Vulnerabilities - Overview

The increased vulnerability of forests (and people) with respect to climate change refers to several impacts (26,32):

• Forest cover: conversion of forests to non-woody energy plantations; accelerated deforestation and forest degradation; increased use of wood for domestic energy.
• Biodiversity: alteration of plant and animal distributions; loss of biodiversity; habitat invasions by non-native species; alteration of pollination systems; changes in plant dispersal and regeneration.
• Productivity: changes in forest growth and ecosystem biomass; changes in species/site relations; changes in ecosystem nitrogen dynamics.
• Health: increased mortality due to climate stresses; decreased health and vitality of forest ecosystems due to the cumulative impacts of multiple stressors; deteriorating health of forest-dependent peoples.
• Soils and water: changes in the seasonality and intensity of precipitation, altering the flow regimes of streams; changes in the salinity of coastal forest ecosystems; increased probability of severe droughts; increased terrain instability and soil erosion due to increased precipitation and melting of permafrost; more/earlier snow melt resulting in changes in the timing of peak flow and volume in streams. The capacity of the forest ecosystem to purify water is an important service, obviating the cost of expensive filtration plants.
• Carbon cycles: alteration of forest sinks and increased CO2 emissions from forested ecosystems due to changes in forest growth and productivity.
• Tangible benefits of forests for people: changes in tree cover; changes in socio-economic resilience; changes in availability of specific forest products (timber, non-timber wood products and fuel wood, wild foods, medicines, and other non-wood forest products).
• Intangible services provided by forests: changes in the incidence of conflicts between humans and wildlife; changes in the livelihoods of forest-dependent peoples (also a tangible benefit); changes in socio-economic resilience; changes in the cultural, religious and spiritual values associated with particular forests.

Productivity

Increasing CO2 concentration can affect tree growth through increased photosynthetic rates and through improved water-use efficiency. There will be complex interactions, however: forest growth rates may well be increased in some cases by rising levels of atmospheric CO2, but rising temperatures, higher evaporation rates and lower rainfall may lower growth rates in other cases (17).

Non-timber products

Increasingly there are concerns about the productivity of non-timber products such as medicines and foods. Relatively little information is available in the scientific literature about the sustainable management of such products, and even less is known about their vulnerability to climate change (26).

Vulnerabilities

Large numbers of Scots pine are dying in the dry inner-alpine valleys of the European Alps; in Switzerland, locally almost half the Scots pine (Pinus sylvestris L.) population has died since 1995. Switzerland’s temperature has increased at more than twice the global average in the 20th century and most of this increase has occurred during the last 20 years. It was shown that the strong climatic warming that has occurred in recent years may well be the indirect cause of the mortality observed in these forests (3). Whereas single drought years had a negative impact on tree growth with a subsequent fast recovery (37), multiple drought years resulted in long-lasting growth depressions, lagged recovery, and increased risk of tree death (38).

Tree mortality was highest following the dry and hot year 1998, and tree defoliation, an indicator of tree vitality, showed a strong correlation with the previous year’s precipitation. While precipitation showed no clear significant trend over time, the number of warm days (mean >20°C, maximum >25°C) and potential evapotranspiration have significantly increased over the last 20 years. Higher temperatures favour pine wood nematodes and bark beetles, both of which are found at the study site, and increasing drought stress reduces tree resistance against pathogens (3). Scots pine mortality in Valais resulted from the combined effects of drought, pests, and diseases (39). Likewise, the ongoing spruce decline in low-elevation Swiss forests was triggered by storms and subsequent bark beetle infestations and aggravated by dry summers (40). 

Climate change will enhance the vulnerability of forests and impair their functions in different ways. According to the Swiss Forests Act, the importance of forests in Switzerland mainly lies in its protective function against natural hazards, its productive function (e.g. wood, drinking water), and its social functions (e.g. for recreation purposes) (1). Weather patterns could develop heavy torrents on exposed slopes leading to erosion and flooding. Erosion would be particularly bad if forests become further weakened by drought (4).

