Beetles transform Canadian forest from carbon sink into carbon source

Blogging on Peer-Reviewed ResearchIn the story of climate change, humans and the carbon dioxide we pump into the atmosphere are the villains of the piece. Now, it seems that we have an accomplice and a most unexpected one at that. It lives in the pine forests of North America and even though it measures just 5 millimetres in length, it is turning these woods from carbon sinks into carbon sources. It's the mountain pine beetle.

The beetle bores into pine trees and feeds from nutrient-carrying vessels called phloem. It also lays its eggs there. Once a beetle has colonised a pine, it pumps out pheromones that attract others, which descend on the tree en masse and overwhelm its defences. The infestation damages the phloem to such an extent that the tree effectively dies of malnutrition within weeks. A single beetle can live for up to a year, giving it plenty of time to damage several trees.

Normally, the beetles target weak, old or dying trees and in this capacity, they speed the growth of young trees. But occasionally, their numbers erupt in large-scale outbreaks and British Columbia is currently facing the largest one ever recorded. They have infected an area of forest the size of Greece and the scale of the epidemic is ten times worse than any previous incident. The damage is even obvious from the air, for the needles of infected trees turn red within the first year and gray as they succumb further (as illustrated after the jump...)

The outbreak is bad news for the local ecosystem and the forestry industry alike. But Werner Kurz from the Canadian Forest Service has found a subtler impact of the beetle's actions - they will transform the local forest from a carbon sink into a carbon source. Dead trees are in no position to soak up carbon dioxide from the air, and their decay will release even more carbon back into their environment.



Kurz modelled the effects of the beetles in the south-central region of British Columbia between 2000 and 2020. He took a wide range of factors into consideration including growth, leaf litter, decay, forest fires and deforestation, and ran 100 different simulations under a range of different conditions.

For the first two years of the simulation, the forest was a carbon sink, soaking in half a million tonnes of carbon every year. After 2002, things changed and the falling forests turned into sources of carbon, giving off an average of 17.6 million tonnes every year between 2003 and 2020.

The beetles' tree-killing antics will result in the release of about 270 million tonnes of carbon over the 21 year period covered in the simulation. The forestry industry is managing the infestation with "salvage-logging", felling infected trees to prevent the spread of the beetles and to recover some economic value from the timber. This harvest will add another 50 million tonnes of carbon to the beetles' direct tally, although only a small proportion of this will be released into the atmosphere.  

The effects of these tiny insects matches those of more usual suspects. Kurz predicts that this single outbreak will release as much carbon dioxide over 21 years as Canada's entire transportation sector does over five. And the annual amount of carbon released by the beetles in this small patch of British Columbia is similar to the amount given off by forest fires over all of Canada during earlier decades.  

Vicious cycle

The beetles hardly absolve Canada of responsibility over our changing climate, for it was probably climate change that triggered their extraordinary outbreak in the first place. Cold snaps kill the majority of beetles. But over the past few decades, global warming has shifted British Columbia's climate toward milder winters and hotter, drier summers - conditions that the beetles have been quick to exploit by expanding their range, and colonising forests further north, and at higher altitudes.

The problem isn't restricted to British Columbia. The mountain pine beetle is wreaking havoc in other parts of North America, and elsewhere, warmer climes have allowed spruce beetles to reproduce in half the time, wreaking havoc on spruce forests. Kurz's results suggest that infestations of tree-boring insects all over the world could be driving vicious cycles of larger infestations, increased carbon emissions and rising temperatures.

There is one positive note. Kurz's simulations suggest that we are currently experiencing the peak of the epidemic. Using data on the beetle populations, the number of remaining host trees and the judgments of entomologists, Kurz predicts that from 2009, the beetles will start literally eating themselves out of house and home. They will have damaged such a large area of pine forest that their own numbers will start to fall.

Even so, the impact of the mountain pine beetles will last much longer than their population. By 2020, the forest's productivity will have started to recover, but it still won't have returned to pre-outbreak conditions. Kurz feels that the forests will eventually recover but he feels uneasy about making projections beyond the 2020 mark, especially since the area is predicted to go through even larger climate upheavals in the future.

Update: Over at The Voltage Gate, Jeremy neatly uses this study as a jumping point for talking about the extent of public concern over climate change. It's well worth your time.

Reference: Kurz, W.A., Dymond, C.C., Stinson, G., Rampley, G.J., Neilson, E.T., Carroll, A.L., Ebata, T., Safranyik, L. (2008). Mountain pine beetle and forest carbon feedback to climate change. Nature, 452(7190), 987-990. DOI: 10.1038/nature06777

Images: Map from Nature

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You beat me to it, Ed! Nature just pops in my inbox and I think "This is cool! I'm going to blog this to get the scoop" and look what happens...

Seriously, though, excellent summary of the paper. I have a feeling that we'll be seeing similar situations as climate continues to warm, though, perhaps even seeing a shift in the ecological makeup of some communities. Now is a good time for ecologists and evolutionary biologists to be looking closely at what happens to organisms when their environment rapidly changes.

Great article, but I think you have "sink" and "source" swapped in the third sentence of the first paragraph.

This harvest will add another 50 million tonnes of carbon to the beetles' direct tally, although only a small proportion of this will be released into the atmosphere.

Er, isn't this essentially a form of carbon sequestration then? The danger of carbon to global warming comes from atmospheric carbon, so as long as the wood is being used for durable goods rather than fuel, then it's only that "small proportion" that would be a concern to global warming. Hopefully that would also allow the regrowing forest to act as a carbon sink again (Provided the beetle growth is checked, naturally)

After all, that seems to be the logic behind biofuels; the carbon emitted from burning biofuels is equal to the carbon removed from the atmosphere from growing those sources int he first place.

Brian - thanks mate. Papers like this which show some of the subtler and more unexpected effects of climate change are some of my favourite ones in the field of environmental science. And sorry, I get a bit of unfair advantage with stuff like this because I have journalist powers :-)

Brad - erm, no I haven't. See?

Left Wing Fox - Guess you're right, although that's still a net release of 220 million tonnes. As I understand it though, the forestry industry is responsible for reforesting the area that they claim for salvage logging.

Dennis - roffle.

Statement is made in the Winter 2008 Tulanian magazine that hurricane Katrina killed some 320 million trees. These trees are releasing more CO2 per year into the atmosphere than all the trees in the USA can absorb.

By Jim Thomerson (not verified) on 23 Apr 2008 #permalink

Yeah. It's still a stark example of the dangers of Global warming and the positive feedback loops that magnify the changes. Am glad to hear that there are some signs that this might be the peak of the devastation though.

I kinda see how this works in theory but I have to wonder.Without the canopy above, wont the ground below become profuse in CO2 gobbling undergrowth? I've often heard it said that the forests themselves arent that productive in producing mass because they're so competitively locked together. As much as I'd like to find as much evidence as possible that our human impacts are screwing up the natural systems, I sometimes feel the impulse to view it all through the lens of global warming is distorting the picture and delaying the kinds of steps we can take to ameliorate the effects we see all around us.