Measured at a global scale, protected forests with legal limits on human activity are significantly cooler than neighboring forests that lack protections, scientists reported in a new Science Advances study today. 

The researchers compared land surface temperatures and warming rates in protected areas to those in unprotected zones across five major biomes—boreal, temperate and tropical forests, grasslands and savannas—and found warming rates across 60 percent of all the protected zones were lower than non-protected areas in the same biomes. 

The temperature reducing effect they documented was most evident in protected forests because they have more vegetation, which gives them a complex structure that “creates more temperature buffering,” said co-author Pieter De Frenne, a climate researcher at the University of Ghent. 

“The reason we think this pattern emerges is because of vegetation structure,” he said. A detailed analysis of forest canopies, he added, showed more leaf area per square meter of ground in protected areas, which means more shade and cooler temperatures that help protect biodiversity near the forest floor.

“The cooling effect is very important for life below the tree canopy near the ground,” he said. Most forest biodiversity is in that zone, including in temperate, mid-latitude forests where “80 percent of all plant species grow in shade of trees.” 

The findings suggest the cooling effect is strongest in the globe-spanning belt of boreal forests at high latitudes in the Northern Hemisphere, the world’s largest land biome, encompassing about 27 percent of total global forest area.

“The warming rate in protected boreal forests is up to 20% lower than in their surroundings, which is particularly important for species … where warming is more pronounced,” the scientists wrote in the study. The fact that unprotected areas with the same type of vegetation show reduced capacity to buffer warming “highlights the importance of conservation to stabilize the local climate and safeguard biodiversity.”

Oregon State University forest ecologist Matthew Betts said the global scope of the study is impressive and “emphasizes the importance of protected areas … from a climate perspective.”

“My very first reaction was, why didn’t I do this? It’s a really good idea,” said Betts, who was not involved in the new study. “We’ve known for decades, if not longer, that protected areas are critical for biodiversity conservation, just in terms of reducing anthropogenic disturbance. This paper shows there’s an additional benefit. And that’s a cooling effect.” 

Getting more temperature data from forest understory would help show the climate benefits of forest protection in even greater detail, he added.

“At the moment we don’t have under-canopy data for large tracts of the planet,” he said. “Peter has done a great job of implementing a network of under-canopy climate stations across Europe but we don’t have anything like that in North America, and I’m sure there’s nothing like that in China or Southeast Asia. And so to do a really good job of this we need international collaborations focused on quantifying microclimate underneath the forest canopy.”

He said the paper also left him curious about the effects of different levels of protection, an important consideration in a world with a growing population that makes it impossible to completely ban human activities in many regions.

Betts said a previous study he worked on showed that higher levels of protection lead to less deforestation.

“None of them worked as well as we hoped. I think on average, protected areas reduce deforestation by something like 40 percent,” he said. “So, as was pointed out in this paper, they’re not perfect. We can’t eliminate human use, and there are limits to monitoring and measuring impacts,” he added.

At the same time, there are some non-protected areas where more sustainable forestry management is practiced that can maintain forest canopy. It would be “very interesting to know what role those sorts of forests play in moderating climate,” he said.

A Good News Story, But There are Wild Cards for the Climate

The new study, he said, is good news, for the most part. “But there are a number of elements that could shift that finding,” he cautioned. “Number one, in the northwestern U.S., it’s painfully apparent that increased warming and fuel loading is driving fires, and fires removed canopy, at least in the short term.”

Fires don’t know the boundaries between protected and non-protected areas, he noted. “They might burn a little bit less voraciously through old growth,” he said. “But it’s still going to burn, as we found out here with the multiple hundreds of thousands of acres that burned in Oregon over the last couple of years.”

More warming means more forest disturbance, which will “detract from the capacity of those protected areas to buffer climate,” he said. “And we’re going to reach a cap in terms of the total amount of area we can feasibly conserve on the planet, and keep humans out, from a management perspective, which means we really need to think about what we call the matrix, the areas in between the protected areas,” he added. 

The biodiversity benefits of protected, temperature-buffering forests also have limits. “Within protected areas temperatures will exceed the tolerances of some species,” he said. “So in an ideal world, they move. They move north, and they move upslope. But if the intervening areas are really hot, or if they’re just very difficult to cross, because they’re heavily managed, that will make it difficult for them to do that movement.”

That can make management of the zones between protected areas as important as the protected areas themselves. He said the new paper doesn’t address that, but a holistic view of forest management will “help biodiversity under a changing climate.”

If the average global temperature increase approaches 2.5 degrees Celsius, as predicted in some of the most recent projections, it will “test the limits of protection,” he said. “Some species just won’t be able to pull it off anymore.”