The protective function refers to protecting settlements, traffic infrastructures and cultivated land from erosion, landslides and rock fall. Rapid climate change and more intensive extreme events may impair the stability of natural or semi-natural ecosystems, thereby reducing their protective functions. In Switzerland 80% of the forests have medium to high protective effects against avalanches. Less than half of the evaluated forest stands assure sufficient protection against rock fall. Problems for protective forests arising from climate change are the appearance of large gaps originating from storm events, insect outbreaks, or dieback as a consequence of drought (1,2).

Dry and hot periods lead to the weakening of trees. Such periods increase the susceptibility to insect attacks and pests and promote the outbreak of diseases. This is even more so if climate change leads to the spread of new varieties of potentially damaging organisms. … Also forest fires are affected by drought. In regions like the dry valleys of the canton of Valais, the drought-induced dieback of pines enhances the amount of dead wood in forests and therefore the risk for forest fires (1).

In the long term, the composition of many Swiss forests will change. This will be the combined result of changing climatic conditions and case-by-case human intervention induced by the impacts of climate change, e.g., afforestation with better adapted tree species after storm, fire, drought events or insect calamities. It is expected that the share of deciduous trees will increase and coniferous trees decrease. This will have consequences for the timber industry, which is mainly equipped for processing softwood (1). Drought resistant species may expand rapidly by the end of this century (41). This concerns extant species, species arriving from the neighbouring Mediterranean region, and also exotics that have progressively spread in the Southern Alps since the 1970s due in part to increasing winter temperatures (42). Furthermore, broadleaf trees that are sensitive to late frosts during bud-break (43) will extend their range upward. 

Most vulnerable regions are low-elevation belts on low-elevation plateaus and xeric inter-alpine valleys that undergo the risk of steppification, and high elevation sites due to terrain instability (retreat of permafrost, avalanches). For the Alps timber production is decreasing in its commercial value. Groundwater depletion is a serious problem, as well as soil erosion and retreat of permafrost. Under the current climate conditions approximately 25-30% of all Forest Inventory Points must be considered as poorly adapted. Moderate warming increases this percentage by 5-10%, strong warming by 10-30% (5).

The warming projected for Switzerland by 2050 would lengthen the growing season of subalpine forests by 2-3 weeks (36).

Forest insect pest species 

Future temperature conditions in the course of this century can be expected to favour some crop pests, by enabling them to overwinter more easily on the Swiss Plateau, as well as some forest pests, which will likely reach higher elevations (44). 

Vulnerabilities – Temperate forests in Europe

Present situation

In parts of Europe with temperate forests, annual mean temperatures are below 17°C but above 6°C, and annual precipitation is at least 500 mm and there is a markedly cool winter period (6). Temperate forests are dominated by broad-leaf species with smaller amounts of evergreen broad-leaf and needle-leaf species (7). Common species include the oaks, eucalypts, acacias, beeches, pines, and birches.

Many of the major factors that influence these forests are due to human activities, including land-use and landscape fragmentation, pollution, soil nutrients and chemistry, fire suppression, alteration to herbivore populations, species loss, alien invasive species, and now climate change (8).

Forest productivity has been increasing in western Europe (9). This is thought to be from increasing CO2 in the atmosphere (10), anthropogenic nitrogen deposition (11), warming temperatures (12), and associated longer growing seasons (13).

Trends

Most models predict continuing trends of modestly increasing forest productivity in Western Europe over this century (14). Projections for the time near the end of the next century generally suggest decreasing growth and a reduction in primary productivity enhancement as temperatures warm, CO2 saturation is reached for photosynthetic enhancement, and reduced summer precipitation all interact to decrease temperate zone primary productivity (15). The projected increased occurrence of pests, particularly in drought-stressed regions, also contributes to decreased long-term primary productivity in some regions of temperate forests  (16).

Sensitivity to increasing air pollution loads, particularly nitrogen deposition and tropospheric ozone, will impact large areas of the northern temperate forest over the next century. In the temperate domain, air pollution is expected to interact with climate change; while the fertilization effects from nitrogen deposition are still highly uncertain, pollutants such as ozone are known to diminish primary productivity (17).

