Germany Germany Germany Germany

Forestry and Peatlands Germany

Forestry in numbers

In 2004 the area under forest in Germany was 29.8% (106,488 km2) (1). According to the findings of the second Federal Forest Inventory, 35.2% of the forests in Germany are to be classified as near-natural or very near-natural on the basis of the tree species composition of the natural forest community. It can be assumed that these forests are most likely to be relatively stable in the face of shifts in climatic factors. By contrast, 64.7% are only classified as not very natural, fairly artificial or predominantly artificial (2).


Almost three-quarters of the forests (73%) are mixed stands. Norway spruce is grown on a little more than a quarter (28%) of the forested land and is therefore the most common tree species in Germany. This is followed by pine with 23%, beech with 15%, and common and sessile oak with 10% (3).

Forty-six percent of the forest is privately owned, 34% are owned by the federal states or the federal government, and 20% are owned by towns, communities and other corporate bodies (3).

The forestry sector employs approximately 175,000 people and accounts for approximately 3% of gross national product (8).

Vulnerabilities - Overview

The increased vulnerability of forests (and people) with respect to climate change refers to several impacts (37,43):


  • 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 (28).

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 (37).

Vulnerabilities - Drought stress

As summer temperatures increase, and dry periods become more prolonged, forests become subject to heat and drought stress. Such risks are especially great for dry and relatively warm regions in eastern and southwestern Germany, as well as for sites with a generally poor water supply or with stands that, for various reasons, are poorly adapted as it is (1). What is more, such warmer and drier conditions can increase the risk of forest fires.

In the North, especially in the Northeast, sandy soils with low water holding capacity increase the risk of drought stress. Furthermore, stands close to groundwater table (lowland riparian forest) are at risk through declining groundwater tables (13).

European beech Fagus sylvatica (L.) is one of the main tree species in Central Europe. Without human interference, beech would probably cover more than two thirds of the forest area in Central Europe (47) instead of their current share of less than one fifth (48). For example, in Germany, beech covers approximately one-third of all forested areas (49). The impacts of climate change on a beech forest in Germany (the federal state of Rhineland Palatinate) was studied for climate change projections based on a larger number of climate models and the A1b emissions scenario (46). Results show that net primary productivity of beech decreases by 30% in 2071− 2100 compared with the reference time span 1961−1990. This is caused by higher mortality rates, lower water availability and higher drought stress, though partly counterbalanced by longer growing seasons.

Vulnerabilities - Pests and Pathogens

When temperatures are high and vegetation period is long, animal pests, such as e.g. the bark beetle, can produce several generations per year, leading to increased abundance over longer periods of time. Mild winters also increase their reproductive success. Moreover, insect pests can spread further North and higher in altitude. New invasive pest species are expected (3). Large swarms of certain pests, such as the nun moth or the cockchafer (May bug), can occur more frequently, and previously insignificant or ignored pests can multiply (1).

Vulnerabilities - Shifts in Species Distribution and Tree Species Composition

For Norway spruce in the Northern Limestone Alps (Germany and Austria), neither growth suppression at the lower elevation sites nor growth increase at higher elevation sites was observed in a dataset covering more than 150 years (until 2003), despite a sharp temperature increase of ~1°C since the 1990s (50). According to the authors, these findings reveal the ability of mountain forests to adapt to an unprecedented temperature shift, suggesting that adaptation to forthcoming climate changes might not require a shift in tree species composition in the Northern Limestone Alps (50).

Beech and pine will be particularly impacted. Stand conditions will shift towards oak-hornbeam and alternatively oak-pine-forests (14,45). … On the other hand, changing environments may offer the opportunity to introduce new species and to diversify the range of suitable species. In the alpine region it is for example expected that the altitudinal limit of beech will incline and therefore the proportion of mixed forests in this regions may increase (3). If Mediterranean species will be able to migrate north is still a matter of debate. One limiting factor could be the soil conditions (e.g. soil pH) (14).


