Under very rare circumstances a spatial mismatch caused by climate change can be a good thing. But before you start betting on compensating one ecological disturbance with another that’s possibly even more dangerous, we think perhaps we should discuss another solution now that forest plagues seem to be increasing everywhere around us.

It’s time you start recognising this tree. Although ‘everyone’ can tell an oak from a beech or a poplar, this hugely important European endemic tree is somewhat anonymous: the ash tree – actually a family member of the olive (Oleaceae). Young trees look a bit spindly, while mature trees develop a very strong oak-like appearance (as if ash trees go through puberty). The European ash is hugely important for biodiversity, with a thousand associated species, from mosses to mammals, indirectly depending on its presence [here’s just the list of moths and butterflies]. Oddly though, this widespread age-old tree, is suddenly at risk of becoming extinct, threatened by two simultaneous plagues – fitting a global trend of increasing forest plagues and threatened tree species. Image: drawing of flowering ash twig, Franz Eugen Köhler, 1862.

Pathogens are part of the natural world. In fact most of Earth’s 100 billion to 1 trillion species’ biodiversity is microbial – including bacteria and fungi (and depending on definition you may even include viruses). Therefore it’s not an unnatural thing for a tree to catch a tree disease. What is however a sign that something is wrong, is when all trees of a single species catch the same disease, especially when it’s a lethal infection.

That’s when you’re talking about a plague – and plagues are usually helped in place by a bigger, deeper ecological disturbance – or several, working together to create an imbalance that weakens a species’ natural defence against a pathogen that it previously managed to co-exist with (or at least co-survive) for probably thousands of years.

Climate change is linked to many different forest diseases and tree dieback crises

When we talk about tree plagues and climate change, there are many examples; both from paleostudies into Earth’s past extinction events when climate forest plagues reached mass proportions, as from our current real-world observations:

We could talk about the Atlas cedar of the Maghreb mountain ranges in Mediterranean North Africa, cedar forests that (apart from direct deforestation) are declining after individual trees are first being weakened by persistent drought, then killed by a combination of fungi, insects and wildfires, we learned from a special publication about drought and heat-induced tree mortality in Forest Ecology Management from 2010.

Or we could talk about either the spread of Armillaria root disease on conifer trees (caused by the parasitic ‘honey fungus’), Yellow-Cedar decline (paradoxically triggered by frost damage to the roots as winter snow cover decreases), dwarf mistletoes (parasitic plants from the genus Arceuthobium, living mostly under the bark of (pine) trees) and Cytospora canker, a pathogenic fungus on Aspen and spruce trees – forest diebacks that are all promoted under warming & drying conditions – or tree diseases caused by the Phytophthora water mold, culprit of ‘sudden oak death’, deadly pathogens that multiply quickly where the climate becomes warmer & wetter, as described in a 2011 report by the USDA Forest Service titled ‘A Risk Assessment of Climate Change and the Impact of Forest Diseases on Forest Ecosystems in the Western United States and Canada’ (PDF).

The most well-known climate plague is not 100% climate-caused

But the example that jumps to many people’s minds is the pine beetle plague that wrecks havoc in part of the US Rocky Mountains and the western boreal forests of Canada, primarily Alberta and British Columbia.

Indeed one of the deeper causes of this plague is climate change – predominantly the effects of winter warming. The larvae of the pine beetle that is actually endemic to these forests die after a couple of nights with temperatures below minus 35 degrees Celsius, winter temperatures that used to occur most winters. When however you have too many mild winters in a row, you have a problem. The Canadian pine beetle plague is even seen as a positive climate feedback, because the large-scale forest dieback it causes leads to megatonnes of CO2 emissions to the atmosphere, feeding back on the warming that drives the plague.

But as is often the case under plague situations, it was put in place by more than one factor. Another very important one is the forestry industry. Large-scale clear-cutting and replanting pine trees as is common practice leads to large areas with trees of the same age and therefore same vulnerability, creating an ideal breeding ground for any plague agent. It also leads to a forest monoculture, while natural boreal forest biodiversity inhibits pine beetle outbreaks.

European ash dieback – invasive species, or decreasing genetic diversity?

Now there are also tree plagues where the role of human (re)planting may be the largest underlying cause. This may be the case for a chronic tree disease that is currently rapidly spreading across Europe: the ash dieback crisis, caused by the pathogenic fungus Hymenoscyphus fraxineus or (in its asexual stage) Chalara fraxinea. As the name suggests, this fungus depends on the ash tree (genus: Fraxinus).

Although the pathogen was only first described by science in the year 2006, it has probably been around (ash trees) for a very long time – as it resembles the harmless European ash tree fungus Hymenoscyphus albidus that is not pathogenic to living trees and feeds on dead ash tree leaves. An even closer relative or possibly the identical fungus is known as Lambertella albida in Japan, where it lives harmlessly on the tiny stalk between the leaf and the stem of the Manchurian ash, a tree that has immunity to deeper infection of the fungus. From this resemblance it is thought that the fungus may originate from Asia, and could be regarded as an invasive species in Europe.

Now in Europe, where the European ash tree (Fraxinus excelsior) has large ecosystem importance in forests [at least a hundred species of plants, insects, mosses and lichens directly depending on its presence and another thousand species associated with ash woodland, including 12 birds and 55 mammal species] H. fraxineus is slowly reaching plague proportions, first starting as a large-scale infestation in Poland in the early 1990s and since spreading across the continent.

