Boreal Riparian Vegetation Under Climate Change - Springer Link

Ecosystems (2013) 16: 401–410 DOI: 10.1007/s10021-012-9622-3 Ó 2012 Springer Science+Business Media New York

Boreal Riparian Vegetation Under Climate Change Christer Nilsson,* Roland Jansson, Lenka Kuglerova´, Lovisa Lind, and Lotta Stro¨m Landscape Ecology Group, Department of Ecology and Environmental Science, Umea˚ University, 901 87 Umea˚, Sweden

ABSTRACT Riparian zones in boreal areas such as humid landscapes on minerogenic soils are characterized by diverse, productive, and dynamic vegetation which will rapidly react to climate change. Climate-change models predict that in most parts of the boreal region these zones will be affected by various combinations of increased temperature, less seasonal variation in runoff, increased average discharge, changes in groundwater supply, and a more dynamic ice regime. Increasing temperatures will favor invasion of exotic species whereas species losses are likely to be minor. The hydrologic changes will cause a narrowing of the riparian zone and, therefore, locally reduce species richness whereas effects on primary production are more difficult to predict. More shifts

between freezing and thawing during winter will lead to increased dynamics of ice formation and ice disturbance, potentially fostering a more dynamic and species-rich riparian vegetation. Restoration measures that increase water retention and shade, and that reduce habitats for exotic plant species adjacent to rivers can be applied especially in streams and rivers that have been channelized or deprived of their riparian forest to reduce the effects of climate change on riparian ecosystems.

INTRODUCTION

and more sensitive landscape units deteriorate (Naiman and others 2006). Second, when organisms reorganize as a result of changing environmental conditions, riparian areas can serve as early warning systems (Johnson and others 2006). This role is particularly relevant in the face of climate change which affects both temperature and hydrologic regimes. This review focuses on riparian ecosystems in the boreal zone, that is, the subarctic part of the northern hemisphere. The boreal zone is dominated by taiga and tundra and covers about 15% of the earth’s land area and comprises northern Russia, northern Europe, Canada, and Alaska. Boreal regions offer numerous streams and rivers and impressive examples of riparian landscapes, some being spatially extensive with high structural and functional complexity (Sokal and others 2010;

Key words: boreal landscapes; climate change; free-flowing rivers; groundwater; ice dynamics; plant invasion; riparian zones; water-level fluctuations.

Riparian zones occupy the interface between land and running waters and are naturally adapted to variable environmental conditions (Gregory and others 1991; Naiman and Dećamps 1997). Their intermediate role has two functional implications. First, they are able to adjust to environmental change, which gives them an important landscapescale stabilizing function when conditions change

Received 21 May 2012; accepted 13 November 2012; published online 8 December 2012 Author Contributions: Christer Nilsson and Roland Jansson conceived of the study and wrote the paper, Lenka Kuglerova´ wrote the sections about groundwater, Lovisa Lind wrote the sections about ice dynamics, and Lotta Stro¨m performed research. *Corresponding author; e-mail: [email protected]

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Dickens and others 2011). The majority of boreal streams and rivers have permanent, although seasonally varying flows, due to variation between cold and warm seasons (Woo and others 2008). During late winter, flows and water levels are at their lowest—and the channels are more or less covered by ice—and during the spring flood following the melting of snow and ice they usually reach their annual highest levels. Sometimes, however, ice events can cause water to rise even higher than during open-water conditions (Prowse and Beltaos 2002). This hydrologic variation among seasons has formed and maintains riparian abiotic and biotic conditions, and is the driver of related processes such as redistribution of sediment, litter, and plant propagules (Gregory and others 1991; Nilsson and Svedmark 2002; Ward and others 2002). Typically, the riparian vegetation along larger streams and rivers is zoned going from forest at the top of the riparian zone to shrubs in the middle to a dominance of graminoids and amphibious plants in the bottom of the riparian zone (Figure 1). In a Swedish example, the riparian vegetation locally harbored most plant species in its middle portion but regionally, the upper area included most species reflecting that the terrestrial species pool is larger than the aquatic one (Nilsson 1983). Riparian vegetation is usually highly productive (Tockner and Stanford 2002), and in boreal, nutrient-poor taiga regions dominated by conifers

