Predictions – of snow and other changes

I’m sorry for a delay in adding to this blog – Christmas festivities here at Harvard Forest, along with a trip back to Scotland over the holidays, topped off with 10 days of flu which I guess I collected at Boston Airport along with my luggage.

Prospect Hill Road

It’s noticeably colder. My thermometer recorded -20C at new Year, and tonight, after a fresh fall of snow yesterday, we’re expected to reach -16C. In the basement next to the wood boiler is the Equipment Room, full of strange bits of kit, plus a pile of snowshoes. Poking around one weekend (as a distraction from working) I uncovered the stratified layers of snowshoe technology. The upper layer are newish aluminium and fabric, but the lower layers start to include wood and canvas, and right at the bottom are pairs of traditional teardrop ‘tennis racket’ designs. Never throw anything away…Anyway, this weekend I shall try out a pair, and report back on my elegant progress.

Avenue of Sugar Maples

Looking down to Highway 32
I slipped spectacularly on the ice underneath the snow about 30 seconds after taking this picture

Cold for the cows...

It occurs to me that I haven’t fully explained what I’m actually trying to do here. I’ve spent much of my time so far honing my central argument, and now feel confident enough to release it into the wild – and start to talk to forest and conservation managers. So this isn’t a bad time to try and summarize it in the blog. Here goes:

Many of the things we appreciate about trees are provided best (or only) by big, old trees. There are exceptions, of course - nobody wants a 200 year old Christmas tree, and plantation forestry is mostly about growing trees until their early middle age. But for wildlife, shade, water management, landscapes, carbon storage and perhaps our own wellbeing, we need the presence of big old trees. 

I thus started with the view that maintaining and renewing our stock of big, old trees is perhaps the chief responsibility of forest managers, particularly in conservation or multi-use forests. The difficulty is that growing big old trees takes a long time - and for all that time the tree must find its environment viable - i.e. remain within its limits - drought, cold, wind, resistance to pests/pathogens, or combinations of these. So when we select trees for planting (or allow them to regenerate) with the aim of producing some big old trees, we're essentially making a bet that our efforts will be rewarded in 150-400 years' time with trees that have remained within their limits for all that time. 

Our current policy is to promote native tree species, in part because we have evidence that they can successfully live in the required location for centuries - the evidence of that is in front of us. However, I'm increasingly concerned that by promoting the exclusive use of local native tree species, we're making the mistake of assuming that the trees' environment will be as benign over the next few centuries as it was over the last few.

'Prediction is very difficult, especially about the future' - famously attributed to Niels Bohr

There's been a great deal of work done to model and predict the future changes in climate. In the UK researchers in the Forestry Commission have used this data to predict the viability of many of our native tree species, and the news was mostly good - that the expected range of future climates don't seem to be an overt threat to the health of our trees. But the data they used only went up to 2080, and almost no climate forecasts (and no analyses of future tree health) go past 2100. Making decisions for 150-400 years based on predictions for the next 80 years or so seems to me to be extrapolating well beyond what the data will bear, into a period where we cannot know the climates our trees will have to live through. What we are pretty sure of is that substantial climate change is already inevitable,  and that the response of trees and their pests/pathogens is very hard to predict accurately.

So, what should we do, if we can't answer the question 'what will be the viable tree species for the next few centuries'? It seems to me that the only way to approach that problem is by embracing the risks and uncertainty - and that means not putting all our eggs in one basket - i.e. using a diversity of genetic origins, species and perhaps management approaches.

Some forests are naturally diverse, particularly tropical forests. The great north-eastern forest in which I working is pretty diverse too, with 115 main tree species between the Carolinas and Quebec. Any system with such a lot of diversity has - automatically - significant redundancy - that is, species could be lost and the forest will continue to function. Europe's forests are much less diverse (a function of the orientation of our mountain ranges) and my own forests in Scotland are significantly less diverse again. Our Caledonian pinewoods are dominated by our single native conifer, Scots pine. It follows that the loss of any tree species in these low-diversity forests would have an impact much more devastating than in more diverse systems. 

200 year old oaks, dying from drought. Vierzon, Loire, France

 Which, to me, poses some difficult questions of our central woodland conservation principle of only using local native tree species. If that principle leaves our low-diversity woodlands open to future risks of catastrophic failure, perhaps it needs to be modified to accept a wider range of species, including non-native ones. Of course, such 'new natives' would need to have some functional overlap with existing natives. As an example of this thinking, before I left SNH, I made a recommendation that we should stop advocating the removal of sycamore from woodlands that would naturally be dominated by elm and ash. We expect to lose the elms, and the chances seem high that we'll see significant losses due to Ash Dieback. So, these woods may lose both their key canopy species, ash and elm, in the foreseeable future. Sycamore, with its large shady canopy (analogous to elm) and its basic bark pH (elm and ash) has become arguably an asset, not a menace, despite its non-native status. 

This seems to me a good test of my logic, and I wonder what you think?