From an economic view the most significant impact for central Europe would appear to be the reduction in areas under spruce and beech and the corresponding expansion of pine and oak. The resulting economic consequences for forestry operations depend to a large extent on the time scale of the transition. Rapid transitions imply particularly high costs and serious financial loss (2).

The change in tree species will in the medium and long term result in marked changes in the performance of forestry operations, since the heat-loving and drought-tolerant species can be expected to be economically less profitable than the species prevailing at present. Against this background it is important for the forestry sector to prepare itself in good time for the changes with which it will be confronted in the long term (2).

Vulnerabilities - Soil organic carbon

During the past three decades, soil organic carbon stocks in German Alps forest soils have decreased. Two independent datasets covering different regions and soil sampling methods show almost identical results: mean overall soil organic carbon stock reduction of 14.0% and 14.5%, respectively (51). These data refer to the topsoil: the organic surface layer + uppermost 30 cm mineral soil.

For dataset 1, organic carbon stock was quantified between 1986 and 1991, and again in 2010/2011. For dataset 2 samplings were carried out in 1976 and 2010/2011. Set 1 sampling sites are distributed over the entire German Alps, covering an area of 4,500 km2 , and include all major parent material, soil, and forest types. Set 2 comprises soils under mature Norway spruce forest and under adjacent meadow subject to extensive mountain pasture in the Berchtesgaden region, distributed over an area of 600 km2.

The results agree with projections in the scientific literature: global soil organic carbon stocks, particularly those in high-latitude or mountain ecosystems, are sensitive to climate change and are predicted to decrease in a warming climate (52), which may result in a positive feedback to climate change (53) as well as in decreased productivity and ecosystem service accomplishment (54). The soil organic carbon losses induced by climate warming are likely to be due to accelerated microbial organic carbon decomposition with increasing soil temperature (55).

The German Alps are characterized by a cool, moist climate. Altitudinal gradient analysis studies report increasing organic carbon stocks of forest soils in the Alps with increasing elevation and concomitantly decreasing mean annual temperature (56). Hence, the increase of mean annual (and mean summer) temperature during recent decades, associated with increased soil temperatures, is probably a main cause for the observed soil organic carbon losses. During these decades, other factors influencing mountain forest soil organic carbon stocks have remained constant or favour soil organic carbon gains rather than losses, such as more sustainable forest management (57), increased atmospheric N deposition (58), and increased forest growth rates (50). Besides, the loss of soil organic carbon was most pronounced at sites with particularly large mean annual temperature increase.

Across the entire data set, significant soil organic carbon stock losses were identified for sites with altitudes <1,150 m. Losses were statistically insignificant at cooler and more humid sites at altitudes >1,150 m. This difference may be (partly) due to the fact that the growth of Norway spruce (the dominating tree species at the study sites), and therefore litter production, is positively related to increased mean annual temperature only at sites >1,200 m (50). Therefore, at higher altitudes increased soil organic carbon losses by accelerated microbial decomposition may be partially or completely outweighed by increased litter input, whereas this was not the case at low-elevation sites. 

Vulnerabilities - The summer of 2003

In the year 2004, 72% of all trees exhibited distinct crown transparency or were rated as “in stage of alert”. This was the highest level of recorded damage since the beginning of forest damage inventories. For the first time, the main reason for this high level of damage in 2004 is not thought to be pollution, but the weather conditions in the hot and dry “record summer” of 2003, and its side- and after-effects. These are direct damage through drought and radiation; damage through increased ozone content of the air, as a consequence of intensive solar radiation; and the spread of calamities as a consequence of the mild winter in 2003, as well as prior damages through direct weather impacts (9).


The heat wave in the summer of 2003 shows how strongly yield potential can be threatened by drought stress. Drought and high temperatures led to a near total depletion of the water reserves in forest soils available to plants. In August/September, the water uptake of trees in many stands was strongly impaired. The consequent water deficiency had profound impacts in many forest areas.

