Adaptation to climate change has become of growing concern in recent years, in part due to the impact of storms and other events that have raised the awareness of such risks amongst forest owners. While mitigation focuses on limiting emissions or increasing sequestration of greenhouse gases, warming debt from past emissions and the current system inertia leave societies with few options, but to adapt to the accumulating consequences of climate change.
For forest socio-ecological systems, these consequences may include changing growth rates, effects of drought, insect pest and pathogen outbreaks, including potentially increasing depredation by invasive species, changes in fire regimes and impact on forest biodiversity.
As the specific nature of these impacts will vary across bioclimatic regions, so too will variations exist in the adaptation alternatives considered and implemented in different forest management systems.
Sweden is one of Europe’s most densely-forested countries, with forestry playing a relatively large role economically (forest products constituting some three percent of GNP and 10 percent of export value). However, climate change adaptation has, to a large extent, been limited to the provision of recommendations to forest managers, most of which have only been partially implemented.
The most recent regional climate scenarios for Sweden are based on moderate or high emissions of greenhouse gases (representative concentration pathways (RCP) 4.5 and 8.5), assessed using nine different general circulation models (GCM), as collated by the IPCC.
The resulting projected increase in mean annual temperature varies from two deg C to seven deg C by 2071 to 2100, compared to the reference period 1961 to 1990. The increase in winter temperature is greater (twodeg C to nine deg C), than in summer (one deg C to six deg C).
There are also regional differences in projected temperature changes, but the general trend is for northern Sweden to face the greatest increases. An increase in temperature of one def C equates to a mean northward shift in temperature zones of approximately 150km, with an associated altitudinal increase of approximately 150m.
Compared to the same reference period and endpoint, regional climate scenarios project an increase in precipitation by 0 percent to 40 percent, but with considerable variations between years and decades. The increase in precipitation is greatest in winter time. In southern Sweden, some projections indicate decreased precipitation in summer, with potential increases in northern Sweden. Climate projections provide no clear indications of changes in wind intensities, nor of the frequency of high wind events. Projections do, however, predict milder and wetter winters with less soil freezing.
The government Commission on Climate and Vulnerability in 2007 came to some conclusions in terms of the likely impact of climate change on the forestry sector, including:
•The consequences for Swedish forests and forestry will be substantial, including increased growth and increase of timber production by 20 percent to 40 percent.
•Native valuable hardwood trees could be used further north than they are today in a future climate.
•Fire frequency is expected to increase significantly in a changing climate according to the climate scenarios studied. This increase is expected to be greatest in southern Sweden.
•There will be increased risk of fungal and insect attack in forests and these problems will spread north; also, increased necessity of pesticides to prevent large-scale damage.
•More difficult conditions for performing forest operations, caused by less soil freezing in winter and increased precipitation during the winter months, is likely to complicate felling and transport of timber out of the forest stands.
With regard to adaptation to these stresses, the commission report drew upon several sub-reports of relevance to forests. These indicated that Norway spruce, which has the highest production value, is particularly threatened by increases in drought, storm damage and pest outbreaks.
Relevant adaptation could include shortening rotation periods, more severe earlier thinning and avoiding the creation of exposed forest edges liable to wind damage. Other relevant adaptations would also include actions to reduce the risk of pest outbreaks, such as removal of excess dead wood and the setting of traps for pest species, as well as more far-reaching changes aimed at increasing variation in terms of tree species and forest management models and, consequently, spreading risks.
Development To Forest Management
Much of what is now known serves to advance and refine the knowledge presented in the Commission on Climate and Vulnerability. The fields advanced primarily relate to maintenance adaptation (mainly spruce), the effects of introduced species, increased frost risk in stand regeneration, how to select suitable planting material for future climates, climatic benefits to forest growth, the extent of threats from new invasive species, as well as the associated financial and economic implications.
With regard to measures at forest management/stand levels, a considerable amount of research has resulted in findings with direct implications for effective adaptation. Potential measures at forest management levels may include changes in forest characteristics to increase diversity and forest management approaches in response to disturbances, such as fires and storms.
The potential for increased growth and yield has been an important area of study given the substantial export value of the Swedish forest industry and may be regarded as related to the focus of research for most factors concerning measures at forest management levels. A warmer climate provides an extended growing season and means that more sunlight can be used for biomass production through photosynthesis.
At the same time, an increased level of carbon dioxide in the atmosphere may lead to increased forest growth (depending on other conditions). Increased temperatures may also lead to increased nutrient availability in the soil as a result of increased biological activity and decomposition of organic matter.
