Vulnerabilities forests

Drought and heat-induced tree mortality is accelerating in many forest biomes as a consequence of a warming climate, resulting in a threat to global forests unlike any in recorded history (8). It has been stated that tall plants with low hydraulic conductance and high leaf area are most likely to die from future drought stress, implying that tall trees of old-growth forests are at the greatest risk of loss (9). Thus, it was concluded with high certainty that today’s forests are going to be subject to continued increases in mortality rates that will result in substantial reorganization of their structure and carbon storage. As a result, today’s forests generally should be replaced by shorter and more xeric plants, owing to future warmer droughts and associated wildfires and pest attacks (9). Forest types are expected to shift northward, which may have substantial negative impacts on forest economy, and alter species composition in natural forest stands (66,70).

Temperate forests

Increasing temperatures, longer growing seasons, higher atmospheric CO2 concentrations, and in the north, increasing N-mineralisation, are likely to increase the potential forest productivity where summer precipitation does not decline (7,19). It is uncertain to what extent this potential can be realised as forests will increasingly face a climate to which the planted species or provenances are not adapted, which might increase their susceptibility to pests and pathogens, such as bark beetle outbreaks, which can lead to major forest die-back events particularly in Norway spruce stands (20). 

Several studies find that tree growth rates in temperate forests passed their peak in the late 20th century and that the decline in tree growth rates can be attributed to climatic factors, especially drought or heatwaves (1). Extreme climate events have had a major impact on temperate forests over the last decade (2). Many species will likely suffer from drought stress at the southern edge of their current distribution in Europe and will probably shift their range northward (18). The complex interactions between climate and forest management in determining susceptibility to extreme events make it difficult to unequivocally attribute these events to recent climate warming (3).

Vulnerabilities peatlands

Peatlands are areas with a naturally accumulated peat layer at the surface, mires are peatlands where peat is currently being formed (21). About 80 % of the peatlands in Sweden and Norway are still mires. In countries such as the Netherlands and the UK, the contribution of mires to the total peatland area is only 1 - 7 % due to widespread drainage and intensive land use (21).

Although peatlands cover only 3-4% of the earth land surface, they hold the equivalent of half of the atmosphere’s carbon. Peatland areas are decreasing worldwide. In Europe, peatlands (> 30 cm peat) cover 5.4% of the total surface area. In about 54% of these areas peat is now being formed (mires). If shallow-peat lands (< 30 cm peat) in European Russia are also taken into account, the total peatland area in Europe is almost 10 % of the total surface area. Finland is the country in Europe with most peatland (26.7 % of the land area).The extent of mire is highest in European Russia (53). Europe’s southernmost blanket bogs, peatlands where a continuous mantle of peat covers entire landscapes, occur in in the Cantabrian Mountains of northern Spain (71). 

Drained and degraded peatlands are hotspots of greenhouse gas emissions (CO2, methane (CH4), nitrous oxide (N2O) and thus influence climate (22). Mineralisation of peat organic matter and the respective CO2 emissions are strongly related to drainage depth and management intensity (23). Higher temperatures will generally increase microbial peat decomposition and carbon mobilisation (24). However, change in land use is the primary driver of changes in peatland hydrology, and probably has a stronger impact than climate change (25).

Greenhouse gas emissions from degraded and agriculturally-used peatlands can be significant when compared to their total national greenhouse gas emissions, with contributions of about 5 % in Germany and Denmark, 2–3 % in the Netherlands and about 1 % in the UK (26).

Models predict considerable increases in CH4 emissions from peatlands over the coming century due to warming as long as wetland area and soil moisture conditions remain unchanged (27). However, wetland area may decrease in response to higher temperatures, leading to lower CH4 emissions (28). Peatland water tables may become significantly lower due to increased evapotranspiration and/or decreased summer precipitation. In that case, peat decomposition and the release of CO2 may be enhanced (29), and so will N2O release (30), and CH4 production and emission will decrease (31).

Peat soil degradation causes land subsidence by a combination of peat oxidation and compaction after drainage. Historical subsidence, caused by drainage since medieval times, often combined with peat extraction for fuel, in coastal peatlands of the Netherlands, Germany and eastern Britain may have resulted in up to several metres of subsidence (32). In the eastern British fenlands, compaction and peat oxidation has resulted in up to 4 m of subsidence in 150 years. In Dutch managed peatlands, subsidence is ongoing at up to one centimetre per year. Under a warmer climate, peat decomposition would be even faster, particularly in drained peatlands. The need for increasingly deeper drainage enhances the upwelling of sulphate-rich brackish or salt water (33). 

