Climate change induces multiple risks to boreal forests and forestry in Finland: A literature review

Abstract Climate change induces multiple abiotic and biotic risks to forests and forestry. Risks in different spatial and temporal scales must be considered to ensure preconditions for sustainable multifunctional management of forests for different ecosystem services. For this purpose, the present review article summarizes the most recent findings on major abiotic and biotic risks to boreal forests in Finland under the current and changing climate, with the focus on windstorms, heavy snow loading, drought and forest fires and major insect pests and pathogens of trees. In general, the forest growth is projected to increase mainly in northern Finland. In the south, the growing conditions may become suboptimal, particularly for Norway spruce. Although the wind climate does not change remarkably, wind damage risk will increase especially in the south, because of the shortening of the soil frost period. The risk of snow damage is anticipated to increase in the north and decrease in the south. Increasing drought in summer will boost the risk of large‐scale forest fires. Also, the warmer climate increases the risk of bark beetle outbreaks and the wood decay by Heterobasidion root rot in coniferous forests. The probability of detrimental cascading events, such as those caused by a large‐scale wind damage followed by a widespread bark beetle outbreak, will increase remarkably in the future. Therefore, the simultaneous consideration of the biotic and abiotic risks is essential.

damages in European forests, such as those caused by windstorms, heavy snow loading, drought, forest fires and major insect pests and pathogens of trees (Bentz et al., 2019;Jactel et al., 2011;Reyer et al., 2017;Seidl, Schelhaas, Rammer, & Verkerk, 2014;Seidl et al., 2017). During the past few decades, windstorms have damaged a significant amount of timber and caused substantial economic losses for forestry in central and northern Europe (Gregow, Laaksonen, & Alper, 2017;Reyer et al., 2017;Schelhaas, Nabuurs, & Schuck, 2003;Seidl et al., 2014). The increasing amount of wind damage in past decades can, at least partly, be explained by an increasing volume of growing stock and changes in forest structure (Schelhaas et al., 2003;Seidl, Schelhaas, & Lexer, 2011). At the all-European level, snow-induced damages have typically been minor compared to the damage caused by windstorms (Schelhaas et al., 2003). Snow damages have been most frequent in northern Europe and at high altitudes (Nykänen, Broadgate, Kellomäki, Peltola, & Quine, 1997). However, under the warming climate both the risks of wind and snow-induced damage may increase, due to the decreased duration of frozen soil (Kellomäki, Maajärvi, Strandman, Kilpeläinen, & Peltola, 2010;Lehtonen et al., 2019;Venäläinen et al., 2001), which weakens the anchorage of trees during the windiest season from late autumn to early spring. Warming climate and an increase in drought events may also increase the frequency of forest fires, which has already been observed particularly in south-eastern Europe .
The widespread and devastating fires in Sweden in 2014 and 2018 demonstrated that such calamities are possible also in the cool climate of the Nordic countries.
Warmer climate also favours many current and potential new insect pests and pathogens of trees, both directly (e.g. through the expansion of living area and abundance of pests) and indirectly (e.g. as a consequence of physical damage caused by storm and forest fires). The most significant insect pest in the European forests is the European spruce bark beetle, Ips typographus (e.g. Christiansen & Bakke, 1988;Grégoire & Evans, 2007). Extensive wind damage or drought increases the risk of intensive spruce bark beetle outbreaks (Marini et al., 2017), as the spruce bark beetles can then attack healthy spruce trees as well. In recent years, insect pests have caused a lot of damage especially in central Europe (Hlásny et al., 2019). Over the last four decades, spruce and pine timber damaged by bark beetles have increased in Europe, from 2.2 million cubic metres of wood (1971)(1972)(1973)(1974)(1975)(1976)(1977)(1978)(1979)(1980) to 14.5 million cubic metres (2002Seidl et al., 2014). Also, in the southern boreal zone (e.g. in Finland), warm summers have increased the populations of spruce bark beetles, together with unharvested wood left in forests after wind damages (Siitonen & Pouttu, 2014). The insect pest damages are expected to increase in the boreal zone in Northern Europe along with climate warming. In addition, other biotic damaging agents, such as wood decaying Heterobasidion root rot, have frequently caused significant economic losses in different areas of Europe (Garbelotto & Gonthier, 2013;Woodward, Stenlid, Karjalainen, & Hüttermann, 1998). Under a warmer climate, a risk of such damage can be expected to increase in coniferous boreal forests as well.
