Encountered today an article from the guardian about ash dieback in the UK (http://www.guardian.co.uk/environment/2013/mar/26/ash-dieback-landowners-restock-woodlands) which reminded me about the fact that I‘ve heard about this disease last summer semester as well even though in German we call it „Eschentriebsterben“ (yep, all German readers finally know what I‘m talking about). It also reminded me about a very valuable lesson about forests and species in context with climate change and is (even though I admit I kinda forgot about that) one of the main reasons I do not agree with single or few species strategies even when they might be the potential natural vegetation of a stand (let‘s talk about PNV another time :P).
So short excursus so you don‘t have to get stuck up in the details of the disease:
Ash dieback is a disease which is comparatively young – in Germany it occurred for the first time around 2002. What is basically happening is that because of a certain fungus called Chalara fraxinea the shoots of the ash tree (Fraxinus excelsior) are dying off. This concerns young and older trees and it also causes bark necrosis (looks kinda like scars on the bark) and leads eventually to death. Issue is that it concerns the majority of ash trees by now and the success rates of newly planted trees is so low that most state ministries either advise forest owners to abstain from planting ash trees (for Germany) or pay them to not do it (UK). The death rate of pole crop ash stands is alone because of Chalara fraxinea around 20% now.
Chalara fraxinea is the anamorph (meaning the asexual form of the mushroom) form of Hymenoscyphus albidus („Weißes Stängelbecherchen“) which was unknown till 2006. The mushroom itself is actually harmless and only decomposes litter in the forest. It‘s still kinda important to know it since this one is producing the spores Chalara fraxinea is originating from. But that‘s already too nerdy, let‘s stick with the important talk without drifting off to mycology.
So two things are important here: The mushroom H. albidus and it‘s anamorph have been around for quite a while – just like the ash. Only few years ago the little guy didn‘t seem to be as much of a bother to the ash trees as he is nowadays – because today it‘s pretty much lethal. We can only guess how long those two have been around but it‘s gotta be at least a couple of centuries, I bet even some thousand years at least. Studies figured that H. albidus had mutated to a new form which is morphologically not distinguishable but as said way more lethal and by scientists called H. pseudoalbidus („Falsches Weißes Stängelbecherchen“).
So why did the situation get so bad now? Climate change? Other human influences? Or pure (bad) luck? Even though there‘s surprisingly much coincidence in nature I‘d like you guys to keep in mind that all those new factors and stresses we put on nature might – just might – lead to some more mutations than we (or nature) is used to. Can‘t prove it but can‘t counter-prove it either.
Ok, so for one climate change could cause more mutations leading to more diseases tackling trees. Not good. However, the other thing is concerning the current course of nature conservation at least to me even more worrying:
Recently there has been a huge call from nature conservation NGOs for close-to-nature forestry with only native tree species. That‘s actually a pretty good idea – who would still want pine monocultures or douglas fir stands? Let‘s have the forests that belong here back!
Unfortunately this leads within those campaigns to a rarely known intolerance for ANY amount of ANY foreign tree species meaning usually the call is to leave them out systematically. Now in most parts of Germany we have a beech forest type which means because of the attributes of beech as a species naturally we got mainly beech (let‘s say at least 60%) and one to maybe 3 (rarely more) side species. In combination with the demand to increase the stand volume which results in less thinning it means the number of tree species in a forest stand does rarely exceed 4 and is mainly depending on beech.
So imagine for a second we follow through with this demand and do this kind of forestry. Our climate change adaption strategy would be to reduce the amount of species and therefore rely on single species – especially beech – more heavily than ever before. In exchange forests would theoretically develop naturally in this direction anyways so fewer forestry measurements are necessary which also means cost reduction.
So what role plays the ash here? Well, ash was because of many attributes like wide natural distribution, high tolerance against pests or drought tolerance one of the tree species forestry had it hopes high concerning climate change – just like those NGOs do now in beech. What if the same happens to one of those few species in a stand reducing it‘s biodiversity and stability greatly (especially if it was beech)? Nobody ever thought it could happen to ash, who are we to claim the same might not happen to beech?
As much as I understand the wish for natural forestry in terms of risk management I cannot understand this strategy and certainly not support it. There‘re two things we should understand as soon as possible concerning climate change: 1) we do not know what will happen and how it will influence our ecosystems and 2) therefore we need a forestry strategy which has a good backup plan and non-knowledge management strategy which distributes the risk as much as possible. I do not want 20% douglas fir in a forest. But neither do I necessarily favor 80% beech.
Because the only thing we know is that we don‘t know anything.