Climate induced changes in precipitation will probably be an important factor altering peatland vegetation in temperate and boreal regions, with decreasing wetness during the growing season generally associated with a shift from a Sphagnum dominated to vascular plant dominated vegetation type and a general decline of C sequestration in the long term (4). Mire ecosystems (i.e. bogs, transition bogs and fens) in Central Europe face severe climate-induced risk, with increased summer temperatures being particularly important (5). Although peatlands cover only about 3% of the land surface, they hold the equivalent of half of the atmosphere’s carbon (as CO2), or one third of the world’s soil carbon stock (400-600 Pg) (6).

Permafrost peatlands in Europe and Western Siberia

Permafrost peatlands in Europe and Western Siberia will soon surpass a climatic tipping point under scenarios of moderate-to-high warming (the scenarios SSP 2-4.5, SSP3-7.0 and SSP5-8.5). The total peatland area affected under these scenarios contains 37.0–39.5 Gt carbon. This equals twice the amount of carbon stored in European forests. According to this assessment all of Fennoscandia will no longer be cold and dry enough to sustain peat permafrost by 2040. However, strong mitigation could retain climates capable of sustaining peat permafrost in northern parts of Western Siberia by 2100; this landscape contains 13.9 billion tonnes of peat carbon (74).

Global assessment - Changes forest carbon stock

The results of a global assessment show that climate change stimulates forest productivity on a global level (52). This is due to the fertilization effect of CO2. Global warming itself does not necessarily increase global forest carbon; it has a negative impact on global forest carbon, likely caused by more wildfires, and climatic effects like droughts. This study was based on two scenarios of climate change: a high-end scenario (analogous to the IPCC RCP8.5 scenario), and a scenario where climate change is mitigated such that a 2 °C global mean warming from pre-industrial by 2100 is not exceeded. The latter agrees with a low scenario of climate change (in between the IPCC RCP2.6 and RCP4.5 scenarios). Changes were studied from 1980-2009 to 2070-2099.

Change of carbon dynamics

Climate change will impact the major forest regions of the world in several ways (72). Forests productivity will change due to higher temperatures and changes in rainfall; in some parts there will be a loss of productivity, in others an increase. Forests will benefit from the higher CO2 concentration in the atmosphere that has a fertilization effect. Climate change will drive migration of forests: they will expand in some regions, and contract in others. Wildfires will increase in most of these major forest regions. Competition will change between different types of vegetation. As a result, carbon dynamics will change (52).

More wildfires versus higher productivity 

Future potential changes were assessed with models that simulate potential future forest growth and decline under different scenarios of climate change and CO2 emissions. The assessment focused on the combined effects of wildfire, climate-induced vegetation migration and productivity in relation to climate change scenarios on a global scale. Main outcomes are: (1) At the global scale mitigating climate change can be beneficial in terms of reducing the impact of wildfires, and costly since it reduces primary production and thus forest carbon; (2) The interplay between direct climate change impacts (changes in temperature and precipitation) and the fertilization effect of CO2 on the world’s forests is complex (52). 

Poleward migration of forests

Under both scenarios of climate change a poleward migration of forests was simulated: in the leading-edge of the migration, grassland and woodlands convert to forests while at the trailing edge, forests convert to shrubland, grassland, or woodland due to lower productivity or frequent fires. Large expanses of boreal forests in Canada and Russia shifted northward, especially under the high-end scenario of climate change. In the southern hemisphere, forest expanded southward in Southern Africa. Poleward migration of forests was not distinct in Western South America, where there the forest contracted along elevation gradients. In Australia, the tropical forests in the north contracted northward as they lost productivity and became woodlands; simultaneously, increased growth of trees was simulated in the woodlands in western Australia, converting those areas to forests (52).