Although there is no known cure once an ash tree is infected, it remains important to monitor the spread of the plague across Europe. Land owners and reserve keepers are encouraged to leave both healthy and infected trees standing. Unlike other fungal tree diseases, ash dieback cannot spread from dead wood. For practical monitoring tips, please visit the website of the UK Woodland Trust. As their above picture beautifully illustrates, mature ash trees can grow to majestic proportions. Ash dieback is therefore not only a loss for ecology, but also for the European landscape.

The species as a whole is considered to be highly vulnerable to deep infections with the fungus, leading to a slow but progressive die-off of branches and ultimately the tree as a whole. However, a small fraction of individual trees (possibly including some cultivars) seems less vulnerable, prompting the thought that some trees may contain genes that improve their immune response to the fungus. Yes, we think of many European tree species as single species and that would be it – but ‘the real biodiversity’ lies underneath: intraspecific biodiversity, and that tree diversity on the gene level actually used to be relatively high in Europe’s endemic species, we learned in our special long article about the effects of climate change of temperate zone biodiversity.

The European landscape is an intensive human-dominated landscape, including its semi-natural forests, which are often planted – especially in the case of ash, a tree that is popular in the forestry industry. Could it be that we have accidentally decreased European ash tree genetic diversity by cutting down the old forests and planting trees from tree nurseries, promoting a form of genetic monoculture?

Meanwhile the Emerald ash borer (a metallic green, 9 millimetres long beetle – another invasive species originating from East Asia) aggravates the European ash dieback, potentially creating a second plague on already weakened, chronically fungus-infected trees – so the situation is looking quite grim. According to a publication from March 2016 by Peter Thomas of Keele University in the Journal of Ecology there’s even a real risk of extinction of the European ash tree:

“[…] ash health and survival is currently seriously compromised by ash dieback caused by the fungus Hymenoscyphus fraxineus (Chalara fraxinea) that has the potential to kill all but a very few resistant trees. Moreover, the emerald ash borer beetle Agrilus planipennis, a serious pest of ash species in N. America, has reached Europe (though not yet the British Isles) and poses an equally if not more serious long-term threat to ash.”

And another recent publication, from October 2016 in Nature’s Scientific Reports agrees, stating ‘the ash tree dieback raises a concern for the persistence of this widespread European tree’.

To the rescue… climate change?

The latter study, performed by a group of six scientist led by Eric Goberville of the University of Lille, investigates the role climate change may play in the ash dieback. Unlike the examples on top of this article we wouldn’t call the ash dieback a climate plague, but continuation of 21^st century climate change will affect its spread, we learn from the French researchers – and possibly not all bad.

We found the study as we’re interested in how the ecological consequences of climate change are often not a product of direct climate interaction on a single species, but instead the differential climate effects on interacting species.

In our last two articles of this series we’ve first presented examples of what ecologists call ‘a temporal (trophic) mismatch’ – the example of birds showing a differently timed response to climate change than the spring species their successful nesting depends on – and then after that of a similar ‘spatial mismatch’ – with the example of how climate change may separate the geographical spread of butterflies and their host plants.

Such spatial mismatches may however become far more common, as entire biomes will have to migrate in order to survive climate change – at timescales that will create large differences among interdependent species according to their respective dispersal abilities.

Trying to compensate a disturbance with another disturbance – when a spatial mismatch can be a good thing

In very rare instances a developing spatial mismatch may be a good thing – with the obvious example of separating a plague-causing pathogen and its otherwise possibly doomed host: we’re still talking about Hymenoscyphus fraxineus and Fraxinus excelsior.

The French research group paired two ecological models with seven general circulation climate models for the four different warming scenarios used by the IPCC (RCP2.6 to RCP8.5) and found the geographical spread of the ecological niche for both the European ash and its pathogenic fungus will be affected by 21^st century climate change, but possibly in different ways:

“[…]we show that climate change, by affecting the host and the pathogen separately, may uncouple their spatial distribution to create a mismatch in species interaction and so a lowering of disease transmission. Consequently, as climate change expands the ranges of both species polewards it may alleviate the ash dieback crisis in southern and occidental regions at the same time.”

However, looking at the projected geographical spread of both the ash tree and the fungus – some caution is warranted against unfounded climate change optimism, as the modelled spatial mismatch seems largely limited to the southern margin of the current spread of the fungus, France in the Southwest and the northern Balkans and Belarus on its southeastern flank, and the overlap between host and pathogen actually remains large in central-northern Europe, roughly from the United Kingdom to Poland.

The above image shows the current spread for (a) the European ash tree and (b) the plague-creating pathogenic fungus. Maps d, e, f and g are projections under future warming scenarios, indicating an already existing spatial mismatch along the southern margin may increase.

Meanwhile an ecological niche develops for both the common ash and H. fraxineus in parts of Scandinavia that were previously too cold – actually promoting spread of the plague to a region where both the fungus and the tree did not occur in the pre-industrial climate.

The good news therefore may not be for European ecosystems as a whole, but for Fraxinus excelsior as an individual species: while continuation of the plague in Central and northern Europe places these ash tree populations are at increasing extinction risk, locally the tree may find a friendly environment to survive.

If however you share our overall concern of decreasing forest health throughout Earth’s temperate ecozone, then climate change is not what you should hope for. Instead of extra warming the utopian solution is promoting extensification of human land use and improving ecosystem health through increase of endemic natural diversity. Perhaps tree nurseries can play their role, by re-entering old and now rare tree genes into the tree pool of the landscapes of Europe, Asia and North America. Then you’ll see those forests can usually fend for themselves, like they have managed for thousands of years before us.

© Rolf Schuttenhelm | www.bitsofscience.org