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I started my academic career in 2008 with a BSc project on the spatial population dynamics of parasitoids of phytophagous geometrid moths in northern-boreal mountain birch (Betula pubescens var. pumila) forest. Due to my growing interest in this study system, encouragement from mentors, and the arrival of timely opportunities, I also went on to do MSc (2009-2010) and PhD (2011-2015) projects on the interactions between moths and other animal species, including parasitoids, passerine birds and saproxylic insects.

Historically, studies concerned with geometrid moths had mostly been motivated by the fascinating high amplitude population cycles of these animals. Much of my early work as a student also tied in with this tradition. However, throughout my student years, moth outbreaks were becoming a cause of increasing concern in the mountain birch ecosystem. While outbreaks by the native autumnal moth (Epirrita autumnata) were well known to be a natural feature of the system, the climatically facilitated northeast range expansion of the winter moth (Operophtera brumata) was increasingly causing outbreaks of longer duration, due to increasing overlap between the outbreak ranges of the two moth species. This resulted in historically unprecedented mortality rates of mountain birch in northeast Norway during the mid to late 2000s, with an associated shift from dominance of dwarf shrubs to graminoids in the ground vegetation.

The emergence of novel outbreak regimes sparked pressing questions about the resilience of the mountain birch system to this type of disturbance. Seeing the devastation wrought by the novel outbreaks, it was natural to wonder if the system could withstand this level of punishment without undergoing major structural change, i.e. a persistent loss of tree cover. This question also tied in to a larger narrative concerning the boreal biome as a whole. In 2012, Scheffer and colleagues published a seminal paper showing that tree cover in the boreal biome exhibited a bistability, where areas with high and low cover of trees appeared to co-exist as persistent alternative states. This type of bistability is characteristic of systems that respond to stress with abrupt non-linear changes in state (ecological regime shifts). Thus, throughout the years of my PhD and first postdoc (2015-2018), my research group paid increasing attention to the possibility of outbreak-driven state transitions in the mountain birch ecosystem.

The study of Scheffer et al. (2012) was a pattern-oriented description of tree cover states based on remote sensing data. However, the underlying theoretical framework clearly predicted the mechanisms that would need to be in place for a state transition to occur. First, the theory posits that systems vulnerable to state transitions respond to stress by way of tipping points, where system state changes abruptly when a certain critical threshold of forcing is exceeded. Second, alternative system states are expected to be maintained by positive feedback mechanisms that cause each state to be self-facilitating. This prevents the system from gradually reverting to its original state after a tipping point has been crossed.

The featured paper by myself and colleagues (Vindstad et al. 2019) is the culmination of an almost decade-long research agenda to test the aforementioned theoretical predictions for the mountain birch ecosystem. In accordance with the theory, we confirm that tipping points and positive feedback mechanisms are indeed prominent features of the response of this system to moth outbreaks. Concerning tipping points, we show that the relationship between forcing (defoliation pressure derived from MODIS-NDVI) and ecosystem response (birch stem mortality rate) is strongly non-linear, with stem mortality rates abruptly increasing from about 20% to 70% when a certain critical defoliation threshold is crossed. We also document positive feedbacks between surviving mature birch stems and trees and the production of new recruits. The polycormic mountain birch has a high capacity for replacing lost stems by way of basal sprouting, but we show that the efficiency this regenerative mechanism is highly dependent on the survival of at least some mature stems. Once a tree loses its last stem, the probability of producing sprouts drops from about 60% to only 20% on average. Accordingly, 4 out of 5 trees that lose all of their mature stems will be permanently lost from the population. In the absence of basal sprouting, new trees have to be produced from seed. However, we also show that the rate of sapling production is more than twice as high in the presence of some surviving mature trees as when all trees are dead. This is another indication that mature survivors facilitate the production of recruits.