Figure 1. The vertical zonation of vegetation in riparian zones along boreal forest rivers on minerogenic soils, delimited into the major vegetation belts. Hatched lines mark the limit between vegetation belts under present and predicted future (at the end of the twenty-first century) hydrologic conditions. The average position of the spring flood peak and summer low water levels are also marked. Modified from Stro¨m and others (2012).

with an understory of ericaceous dwarf shrubs, it often stands out in stark contrast to the surrounding vegetation (Nilsson and Jansson 1995). Riparian zones also offer valuable habitat and food for numerous animals, such as ungulates (Brookshire and others 2002), beaver (Naiman and others 1994), birds (Larue and others 1995; Wiebe and Martin 1998), and beetles (Andersen and Hanssen 2005) throughout the year, and serve as major dispersal and migration corridors for organisms across landscapes (Johansson and others 1996; Machtans and others 1996). Beavers represent a disturbance because they are able to modify streams and their edges by building dams and dens, cutting wood, and digging tunnels and furrows in the banks (Gurnell 1998; Collen and Gibson 2001). Plant dispersal in riparian zones may occur throughout the year; however, mostly during the ice-free period and especially during major (spring) floods when plants can be waterborne over substantial distances (Nilsson and others 2010) and extend the distribution areas of many plant species (Danvind and Nilsson 1997). Although boreal riparian zones are dynamic systems adapted to variable geological, hydrologic, and climatic conditions, current and predicted future climate change presents and will present difficult challenges. In general, free-flowing rivers will be able to adjust to new conditions whereas regulated, confined rivers may require extensive management intervention to avoid climate-change related catastrophes (Palmer and others 2008). In this paper, we do not repeat the conclusions by Palmer and others (2008) for dammed rivers but restrict the discussion to anticipated changes in the remaining free-flowing and little developed rivers (Nilsson and others 2005), with several examples taken from humid, bedrock dominated landscapes such as northern Europe. Based on climate-change models for the next 4–9 decades, three major drivers of ecosystem change can be identified for those rivers: increased temperature (IPCC 2007), increased but seasonally less varied runoff, and more dynamic ice regimes (Table 1). The aim of this paper is to summarize these drivers and describe anticipated changes in riparian vegetation. The large boreal zone comprises much variation that is impossible to describe in detail in a short paper and major areas, such as Russia, seem to lack detailed predictions of climate-change effects. We, therefore, discuss more general changes in the boreal zone and provide some specific examples, well aware that they are just examples and may not be applicable to the entire boreal area.

Boreal Riparian Vegetation Under Climate Change Table 1. Change

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Selected Examples of Predicted Changes in Boreal, Riverine Ecosystems, Following Climate

Climate change variable

Riverine change

Increased temperatures

Immigration of temperate species and potential loss of boreal species Expansion of existing populations of exotic plants Drought stress during summer following increased evaporation Increased precipitation and permafrost and glacier thaw leading to increased discharge in streams and rivers Earlier and smaller spring floods and increased winter discharge implying narrower and less species-rich riparian belts Increased frequency of heavy rains and flood erosion events Reduced duration and thickness of surface ice, favoring availability of aquatic habitat during winter More shifts between freezing and thawing stimulating formation of frazil ice, anchor ice, and hanging ice dams, favoring production and species richness of riparian vegetation Flow obstructions because of extensive ice formation, leading to flooding and Aufeis formation

Changes in hydrology

Increased ice dynamics

RESPONSES

TO

CLIMATE CHANGE

Increased Temperatures The mean annual temperature in boreal regions is predicted to increase during the rest of the century, implying longer growing seasons and shorter winters (Rouse and others 1997; IPCC 2007). Such changes may facilitate the immigration of ‘‘new’’ species primarily from temperate regions (Saetersdal and others 1998), but may also lead to the extinction of species that are currently adapted to boreal regions. However, the potential number of colonizing species greatly exceeds the potential losses, because the number of species specialized on cold, disappearing habitats is relatively small (Saetersdal and Birks 1997), and the number of species increases to the south. Given that most boreal plant species adapted to wet and mesic soils are at least occasionally found in riparian zones (Reno¨fa¨lt and others 2005), many of the new species are likely to appear in riparian zones, altering their species composition and potentially enhancing competition. Although the number of potential invaders with a warmer climate is large, predicting when and which species will be able to colonize is fundamentally difficult. As a result of geographic barriers, habitat fragmentation, and declining population sizes, many southern species may not have the capacity to expand their ranges into boreal areas that become climatically suitable. In contrast, many exotic species are already present in boreal landscapes, but are confined to marginal habitats because they are not well adapted to current climatic conditions, originating in temperate areas (Mandryk and Wein 2006; Sumners and Archibold 2007; Jauni

