based on climate projections, the team found that the increased demand during the warm seasons would subject the Southwest's forests to higher levels of stress than levels triggered by any drought in the past 1,000 years. Similar conditions could prevail in other water-stressed forests globally, the team added.

The new study takes that conclusion a step further by using Darcy's law – developed in the 19th century and adapted in 1981 to describe the flow of fluids through plants – to see in general terms which types of plants would be the winners and losers.

Taking into account features such as tree height and leaf area as well as evaporative demand, McDowell and Dr. Allen found that tall trees with large leaf areas and a relatively slow movement of water through them are the most vulnerable.

These trees are replaced by short, shrubby plants capable of surviving hotter, drier conditions. This would change the nature of the forest ecosystems and the services they provide – sequestering less CO2 and altering the landscape's hydrology, which can affect community water supplies.

The study is important not just for the application of Darcy's law as a tool for predicting the arboreal winners and losers, notes Neil Pederson, a senior ecologist at Harvard Forest, an outdoor lab spanning more than 3,700 acres of woods in central Massachusetts and one of the National Science Foundation's long-term ecological research stations. It also is a reminder that Darcy's law doesn't let wetter regions, like the Northeast, off the hook.

"Some people don't think our trees are susceptible to drought because it rains so much," he says. But trees' competition for light, driving them to their maximum height, "puts them closer to the edge of drought stress."

The eastern US has experienced a wetting trend over the past 100 years. Some places, such as New England, haven't experienced a serious, historic drought in more than 40 years. 

"I think this masks the vulnerability of the forests" in the region, Dr. Pederson says. But, he adds, the diversity of tree species in the region may help buffer it from experiencing the wholesale landscape changes that the Southwest appears to face.

In identifying the major types of plants likely to lose out, the new study also hints at broad, if potentially controversial, approaches that could help forests adapt to the changing conditions, at least for a while.

"Beyond the obvious but at present improbable mitigation solution of immediate massive reductions in greenhouse gas emissions, there are adaptation options for managing forests," McDowell and Allen write.

These include thinning some forests to increase the amount of soil moisture available to the remaining trees. That would reduce the risk of fire so intense that entire stands of trees are destroyed. It would also strengthen trees' biochemical defenses against pests, the researchers say.

In addition, forest managers could plant trees more resilient in the face of persistent droughts. 

It would be up to local or regional forest-management specialists to determine whether and where to apply such approaches, based on local conditions, the researchers say.