Increase total live forest carbon stock

In these simulations, total live forest carbon stock increased dramatically and consistently under both climate change scenarios, gaining 59% and 54% under the high- and low-end scenario, respectively. The vast majority of the total live forest carbon gain was simulated to occur in the southern hemisphere: Western South America, South America, and South Asia. For Europe only small increases were projected, while Russia was projected to see a significant decline, mainly due to forest contraction and more wildfires. Both positive and negative effects are generally higher for the high-end scenario compared with the low-end scenario of climate change: a higher increase of both carbon stocks and productivity, and of the impact of wildfires (52).

Limitations of this study

This study focused on evaluating the role of wildfire as a major disturbance regime. There is an array of disturbance regimes, however, including land cover change, logging, and insect and pathogen outbreaks. These disturbances were not included in the simulations. Also, current developed and agricultural areas, land use change, and forest management practices were left out. Besides, the study’s results are based on a single climate model; the effects of mitigation policies on the forest carbon stock may be sensitive to climate model selection (52). 

Global assessment - Changes peatlands carbon stock: since year 850

The world’s peatlands have been a global carbon sink for millennia. The amounts of carbon they contain equal the amount of carbon in the preindustrial atmosphere. Carbon storage in peatlands is the result of the balance between carbon uptake by plants and microbial decomposition. A disturbance of this balance changes the carbon sink potential of peatlands, thus affecting the concentration of atmospheric green houses gases and, in the end, climate change.

In a warmer climate, decomposition in peatlands increases and more carbon is lost to the atmosphere. On the other hand, the accumulation of peat may increase when primary productivity of plants increases due to higher temperatures and a longer growing season length. Studies on high-latitude northern peatlands data have shown that in warmer climates, increases in plant productivity overcome increases in respiration and that these peatlands will probably become a more efficient sink if soil moisture is maintained (55). This is good news for climate change. However, peatlands at lower latitudes may tell a different story.

In a recent study, the data for northern high latitudes was improved and expanded to low latitudes and southern high latitudes. This way, a more globally distributed peatlands dataset was obtained. From these data, global carbon accumulation over the last millennium (period 850 - 1850) could be quantified. From these insights, future projections of the world’s peatlands carbon sink potential were made (54).

These future projections indicate that until 2100 carbon sink potential of the world’s peatlands slightly increases, both under a low-end and a high-end scenario of climate change (RCP2.6 and RCP8.5 scenarios). This is good news: this increase will lead to a small negative feedback to climate change. The bad news is, this negative feedback does not persist in time. After 2100, carbon sink potential decreases and the initially negative feedback shifts to a positive feedback to climate change. According to the authors of this study, this shift is a plausible and robust result of their study (54).

The shift results from the combined changes in the peatlands at different latitudes. Over time, the balance between an increasing high-latitude sink and a decreasing low-latitude sink changes such that the global sink eventually begins to decrease. At high latitudes the carbon sink increases continuously far beyond 2100. At lower latitudes, higher temperatures drive increased microbial activity and decomposition rates in the peat and surface litter, but this is not fully compensated by increases in plant productivity. As a result, carbon sequestration decreases. At mid-latitudes, peatlands gradually move into the decline phase when decomposition rises faster than net primary productivity (54).

Uncertainties in this study are partly due to assumptions that have been made. One of these assumptions is that no changes in deeper peat layers were taken into account. Only the changes in the surface accumulation rates were considered, whilst deeper peat may also warm and provide a further source of carbon release (54). According to the authors, the most important uncertainty is human impact. Human impact is likely to be the most important determinant of global peatland carbon balance over the next century. Ongoing destruction of tropical peatlands is the largest contributor at present and at current rates, the losses from this source outweigh carbon sequestration rates in natural peatlands (56). Reducing anthropogenic release of peatland carbon is the highest priority in mitigation of peatland impacts on climate change, the authors state (54).

Global assessment - Changes peatlands carbon stock: period 1850-2100

Ever since the last ice age, natural peatlands have been important carbon sinks, and intact peatlands still are today. However, drainage of peatlands for agriculture and forestry has transformed large areas from former sinks into net sources of greenhouse gasses. Especially in temperate and boreal regions, but increasingly also in the tropics. On a global scale, peatlands turned from a net sink into a net source of greenhouse gasses in 1960. Rehabilitation of these peatlands may greatly reduce additional emissions in the future (67).