and Hyvo¨nen 2010). Such exotic species adapted to more temperate climatic conditions should preferentially benefit from warming and spread. Even though boreal regions so far have very low proportions of exotic species (Dynesius and others 2004; Sumners and Archibold 2007), riparian zones belong to the habitats most frequently invaded (Rose and Hermanutz 2004). In many temperate regions exotic species already make up a substantial part of the riparian flora (DeFerrari and Naiman 1994; PlantyTabacchi and others 1996). Moreover, riparian zones are efficient conduits for the dispersal of invading species across landscapes (Naiman and Dećamps 1997). Thus, although many temperate species are likely to eventually colonize riparian zones along boreal rivers, the number and identity of species colonizing as well as the timing of arrival is difficult to predict. The safest prediction is an increase in the number of exotic species. Increasing temperature will also cause decreased soil moisture and groundwater tables, especially during summer when vegetation is most active and precipitation is low. It has been suggested that increased evapotranspiration during summer might cause a soil moisture deficit of up to 30% in boreal ecosystems (Okkonen and others 2010). This may induce drought stress given that riparian zones are groundwater-dependent ecosystems (Bertrand and others 2012), thus being sensitive to decreases in water table and soil moisture. Significantly lowered summer groundwater tables would reduce runoff to streams and rivers during these periods which in turn would imply less headwater supply. Since headwater streams harbor distinct riparian communities (Nilsson and others 1994) and the total

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length of headwaters far exceeds the total length of major rivers (Bishop and others 2008) this may result in dramatic changes of riparian communities on the landscape scale. However, global and regional circulation models of climate suggest that runoff at regional scales will increase in most boreal areas as a result of higher precipitation (Dankers and Middelkoop 2008; Woo and others 2008), making the net effect uncertain. Higher temperatures during winter will also result in less snow accumulation and this can significantly alter soil conditions (Okkonen and others 2010). Snow insulates soils during winter and the thickness of the snow layer determines the depth of soil frost and thus controls the groundwater table (Okkonen and others 2010). Enhanced soil frost can mechanically damage soils and plants (Freppaz and others 2008). This can be destructive for perennials because their root systems and organs for vegetative reproduction are typically situated below ground. However, the decrease in snow cover may be balanced by increased winter temperatures (Okkonen and others 2010), thus making it difficult to predict the most likely responses of riparian plant communities.

Changes in Hydrology The mean annual precipitation is predicted to increase in the entire boreal zone and more rain will fall instead of snow during winter in high northern latitudes (Christensen and others 2007; IPCC 2007). These changes will lead to increased discharge in boreal rivers (Falloon and Betts 2006), a phenomenon that is in fact already underway (Peterson and others 2002). For example, a hydrologic model for Sweden (Andreásson and others 2004) predicts mean annual runoff to increase by up to 24% in its boreal region with decreased spring-flood peaks, increased frequency of high flow events during fall and higher winter flows for the period 2071–2100 (Andreásson and others 2004). Similar increases have been predicted for boreal Russian and North American rivers (Falloon and Betts 2006; Palmer and others 2008; Woo and others 2008). Earlier snowmelt and thinner snow packs will also result in earlier and smaller spring floods (Andreásson and others 2004). Lower spring-flood peaks, and especially a decrease in the magnitude of extreme flood events (Andreásson and others 2004) are likely to make the rivers geomorphologically less active and will restructure ecological communities. Hydrologic changes can result also from other, climate-changerelated effects. In northern Canada, for example,

