Are large, old trees in decline?

I’ve banged on enough about the crisis in forests for people here to know what the deal is.

Anyway, there has recently been a bit of back-and-forth regarding the state of large, old trees at a global scale.

These trees are key in both forest and non-forest ecosystems. The definition of what is ‘old’ and ‘large’ is specific to each region but it is widely accepted that these trees tend to store lots of carbon and are valuable for many species because of their structural complexity.

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David Lindenmayer and colleagues published a note last year on the importance of large, old trees and evidence for their declines, and they expanded on that with an article discussing policy options to deal with these declines.

This is all important stuff and they had me convinced. It makes sense. Large, long lived species are disproportionately vulnerable to threats because they take a long time to reach maturity and they are targeted simply because they are large – for animals see hunting of ungulates, for trees see selective logging.

However, a recent letter by Edward Faison has made me doubt the claims of Lindenmayer. Faison points out that there have been increases in the abundance of large trees in forests in Sweden, Spain, Hungary, Italy and Switzerland. In addition there have apparently been relatively few declines of large trees in North America.

Lindenmayer and colleagues have since rebutted this letter, saying that there is a difference between large old trees and simply large trees.  They point out that there have been increases in Europe and North America but that these increases have been from a very low point since both regions have historically cleared large swathes of forest for agriculture. They also point out the loss of large trees as a result of logging in the tropics as well as in Australia, North America and Siberia.

And yet I am still not entirely convinced.

Don’t get me wrong, I believe that large old trees are probably in decline in the ecosystems they talk about, but is this a general trend all over the place?

I am also a little scared that all of the discourse on this so far has been in the form of reviews/essays that could easily cherry-pick some cases and then craft a nice narrative around them. It is easy to believe the stories we tell ourselves and this is where we as scientists should be most self critical. After all how many beautiful sounding theories have been seen to have nothing to do with how things work in the real world? The only way to confront such problems is with cold, unemotional statistical analysis.

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5 thoughts on “Are large, old trees in decline?

  1. Thanks for this summary of some interesting recent papers. My sense is that a regional approach is needed here … I mean, even if, on average, large old trees are in decline worldwide, we still need to solve this problem on a regional scale, ultimately. The debate as to whether or not this decline is therefore “global” thus strikes me as somewhat academic. Large, old trees are disproportionately important — that much is certain — and where they are declining, this should be a cause of conservation concern. To me, that’s the key point, and David Lindenmayer’s recent papers have pointed out that the phenomenon of tree decline is not an isolated occurrence — and hence needs to receive attention. That said, I suspect, like in most cases, examples can be found where large (or even large, old) trees have increased, in which case those are nice situations we should be happy about (and perhaps learn from).

    1. I agree wholeheartedly.

      The question of whether things are in decline is as you say a bit academic.

      I think the best way would be to look at the problem is, as you say, at a regional scale and also by looking at different human pressures. In this way the question is a bit similar to that in the recent Vellend et al paper. They found that on average plant species richness across many plots throughout the globe subject to different drivers of change shows no net change. However, the drivers that are what is really important for explaining variation in any species richenss changes.

      If we can say how much of each region is subject to a particular pressure and how that pressure generally affects large trees then we can estimate the broader impacts.

  2. Thanks for this.

    I’m skeptical of Lindenmayer et al’s claims also and definitely want to see their reasoning as to why they think big and old vs just big is an important distinction. From a C sequestration point of view, the opposite is true because big but younger means faster growth than does big and old, for equivalent sized trees (I do not necessarily want to argue from that point of view, just making a logical point).

    Have you seen the new paper by Stephenson et al in Nature? I’m not so sure about that one either. Methodological concerns.

    1. Thanks for the comment Jim.

      I think the big vs big and old is largely seen from a biodiversity perspective – old trees are more complex, have holes and generally more nooks and crannies for species to inhabit.

      I have seen the new Nature paper – I have given it a skim. What are your concerns about the methods?

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