Boreal and temperate regions versus the tropics

The distinction between boreal and temperate peatlands, and peatlands in tropical regions is important. Where the first group of peatlands experienced widespread land conversions by the nineteenth century, drainage onset in the tropics commenced only from 1960 (68). Between 1850 and 2015, temperate and boreal regions lost 26.7 million ha, and tropical regions 24.7 million ha, of natural peatland. By 2100, the loss of natural peatlands in tropical regions may have increased to 36.3 million ha, corresponding to 62% of the initial peatland area in the tropics in 1850. The additional loss of peatlands in temperate and boreal regions will be small; total loss by 2100 is projected to be 7.0% of the initial peatland area in these regions in 1850 (67).

Global balance 1850 - 2100

Drained peatlands not only emit CO2 but NH4 and N2O as well. These are all greenhouse gasses. The emission volumes of the latter two are also expressed in terms of CO2 by translating their volumes into volumes of CO2 that have the same global warming impact. These volumes are expressed in volumes CO2e, where ‘e’ means carbon dioxide ‘equivalent’.

A reconstruction of the past and projections for the future were combined to estimate changes in the global balance of peatland greenhouse gasses for the period 1850 - 2100. For boreal and temperate peatlands annual uptake rate of greenhouse gasses by intact peatlands is projected to decrease from 0.31 ± 0.06 in 1850 to 0.29 ± 0.05 Pg CO2e in 2100, along with an increase in the drained area. For tropical peatlands a decrease from 0.10 ±0.06 to 0.04 ± 0.01 Pg CO2e is projected (67). 1Pg = 1012kg.

In the context of the global budget of greenhouse gasses this change in uptake (sink) is relatively small. The change in emissions (source) is much more pronounced, however. Estimates for the period 1850 - 2100 show a steady increase of emissions from peat decomposition. For 2015, a release of greenhouse gasses from drained peatlands of 1.53 ± 0.80 Pg CO2e is estimated, with 1.26 ± 0.77 of that total emitted in the tropics (67).

Dominant contribution tropics since 1980

Emissions from temperate and boreal systems dominated the overall picture until around 1980. Since then, annual emissions from the tropics exceed those from temperate and boreal regions. By 2100, tropical regions are projected to completely dominate greenhouse emission from peatlands. Without counteracting measures having been taken, the annual emissions rate will then reach 2.23 ± 0.69 Pg CO2e in the tropics, compared with ‘only’ 0.07± 0.16 Pg CO2e in boreal and temperate regions. In the latter regions, most of the carbon will have disappeared by oxidation by then. In drained tropical peatlands this exhaustion will start after 2100 because of the later drainage onset (67).

Between 1850 and 2100, drained peatlands will have released 249 ± 38 Pg CO2e by biological peat oxidation. From 2015 onwards, the release will total 169 ± 44 Pg CO2e, dominated by tropical emissions (67).

No more net emissions from peatlands?

These results illustrate the strong dominance of sources over sinks. What would it take for global peatland sources to equal peatland sinks, and thus for peatlands to become neutral in terms of emissions of greenhouse gasses by 2050 and thereafter? Almost all drained peatlands need to be rehabilitated. Full rehabilitation of all degraded peatlands in boreal and temperate regions, and of 98% of degraded peatlands in the tropics is needed for this (67).

Stop global warming at +2 °C?

The IPCC has estimated in 2014 that a net emission of about 2,550 - 3,150 Pg CO2e since 1870 would allow global mean air temperatures to remain below +2 °C. Without counteracting measures, the emissions from drained peatlands will consume 7.9 to 9.8 % of this amount. Most of this ‘allowed’ budget has already been used, however, due to the already high anthropogenic emissions accumulated since 1870. The available budget left for 2020 - 2100 is only about 400 - 1,600 Pg CO2 (69).

Rehabilitation of peatlands

Without rehabilitation of peatlands and with continued land conversion in the tropics, the emissions from peatlands may comprise 12-41% of the total remaining budget (from now on) of greenhouse gasses that would keep global warming below +2 °C. Widespread rehabilitation of formerly drained peatlands would substantially reduce these values to 3-12% of the budget. If we include extant peatlands in the overall calculation, the contribution of peatlands to this total ‘allowed’ budget would reduce to ‘only’ 1-5% (67).

Impacts of forests on climate change mitigation and adaptation

Forests provide multiple water and climate-related services, including precipitation recycling, cooling, water purification, infiltration and groundwater recharge. These services may be far more important, and are often underrated, when compared with traditional benefits such as food, fuel and fiber, and carbon storage. In addition, these services benefit and impact people well beyond the local or catchment scale, often far from where actual decisions on tree planting or removal are made (34,37).