increased winter flows in streams and rivers have been observed and attributed to increased permafrost thaw (St. Jacques and Sauchyn 2009; Quinton and others 2011). Permafrost in the northern boreal zone is common in central and eastern Siberia and more discontinuous or sporadic in North America, but represents a substantial amount of water (Quinton and others 2011). In northern Europe permafrost is largely lacking (Slaughter and others 1995). It is suggested that if permafrost disappears headwater streams can more easily become ephemeral because of a shrinking groundwater supply (Jones and Rinehart 2010). Glacier melting will also contribute to temporarily increased flows (Aðalgeirsdo´ttir and others 2006; Hannah and others 2006), even to major torrential flood events (Wilhelm and others 2012). Higher winter precipitation in the form of rain will increase the water supply to streams and rivers and raise groundwater tables during the season when most trees and understory plants lack leaves and are physiologically inactive (Rood and others 2008). Therefore, increased winter flows will most likely have small or no direct effects on ecological communities along streams and rivers but will dramatically influence the ice dynamics and its effects, as discussed below. More intense rain events during fall may alter groundwater paths across landscapes and increase groundwater discharge areas along streams and rivers. Such discharge areas are important, increasing vascular plant species richness in both riparian and non-riparian settings (Giesler and others 1998; Zinko and others 2005, Jansson and others 2007); therefore, the higher precipitation during fall could favor local plant species richness and the extent of groundwater-fed vegetation along streams. Heavy rains are predicted to become more frequent, especially in the fall (Andreásson and others 2004), and this will increase the frequency of high flow events. Additional flood disturbance can have different effects on the development of riparian vegetation depending on its timing. For example, flood events toward the end of the growing season may be more destructive to vegetation than those in the beginning when many species are still dormant. Flood erosion provides new habitat for colonization. However, seedling establishment in the fall may be hindered by low temperatures and dormant seeds. Plants that are damaged or uprooted by floods in the fall may, therefore, not be replaced, although such floods can distribute propagules that can overwinter in vacant habitats and be the first to germinate in the following spring (Nilsson and others 2010).

Boreal Riparian Vegetation Under Climate Change The hydrologic changes are likely to have both regional and local effects on riparian vegetation. The reduction of spring-flood peaks is likely to reduce the distance at which propagules can spread during single flood events, thus reducing the regional (landscape) influence on the local species pool. On the other hand, higher flood peaks during fall caused by rain events (Andreásson and others 2004) might increase the density and distance traveled of water-dispersed propagules during the fall. We know of no examples modeling potential changes in the timing and dispersal distance of plant propagules. Floristic effects of the narrowing of the riparian zone have, however, been predicted for one river. Thus, Stro¨m and others (2012) modeled the local floristic responses of the predicted hydrologic changes toward the end of the twenty-first century, using the northern Swedish Vindel River that runs over minerogenic substrates as their model river. They found that the two most species-rich vegetation belts, riparian forest and willow shrub, are likely to decrease the most in elevational extent, up to 39 and 32%, respectively (Figure 1). The graminoid belt below the shrub belt was predicted to mainly shift upwards in elevation while the amphibious vegetation belt at the bottom of the riparian zone was expected to face an increase in size. The riparian forest and willow shrub zone will lose most species whereas the predicted loss from the entire riparian zone is lower (on average 8–14%) because many species occur in more than one vegetation belt. Most individual plant species are predicted to grow within a narrower belt, losing on average 13–28% of their elevational extent depending on scenario. Species with narrow hydrologic niches occurring in the riparian forest and willow shrub belts are expected to decrease the most (L. Stro¨m and others, unpublished). In addition to changes in dispersal patterns of propagules due to altered hydrologic regimes, Greet and others (2011) recognized the importance of timing of different hydrologic events for sustaining riverine ecosystems over the range of climatic conditions. Different flow components usually affect different stages of the plant life cycle and a mismatch between these can results in diminished seed germination, plant growth, or survival. Even though the work of Greet and others (2011) focuses on river regulation as the cause of changes in the timing of flow events we believe that climate induced hydrologic changes can have similar effects. Biomass from riparian and adjacent terrestrial vegetation is an important driver of aquatic communities