Damages through extreme weather conditions such as in the year 2003 can continue to have an effect over more than 10 years, and can lead to changes in growth trends in the long term, beyond actual reduced growth rates (11). In 2003, the connection between drought, heat and risk of pest infestation became also apparent. An explosive propagation of pests, particularly bark beetles and nun moths, was a consequence of high temperatures and decreased vitality of forests in 2003 (10).

Vulnerabilities - Additional stresses

Climate change is only one of a number of stress factors for forests. Many forests are in poor health as a result of air pollution – now, especially, in the form of large depositions of atmospheric nitrogen. Since the 1970s, that phenomenon has been referred to as "new types of forest damage". The impacts on soils and vegetation will persist for long periods of time (1).

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 (17). Temperate forests are dominated by broad-leaf species with smaller amounts of evergreen broad-leaf and needle-leaf species (18). 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 (19).

Forest productivity has been increasing in western Europe (20). This is thought to be from increasing CO2 in the atmosphere (21), anthropogenic nitrogen deposition (22), warming temperatures (23), and associated longer growing seasons (24).


Trends

Most models predict continuing trends of modestly increasing forest productivity in Western Europe over this century (25). 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 (26). 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  (27).

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 (28).

Migration

The ranges of northern temperate forests are predicted to extend into the boreal forest range in the north and upward on mountains (29). The distribution of temperate broadleaved tree species is typically limited by low winter temperatures (30). 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 (31) are known to be robust carbon sinks, although increased temperature may reduce this effect through loss of carbon from soils (32). Weaker carbon sinks or even carbon losses are seen for temperate forests in areas prone to periodic drought, such as southern Europe (33).

Models suggest that the greatest climate change threat to temperate forest ecosystems is reduced summer precipitation, leading to increased frequency and severity of drought (34). 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 (35). Together, these related effects can potentially change large areas of temperate forest ecosystems from carbon sinks to sources.

Benefits from climate change

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 (44).

In principle, climate change is expected to increase yield potential in forestry. This is on the one hand due to the “fertilising” effect of the increased CO2-content of the air. CO2 enhances photosynthetic activity and water use efficiency. On the other hand, rising temperatures increase photosynthetic rate and other metabolic processes until a certain temperature optimum. Furthermore, a temperature increase leads to a longer vegetation period and therefore to a longer growth phase (3).


In experiments, a doubling of CO2-content of the air led to a growth increase by 20% in trees on average (4). However, in case of a temperature increase of much more than approximately 2ºC negative effect will prevail in most native tree species (5). It strongly depends on water and nutrient supply if an increase in temperature and CO2-content will actually increase yields in a specific location, with water supply probably being the future limiting factor (6). … According to an experts’ survey, drought stress is the most relevant impact of climate change on forest growth (7).

Of special importance are water supply and therefore precipitation and its seasonal patterns. Besides this, yield potential will continue to also depend strongly on the chosen management options (targeted tree species, type of management). According to experts, productivity (annual increment) could be increased by 5-20%, depending on tree species and region, under the assumption of a warming by 1-2ºC and an increase of precipitation by 0-20% in the next 60 years (12).

Germany can expect a further increase in stocks of wood and also carbon stocks in the next 100 years under all scenarios. According to these scenarios, German forest would continue to be a sink of carbon in future. However, this goes along with an aging of the forest stands, lower increments, and a higher susceptibility to weather extremes and calamities. Reasons for this trend are not so much climate changes, but the present trend in management of low wood extraction. Accordingly, the results depend more upon different socioeconomic conditions (SRES-Scenarios) than on different climate scenarios calculated by different climate models (3).

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% (36).