Forests are intimately linked to rainfall and water availability

Forests contribute to atmospheric moisture and rainfall patterns over land through evapotranspiration: evaporation from soil and plant surfaces and transpiration of water by plants. On average, at least 40% of rainfall over land originates from evapotranspiration. The resulting atmospheric moisture is circulated by winds across the Earth’s continents and oceans. This cross-continental production and transport of atmospheric moisture, called “precipitation recycling”, can promote and intensify the redistribution of water across terrestrial surfaces (34).

Forest loss and degradation reduce evapotranspiration, with important implications for rainfall thousands of kilometres downwind (35). In addition, forests affect the Earth’s surface albedo, temperature, and surface roughness, and thus also alter moisture and heat fluxes between terrestrial surfaces and the atmosphere. Large-scale deforestation may reduce rainfall in some regions by as much as 30% (36). Trees and forests also lead to more intense rainfall through the biological particles they release into the atmosphere (37). Satellite observations suggest European forests are a major influence on cloud formation (38).

The impact of deforestation on altered rainfall patterns can lead to feedback effects on remaining vegetation, reduced biomass accumulation, drought, die-off and fires (39).

Forests transport water locally and globally

Large, continuous areas of forest drive the atmospheric circulation that brings rainfall to continental interiors, according to the so-called “biotic pump theory” (40). It explains that, through transpiration and condensation, forests actively create low pressure regions that draw in moist air from the oceans, thereby generating prevailing winds capable of carrying moisture and sustaining rainfall far within continents (41). Reforestation may re-activate such pumps, returning rainfall to continental interiors (42).

Forests cool locally and globally

Forests influence local and global temperatures and the flow of heat. Individual trees can transpire hundreds of litres of water per day. Every 100-litre of water transpired equals a cooling power equivalent to two average household central air-conditioning units per day (43). Additional regional and global cooling derives from the fact that forests can increase low-level cloud cover and raise reflectivity (44).

On the other hand, forests may contribute to warming. They may stimulate the formation of clouds that trap long wave radiation beneath. Under more cloud-free skies (at high latitudes and particularly in winter) they reduce the earth’s albedo and thus contribute to local warming (45).

Tropical and, to a lesser extent, temperate forests very likely provide net regional/global climate cooling. At higher latitudes, forests may warm regional and global climate (46).

Higher CO2-levels complicate the cooling potential of forests

Vegetation has an impact on extreme high temperatures through a combination of three processes. First, at a higher CO2 concentration in the atmosphere the stomata in plant leaves are closed more than at lower CO2 concentrations. As a result, less water is being transpired by the plant, which increases the plant’s water use efficiency but also reduces the latent heat flux, and thus cooling potential, through transpiration. Second, at a higher CO2 concentration a plant’s leaf area increases (the “fertilization effect” of CO2), which increases the latent heat flux and thus cooling potential, and (partly) offsets the stomatal reduction of transpiration (58). Third, vegetation influences soil moisture: because of the water taken out of the soil and because more biomass leads to more wind turbulence that cools the surface and near-surface air. If the soil dries out, the strong reduction in evaporation limits the cooling potential of the soil and increases extreme air temperature. In addition to these 3 processes, evapotranspiration can also increase as a result of enhanced evaporative demand and the lengthening of the growing season (59).

As a result of this combination of processes, vegetation modulates the latent heat flux and influences surface temperature. Model simulations for a high-end scenario of climate change (RCP 8.5) show that vegetation contributes to the projected global increase of extreme high temperatures with rising CO2-concentrations. In fact, on a global scale, vegetation may account for around 13% of the projected increase of maximum temperatures by 2100 (57).

Forests regulate water supplies

Forests regulate water supplies in many ways. High altitude forests can intercept fog and cloud droplets, which may account for up to 75% of total catchment runoff (47). Where such forests have been removed, the atmospheric moisture present in clouds may move on to other locations. This could represent an important loss to local, downstream water supply (48). Forest clearing may have several, sometimes opposing, effects on water supply, however. Less trees means less water is being evaporated and more groundwater feeds as stream flow into water supplies downstream (49). Loss of tree cover promotes soil degradation that leads to reduced soil infiltration and water retention capacity (50), and in turn reduces groundwater reserves that maintain dry season base flows.

