People are killing big old trees.
Across the planet, old growth is being lost to logging and land clearing, and a lethal list of disturbances stoked by climate warming that disproportionately take out big old trees, including drought, heat, bugs, lightning, wind storms, floods and fire.
The world's forests are getting younger and shorter as a result.
Already reduced by about a third in area by logging and land use change since 1900, there will be fewer big trees 140 years old or older as the climate warms, scientists found in a paper published May 29 in the peer-reviewed journal Science.
"We are losing and probably are going to be continuing to lose old-growth forests, globally," said Nate McDowell, lead author on the paper and staff scientist at the Pacific Northwest National Laboratory.
The climate these trees sprouted in is not the climate in which they endure today. The extremes are more extreme -- whether heat, bugs, drought, windstorms or fires. For these big old trees, the world has literally changed right out from under them. Big trees are caught in a downward spiral: Losing the world's big carbon sinks in old-growth forests is further stoking climate warming that is amping up all the forces particularly lethal for big trees.
A big, tall tree is a lightning rod and a sail for high winds. The plumbing within a tree that enables it to lift water hundreds of feet in the air is more susceptible to breakage under drought stress. Bugs preferentially target big old trees. And while larger trees with their thick bark tend to be fire resistant, bigger hotter fires are roasting the survivors of what used to be typical blazes.
The more these trees die, the more carbon goes into the atmosphere, stoking global warming even more. "It is a feedback loop that makes it worse," McDowell said. In the Klamath region of northern California and southwest Oregon, the mature forests have been so besieged by repeated fire they are lost forever: they will not regrow to big trees, or even forests at all.
There are exceptions, of course, to the rout of big old trees, including the old growth in the gloaming of the Pacific Northwest's rainforests. But these forests are not forever immune from the trends scientists identified. The drought of 2015 saw even parts of the rainforest in Olympic National Park burn in the ironically named Paradise fire.
Droughts are punishing
The very structure of big trees puts them at risk, said Kristina Anderson-Teixeira, another author on the paper. Her lab at the Conservation Ecology Center at the Smithsonian Conservation Biology Institute in Front Royal, Va., in 2015 documented how drought particularly punishes big trees in forests worldwide.
"The assumption of a lot of people was that the larger trees have deeper roots and better access to water, and therefore they are going to do better in drought," Anderson-Teixeira said. But it's just the opposite, because of how trees work.
Trees move water from the ground to their crowns by continuous vertical piping in their trunks in cells called xylem.
Water moves up these cells both because of the sticky property of water molecules, and the pull of evaporation to the atmosphere, through tiny pores on the bottoms of leaves, called stomata. On a hot day, in dry conditions, moisture can be actively sucked out of a tree to the point it can't at great height, against the forces of friction and gravity, lift enough water fast enough. Air bubbles result -- a catastrophe in the tree's plumbing.
In a drought, the damage is worse because a tree also can't replace the water taken from it. It shuts its stomata in defense, but then it can't make food from carbon dioxide in the atmosphere, through photosynthesis. With no food or water, the tree first stops growing. Then it starts to shed leaves or needles. Then, it starts to die. First by degrees. Eventually, to the ground.
In some forests, the carbon dioxide people are pumping into the air by fossil fuel burning, combined with longer growing seasons caused by climate warming, is helping trees bulk up and grow.
But more carbon in the atmosphere -- the so-called CO2 fertilization effect -- doesn't work for trees that are so drought and heat stressed they can't breathe. That's when big trees die fast, and first.
Not every stressed or injured tree will die; a lot will die back, but persist, and survive, notes Neil Pederson, senior ecologist at the Harvard Forest and a dendrochronologist who has studied tree stress all over the world. He warned against an overly bleak interpretation of how trees are managing in a warming world. He has documented trees that are masters of resilience, dying back and recovering, over hundreds of years. Shedding limbs and dying back from the top is an adaptive strategy for trees in tough spots and tough times.
But to the extent forests can't age toward larger size trees, the braking effect of big trees on climate warming is diminished, notes Dave Orwig, senior ecologist and forest ecologist at the Harvard Forest.
The largest trees hold 50% of the aboveground biomass in mature forests, worldwide. Overall, the terrestrial biosphere, including forests, soaks up about a third of the carbon emissions produced by people, and big old trees are the biggest carbon sinks, making them critically important to the health of the planet.
Carbon storage is just one of the essential benefits of big old trees, Orwig notes. They are also havens of biodiversity.
Value to the planet
Old are different from young trees, not just bigger.
Old trees provide habitat for many lives, including insects, epiphytes and other plants and animals that shelter in their unique snaggled tops, big branches, canopies and cavities, and layered, shredding bark, said Jerry Franklin, professor emeritus at the University of Washington. Franklin, 84, is a bit of an old-growth tree himself: Over his long career he led the way to ecological forest management in the Pacific Northwest, including a ban on logging of old growth on federal lands. Franklin opposes any logging of old growth anywhere because of its value to the planet and so much has already been lost.
In British Columbia, productive old-growth forests today comprise less than 1% of forest area, according to a new report by independent scientists. Yet logging of old growth contracted by the B.C. government continues both on Vancouver Island and in rainforests of the Selkirk Mountains, where the logging of old-growth forest is driving the mountain caribou to extinction.
Gone with the giants are all their unique benefits.
"They have so much value, they offer such an array of niches for organisms," Franklin said. "The old trees are just much richer in the variety of habitats and conditions they provide."
Big trees also bring water from deep in the ground to the subsurface on a hot summer day, irrigating the understory. Big trees cool the microclimate around them with the water they emit from their leaves, and their shade.
Franklin cautioned against any sweeping predictions about how climate warming will affect the forests of the future, because of the complexity of forests. "The one thing we know is that whatever happens, it will be a surprise."
But there's no question the effects of climate warming are punishing big trees most, and hammering forests worldwide, said Craig Allen, another author on the paper, who noted the rate of tree mortality in the Western United States already is outside anything in recent history.
Allen has spent 40 years at Los Alamos, New Mexico, in his career at the United States Geological New Mexico Landscapes Field Station, basically watching trees die.
He has become a student of what happens when trees in good times surge in growth -- only to produce more physical mass than they can support when heat and drought hit. "First they die," Allen said. "Then they burn."
The death of forest giants is hard to watch, even for scientists.
"Everywhere on the planet people care about big old trees. They have monumental status. It is profoundly disturbing when they go down," Allen said.
"To get a big old tree takes centuries or longer. And they can die in a very short window of environmental stress."