SLU news

Globalization and climate change paves the way for invasion and mass death

Published: 11 January 2017

Multiple reports tell of how tree species prevalent for millions of years fall victim to mysterious illnesses and succumb at an alarming rate. This is caused by globalization and climate change; two factors created by mankind. A new review study from SLU illustrates how the lack of co-evolution between host and environment, just as well as the better known evolution between host and pathogen, enables these outbreaks.

All forests are under constant attack from pests and pathogens, among which the greater part consists of fungi. The outcome of such attacks is decided by the nature of the interaction between host and attacker. Interaction between a domestic pathogen and its host is usually balanced. These species have evolved together, probably during millions of years, and the host has acquired a certain resistance against the pathogen whereas the pathogen has a certain virulence against the host. A lesser part of the infected hosts are lost every year, although this part varies between seasons with, among other things, how changing weather conditions benefit or detriment either part of the interaction. Usually however, the balance is not affected and domestic pathogens thus rarely constitute an existential threat against domestic hosts. Notwithstanding, they can still be highly significant. Heterobasidion annosum, for example, is a domestic pathogen causing root rot in conifers that may be considered in balance with its hosts pine and spruce, but the portion of harvested lumber lost yearly still cost the Swedish forest industry roughly 1 billion SEK.

Invasive and emerging species

Contrasting the balanced interaction is that between a foreign, or invasive, species and its host. These species have not evolved together, which is why the host does not carry a specific protection. Quite the contrary, it might carry an inherent sensitivity that has never been subjected to any evolutionary pressure, but is dramatically exposed as an Achilles’ heel threatening the very existence of the species when the invasive species arrives, usually transmitted through mankind’s transport of plant material. Ash decline in Europe is an example of the result of such an invasion. The casual fungi originates from China, where it lives in the leaves of Manchurian ash without causing any detectable harm to the tree; probably in order to live off the fallen leaves once shed. European ash, however, has proved to be exceedingly sensitive to this behavior, in spite of the fungus apparently not having evolved as a pathogenic parasite. Such effects occur due to the mere chance that some microbe genotypes happen to be very harmful against some plant genotypes. It is possible that virtually every plant or animal carry such an Achilles’ heel; a random sensitivity to a gene in some microbe somewhere. As long as they are geographically separated nothing happens, but globalization and its extreme increase in transports of people and goods has made such unfortunate encounters infinitely more likely. The microbe may then transform into aggressive pathogen and the plant into super sensitive host, with mass death as the inevitable outcome.

However, not all sorts of dramatic outbreaks of disease can be explained by such lack of co-evolution. For example, Sweden has not yet been struck by any serious outbreaks of Dothistroma needle blight on pine, in spite of several singular observations of symptoms and of multiple examples of devastating attacks in other countries. This occurs because the disease is regulated by weather conditions, which either stress the host and make it more susceptible, or benefit the pathogen and make it more aggressive. Random weather changes constantly make some years more conducive for outbreaks than others, but the ongoing climate changes could transform temporary conditions into permanent ones. If Swedish spring and early summer would get wetter, it would be dramatically beneficial for outbreaks Dothistroma needle blight. Concerns in regard to climate change thus lies in the risk that some domestic species in balance with their hosts, or some invasive species not yet able to cause any greater havoc, might suddenly grow significantly more viable, reproducing or active if environmental factors such as humidity, temperature, light and wind should alter. This could enable a greater production of inoculum, i.e. the pathogen’s infecting agent, as the optimal weather conditions during which the fungus may produce fruiting bodies and spores might occur more often and for longer periods of time, but may also allow the pathogen to complete more life cycles per year, or more spores to survive through the winter into the next growing season.  In addition, new climate conditions may open new areas for many pathogens previously limited by too cold winters or too dry summers. Obviously, the opposite can also be true; a pathogen that only infects the freshest shoots has evolved to produce as much inoculum as possible during shooting. If climate changes would propel or delay this time point, the pathogen would be deprived of its substrate.

Model for risk assessment

If one’s target is to understand how, when and why an invasive or domestic species has any important impact on a certain plant species, these factors are highly important to consider. This is discussed in a new review study from the Department of Forest Mycology and Plant Pathology at the Swedish University of Agricultural Sciences. Professor Jan Stenlid is one of its authors.

“Two external factors act as the main drivers for the many outbreaks of invasive or suddenly aggressive domestic species that we see today”, Jan Stenlid says. “Both of these are created by man. One is globalization and the ever increasing transports it brings. These constantly introduces new species throughout the globe, and the effects of this, and what hosts are potentially affected, is impossible to foresee. One year it is the ashes of Europe, the next the avocado plantations of Mexico; the only thing we know for sure is that risks increase linearly with transports. The other factor is global warming, which continue to make new areas susceptible to invasion by species that previously couldn’t establish themselves there.”

Jan Stenlid and his co-author Jonas Oliva have constructed a theoretical model to visualize how these factors affect the magnitude of an attack. In recent years, the lack of co-evolution between host and pathogen has usually been suggested as the primary cause of greater outbreaks, but the model also takes into account the lack of evolution between host and environment.

Scleroderris canker as a case study

A good example of how the impact of an invasion is affected by the lack of host adaptation to a new climate, is the disease Scleroderris canker. It is transmitted by an Ascomycete fungus called Gremmeniella abietina and attacks first and second year shoots on the domestic Scots pine.

“If pines are planted on more fertile soil, they will grow more susceptible to Gremmeniella attacks”, Jan Stenlid says. “The reason is unclear, but it has been suggested that since trees on fertile soil produce less needle cohorts, a greater part of those will sit on the first and second year shoots that gets attacked. We also know that when pines of southern descent are planted in northern areas, damages caused by Scleroderris canker grow more serious. This is probably due to the fact that trees adapted to a southern climate will winter harden later, that is, go into a form of cold and drought resistant torpor in order to survive the winter months. Thus, they attract frost damages in the early northern winter, which decreases resistance against Gremmeniella.”

“Maladaptive behavior like late winter hardening is common when an economy crop is introduced to a new environment”, Jan Stenlid says. “They may grow much more productive than in their original environment, partly because they are removed from their natural parasites and end up in an ecosystem where no species has adapted to live as a pathogen on them. But there is a back side to this perk, since the tree has not adapted to the local environment either and thus will not allocate its resources in a way that protects it in the long run.”

Gremmeniella is also a good example of a disease that may be drastically affected by climate change. The disease is usually not dramatically prevalent, but the fungus may during certain conditions wander from the needles to the branches and cause mortality in the trees. This only happens when the amount of inoculum is high enough, which usually occur in rare three-year-cycles where one conducive year, with cold and rainy summers and cold and wet winters, are followed three years later by a similarly beneficial year. Such winters and summers are predicted to increase rampantly in wake of the climate changes. These very conditions are, by the way, also conducive to a wide range of other pathogens.

“Scientifically founded risk assessments and estimations of damage are essential for us to fully understand the effect globalization and climate change has on the Earth’s ecosystems”, Jan Stenlid says. “Case studies like the one on Scleroderris canker will be important components in this work.”


Contact

jan.stenlid@slu.se, 018-671807