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in many streams (Wallace and others 1997; Richardson and others 2010). Although riparian biomass production is heavily dependent on the hydrologic regime, no dramatic changes are expected in response to climate change according to a reciprocal transplant experiment in the Vindel River in Sweden (Stro¨m and others 2011). Despite this, changes in the abundance of individual species may have significant consequences. For example, leaf litter of Alnus incana L. is important for aquatic macroinvertebrates, and A. incana is projected to lose 43–51% of its present elevational extent in riparian zones along the Vindel River (L. Stro¨m and others unpublished). When water levels fall, upland vegetation with low diversity and biomass production will likely encroach on the former upper parts of the riparian zone (Stro¨m and others 2011, 2012) because places which are no longer regularly flooded will not support riparian species. The changes in riparian vegetation extent and composition projected for the Vindel River are considerable, but in other boreal regions alterations may be more severe. For example, in Scandinavia as a whole, the most extensive reductions of the riparian zone, and consequent loss of species, are expected in the southern boreal zone where the largest spring-flood reductions are projected and only minor increases in mean annual discharge. Hydrologic effects of climate change cannot be discussed without also mentioning compounding effects of climate change and anthropogenic disturbance. For example, clearcut harvesting can affect headwater stream-bed characteristics directly due to increased water yield from snow (Winkler and others 2005) and rainfall without canopy interception, reduced evapotranspiration and slash loading into streams, or indirectly via logging slash transport and accumulation on channel beds. This can cause multiple channel formation and widen the riparian zone (Mallik and others 2011). Some forestry operations in Scandinavia have implied extensive drainage of forested areas. By directing groundwater flows to straight and narrow ditches groundwater recharge and discharge flow paths are altered and impacts of forest drainage on floods have been documented (Iritz and others 1994). Therefore, restoration of drained forest land to increase water retention capacity may be a method to counteract the effect of climate change.

Increased Ice Dynamics The weather in most parts of the boreal zone is expected to become more variable, probably also with an increased frequency of extreme temperature,

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precipitation, and wind events (Thorne 2011). Effects of climate change such as high winter flows and more rain-on–snow events are projected to cause considerable hydrologic alterations, not the least with regard to ice conditions. Among others, the duration and thickness of ice are projected to decrease, and this is likely to alter both thermal and radiative regimes, favoring winter open-water productivity and availability of under-ice habitats (Wrona and others 2006). For example, boreal Canada is experiencing large temperature variations between winters (Thorne 2011), and in Scandinavia intra-annual winter temperatures are predicted to fluctuate more, leading to a higher number of zero crossings on a Celsius scale (SOU 2007). In northern Scandinavia, the annual number of days with zero crossings is predicted to increase from 10 to 30 toward year 2100, and yet winters are expected to become considerably shorter (Persson and others 2007). More shifts between freezing and thawing and vice versa may stimulate a dynamic formation of frazil ice, anchor ice, hanging ice dams (Beltaos and others 2006; Stickler and others 2010), and Aufeis (Li and others 1997). Frazil ice is tiny ice particles that have adhesive features and are formed under supercooled conditions. If these particles adhere to large enough gravel or boulders they will remain at the stream bed and form anchor ice. Anchor ice affects the riverine environment (Prowse 2001; Hicks 2009; Stickler and Alfredsen 2009), for example by forming ice dams that alter the flow regime (Devik 1944; Stickler and others 2010). Ice dams can then cause flooding and ice formation in riparian areas (Stickler and others 2010). By reducing current velocity and increasing water depth, anchor ice formation can transform a reach from turbulent to slow flowing (Hicks 2009; Stickler and others 2010). In shallow watercourses with low winter flows freezing can extend down to the bed. Aufeis is then formed by water pushing through cracks in the ice up onto the surface, creating different layers of ice that might obstruct flow (Hicks 2009), and sometimes Aufeis is created in riparian areas (Li and others 1997). Thickening of an ice cover can occur both from above and below. An increased temperature will reduce the snowpack on the ice leading to a loss of thermal heat through the ice and growth of ice—sometimes in the form of hanging dams—underneath the ice sheet (Hicks 2009; Brown and others 2000). Hanging ice dams fill up pools and can cause flooding of riparian areas during early winter (Hicks 2009). Such ice dynamics, which are usually most pronounced in early winter before the watercourses freeze over, is likely to cause considerable