The simultaneous occurrence of multiple hazardous events can make the adverse impacts even manifold. For example, wind and snow-induced damages may increase the breeding material for bark beetles and increase attacks by Heterobasidion species through tree injuries (Honkaniemi, Lehtonen, Väisänen, & Peltola, 2017).
Furthermore, the outbreak of bark beetles can increase the amount of easily flammable dead wood exacerbating the forest fire risk (e.g. Jenkins, Runyon, Fettig, Page, & Bentz, 2014). Wood decay also increases the risk of wind damages due to poorer anchorage and stem resistance for trees (Honkaniemi et al., 2017). In the future, the increase of many abiotic and biotic damages in forests may at least partially counteract the positive effects of climate change on forest growth and productivity . Accordingly, climate change may cause severe economic losses in European forests (Hanewinkel, Cullmann, Schelhaas, Nabuurs, & Zimmermann, 2013).
Similar results have been observed elsewhere in the world. For example, Watt et al. (2019) found that in New Zealand plantation forests, the growth will increase, but also the wind damage and fire risks exacerbate. Also, studies from North America (Gathier et al., 2014;Halofsky et al., 2018;Peterson, Vose, & Patel-Weynand, 2014) indicate notable climate change risks to forests requiring efficient adaptation measures.
In order to ensure management of forests for different ecosystem services, better preparation is needed in forest management and forestry. This requires a holistic understanding on how the climate change may affect various risks in different spatial and temporal scales. For this purpose, our review article summarizes the most recent findings on major abiotic and biotic risks to boreal forests and forestry under the current and changing climate, with focus on windstorms, heavy snow loading, drought, forest fires and major insect pests and pathogens of trees. Our boreal case study area is Finland, which is an excellent 'laboratory' for studying the impacts of climate change in the boreal zone. This is because Finland is the most heavily forested country in Europe and its forests cover about 70% of the land area. Also, the Finnish forests belong to those most intensively monitored in Europe and even globally. Accordingly, even though our review focuses mainly on boreal forests in Finland, the findings evidently have substantial importance also globally and especially in boreal forests, which represent the largest terrestrial biome in the world ( Figure 1) and have a significant role in the mitigation of climate change.

| OVERVIE W OF PROJEC TED CLIMATE CHANG E IN FINL AND
In Finland, the annual mean temperature is now about 2.3°C higher than it was in mid-19th century (Mikkonen et al., 2015). In the future, according to a multimodel mean, the annual mean temperature is projected to increase by 1.9, 3.3 and 5.6°C by the 2080s under the RCP2.6, RCP4.5 and RCP8.5 scenarios, respectively, compared to the period of 1981-2010 (Figure 2). At the same time, the mean annual precipitation would increase by 6%, 11% and 18% under these RCPs. The changes are projected to be larger during the winter than the summer months. During the potential growing season (April-September), the mean temperature is expected to rise by about 1-5°C and precipitation by 5%-11%, depending on the RCP scenario (Ruosteenoja, Jylhä, et al., 2016).
As a result of the projected warming, higher growing degree day (GDD) sums will accumulate during the longer growing seasons ( Figure 2, lower panels). At the end of this century under RCP4.5, the thermal growing season would be 1-1.5 months longer and the GDD sum about 500°C days larger than in 1971-2000 (Ruosteenoja, Räisänen, Venäläinen, & Kämäräinen, 2016).
Compared to other areas of the boreal zone (Figure 1), the summer season temperature increase projected for Finland is of a similar F I G U R E 1 Global biome-map (Dinerstein et al., 2017). Boundaries of Finland (about 60°N-70°N and 20°E-30°E) are marked in the map magnitude (IPCC, 2013). In winter, the modelled warming is stronger in northern Russia, Siberia and Alaska than in Finland, particularly in the northern parts of these areas. Moreover, in eastern Siberia and Alaska, summer precipitation is anticipated to increase somewhat more than in Finland, while in southern Canada summers may even become dryer.