Adaptation strategies

A number of measures and strategies can be implemented to adapt German forests to the impact of climate change (3):

  • Measures of forest conversion – increasing diversity. Diverse forests with nature-oriented species compositions and wide genetic amplitude are the best precondition for adaptive and therefore henceforth stable forest ecosystems (15).
  • Increasing genetic diversity. Besides enhancing species diversity, increasing genetic diversity plays an important role. Adaptation on the genetic level includes favouring trees from well-adapted, e.g. drought-tolerant populations.
  • Plantation of non-native tree species. It is currently a matter of controversial debate if non-native species that are well-adapted to the impacts of climate change (e.g. Douglas fir) should be increasingly used.
  • Management strategies. Only about 70% of the annual increment is harvested in German forests. This goes along with an aging of the stands, a decrease in biomass increment, and a decrease in carbon absorption. Harvesting wood sustainably and in alignment with wood increment is therefore an important contribution to the protection of German forests. Rejuvenation of stands on the one hand leads to an increased adaptability of (young) individual trees, and on the other hand promotes natural selection toward climate-adapted populations.
  • Increased prevention of forest fires. Such measures include mainly improved forest fire early warning systems via video surveillance or satellite-based systems, a better integration of planning levels (forest owners, communities, regional authorities, forestry departments, fire brigades, road constructions), and improvements in technical infrastructure. Moreover, a conversion to mixed forests, which usually exhibit a moister forest internal climate, decreases the risk of forest fires (16).
  • Changes in water management plans. This refers to measures that counteract an additional decrease in water supply, mainly through declining groundwater tables. Examples for this are the re-wetting of floodplain forests and the deactivation of melioration systems (draining systems).
  • Reducing additional threats. This mainly includes the further decrease of pollution, the maintenance of soil fertility (mainly protective lining of soils, minimising soil compaction), as well as avoidance of the disturbance of sensitive forest ecosystems, e.g. through decreased traffic.
  • Improved risk management. In general, consistent risk management of forestry enterprises should gain importance and be supported, e.g. through training courses. This includes the identification, prevention and defence against risks, as well as the management of damages.

The forestry sector should have a high capacity to adapt to the impacts of climate change more than today, since a range of effective adaptation measures are available, even if these are often rated as “complicated”. The shift to mixed forests and the maintenance of genetic diversity were seen as broadly effective in responding to a range of uncertain risks and opportunities of climate change, since these measures maintain or broaden the capability to adapt (3).

To date, the forestry sector is adapted to the impacts of climate change only to a certain degree. On the one hand, the discussion about climate change is most intense compared to the other examined climate-sensitive sectors, but on the other hand, the full implementation of planned adaptation measures to climate change often takes several decades in the forestry sector. In some regions, measures still need to be planned. The vulnerability of the forestry sector to climate change without further measures has been rated as “moderate” (3). Only the drought prone regions (Eastern Germany), as well as regions with strong temperature increase and a high proportion of out-of-natural-habitat spruce stands (lower regions in Western and Southern Germany) are rated as presently “highly vulnerable” (3).

The economic pressure that each forest owner has to bear will also be decisive for the adaptive capacity of the forest sector. In this regard, the adaptation of privately owned forests relies on special support. The vulnerability of the forestry sector to climate change has been rated as “low”, if the suggested adaptation measures are implemented (3).

Adaptation strategies - Forest management measures in general

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 (41).

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 (38). Adaptive management is a systematic process for continually improving management policies and practices by learning from the outcomes of operational programmes (39). 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 (40).


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 (37).

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 (37).

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 (41).

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 (42).

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 Germany.