Migration

The ranges of northern temperate forests are predicted to extend into the boreal forest range in the north and upward on mountains (18). The distribution of temperate broadleaved tree species is typically limited by low winter temperatures (19). Since the latter are projected to rise more rapidly than summer temperatures in Europe and North America, temperate broad-leaved tree species may profit and invade currently boreal areas more rapidly than other temperate species.

Carbon sinks/sources

Temperate forest regions in the highly productive forests of western Europe (20) are known to be robust carbon sinks, although increased temperature may reduce this effect through loss of carbon from soils (21). Weaker carbon sinks or even carbon losses are seen for temperate forests in areas prone to periodic drought, such as southern Europe (22).

Models suggest that the greatest climate change threat to temperate forest ecosystems is reduced summer precipitation, leading to increased frequency and severity of drought (23). This will probably be most prominent in temperate forest regions that have already been characterized as prone to drought stress, such as southern Europe. Drought-stricken forests are also more susceptible to opportunistic pests and fire (24). Together, these related effects can potentially change large areas of temperate forest ecosystems from carbon sinks to sources.

Benefits

Globally, based on both satellite and ground-based data, climatic changes seemed to have a generally positive impact on forest productivity since the middle of the 20th century, when water was not limiting (33).

Timber production in Europe

Climate change will probably increase timber production and reduce prices for wood products in Europe. For 2000–2050 a change of timber production in Europe is expected of -4 to +5%. For 2050–2100 an increase is expected of +2 to +13% (25).

Adaptation strategies

Near-nature forest management and a move away from monocultures toward mixed forest types, in terms of both species and age classes, are advocated. In addition, natural or imitated natural regeneration is indicated as a method of maintaining genetic diversity, and subsequently reducing vulnerability. For management against extreme disturbances, improvements in fire detection and suppression techniques are recommended, as well as methods for combating pests and diseases. It is reported that through stricter quarantine and sanitary management, the impact of insects and diseases can be minimized. The establishment of migration corridors between forest reserves may aid in the autonomous colonization and migration of species in response to climate change (30).

In recent years, some Swiss cantons have published guidelines for operational adaptive forest management under climate change. The overall recommendation is the strengthening of silviculture in respect of site-specific natural conditions and the enhancement of species diversity. This will strengthen the resistance of forest stands and trees and distribute and minimize potential risks (1,3).

One of the main benefits of alpine forests is the protection from natural hazards. In Switzerland, 49% of the total forest area acts as protective forest. Less days with high snow depth means less avalanches in forested terrain (34). This would imply a change in forest management from avalanche protection to other priority functions such as rock fall or erosion control. In fact, forest management should already adapt to changes that are underway, for instance by planting other tree species (35). 

Adaptive management

The terms adaptation and adaptive management are often incorrectly used interchangeably. The former involves making adjustments in response to or in anticipation of climate change whereas the latter describes a management system that may be considered, in itself, to be an adaptation tactic (27). Adaptive management is a systematic process for continually improving management policies and practices by learning from the outcomes of operational programmes (28). It involves recognizing uncertainty and establishing methodologies to test hypotheses concerning those uncertainties; it uses management as a tool not only to change the system but to learn about the system (29).

Both the climate and forest ecosystems are constantly changing, and managers will need to adapt their strategies as the climate evolves over the long term. An option that might be appropriate today given expected changes over the next 20 years may no longer be appropriate in 20 years’ time. This will require a continuous programme of actions, monitoring and evaluation – the adaptive management approach described above (26).

There is a widespread assumption that the forest currently at a site is adapted to the current conditions, but this ignores the extent to which the climate has changed over the past 200–300 years, and the lag effects that occur in forests. As a result, replacement of a forest by one of the same composition may no longer be a suitable strategy (26).

Adaptation to climate change has started to be incorporated into all levels of governance, from forest management to international forest policy. Often these policies are not adopted solely in response to climate, and may occur in the absence of knowledge about longer-term climate change. They often serve more than one purpose, including food and fuel provision, shelter and minimizing erosion, as well as adapting to changing climatic conditions (30).