| IMPAC TS OF CLIMATE CHANG E ON FORE S T G ROW TH AND TIMB ER SUPPLY
Based on recent process-based forest ecosystem model simulations under the RCP2.6, RCP4.5 and RCP8.5 scenarios , the forest growth evidently increases significantly more in the northern than southern boreal zone, regardless of the RCP scenario ( Figure 3a). In the northern boreal zone the growth will also increase clearly more in birch (Betula spp.) and Scots pine (Pinus sylvestris) than in Norway spruce. On the other hand, when climate change proceeds, in the southern boreal zone, the forest growth may substantially decrease (and mortality increase) under RCP4.5 and especially under RCP8.5, particularly in spruce and to some extent also in pine (RCP8.5), in contrast to birch. The differences in the responses of the tree species and boreal zones increase along with the severity of climate change, which tend to make growing con-  2000-2085 (relative to the period 1981-2000), both corresponding to the mean of 28 global climate models (Ruosteenoja, Jylhä, et al., 2016). The growing degree days sum (°C day) in (d) 1971-2000, (e) 2010-2039 and (f) 2040-2069 under the RCP4.5 scenario (Ruosteenoja, Räisänen, et al., 2016)    maintenance ditching (in drained peatlands). If drained peatland sites are included in these calculations, the positive growth responses are slightly higher and the negative ones lower .
The possible increase of various abiotic and biotic damage risks to forests and forestry were not considered in the simulations.

| Observed variation of windstorms and strong winds
In Finland, the year-to-year fluctuation in the frequency of windstorms is quite high. Southerly and westerly winds are more common than northerly and easterly ones, typically causing wind damage in late autumn when there is no soil frost on ground. However, also in summer months downbursts associated with thunderstorms have sporadically caused large forest damages. Another estimate for the spatial and temporal distribution of devastating winds can be obtained by analysing gust wind speeds that are typically one to two times higher than the 10 min average  Figure 4b shows the annual probability for the gust wind speeds exceeding 25 m/s that is comparable to a 31-32 m/s wind speed obtained from point observations. Probabilities were derived from hourly data for the years 1979-2018 applying the generalized extreme value distribution of extreme value theory (Coles, 2001) using the R package 'evd' (Stephenson, 2002). The annual probability of the 25 m/s exceedance varies from approximately 60% on the Baltic Sea coast to values close to 1% in eastern Finland.
According to Laapas and Venäläinen (2017), in Finland, the occurrence of strong winds has been decreasing during the recent years.
During the period 1959-2015, the mean linear trends for the an-

| Projected future changes in windstorms and strong winds
Recent simulation studies (Groenemeijer et al., 2016;Kjellström et al., 2018;Ruosteenoja, Vihma, & Venäläinen, 2019), do not indicate any robust climate change signal in the future occurrence of strong winds in Nordic countries. Ruosteenoja et al. (2019) derived the projected changes in geostrophic winds from 21 global climate models (GCMs). These estimates can be regarded as more reliable than the projected changes in near surface winds, which are very dependent on the surface characteristics specified in the climate model. In Figure 5 an estimate for the change of the 99th percentile of the frequency distribution of geostrophic winds is given, that is, for the highest 1% of wind speeds. According to this estimate, strong winds may increase slightly, by 0%-2%, in summer and autumn. In spring and winter, the projected change is even more negligible. Moreover, these projected changes are subject to significant intermodel scatter. In addition, the proportion of westerly winds can increase slightly at the cost of easterly winds.
Soil frost that is regarded as beneficial for forestry as it anchors trees to the ground is predicted to decrease, both in depth and