For all the reasons noted above, transpiration, interception, evaporation, infiltration and groundwater recharge, tree cover can either store or recycle substantial amounts of water downwind, providing a positive impact on (and protection of) the local catchment, thereby moderating floods (34).

Mixed species forests are more effective in regulating water supplies and moderating floods than monocultures. Through variation in rooting depth, strength and pattern, different species may aid each other through water uptake, water infiltration and erosion control (51). 

The consequences of global tree restoration on regional water availability have been studied (75). On the one hand, more trees would increase evaporation and reduce stream flow. On the other, following tree restoration, the increase of evaporation would increase terrestrial precipitation due to recycling of increased evaporation, but often largely in other parts of the continent. The global hot spots for forest restoration could face a strong reduction in the water availability because water supply by evaporation recycling is not enough to compensate for water loss through evaporation. However, regions with no or limited tree-restoration potential could see an increase in water availability due to the lateral transport of evaporated water into the region. The ratio between the direct effect of tree restoration on reducing water availability and the indirect effect of increasing precipitation recycling and hence water availability shows a global net loss of water availability over the continents. These conclusions are consistent with other data-driven or model-based studies of the effects of land-cover change on water fluxes (76). The authors of this study stress that future tree-restoration strategies should consider the hydrological effects. Smart tree-restoration strategies are necessary to secure the water supply locally and remotely or enhance the water supply in water-scarce regions (75).

Adaptation strategies - Forests

Climate change is expected to have a diverse range of effects on forests, such as changes in distribution of tree species, effects on forest productivity, increased risk of storms, fires, insect pests, and drought (10). Evidence of such impacts has already been reported for scots pine (11) and beech forests (12).

Climate change-induced productivity and species suitability changes 
in Europe have been assessed (13). This was done for several tree species (oaks, beech, spruce, scots pine) under the A1B scenario of climate change (for three different combinations of regional climate models and general circulation models). With respect to these changes, the value of alternative forest management strategies to compensate for negative effects has been assessed. These alternative strategies are: (a) reduce the rotation length of harvesting and (b) adapt species composition to those species that are expected to perform better under a changed climate (30 years from now).

Reductions in rotation length decrease the time the timber crop is at risk (14), limit the top height reached, reducing windthrow risk (15), and generally reduce uncertainty, allowing another more suitable species to be replanted. A change in species composition avoids risks associated with specific species, as for example, windthrow and bark beetles in Norway spruce (16) or drought-intolerant species, but also to spread risk in general by using more species, the so-called insurance hypothesis (17).

Results show that the European forest system is very inert and that it takes a long time to influence the species distribution by replacing species after final felling. By 2070, on average about 36 % of the area expected to have decreased species suitability will have changed species following business as usual management. Alternative management, consisting of shorter rotations for those species and species planting based on expected trends of climate change (looking 30 years ahead), will have increased this species transition to 40 % (13).

Northern Europe is projected to show the highest production increases under climate change and can also adapt its species distribution faster. The forest in Northern Europe is generally distributed rather equally over the age classes, and lowering the rotation length makes a considerable additional amount of area available for harvesting, resulting in fast adaptation. Besides, in Northern Europe only a few species show a lower suitability due to climate change. Southwest Europe is expected to face the greatest challenge by a combination of a predicted loss of production and a slow rate of management alteration under climate change. In Southwest Europe, the age class distribution is less favourable for adaptation with a large share of relatively young stands, and the suitability due to climate changes affects almost all (nine out of 10) species (13).

Although alternative management can increase the rate at which species with declining suitability are replaced, it comes at the cost of a lower increment. Generally, the species with declining suitability are productive coniferous species like spruce, whereas the species to plant are mostly broadleaves with lower increment rates. Moreover, increased final felling leads to a temporary reduction in increment due to the increased replacement of mature forest by low-productive young forest. At longer timescales, the increased regeneration effort might lead to higher increment rates under adaptive management (13),

The results show that forest management cannot keep pace with the projected change in species suitability. As a consequence, larger parts of the forest will be exposed to climate regimes they are not adapted to, thereby increasing the risk of large-scale mortality events due to for example drought and pests (13). 