Changes in rainfall, mainly during the winter, as most climate scenarios indicate, would not provide any significant effect on growth in Sweden, except possibly in the spring and in southern Sweden. A change in rainfall patterns during the summer, however, would affect growth. Growth is likely to increase for most tree species, and different model estimates indicate increased tree volume growth of 10 percent to 40 percent in 100 years.
Results related to growth issues thus fall under a number of categories. For example, one issue related to forest yield and the potential for increases in forest growth has involved the higher potential use of introduced/non-native tree species. Following up on the 2007 Forest Bill (Swedish Ministry of Agriculture 2007), a government study was launched to examine the potential for increasingly intensive forestry, including the use of fast-growing trees on abandoned agricultural land, that highlighted the necessity of clarifying and potentially changing regulations to allow such measures.
The government study concluded that opportunities for increasingly intensive forestry using introduced, fast-growing tree species was limited by the availability of abandoned agricultural land. Nevertheless, a large area of forest land could be used for such forestry; however, potential negative environmental effects could result due to nitrogen leakage and reduced forest biodiversity.
Beyond General Forest Management
Given the focus in Sweden on growth and yield, genetic adaptation to climate change has been readily incorporated into the pre-existing focus on identifying high-performing and robust genotypes and provenances for establishment in different regions of the country.
Given current climatic scenarios, and their abiotic and biotic consequences, interesting traits for genetic adaptation range from abiotic tolerance, such as tolerance against drought, and traits beneficial to enduring strong wind and heavy snow loads, to biotic resistance, such as resistance against pest, pathogen and fungal infection: the variation of factors discussed above. Further, sustainable management should include considerations for ecological values, such as biodiversity and ecosystem function, which may rely on forest genetics.
Also, given that the risk of extreme events is predicted to increase in the future, plasticity or an inherent ability for acclimation will likely be beneficial. Genetic adaption to these changes in climatic conditions may be either passive, relying on natural migration, plasticity and evolution of the current genetic resources, or active, by implementing assisted migration, or through actions that maintain or enhance the diversity of gene pools.
Research on genetic adaption suggests that natural processes might not be enough to keep up with current climatic trends. Artificial regeneration and assisted migration with suitable seed sources may thus be a way of increasing the proportion of adapted genotypes. Enhancement of relevant traits through breeding could also be beneficial, even if care needs to be taken in terms of maintaining genetic variation.
The Swedish tree breeding programme, started in the 1940s with its current strategy developed in the late 1980s, has three main aims: (i) to manage and maintain the genetic diversity of production tree species; (ii) to develop a preparedness for climate change; and (iii) to breed for general purpose objectives.
Climate change has therefore been included as an important consideration in this program over a considerable period of time. By carefully designing the number and size of the breeding populations, long-term genetic gain can be achieved without eroding the genetic variation of the specie. Furthermore, the separate breeding populations are allocated to different adaptation targets (including matching of growth rhythm) defined by photoperiod (latitude) and temperature conditions.
For a given photoperiod, breeding populations are adapted to both colder and warmer conditions by directed selection of suitable test sites, resulting in genetic material adapted to different climatic conditions. The general purpose objectives currently used consist of improvements in growth, vitality, quality and that the selected genotypes display robust behaviour. As genotypes are tested at several sites experiencing different climatic conditions within an intended target zone, only those showing high, stable performance in the most important traits over all tested sites are selected (selection of generalist genotypes).
In Sweden, a long history of provenance testing has shown great variation among provenances, with strong climatic and photoperiodic gradients in the performance of both Norway spruce and Scots pine. In addition, considerable variation has been found in growth rhythm traits between and within populations.
Results from both provenance studies and studies of growth rhythm traits have been used to develop current transfer functions and deployment recommendations to facilitate the use of highly-productive and well-adapted material. Recently, provenance trials have been re-examined using new climatic data to predict tree performance in the climate change context for several different species worldwide.
New transfer functions for Scots pine in Sweden and Finland have been developed using a comprehensive set of field data (provenance and genetic field tests) and state-of-the-art climate indicators. For Norway spruce, a similar project is underway; however as a secondary tree species, growth rhythm is a particularly important factor for adaptation to a changed climate.
With regard to other adaptation beyond forest management and not purely focused on genetic adaptation, there is a long tradition of approaches to ecosystem-based management, as well as protected forest. For example, conservation ecologists have long argued that the role of forest management must include protected area management over areas larger than single properties.
However, the interaction between such networks and other risks, such as those described above, is not clear. For example, the role of protected forests in the build-up of populations of insect pests, not the least of which is bark beetles, is still an issue under debate. Some evidence indicates that there is a local build-up, but to what extent these beetles increase the risk of damage in adjacent production forests and at what spatial scale this may happen are still unclear. Interrelationships can also be found with genetic tree breeding, where the influence of tree genetics on their environment and dependent communities can be substantial.