Predicting Forest Tree Responses to Climate Change: Some Humility Required

 

During the previous discussion on the Northwest Forest Plan Study here,  Lance posted a link to this article (titled “Climate modeling shows significant shifts in 21st century Pacific Northwest coastal forests” YOA (Yay Open Access). The study talks about how climate will change forests composition.  As a forest geneticist, I have always questioned the conclusions of these kinds of studies.  The first time I heard these kinds of predictions, I believe it was a Forest Biology Workshop in Baton Rouge, Louisiana in 1994 (yes, 25 years ago).  At the time, I thought “if we don’t know how much of a change in climate mature trees can handle, and we don’t know how much genetic variation are in their offspring, how can we possibly predict where trees will live or die?”

One of my tasks in Forest Service R&D was to be on reviews of the Research Stations. I remember one particular Assistant Director, who felt that systems thinking and research was the wave of the future. One of the scientists at the  Station had done some excellent work (IMHO) on how far fish move in streams, that was utterly surprising and important for many reasons.  This AD seemed to feel that this work, and indeed organismal biology, was passe´.  Again, you might ask, if you don’t understand how an organism works, how can you possibly predict what it’s going to do under changing conditions?  Let’s look at a couple of ways of approaching the same topic- how changes in climate will affect tree species in the Northeast.

First we have this newspaper story citing a variety of researchers who talk about climate impacts on forest tree composition (from this interview, it’s not clear if these predictions are based on general principles or specific models), e.g.,

“As the climate warms, we expect to see some of the iconic species of New England, like maples and beeches, pushed out. Those will no longer be there,” he said. “We are going to see things we never even thought of — we’ll see a shift in the face of the forests of New England. You can’t change one thing, and expect nothing else to change along with it.”

We might call this the “have idea and model” approach.

And here is a study by Canham et al. here (YOA)  that may yield information to improve models from using FIA data and measuring tree growth.  They seem to arrive at the tentative conclusion that there are some phenotypic (not genetic) responses to climate change. We might call this the “observe how organisms operate” approach.

Ultimately, it will be critical to understand whether the patterns we have observed represent genetic adaptation or phenotypic acclimation, or some combination of the two. The consequences for responses of these tree species to climate change could be very different depending on that balance. If the results are genetically based, trees within a given location could be much more sensitive to climate change than indicated by the very broad geographic distributions of these temperate tree species. But if the results are phenotypic, this would represent local acclimation that could help buffer species in the face of climate change. Adult tree growth is a dominant term in interannual variation in forest productivity and the attendant ecosystem properties associated with primary productivity, including carbon sequestration and nutrient retention. But from a demographic perspective, adult tree growth is much less important to the geographic distribution and successional dynamics of these temperate tree species than are other life history stages, particularly seedling recruitment and survival, and adult tree mortality (Pacala et al. 1996, Canham and Murphy 2016ab2017). It remains an open question whether the ubiquitous local adaptation to long‐term climate conditions we have documented for adult tree growth in these temperate tree species is present in those other critical life history stages.

Like I said in 1994, if we don’t know how trees adapt, either phenotypically or genetically, we can’t really place much faith in models that try to predict what will happen in the future.  And that’s leaving aside changes in diseases, insects, pollinators, seed spreaders, mycorrhizal associations and so on. But that’s OK- humility in the face of the complexity of Nature is, perhaps, the most appropriate response.

4 thoughts on “Predicting Forest Tree Responses to Climate Change: Some Humility Required”

  1. One drought response I noticed, in incense-cedar, was that the foliage looked droopy. They tried to retain their entire crowns, at the expense of summertime growth. Some species of trees will self-prune, via drought. Pines seem to be able to dig deep enough to find water. True firs often will be top-killed.

    It does seem like it would be worthwhile to limb trees up (especially incense-cedar), to reduce their uptake of water. There would also be a benefit of fire resilience. This is something volunteer groups could do, and have a real impact on their local forests.

    Reply
    • Larry that reminds me.. on my laundry list of complexities, I forgot to mention competition from other plants, including trees, as a reason for trees being where they are (some places they are out competed and others not), and of course fire regime, which may change due to climate and/or due to our responses to climate, or just due to changes in human culture around living with fire. That’s not so important in the NE, (although in the article one author suggested climate change would lead to forest fires there) but certainly in the ponderosa, true fir, incense cedar mix.

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