| Magnitude of observed and predicted wind damage in forests
In Finland, over 24 million m 3 of timber in total has been dam-  Figure 6). Furthermore, they showed that preferring Norway spruce or birch in tree planting clearly increases the probability for wind damage compared to preferring either Scots pine or the business as usual (baseline) management, in which the same tree species is planted after the clear-cut. This is because much lower wind speed is required to damage Norway spruce compared with Scots pine or birch of same tree size.

| Snow damage
After windstorms, heavy snow loading is nowadays the most important abiotic cause of damage in the Finnish forests. Snowinduced forest damages include stem breakage and bending or leaning of stems. If the soil consists of unfrozen trees they can also be uprooted (e.g. Nykänen et al., 1997). In Finland, snow damage has been detected at 7.1% of forested land, less extensively in the south (3.4%) than north (12.1%; Korhonen et al., 2017). Snowinduced damage has typically occurred when the load of wet snow on tree crowns has exceeded 30 kg/m 2 (Nykänen et al., 1997).
Young Scots pine and broadleaf stands, especially those with a high height to stem diameter ratio, are particularly susceptible to snow damage.

| Drought and forest fires
Climate warming is expected to increase the occurrence of drought in northern Europe, despite the simultaneous increase in precipitation, because higher temperatures act to increase potential evaporation, which will overcome the impact of increased precipitation. In spring and early summer especially, the average moisture in the soil surface layer will decrease under the climate warming (Ruosteenoja, Markkanen, Venäläinen, Räisänen, & Peltola, 2018). Also, the probability of anomalously dry events will increase.

| Spruce bark beetle outbreaks
Nowadays the most devastating insect pest in Finnish forests is the

| Other insect pests
Large pine weevil (Hylobius abietis) is a major problem for the regeneration of coniferous forests in Europe (Långström & Day, 2004).

F I G U R E 8
The impact of climate change on the number of potential forest fires larger than 10 hectares under the RCP4.5 scenario, compared with the period 1980-2009. Bars indicate the estimates based on different climate models used in the study  0 2 4 6 8 10 12 1980-2009 2010-2039 2040-2069 2070-2099 Projected number of large fires In Finland, it is the only forest insect pest which needs continuous active measures in order to minimize its damages on planted tree seedlings. The damage caused by the large pine weevil is expected to increase in the future. Warmer summers and shortening of the period when soil is frozen shortens the development time of immature weevils, increases feeding and prolongs feeding period. Historically, the development time of the large pine weevils has been 2 years in southern and more than 3 years in northern Finland, and the abundance of weevils in clear-cuts decreases from south to north (Långström, 1982). In a warmer climate, a new generation of weevils will emerge sooner, at the time when planted seedlings are still small and vulnerable to serious feeding damage.
The study for northern Sweden also suggested that warming of climate will increase the weevil damage risk (Nordlander, Mason, Hjelm, Nordenhem, & Hellqvist, 2017).
Warming of climate is anticipated to benefit several other insects which have frequently caused significant defoliation on trees, because many outbreaking insect defoliators overwinter as eggs exposed to extreme low temperatures. So far, frequent cold winters have limited their populations, but an increase of the winter temperatures will favour their reproduction. A well-known example is the outbreak of autumnal moth (Epirrita autumnata) and winter moth (Operophtera brumata) in mountain birch forests of northern Fennoscandia; these moths have spread to new areas because of the increase of winter temperatures (Jepsen, Hagen, Ims, & Yoccoz, 2008). In addition, the populations of the pine sawfly F I G U R E 9 The probability of the growing degree day sum exceeding 1,500°C days for the periods 1971periods -2000periods , 2010periods -2039periods , 2040periods -2069periods and 2070periods -2099.5, derived from simulations performed with 23 global climate models (Ruosteenoja, Räisänen, et al., 2016) (Neodiprion sertifer), which has been historically the most significant defoliator of pines in Finland (Juutinen & Varama, 1986), may increase in eastern and northern Finland due to the rising winter temperatures and in southern and western Finland due to the increasing summer drought (Nevalainen, Sirkiä, Peltoniemi, & Neuvonen, 2015;Virtanen, Neuvonen, Nikula, Varama, & Niemelä, 1996).
In addition to insect defoliators overwintering as an egg, some defoliating species overwinter as a pupa. This group includes several species which have caused only occasional severe outbreaks in Finland, but which frequently cause severe defoliation in central Europe. If the future temperature sums in Finland will be close to those currently recorded in central Europe, such species will have a high risk to cause damage in Finland. These species include Pine beauty moth (Panolis flammea), pine lopper moth (Bupalus piniarius) and common pine sawfly (Diprion pini; Sierota, Grodzki, & Szczepkowski, 2019).

