Arboreally Speaking, the ‘Good Old Growth Curve Is a Delusion’

In the previous post, I outlined the argument lighting up parts of the New Jersey legislature and the human elements of its ecological communities. Briefly, one reason some people are using to promote logging on public lands is the perception that old trees and forests are dying of old age. While there are other arguments as a part of the bill, like the fact that because forested ecosystems are maturing, species that use younger forests are declining, this “old trees are in decline” argument has led to much logging of old forests. I would argue it doesn’t have to be that way.

I will spare you many of the details from the scientific literature. But there is a plethora of papers indicating old trees and forests are dying of anything but old age.

An early paper that cemented the age-related decline concept was put forth in the journal Science by the excellent ecologist, Eugene Odum. The main figure of the paper that sticks in many people’s mind that summarizes the concept can be found here. Google Scholar estimates this paper has been cited 3,349 times. The idea was mostly based upon a paper focusing upon short-term leaf biomass production of two coniferous plantations (~18 & 36 yrs) and a 70-year-old forest. Odum described the idea that forest respiration increased as the forest aged such that they released more carbon dioxide than what they took in. It makes sense when you think about biological aging.

Eileen Carey pretty much put the first, evidenced-based blow into the aging forest concept. She strapped monitors onto young and old trees and found that increased respiration might not be the case in old trees. In a wonderfully titled article published in 2001, “Are old forests underestimated as global carbon sinks?,” she put forth the idea that the concept of ecosystem decline with time in the Odum paper might be flawed. Unfortunately, Carey’s article has only been cited 100 times, according to Google Scholar.

Testing the concept at the ecosystem level, which is an important consideration for the argument here, is that there is growing evidence that: 1) ecosystems accumulate carbon as they age, especially temperate forests, and 2) old-growth forests actively take up more carbon every year. Sebastiaan Luyssaert noted in the journal Nature, “Old-growth forests therefore serve as a global carbon dioxide sink, but they are not protected by international treaties, because it is generally thought that aging forests cease to accumulate carbon.” His 2008 article was a part of a small groundswell indicating that old-growth forests are much more important to the carbon cycle than previously presumed. Want more proof? Try here and most of the chapters in this 2009 book. There is more out there.

Better, if you live near Black Rock Forest (a consortium including Columbia University) in the southern Hudson Valley, you can see physical proof that age does not slow carbon uptake. By studying almost every facet of the trees and forest, Cheng-Yuan Xu and others found that, contrary to the long-standing belief that growth (productivity) was the cause of the age-related decline in forest biomass accumulation, it might be increased tree mortality that reduced ecosystem productivity, especially with the mortality of big trees. It might be the ecology, not the physiology of trees, silly.

Of course, there were hints to the idea that old trees might not age as people expect.

Tree rings, for quite some time, have shown that old trees are capable of “good” growth rates. Val Lamarche and others found evidence of increased growth rates in old bristlecone pine and speculated in a 1984 Science magazine article that the cause was the result of carbon dioxide (I will not get into the CO₂ fertilization controversy here). This was followed up in 1989 by Lisa Graumlich and others that showed a trend of increased ecosystem-level productivity in the Cascade Mountains. Soon the floodgates opened here, here, here, here, pgs. 143-156 here, here, here, here, here, here. Most recently, a paper by Matthew Salzer indicates that bristlecone pine 1,000 yrs old or older are growing faster than perhaps ever in their entire life and that this is likely the result of warming. These are but a few papers showing this pattern.

Complementing these studies is an ecophysiological study that indicates there is no difference between the physiological processes in young vs old bristlecone pines. In a study in Experimental Gerontology by Ronald Lanner and Kristina Connor, the question is asked, “Does bristlecone pine senesce?” They “conclude that the concept of senescence does not apply to these trees,” which is very different than us humans.

One new finding in plant ecophysiology that makes trees more like humans is that size matters. It seems that it might be difficult for bigger trees to maintain physiological processes. In a nice series of papers, Maurizio Mencuccini provides some evidence that size mediates tree vigor and that “after the first few years of a tree’s life, size-mediated factors largely prevail over age-mediated factors in determining tree growth rates.” Bigger trees, in this way, do not grow as well as smaller trees.

We find that in nature as well. Bryan Black reviewed of hundreds of trees of various types in North America and found that the oldest trees currently in the landscape were smaller at 100 years of age than younger trees currently in the landscape. A brand new study in Italy by Alfredo Di Filippo finds that lifespan in beech is greater in trees with slower growth and for trees living in areas with shorter growing seasons. They go on to suggest that global warming might reduce longevity of trees. I’m not ready to go there, but if caloric intake is related to biological longevity, perhaps that is correct. Size matters.

Why might size matter for trees? Large, heavy limbs catch much snow, ice and rain. The tallest trees literally become lightening rods. There are advantages to being a large tree, but there are disadvantages when tough times arrive.

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You get the point, correct? There is much evidence here against the claim that old trees die of old age. Big trees have some longevity-related issues. Old trees do not seem to be threatened by their great ages. Why trees die is a complex, perplexing and a very active field of research.

So, I promised I’d take you on the way-back machine and show that the concept that old trees decline with age is not even supported by “ancient” forestry literature. Whenever I get on this subject, I always let the late, great Bob Marshall have the last words (not only did they name a wilderness after him, he was a Stumpy, dendrochronologist and A1 scientist with ideas that were decades ahead of his time). After conducting some pretty radical tree-ring analysis for his times, he created the following figure:

Towards the end of his 1927 paper in the Journal of Forestry, Vol 25 (yes! it is finally online!!), he wrote:

It does not take long to see from this picture that the good old growth curve is a delusion.