Climate change is harming a vital tool to combat it

• It may also mean selling forest carbon offsets to reduce emissions may not be worth as much as companies think.
• "We could be betting a lot on forest offsets that may not have a lot of benefit by end of the century," says William Anderegg, an ecologist at the University of Utah.

The big picture: Earth's temperate, boreal and tropical forests capture a massive amount of carbon — roughly 20%-25% of the CO2 humans emit each year by one estimate.

What's new: The death rate of trees in the tropical forests of North Queensland, Australia, doubled between 1984 and 2019, according to a study published this week in Nature.

• The researchers suspect that's linked to climate change: Over that same period the vapor pressure deficit (VPD) — a measure of how quickly the air robs plants of moisture — increased in this region.
• An increase in VPD has been linked to a spike in wildfire risk, in which it plays the "role of a sledgehammer," study authors Sean McMahon and David Bauman of the Smithsonian Environmental Resource Center tell Axios in an email.
• But in forests, some tree species may be resistant, tolerant or resilient to VPD increases, whereas it can stress others, they added. Increases can directly kill trees or make them more vulnerable to drought, disease and pathogens. But, the authors caution, it is difficult to ascribe tree death to drought.
• "Our study indicates that climate in general, and VPD in particular, need to be understood not simply physiologically, but how different species differ in their vulnerability to this stress," they said.

In another study, Anderegg and his colleagues found the risks posed to trees by wildfires, drought and insect outbreaks through the end of the 21st century may be pervasive, especially in the western U.S.

• "In a scenario with medium emissions, wildfire risk is projected to increase by a factor of four. Drought and insect risks increase by about 50% to 80%," they wrote in Ecology Letters.
• One limitation is the study doesn't look at the regrowth of trees over time. "It could be there are scenarios where species composition changes or forests lose enough biomass that there isn’t fuel for fire. But that is really more important for [the] end of century and less in the next 30 to 40 years."

Zoom in: More CO2 in the air and higher temperatures are expected to spur the growth of trees from supercharging photosynthesis.

• But it's an open question whether photosynthesis or the expansion and division of cells that make up wood drives tree growth.

Using tree ring data from thousands of trees in the U.S. and Europe and measures of changes in CO2 in the air that are a proxy for photosynthesis, Anderegg and his colleagues tried to answer that question. If rates of photosynthesis and tree growth increased or decreased at the same time, it would point to photosynthesis as the driving force.

• But they found little evidence of that in the study areas, which are located in temperate and boreal forests. That may mean current models, which funnel CO2 increases into tree growth and predict trees can lock up more carbon, may be overestimating how much carbon forests can — and will — sequester, Anderegg says.

• On top of that, the study suggests tree growth rates may be influenced heavily by the water-dependent process of cell growth. Drought then may be slowing tree growth in many regions of the world that we don't realize, he says, adding the mechanism needs to be tested in models.
• The study is limited to boreal and temperate forests as well, underscoring the need for more data from the tropics, the researchers said.

What to watch: Future forests will remove carbon each year and store it for centuries — but existing forests have already accumulated carbon and could capture more.

• Trees that are harvested can take decades or a century to release the carbon stored in them. But left to grow another century, they could capture even more carbon, says William Moomaw, a professor emeritus of international environmental policy at Tufts University. (Trees grow faster later in their life — and bigger trees store more carbon.)
• Moomaw advocates for what he calls pro-forestation, which essentially means to grow forests and could include increasing the time between harvests so trees can capture more carbon.

The bottom line: "Forests can store CO2 for centuries but will they survive climate stresses in [the] 21st century?" Anderegg says.

Go deeper: How to build forests to combat climate change (Axios)