This chapter deals with the technical, silvicultural and environmental issues that may constrain or otherwise affect the application of sustainable forest management principles in temperate and boreal forests. It is of course impossible to deal with all factors that affect the feasibility of implementing sustainable forest management. Included here are only some of the issues, particularly those that are considered to affect these forests universally.
The portion of forests that, due to cold climates, are growing close to the limit for their ability to survive, is greatest in the boreal forest zone. However, cold conditions also exist in all temperate and boreal forest areas at high altitudes and trees in such areas are also at the climatic limits beyond which they cannot survive.
It is tempting to harvest valuable timber in forests even where they are at their climatic limits. In Nordic countries, for example, harvesting frequently took place in such areas in the past. In such cases, the problem of managing forests in very cold climates, with a very short growing season, soon became apparent when forest managers tried to re-establish forests on these areas. In such areas, the trees natural ability to regenerate by self-seeding is very reduced, in fact close to zero. Natural processes had probably taken centuries to establish forest in these areas through natural regeneration. Under such conditions, these processes are difficult or even impossible to replace with artificial reforestation. In other words, planting is hazardous and unlikely to succeed. Therefore, timber harvesting in these areas should either be avoided altogether or should be carried out very carefully using a selective harvesting system.
The soils in the boreal and colder parts of the temperate forest zones are generally subject to ground frost during winters. In the far north the ground may be frozen permanently, allowing the trees to utilise only a shallow layer of soil which thaws during the summer.
Temporary ground frost conditions are ideal for harvesting during the cold part of the year as it increases the carrying capacity of the soils and results in little soil disturbance from harvesting operations as long as the ground is frozen. This is particularly important for harvesting operations in areas with clay or silty soils (these soil types are widespread in flat terrain in, for example, northern Russia and eastern and interior parts of Canada). On the other hand, when these soils thaw, they are easily damaged by forest harvesting equipment. The construction of permanent roads can also be problematic in these areas. Sustainable forest management in these areas therefore, requires that low intensity harvesting operations be used.
If soils with fine texture are exposed to frost after site preparation, they are easily subject to so-called "needle ice" formation during autumn and spring, when they contain a lot of moisture. Planted and even naturally established tree seedlings tend to be lifted out of the ground by the ice needles and killed when such conditions occur. Sites with these types of soil conditions are often spread among more easily cultivated upland soils. They are often also sites of great value for biodiversity and are ideal for setting aside for nature conservation. If such sites are more widespread, a sustainable forest management approach would be to try to save the naturally established lower tree vegetation during harvesting operations, to form the next forest rotation.
In areas with frequent occurrence of wet snowfall and/or high winds, the use of seed trees and shelterwoods to encourage natural regeneration may be limited, due to the risk of snow-break and windthrow of dispersed trees. On the other hand, using modern harvesting machinery, snow is seldom any hindrance in harvesting operations, except in very mountainous areas or under other extreme conditions. In areas with very steep terrain, as in the European Alps, clearcutting is prohibited or restricted to very small areas (one hectare or less), in zones where a closed forest cover is required as protection against snow avalanches.
High winds have been observed to cause more windthrow damage in recent years in Central Europe than in earlier years. This has been attributed to a higher frequency of severe storms. Another factor, which has often been overlooked, is that the affected forests have often been managed with the aim of producing old and more valuable trees. Consequently, the forests have grown increasingly taller and hence also increasingly more sensitive to windthrow.
Widespread occurrence of wetlands, in the form of treeless peat bogs and forests growing on wet and moist soils, is a typical feature of the boreal forest zone. Such areas are often also covered with a large number of streams and lakes. Even in the temperate zone, forests are frequently found on wet sites.
Wetlands and the areas adjoining wetlands are often areas of tremendous biodiversity. Sustainable forest management therefore requires careful consideration of the potentially negative environmental effects of timber harvesting on such sites. A widespread awareness of these concerns currently seems to exist within the forestry profession all over the world.
Where fragile wetlands occupy only a small or separate portion of the total forest area, it is advisable to leave such areas for nature conservation and concentrate commercial forestry activities on the more easily cultivated soils. This is reflected in most modern silvicultural prescriptions all over the world as well as in the main requirements for forest certification.
Traditionally in the past, wet forest areas have been drained to increase the growth potential of forest sites or to allow new forests to be planted. This was in the past important in the Nordic countries and the former Soviet Union. It has though, currently been more or less brought to a halt for the sake of nature conservation. For the same reason, the conversion to forest plantations of flatlands with high groundwater levels, has been restricted in the Southern United States of America.