Adaptation strategies - Assisted tree migration

The tree composition of European forests will change in response to global warming. The optimal conditions for Mediterranean tree species will shift northwards into the temperate zone, whereas the climatic suitability of temperate species will move into the boreal zone at the expense of boreal species.

The velocity of climate change outpaces the abilities of tree species to migrate north. As a result, in future decades, the distribution of tree species will no longer match climatic zones and the quality and diversity of forests will decline. Detrimental cascading effects on wildlife habitats and biodiversity will propagate through networks of ecological interactions (83), further reducing forests’ tolerance to climate-driven disturbances such as pest, fires, and invasive species. What’s more, as forests change, the ecosystem services these forests provide to society will also change.

These ecosystem services include providing food and medicine, and wood for construction and furniture, for fuel and for pulp in the paper industry, contributing to cultural values and recreation, and protecting the soil from erosion and degradation. When forests are affected by climate change, these ecosystem services decline. This potential impact of climate change has been studied for the entire European land area. In addition, the researchers looked at the potential of human assistance in tree migration so forests can adapt to climate change faster, and the loss of ecosystem services can be limited (82, see also 87).

Two scenarios: the natural and the human-assisted way

In their study, researchers looked at current and future distribution of 66 tree species across Europe under a moderate (RCP 4.5) and high-end (RCP 8.5) scenario of climate change, for three 30-year time intervals centred on 2035, 2065, and 2095. They focused on two scenarios for the future dispersal of tree species across Europe. In the first scenario they looked at future tree distribution under the assumption that seeds are dispersed in a natural way, without human assistance. In the second scenario they assumed human assistance in tree migration would lead to unlimited dispersal of seeds, closely matching the northward shift of climatic zones. In fact, this scenario assumes that species will be able to move to, and colonize, all areas of future climatic suitability, and thus reflects the climatic potential of species dispersal in the case of forestry-based assisted tree migration (82).

The natural way: a loss of ecosystem services

The results indicate that, without assisted migration, the provision of ecosystem services by European forests would decrease on average by 15% or 23% by the end of the century under a moderate or high-end scenario of climate change, respectively. The situation is quite different for the Mediterranean, though, where a decline of ecosystem services is projected of 52% (70%) under a moderate (high-end) scenario of climate change (82).

Human-assisted tree migration: still decline in the South

If we do assist tree migration, the dispersal of European tree species will experience substantial changes, following the rapid shifts of climatic zones. As a result, the decline of ecosystem services can be limited to 10% (15%) on average in Europe. In the Alpine and Boreal regions, forest ecosystem services may actually increase compared to the current situation. Here, global warming could permit tree species colonization into new territories previously not climatically suitable. In most of Central Europe, forest ecosystem services can be maintained near their current level, especially under a moderate scenario of climate change. Services will still be lost in the Mediterranean, however, by 33% (54%) under a moderate (high-end) scenario of climate change (82).

On the positive side, tree migration strategies are effective under both a moderate and high-end scenario of climate change. This shows that this approach is a powerful tool for forest management: tree species migration and management can be planned over the next decades to target a certain forest composition by the end of the century, no matter how global warming will develop (82).

Ecological point of view

One might argue – from an ecological point of view – against the deliberate translocation of species beyond their current occupied ranges. The authors of this study stress, however, that assisted migration is nothing new. It has already been common practice for centuries, millennia even. European forests have been managed through clearance to create croplands and pastures. They have been used for intensive wood harvesting for fuel and construction. According to them, ‘currently, only 2.2% of the forests in Europe are classified as undisturbed by man, 3.8% are classified as plantations, and 94 % as semi-natural’. Besides, the fact that European forests are highly fragmented by human settlements and infrastructure might hamper the natural dispersal of most European tree species (82).

It’s time to act

The authors conclude that natural dispersal will probably play a minor role in future tree distribution across Europe. Assisted migration – defined as the ‘deliberate human-mediated movement of species into areas where their environmental needs better match the future climatic conditions’ (84) – has been and will be key to the adaptation of European forests to climate change. Knowledge on effective assisted tree migration, to preserve forests’ ecosystem services for European economies and societies, is highly relevant as the European Union intends to plant 3 billion trees in Europe by 2030 (85). The experts urge to start quickly, ‘because forests take decades and not years to adapt to climate change’ (82).