| Heterobasidion species
The most devastating forest pathogen in Finland currently is Heterobasidion spp, which is the most important wood-decay fungi for conifers in Europe and even globally (Woodward et al., 1998).
In other decay fungi. . Increasing temperatures increase the spore formation of the H. parviporum, and it is expected that the share of infected spruces will increase in the future (Müller et al., 2015;Pukkala, Möykkynen, Thor, Rönnberg, & Stenlid, 2005). In an infected stand, the vegetative spread of the fungus can be very effective. The higher temperatures will enhance the growth rate of H.
parviporum, since the optimal temperature for the growth mycelia is between 20°C and 30°C (Müller et al., 2015). Higher growth rates of mycelia act to increase the amount of decay in infected trees and the spread of fungus in diseased stand. Furthermore, the decrease of soil frost duration under the warming climate may increase root damages in forest harvesting, making trees more vulnerable for wood-decay fungus.

| Other forest pathogens
Very little is known about the effect of climate change on other major forest pathogens in Finland. An ascomycete fungus Gremmeniella abietina has caused major epidemics in pine forest in Finland and (Nevalainen et al., 2015;Wang, Stenström, Boberg, Ols, & Drobyshev, 2017). The infection of plant tissues by airborne spores of fungi is largely dependent on air moisture at the time of spore dispersal. It is known that successive years with moist and cool springs are followed by major epidemics of G. abietina. We can assume that an increase of spring temperatures and drought periods will limit the epidemics of G. abietina and other pathogens which infect the shoots and foliage of trees.

| Mammalian herbivores
The browsing caused by local high moose (Alces alces) populations is one of the most serious forest damage problems in Finland in Scots pine and birch forests. This has led to an increase of cultivation of Norway spruce on less fertile forest sites that are more suitable for pine. The impact of climate change on the risks of moose-caused forest damages is more difficult to evaluate than the impacts of insect and fungal forest pests. The reduction of the snow depth and duration under climate change may increase the severity of browsing damage (Herfindal, Tremblay, Hester, Lande, & Wam, 2015). It is also possible that in southern Finland dense moose populations will be partially replaced by an increasing deer population (Weiskopf, Ledee, & Thompson, 2019). Indeed, since 2010, the white tail deer (Oedocoileus virginianus) population has increased exponentially, thus already being more numerous than moose in southwestern Finland.
The effect of an increasing deer population on forest damage in Finland is unknown. Elsewhere, such dense populations have led to overbrowsing and severe problems in the regeneration of deciduous trees and other preferred tree species (Côté, Rooney, Tremblay, Dussault, & Waller, 2004;White, 2012).
In addition to cervids, the length of snow cover will affect small mammals. In a warming climate, there can be changes or even dampening in the vole cycles (Cornulier et al., 2013). Climate warming may also increase the abundance of herbivores, including voles.
However, the impact of warming on future vole damage in Finland is still largely unknown.