In Finland, forest drains are however, still maintained over large areas of earlier drained forest sites, in order to take advantage of these investments. In Ireland and Scotland, vast areas of peat covered land and even deep peat bogs have also been converted to forest plantations after draining them. Growing trees in such areas requires the repeated application of fertilisers in most cases. The growth results can be startling, but this kind of plantation forestry is meeting increasing criticism from environmentalists.
Currently, most forest harvesting is highly mechanised in temperate and boreal forests. The development of ground based machines (as opposed to cable extraction systems) is targeted at building lighter, more flexible machines, which have improved economic performance and cause as little damage to remaining trees and the forest floor as possible. Great improvements have already been made in this area. This is particularly important because the trend in most forest harvesting operations is towards continuous harvesting all year round.
For sustainable forest management, soil compaction and damage to tree roots and tree stems should be avoided as much as possible. Modern machines operating on unfrozen ground should have a low specific ground pressure, by using sufficiently wide rubber-tires or tracks that do not penetrate deep into the ground. On sensitive sites (e.g. silty, moist or wet sites) the machines movements should preferably be located parallel to skid-trails and roads and skid-trails should be reinforced with a layer of debris from harvested trees.
Existing forest cover is often a hindrance to the establishment of tree seedlings. This is particularly the case in boreal forests, where a thick humus layer develops during the lifetime of the forest. On top of this layer germinating seedlings frequently die during drought spells. Seeds and small seedlings also fall victim to the many animals that dwell on the forest floor. Site preparation, using different types of cultivators or ploughs, has proved to be an important silvicultural measure in intensive forest management. This encourages a successful start to the regeneration process, regardless of whether planting, direct seeding or natural regeneration is used to establish the next crop.
Many cultivators only create parallel furrows or rows of patches in the otherwise unaffected forest floor. In cold climates so called "tilt-ploughing" and "mounding", producing elevated planting spots, have proved to be very efficient measures that encourage good seedling survival and early growth. A special type of deep ploughing is sometimes carried out on sandy soils where a layer of so called "hardpan" has developed. Hardpan has to be broken up because it is a major hindrance for tree roots that prevents them from reaching deeper down in the ground to secure the water and nutrients that they need. In forest plantations it is often necessary to use a number of major site preparation operations in order to plant bare land.
Site preparation methods have been criticised as being too destructive and as causing unnecessary soil disturbance, erosion and even damaging archaeological monuments. The accusations are justified to some extent, at least in boreal forests, even though serious erosion has seldom been observed. In Sweden, for example, deep tilt ploughing has been banned, partly because of the effects it has on reindeer breeding. Forest managers worldwide should nowadays be quite cautious and selective in the choice of methods for site preparation and their application. On the other hand, it would be difficult for any intensive sustainable forest management system to totally refrain from using at least some basic forest cultivation measures.
Under natural conditions, forest fires constitute the most important rejuvenating factor in boreal forest ecosystems and in many temperate forest ecosystems as well. Millions of hectares of forests still currently burn every year, mostly in the remote far North of the boreal forest zone. When forests are managed for wood production wildfires are of course seen as a great danger and an undesirable and expensive disturbance factor. Therefore, forest fire fighting has been an important forest management activity for decades. Today, most attention to forest fires is given in forest areas with pronounced spells of drought in the summer and in areas with old unmanaged forests that contain a lot of dry and inflammable wood. On the other hand, controlled burning is a tool that has long been used as part of some sustainable forest management systems, to promote forest rejuvenation and control unwanted ground and brush vegetation.
Sustainable forest management generally reduces fire events because it results in healthy green forests and reduces the presence of old stands with dying trees. Consequently, the absence of forest fires in some forest ecosystems has threatened the survival of some forest plant and animal species. In recent years however, as aspects of nature conservation have gained greater influence over commercial forest management, the appreciation of fire as a natural component of many forest ecosystems has started to attract due attention. In Nordic countries for instance, silvicultural prescriptions for large scale forestry operations nowadays require that a certain portion of the annual restocking area should be burnt, even if the controlled burning is not necessary (or is even inconvenient) from the operational point of view. In addition, but on a much more limited scale, fire may be used as a management tool in some stands with live trees.
Outbreaks of insect attacks or fungus infections are a common problem in forestry worldwide. Typically, some of these outbreaks occur if and when trees are subject to stress for some reason (e.g. drought; frost damage; overstocking; lack of nutrients; etc.). There are also pests that start to become more of a problem as trees become very old. These act as a natural rejuvenating factor by killing or weakening trees and making them more susceptible to wildfires or to attack by other organisms.