Socio-economic and political conditions have significant influences on vulnerability and adaptive capacity. Climate change projections are perceived by many forest managers as too uncertain to support long-term and potentially costly decisions that may be difficult to reverse. Similarly, uncertainty over future policy developments may also constrain action. Finance is a further barrier to implementing adaptation actions in the forest sector (31).

References

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

1. Federal Office for the Environment FOEN (Ed.)  (2009)
2. Fuhrer et al. (2006)
3. Rebetez and Dobbertin (2004)
4. Bogatai (2007)
5. Kellomäki et al. (2000)
6. Walter (1979), in: Fischlin (ed.) (2009)
7. Melillo et al. (1993), in: Fischlin (ed.) (2009)
8. Reich and Frelich (2002), in: Fischlin (ed.) (2009)
9. Carrer and Urbinati (2006), in: Fischlin (ed.) (2009)
10. Field et al. (2007b), in: Fischlin (ed.) (2009)
11. Hyvönen et al. (2007); Magnani et al. (2007), both in: Fischlin (ed.) (2009)
12. Marshall et al. (2008), in: Fischlin (ed.) (2009)
13. Chmielewski and Rötzer (2001); Parmesan (2006), both in: Fischlin (ed.) (2009)
14. Alcamo et al. (2007); Field et al. (2007b); Alo and Wang (2008), all in: Fischlin (ed.) (2009)
15. Lucht et al. (2006); Scholze et al. (2006); Alo and Wang (2008), all in: Fischlin (ed.) (2009)
16. Williams et al. (2000); Williams and Liebhold (2002); Logan and Powell (2001); Tran et al. (2007); Friedenberg et al. (2008), all in: Fischlin (ed.) (2009)
17. Fischlin (ed.) (2009)
18. Iverson and Prasad (2001); Ohlemüller et al. (2006); Fischlin et al. (2007); Golubyatnikov and Denisenko (2007), all in: Fischlin (ed.) (2009)
19. Perry et al. (2008), in: Fischlin (ed.) (2009)
20. Liski et al. (2002), in: Fischlin (ed.) (2009)
21. Piao et al. (2008), in: Fischlin (ed.) (2009)
22. Morales et al. (2007), in: Fischlin (ed.) (2009)
23. Christensen et al. (2007); Fischlin et al. (2007); Meehl et al. (2007); Schneider et al. (2007), all in: Fischlin (ed.) (2009)
24. Hanson and Weltzin (2000), in: Fischlin (ed.) (2009)
25. Karjalainen et al. (2003); Nabuurs et al. (2002); Perez-Garcia et al. (2002); Sohngen et al. (2001), in: Osman-Elasha and Parrotta (2009)
26. Innes (ed.) (2009)
27. Ogden and Innes (2007), in: Innes (ed.) (2009)
28. BCMOF (2006a), in: Innes (ed.) (2009)
29. Holling (1978); Lee (1993, 2001), all in: Innes (ed.) (2009)
30. Roberts (ed.) (2009)
31. Keskitalo (2008), in: Roberts (ed.) (2009)
32. Kirilenko and Sedjo (2007)
33. Boisvenue et al. (2006)
34. Bebi et al. (2009); Teich et al. (2012), both in: Schmucki et al. (2017)
35. Temperli et al. (2012), in: Schmucki et al. (2017)
36. Henne et al. (2018)
37. Lévesque et al. (2013), (2014b), both in: Henne et al. (2018)
38. Bigler et al. (2006), in: Henne et al. (2018)
39. Henne et al. (2018)
40. Temperli et al. (2013); Stadelmann et al. (2014), both in: Henne et al. (2018)
41. Bugmann et al. (2014); Henne et al. (2015), both in: Henne et al. (2018)
42. Walther et al. (2002); Berger et al. (2007), both in: Henne et al. (2018)
43. Kollas et al. (2014), in: Henne et al. (2018)
44. Schneider et al. (2021)