| New biotic threats to forest health
In addition to old and familiar problems, new issues will almost certainly emerge as the climate warming proceeds. An example of the potential new threats is the recent northward spread of nun moth, Lymantria monacha. It is nowadays capable of causing large-scale defoliation in coniferous forests in central Europe (Bejer, 1988;Nakládal & Brinkeová, 2015;Sierota et al., 2019). Nun moth has been extremely rare in Finland (Saalas, 1949), but because of warming of winters, nun moth populations have spread northwards (Fält-Nardmann et al., 2018). Melin, Viiri, Tikkanen, Elfving, and Neuvonen (2020) likewise reported that this species is now common throughout southern Finland and can be locally highly abundant.

| D ISCUSS I ON AND CON CLUS I ON S
Warmer and drier summer conditions are expected to increase droughts, forest fires and pest insects, while warmer and wetter winters will lead to increasing damage by windstorms, heavy snow loading and pathogens (Jactel et al., 2011;Reyer et al., 2017;Seidl et al., 2017). Such disturbances are likely to be the most pronounced in coniferous forests, particularly in the boreal zone (Seidl et al., 2017). Therefore, to ensure preconditions for sustainable multifunctional forest management and use of different ecosystem services, a holistic understanding is required on how climate change may affect abiotic and biotic risks in different spatial and temporal scales. For this purpose, our review article summarized the most recent findings on the major abiotic and biotic risks to boreal forests and forestry in Finland under the current and changing climate (see Figure 10).
Depending on the development of the GHG concentrations, the mean annual temperature in Finland is projected to increase in the range of 1.5-6°C by 2100, compared to the period 1981-2000 (Ruosteenoja, Jylhä, et al., 2016). In general, the forest growth and productivity tend to increase, mainly in the northern boreal zone . Conversely, in the southern boreal zone, a large increase in summer temperatures and associated drought (especially under RCP4.5 and RCP8.5) may make growing conditions suboptimal, especially for Norway spruce but partially for Scots pine as well . Accordingly, in this zone there may be a need to avoid the cultivation of spruce, particularly on forest sites with a relatively low water holding capacity ( Figure 10).
In Finland, in recent decades there has occurred less wind damage than elsewhere in Europe, including Sweden. However, the wind damage risk of forests is expected to increase along with climate warming, especially in the southern and middle boreal zones of Finland, mostly in stands dominated by Norway spruce with shallow rooting (Ikonen et al., 2017). This is so despite any significant change in wind climate, because of shortening of the soil frost period, particularly in the southern and middle boreal zones and on peatlands in large parts of Finland . As a result, strong winds in the future will blow more frequently under unfrozen soil conditions (Kellomäki et al., 2010;Laapas et al., 2019;Peltola, Kellomäki, & Väisänen, 1999  in planning of temporal and spatial patterns of thinnings and clearcuts (Heinonen et al., 2011). Moreover, the increasing risk of snow damage, especially in Scots pine and birch stands in the eastern and northern Finland should be considered in forest management, for example, by making timely precommercial and commercial thinnings and avoiding forest fertilization on forest sites located more than 200 m above the sea-level (Nykänen et al., 1997;Valinger & Lunqvist, 1992).
The increase in summer droughts (Ruosteenoja et al., 2018) may also exacerbate the risk of large-scale forest fires, especially in southern and middle boreal conditions Mäkelä, Venäläinen, Jylhä, Lehtonen, & Gregow, 2014) (Hlásny et al., 2019;Schelhaas et al., 2003;Seidl et al., 2014;Woodward et al., 1998), may become more common in the future, especially in coniferous forests in the southern and middle boreal zone of Finland. In this respect, multifunctionality of forest management (Díaz-Yáñez, Pukkala, Packalén, & Peltola, 2019;Seidl & Lexer, 2013) is needed to increase the resilience of forests to multiple disturbances induced by the climate change (Seidl et al., 2017). For example, growing mixed forests on suitable forest sites instead of monocultures may increase the resilience.
