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Thursday, August 4, 2016

How Many Trees are There in the World?

Here's an interesting example of scientific news, that on its surface, sounds good. The video informs us that a new study reveals that there are 3.07 trillion trees in the world...more than 8 times more than scientists previously thought.



So, you would think, then, that the message from the people at Nature would be one of relief. But the story narrated continues to push concern that we're "currently losing about 10 billion trees per year", and that "if we keep going at this rate, a walk in the woods will soon become a lot trickier."

Well, let's do some math here. Ten billion trees is 0.3% of the 3.07 trillion trees they say we have. If we continued to "lose" trees at that rate, we would have 1/3 fewer trees (only 2 trillion) in 100 years. That would still be five times more trees than scientists thought we had until this study was published.

But let's dig a little deeper. I wondered how the folks at Nature.com arrived at the ten billion tree "loss". I don't find any such estimate in the original paper. Here's what the authors actually say:
"Current rates of global forest cover loss are approximately 192,000 km2 each year. By combining our tree density information with the most recent spatially explicit map of forest cover loss over the past 12 years, we estimate that deforestation, forest management, disturbances and land use change are currently responsible for a gross loss of approximately 15.3 billion trees on an annual basis. Although these rates of forest loss are currently highest in tropical regions, the scale and consistency of this negative human effect across all forested biomes highlights how historical land use decisions have shaped natural ecosystems on a global scale. Using the projected maps of current and historic forest cover provided by the United Nations Environment Programme (http://geodata.grid.unep.ch), our map reveals that the global number of trees has fallen by approximately 45.8% since the onset of human civilization (post-Pleistocene)."
- Nature, http://www.nature.com/nature/journal/v525/n7568/full/nature14967.html
So they say that 15 billion trees are the "gross loss" on an annual basis. To claim that the net loss per year is 10 billion trees, one would have to assume that annual forest "loss" is 50% higher than forest growth all across the world. That sounds fishy, since we know that here in the United States,
"Growth has more than doubled on both national forest and other public timber land since 1952 and has increased 13 percent on private timber land since that year.
On a per-acre basis, total net growth averaged 52 cubic feet annually for all timber land."
- U.S. Forest Service, http://www.fs.fed.us/rm/pubs_other/wo_gtr078_064_066.pdf 
Even supposing that the growth to removal rate is negative in tropical forest regions, it's hard to get anywhere near a 50% net loss of trees per year. For instance, according to University of Maryland researcher Matthew Hansen, the average loss of forest acreage in tropical countries of the world was only 3.9% over a ten-year period from 2005 to 2014, or only about 0.4% per year. Furthermore, that was gross loss, so regrowth was not counted.

I suspect the folks at Nature came up with their ten billion tree loss by reducing the 15.3 billion tree gross loss stated by their authors by the 45.8% loss estimated in the same paragraph since the start of human civilization...which is a gross statistical error. Unfortunately, statistical errors of this sort are not uncommon and are frequently used to weave all sorts of false narratives these days. Even if you assume that the tree loss continued at this same actual loss rate, that would mean that we would "only" have about 1.6 trillion trees ten thousand years from now, or still four times more than what scientists thought we had until this study was published.

And there is all sorts of evidence that this rate of tree loss will not occur. Food production, which is the cause of most forest depletion, is becoming much more efficient and utilizes far less acreage per ton of food than it did even fifty years ago. Agricultural advancements promise to continue this trend, and the rate of forest growth in even tropical countries may soon exceed forest removal as it has in temperate countries for some time now.

So, let's be honest...the findings of this study are good news. and bode for an abundance of trees for any imaginable future. Any "concern" that you'll have a hard time finding a forest to walk in is nothing more than an agenda-driven false narrative. Be careful when you view internet video...caveat emptor.

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