Our findings do much to clarify the dynamics that the mountain birch ecosystem can be expected to display under stress. First, the positive feedbacks we have uncovered imply that a healthy or moderately damaged forest will have high capacity for producing recruits and will thus be self-maintaining. On the other hand, a forest that has lost the great majority of its stems to a severe outbreak will likely struggle to recover and may thus exhibit a persistent transition to a non-forested state. This is exactly the kind of dynamic that can cause an ecosystem to exhibit alternative stable states. Further, the tipping point in the response to defoliation means that the difference between negligible forest damage and mass mortality can hinge on relatively small changes in defoliation pressure close to the tipping point. This is admittedly somewhat ominous when we consider the fact that the ongoing range expansion of the winter moth is currently leading to higher cumulative defoliation pressure in areas that historically have seen outbreaks only by the autumnal moth. This can increase the probability of exceeding the tolerance threshold to defoliation, and thereby increase the risk that outbreaks decimate the forest to the extent that positive recruitment feedbacks start to be lost

At this point, it is important to emphasize that persistent losses of tree cover in the mountain birch ecosystem are not purely hypothetical. We know that forested areas that were heavily damaged by unusually severe autumnal moth outbreaks in the past have sometimes failed to recover and transitioned to treeless secondary tundra. Evidence of this can be seen in the Utsjoki municipality in northern Finland, where we can observe large open areas pockmarked by the rotting stumps of trees that were killed by an outbreak during the 1960s. The occasional failure of mountain birch stands to regenerate has usually been attributed to grazing by semi-domestic reindeer. While this is undoubtedly a contributing factor in some areas, our current findings also indicate that that the regenerative pathways of the mountain birch display internal positive feedbacks that can make the system vulnerable to regime shifts.

In the wake of the featured paper, we are left with questions about the degree to which the results can be generalized. These questions must be posed on multiple scales. First, with respect to the mountain birch forest itself, this is a system that covers strong spatial gradients in climate, productivity and browsing pressure from ungulates. All of these variables may influence the relationship between defoliation pressure and forest mortality or the capacity of the forest to recover. Thus, our first priority for the future is to conduct a field survey of forest health that includes the aforementioned drivers as explicit design variables and expands the scope of the sampling outside of the original study region in northeast Norway. We aim to conduct this survey during the summer of 2020 (coronavirus situation permitting).

On a larger scale, questions about generalizability also apply to the boreal biome as a whole. The multimodal distribution of tree cover states across the biome hints at underlying tipping points and positive feedbacks also outside of the mountain birch ecosystem. Confirming the presence of such mechanisms would imply that biome-wide state transitions under sustained climate-induced forcing cannot be ruled out. As the boreal zone is the largest terrestrial biome on earth, this could have profound consequences for ecosystem processes that feed back to the climate system, including carbon sequestration and albedo. The matter is also pressing because the boreal zone is one of the most rapidly warming biomes on the earth, and is expected to approach temperate climate regimes by the end of the century. While the long-term consequences of this are unforeseeable, a good understanding of tipping points can help us to foresee imminent state transitions in the near future. This can facilitate planning and implementation of proactive measures for stakeholders and managers. 

Finally, on an entirely general level, a timely contribution of the theoretical framework of ecosystem regime shifts has been to alert ecologists to the fact that ecosystems may respond to climate change by abrupt state transitions, rather than gradual change. However, although this expectation is theoretically well founded, empirical evidence for climate-driven state transitions has so far been limited and mostly derived from aquatic systems. Thus, it remains to be seen if the non-linear response to climate-induced stress that we have documented for the mountain birch forest is an exception or a rule for terrestrial systems. It is important that this question be answered, as it will largely determine how we should expect our ecosystems to behave under climate change.         

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Ole Petter Laksforsmo Vindstad Researcher at UiT – the Arctic University of Norway and member of the Climate-Ecological Observatory for arctic Tundra (COAT).

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You can read all of the Harper Prize 2019 shortlisted papers in our Virtual Issue.