Several years ago, my colleagues (Navjot Sodhi† and Ian Warkentin) and I wrote a major review in TREE about the fate of the world’s ‘second’ lung of the planet, the great boreal forests of Russia, Canada & Scandinavia. We discussed how fragmentation was increasing at an alarming rate, and that although most species there are still relatively intact, we stand to lose a lot of its biodiversity if we don’t halt the fragmenting processes soon. We wrote more on the subject in a paper to appear imminently in Biological Conservation.
Another component though that we raised in the TREE paper was the boreal forests were very much in danger of turning into a net carbon producer. You see, the ‘lung’ analogy is very pertinent because on average, the growth of the massive expanse of the vegetation in the forest generally takes up much more atmospheric carbon that it exudes through decay and burning (for as we all know, plants take up carbon dioxide to produce sugars during photosynthesis, and produce oxygen as a ‘waste’ product). However, as we fragment, cut down and burn the forest, it can end up producing more than it takes up (i.e., turning from a ‘sink’ to a ‘source’). We highlighted several studies indicating how insect outbreaks and human-exacerbated fire intensities and frequencies could conceivably do this.
Now Zhihai Ma and colleagues have just compiled a paper in PNAS indicating that the danger is well on the way to becoming reality in Canada. The paper entitled Regional drought-induced reduction in the biomass carbon sink of Canada’s boreal forests reports the results from 96 long-term permanent sampling plots spread right across southern Canada – from British Columbia in the far west, to Newfoundland in the far east.
They found some surprising things. First, there were general east-west differences in the trends of biomass change: the west generally declined, and the east stayed more or less constant. Thus, total carbon content was declining in the west. When they looked at the possible drivers, it turned out that mortality was the greatest contributor (with a lesser effect of reduced growth), and this resulted primarily from climate change-induced droughts in the west. Exacerbated by massive insect outbreaks, increasing fragmentation and greater fire frequency, this means that at least the western boreal region of Canada could become a net carbon source in the foreseeable future, thus precipitating the loss of one of the planet’s most important ecosystem services – climate change mitigation via balancing the carbon budget.
CJA Bradshaw
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†Deceased.
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