The simultaneous consideration of multiple risks of forests should also be emphasized in the future forest management and forestry. This is important, in particular, because the frequency of harmful cascading events is likely to increase. One example of such an event chain is a large-scale wind damage producing substantial amount of unharvested damaged wood in the forests, followed by a very warm summer favouring bark beetle invasion (Lindelöw & Schroeder, 2008). Such a cascading damage chain can lead to far F I G U R E 11 The annual probability for climatological conditions optimal for an outbreak of a serious bark beetle invasion. The probability is calculated by multiplying the probability of a large wind damage (Figure 4) by the probability of growing degree day exceeding 1,500°C day ( Figure 9)  imately the 25 m/s in the ERA5 reanalyses) would initiate wind damage exceeding 2-3 million m 3 . This is close to the upper limit of the timber amount that can be collected from the forests with the existing timber harvesting resources (Valta et al., 2019). The total probability for conditions optimal for large bark beetle damages is thus the probability of a windstorm causing large forest damages ( Figure 4) multiplied by the probability of the GDD sum exceeding 1,500°C days during the next summer ( Figure 9). In recent past climate , the risk was approximately 2%-5% on the south-western coast of Finland, less than 1% in the central parts of Finland and in the north virtually zero (Figure 11). At the end of this century, in the southern boreal zone, the annual probability will be about 40% at the coast of the Baltic Sea and about 2%-5% in the eastern part of the zone. In the western parts of the middle boreal zone, the probability is about 5%-20%, decreasing to about 1%-5% in the east. In the northern boreal zone, the probability ranges from about 5% in the southern parts to about to less than 1% in the northern parts of the zone and the country ( Figure 11).
Accordingly, compared with 1971-2000, climate warming will drastically increase the occurrence of conditions favourable for a widespread spruce bark beetle outbreak.
The above narrative demonstrates illustratively how seriously climate change can influence boreal forests in the future. The next step in the damage chain would be an ignition of large forest fires, as has happened in Canada. A robust assumption is that climate change will also manifold the number of forest fires and increase burned area, as shown by . Recent mega-scale forest fires in Canada and Siberia have also released huge amounts of stored carbon into the atmosphere, thus nullifying the potential positive impact of climate change on carbon sequestration in forests (Walker et al., 2019).
According to our study gradual climate change generally tends to accelerate forest growth, however, drought and biotic damages in particular may have adverse impacts. When comparing conditions in Finland with those in North America, the largest difference lies in the magnitude of disastrous events, like forest fires and insect-caused damages. Until today, in Finland the control of forest fires has succeeded well, and the burned area has been only a few hundred hectares annually , while in Canada the annual burned area is typically millions of hectares (Hanes et al., 2019). When climate change proceeds, the environmental conditions are foreseen to become more severe in Finland.
This likewise holds for the biotic damage risks as demonstrated in Figure 11; the risks have already been observed to increase in Central Europe (see Seidl et al., 2014). Luyssaert et al. (2018) have emphasized that the primary role of forest management in Europe in the coming decades is not to protect the climate, but to adapt the forest cover to changing climatic conditions. In the future, the probability of harmful multiple and cascading forest disturbance events may increase remarkably in boreal conditions. Therefore, the simultaneous consideration of multiple risks of forests should be emphasized in forest management and forestry. This is also needed in order to ensure preconditions for multifunctional forest management and simultaneous provisioning of different ecosystem services in a sustainable way. The climate change-induced risks can to some extent be mitigated through forest management practises, like making forest thinning and cutting the way that the wind, snow and fire risks are minimized. One potential, but little studied option is to grow trees in mixed species stands as an attempt to lower the risk of biotic forest health hazards (Jactel & Brockerhoff, 2007).

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the new finding ( Figure 11) of this study are available from the corresponding author upon reasonable request.