Adaptation strategies - Peatlands

By 2023, eight European countries had already developed national peatland strategies in which peatlands are considered a nature-based solution to mitigate climate change (86). 

Drained peat soils used for agriculture can be large greenhouse gas sources. By rewetting these soils and thus restoring the wetlands, significant agricultural emissions are avoided, and soil carbon can be sequestered and protected. Rewetting of peat soils is not a quick win contribution to the reduction of greenhouse gases emissions, however (60).

With respect to rewetting peat soils, the net impact on the emission of greenhouse gases depends on the balance between CO2 and CH4 (the impact of N2O, another greenhouse gas emitted by these soils, has been shown to be negligible, compared to the other two (61)). CO2 is emitted when peat soils are drained and exposed to the air. CH4 is formed under anaerobic conditions, so when peat soils are rewetted again (81). Clearly, wetland restoration comes with a biogeochemical compromise (62): while flooded wetlands have the potential to sequester carbon, the highly reduced (anaerobic) conditions can result in significant CH4 emissions (63). What’s more, CH4 is a much stronger greenhouse gas than CO2! Wetlands contribute around 30% of all anthropogenic and natural CH4 sources (65).

35 years of continuous CO2 and CH4 measurements in the Sacramento-San Joaquin River Delta (USA) showed that restored wetlands effectively sequestered carbon and halted soil carbon loss associated with drained agricultural land uses. It takes some time, however, before this change from carbon source (drained land) to carbon sink (restored wetland) becomes effective in terms of climate change mitigation: restored wetlands do not begin to accrue greenhouse gas benefits until nearly a half century, and become net sinks from the atmosphere after a century. This is related to the fact that after rewetting, the peat soils start to emit CH4 as a result of anaerobic decomposition of carbon. CH4 being a much stronger greenhouse gas, it takes some time for the CO2 accumulation to dominate over this CH4 effect. The latter effect may turn restored wetlands into net sources of greenhouse gases to the atmosphere over decadal timescales (64). The good news in this US study: over time, the cumulative removal of CO2, an extremely long-lived green house gas, from the atmosphere vastly outweighs the short-lived CH4 warming effect (60).

The study shows that land use change from agriculture on drained, degraded peat soils to freshwater, deltaic restored wetlands will result in a net greenhouse gases benefit over multi-decadal timescales, while accreting soil and sequestering carbon from the atmosphere into the ecosystem. Along with climate benefits, these ecosystem services have the potential to halt and reverse soil subsidence and protect the fragile hydrological network of wetland ecosystems. The message to policymakers and planners: take measures that promote the long-term restoration of these kinds of systems to maximize climatic benefit (60).

Adaptation strategies - Paludiculture in peatlands

Rewet drained peatlands would stop the release of CO2 and instead would turn the peatlands into a carbon sink again. Rewetting and cessation of high-intensity land use would significantly improve biodiversity and water quality compared with the current status. What’s more, rewetted peatlands can still be used for commercial application. Commercial land use on rewetted peatlands is called ‘paludiculture’. This includes producing biomass while facilitating peat accumulation and providing many of the ecosystem services associated with natural, undrained peatlands (77). Crops can be grown for a whole range of products and applications, including human consumption and fodder, as raw material for the production of horticultural growing substrates, for bioenergy and as a resource for bio-based materials such as insulation, building materials, paper, bioplastics, and even pharmaceuticals and cosmetics. Besides, wet grasslands can still be used as pastures, be it on a less intensive scale (78,79).

More benefits: restoring ecosystem services

The transition to paludiculture will have a strong impact on current land users, but the advantages are manyfold. In addition to climate benefits, these advantages include several ecosystem services. The release of nutrients to groundwater and surface water is reduced, and water quality and biodiversity are improved. Rewetted peatlands act as sponges that temporarily store water. This reduces flood risk and creates a buffer for times of droughts. These wetlands also reduce flood risk by reducing – or stopping – land subsidence. In fact, peat forming may restart and ground level height may rise again (79).

Barriers to overcome

According to the experts, the market for most paludiculture products – such as raw materials for bioenergy and bio-based materials – is not yet functional and business models are still under development. Incentives should be developed to stimulate the implementation of paludiculture, including the accounting for reduced emissions of greenhouse gases (carbon credits), water purification, climate change-related water retention and storage (blue credits), and biodiversity (79,80).

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The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for Europe.

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