disturbance to riparian zones (Turcotte and others 2011). The response in vegetative composition to ice events depends partly on their severity and frequency and partly on the type of vegetation that is affected. Ice can cause both physiological alterations to plants by exposing them to cold and physical alterations by breaking and up-rooting plants (Scrimgeour and others 1994; Prowse 2001). In addition, during ice break-up streams and rivers can efficiently redistribute sediments and reshape channels (Turcotte and others 2011). Such processes stimulate further erosion and gap formation and likely increase both the overall species richness (Engstro¨m and others 2011) and the proportion of invaders (Stohlgren and others 1998) in riparian areas. If climate change alters the break-up dynamics of ice, this will also consequently affect other riverine structures and functions. A reduction in break-up intensity and thus disturbance will lower the biological diversity as well as the riparian productivity (Prowse and others 2006). Break-up events often result in ice jams which may not only have ecological but also socio-economic consequences. The break-up of ice in streams and rivers usually occurs in spring, but can be triggered by mid-winter thaws. Even minor climate changes can, therefore, have profound effects on ice breakup regimes. This is likely to happen in Russian rivers that run from south to north. Temperature is predicted to increase in both upstream and downstream reaches implying an earlier onset of snowmelt and less severe ice breakups and floods (Prowse and others 2006). Temperature changes will also modify ice-jam regimes. An ice jam can produce very high water levels, upstream of the jam as well as downstream by the surge when it finally collapses, and the effects on the riparian zone can be even more pronounced than during the spring flood (Cunjak and others 1998; Beltaos 2002, 2008; Luke and others 2007). Ice disturbance in conjunction with floods during the fall, which are expected to become more frequent (Andreásson and others 2004), may also facilitate plant invasion and modify vegetation zonation (Stro¨m and others 2012).

CONCLUSIONS With changing climates in the boreal zone we expect the stabilization of flows to reduce the floodrelated, geomorphic activity, but in some areas disturbance of channels caused by ice dynamics is likely to increase, especially in turbulent reaches. Increasing permafrost thaw and glacier melting will

Boreal Riparian Vegetation Under Climate Change temporarily increase water supply to streams and rivers but later hydrologic conditions will become more unstable. The vegetation will respond with species shifts, laterally as well as longitudinally. Most likely, ecological communities will change but in different directions depending on which climate-change factor is involved. Although more stable flows and shrinking riparian zones reduce species richness, increasing temperatures and ice dynamics will likely favor the number of species. Groundwater table fluctuations may be critical for riparian vegetation because of a mismatch between higher groundwater supply and the growing season. In general, the number and abundance of exotic species will also increase. Such community changes are likely to affect species communities also in the surrounding landscape matrix because uplands and riparian zones interact (Naiman and Dećamps 1997; Wiens 2002) and the riparian and upland species pools are related (Reno¨fa¨lt and others 2005). For example, it is likely that exotic species that invade riparian zones will later disperse also to the surrounding landscape. Furthermore, in cases of more extreme weather events following climate change, the riparian zones may receive a greater role as refugia for more sensitive species (Klein and others 2009). Most boreal river reaches will experience higher temperatures, as well as altered hydrology and ice conditions (Table 1), calling for studies to predict the joint effect of these changes on riparian ecosystems. Although we have the ability to predict the consequences for riparian communities of changes in individual factors, projecting net effects of alterations in many variables would require experimental manipulation. For example, lower magnitudes of spring floods are predicted to result in narrower riparian zones, resulting in lower plant species richness, whereas more dynamics in ice conditions might favor plant establishment and diversity. Thus, the loss in overall riparian area might be compensated for by higher species density in the remaining riparian plant communities. The first steps toward understanding such interactions might be to study how processes such as waterlevel variation and ice disturbance interact in present riparian ecosystems, but ultimately, experiments are needed. Several measures can be implemented to reduce the effects of climate change in free-flowing rivers. For example, to reduce the formation of frazil ice and anchor ice—which is most common in turbulent, channelized reaches in northern Fennoscandia (Engstro¨m and others 2011)—boulders and large wood can be replaced in channels to slow

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down flows and stimulate the formation of protective surface ice. Such restoration measures also increase the residence time of water, prolonging duration of riparian inundation and increasing flood frequency (Helfield and others 2007), potentially offsetting the expected hydrologic effects of climate change. Furthermore, built areas and other developments—common starting points for the dispersal of exotic plants—could be avoided in close vicinity to rivers to reduce the spread of exotics to riparian zones. A reduction or cessation of forestry operations in riparian zones such as clearcutting and drainage may counteract climatechange effects on riparian ecosystems. As a final example, the importance of intact riparian zones in providing shade will become increasingly important in smaller streams to prevent water temperatures from increasing to lethal levels for numerous organisms (DeWalle 2008).

ACKNOWLEDGMENTS This study was supported by the Swedish research council Formas and the Nordic Council of Ministers. We thank two reviewers for valuable comments on the manuscript.

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