  1. Government of the Federal Republic of Germany (2010)
  2. Government of the Federal Republic of Germany (2006)
  3. Zebisch et al. (2005)
  4. Norby et al. (1999), in: Zebisch et al. (2005)
  5. Hirschberg et al. (2003), in: Zebisch et al. (2005)
  6. Flaig et al. (2003), in: Zebisch et al. (2005)
  7. Spiecker et al. (2000), in: Zebisch et al. (2005)
  8. DFWR (2001), in: Zebisch et al. (2005)
  9. BMVEL (2004), in: Zebisch et al. (2005)
  10. BMVEL (2003), in: Zebisch et al. (2005)
  11. Anders et al. (2004), in: Zebisch et al. (2005)
  12. Spiecker et al. (2000), in: Zebisch et al. (2005)
  13. Gerstengarbe et al. (2003), in: Zebisch et al. (2005)
  14. VWF (1994), in: Zebisch et al. (2005)
  15. BMVEL (2004), in: Zebisch et al. (2005)
  16. Badeck et al. (2004b), in: Zebisch et al. (2005)
  17. Walter (1979), in: Fischlin (ed.) (2009)
  18. Melillo et al. (1993), in: Fischlin (ed.) (2009)
  19. Reich and Frelich (2002), in: Fischlin (ed.) (2009)
  20. Carrer and Urbinati (2006), in: Fischlin (ed.) (2009)
  21. Field et al. (2007b), in: Fischlin (ed.) (2009)
  22. Hyvönen et al. (2007); Magnani et al. (2007), both in: Fischlin (ed.) (2009)
  23. Marshall et al. (2008), in: Fischlin (ed.) (2009)
  24. Chmielewski and Rötzer (2001); Parmesan (2006), both in: Fischlin (ed.) (2009)
  25. Alcamo et al. (2007); Field et al. (2007b); Alo and Wang (2008), all in: Fischlin (ed.) (2009)
  26. Lucht et al. (2006); Scholze et al. (2006); Alo and Wang (2008), all in: Fischlin (ed.) (2009)
  27. 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)
  28. Fischlin (ed.) (2009)
  29. Iverson and Prasad (2001); Ohlemüller et al. (2006); Fischlin et al. (2007); Golubyatnikov and Denisenko (2007), all in: Fischlin (ed.) (2009)
  30. Perry et al. (2008), in: Fischlin (ed.) (2009)
  31. Liski et al. (2002), in: Fischlin (ed.) (2009)
  32. Piao et al. (2008), in: Fischlin (ed.) (2009)
  33. Morales et al. (2007), in: Fischlin (ed.) (2009)
  34. Christensen et al. (2007); Fischlin et al. (2007); Meehl et al. (2007); Schneider et al. (2007), all in: Fischlin (ed.) (2009)
  35. Hanson and Weltzin (2000), in: Fischlin (ed.) (2009)
  36. Karjalainen et al. (2003); Nabuurs et al. (2002); Perez-Garcia et al. (2002); Sohngen et al. (2001), in: Osman-Elasha and Parrotta (2009)
  37. Innes (ed.) (2009)
  38. Ogden and Innes (2007), in: Innes (ed.) (2009)
  39. BCMOF (2006a), in: Innes (ed.) (2009)
  40. Holling (1978); Lee (1993, 2001), all in: Innes (ed.) (2009)
  41. Roberts (ed.) (2009)
  42. Keskitalo (2008), in: Roberts (ed.) (2009)
  43. Kirilenko and Sedjo (2007)
  44. Boisvenue et al. (2006)
  45. Hlásny et al. (2011)
  46. Rötzer et al. (2013)
  47. Bohn et al. (2003), in: Rötzer et al. (2013)
  48. Fischer and Fischer (2012), in: Rötzer et al. (2013)
  49. BMELF (2005), in: Rötzer et al. (2013)
  50. Hartl-Meier et al. (2014)
  51. Prietzel et al. (2016)
  52. Davidson et al. (2000); Knorr et al. (2005); Hagedorn et al. (2010), all in: Prietzel et al. (2016)
  53. Cox et al. (2000), in: Prietzel et al. (2016)
  54. Brang (2001); Hagedorn et al. (2010), both in: Prietzel et al. (2016)
  55. Hagedorn et al. (2010); Schindlbacher et al. (2009); Davidson and Janssens (2006), all in: Prietzel et al. (2016)
  56. Rodeghiero and Cescatti (2005); Djukic (2010); Hagedorn et al. (2010); Prietzel and Christophel (2014), all in: Prietzel et al. (2016)
  57. Thuille and Schulze (2006); Pötzelsberger and Hasenauer (2015), both in: Prietzel et al. (2016)
  58. Nave et al. (2009); Janssens et al. (2010), both in: Prietzel et al. (2016)
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