Large areas of even-aged monocultures are particularly sensitive to such damages and experience shows that continuous pest control may be necessary in order to maintain forest health and vitality in forest plantations. For example, Radiata pine plantations in New Zealand and Chile are sensitive to Dothistroma needle blight during one stage of their development, but aerial spraying of a copper agent has proved effective at keeping this pest under control.
All over the temperate and boreal forest zone, the rule-of-thumb is that a green forest (i.e. one with relatively few dead and dying trees) is a healthy forest and will be most resistant to pests and diseases. Even so, pest outbreaks can not be avoided entirely, especially if the forest is under some type of stress. The general solution to this problem is therefore, to reduce susceptibility by utilising a mixture of tree species that are appropriate for the site, to regularly tend the stands and thus avoid overstocking and, finally, to harvest the trees at an appropriate time (i.e. before they become susceptible to age related diseases).
Nowadays, the forest manager can also rely on some rather effective biological means of pest control, such as parasitic micro-organisms and pheromone traps, instead of using chemicals. However, in many cases, the application of chemicals to control pests can not be avoided. There is a noticeable trend though, towards the development of more biological means of control as an alternative to the greater use of chemicals.
Trees currently planted in forests, largely originate from seeds produced in seed orchards. Such new forests consequently consist of trees with properties that are genetically improved in terms of their yield and overall performance.
There is currently a major debate about this subject and it is sometimes claimed that the use of improved planting stock is irresponsible, because it changes the natural and/or original gene pool in managed forest areas and doesn't preserve natural variety. The alternative argument is that forestry serves two purposes: nature conservation and the production of wood raw material supplies for expanding forest products market and, consequently, the benefit of mankind. Another argument in favour of the use of genetically improved planting stock is that this has been done for centuries in agriculture without, until recently, questions about the appropriateness of crop breeding and improvement.
To compensate for intensive forest management, most countries are also currently actively trying to preserve biodiversity in a number of other ways, such as: utilising landscape planning techniques; encouraging the use of selective harvesting systems; protecting sensitive sites; and setting aside some forest areas as forest reservations.
Within the forest sector, there has been a long debate over many centuries about whether forest sites in temperate climates, with a long history of intensive cultivation (especially where monocultures are being grown), slowly loose their fertility. In particular, certain tree species, such as: spruces; pines; and eucalypti, have been accused as causing losses of site fertility over the long-term.
However, scientific studies, as well as practical experience, do not lend much support to this hypothesis. In Europe, for example, Norway spruce has been cultivated with startling results for two hundred years on sites that originally contained mixed deciduous forests. Some Radiata pine plantations in New Zealand are also now into their third rotation, without any measurable loss in site fertility.
Where site impoverishment is observed, it can usually be explained by other factors. A classic example in Europe, was the situation whereby farmers, in the past, tended to repeatedly collect the decomposing forest ground cover (i.e. the humus layer) in pine plantations. Because this ground cover provides the trees with many of their nutrients, this resulted in slower tree growth over time. It seems highly likely that, at least in terms of sustained wood yield, there is little evidence that intensive sustainable forest management in temperate and boreal forests will not be sustainable in the long term due to a gradual loss of soil fertility.
The significant decline in forest health that became apparent during the 1970´s in Central Europe was identified as being related to the burning of fossil fuels in these and neighbouring countries. Further studies showed that the large areas of dying spruce forests in Central Europe were just the tip of an iceberg. Similar situations were discovered all over the world in densely populated and industrialised regions, wherever pollution from industry, cars and homes was significant and widespread. As a result, policies started to be introduced in many countries, to halt and decrease the amount of dangerous pollutants emitted into the atmosphere each year.
The concerns expressed at this time, that even larger forest areas were going to collapse and die in Europe, have not been fulfilled so far. In central and eastern Europe, parts of North America and elsewhere, trees in exposed areas and positions show clear signs of stress but, apart from this, most forests do not show any evidence of forest decline, even in areas that suffer from high levels of air pollution. However, the problem of forest decline is still present in many places even if trees are not dying in large numbers.
Eliminating the major causes of forest decline will take time, or may never be accomplished. In recognition of this, forest managers in the regions most affected have been forced to deal with both known and hypothetical effects of air pollution. A major problem however, is that there is very little knowledge or experience about how to best handle such situations. Some experts believe that conifers grown in even age class stands are especially sensitive to damage and decline from air pollution. They suggest that, at least in Central Europe, the forest should be gradually converted back to its original composition (i.e. mixtures, often dominated by broadleaved trees). However, even forests with their original composition are currently suffering from decline due to air pollution and stress, in particular, some of the most valuable climax trees (i.e. the oaks and beeches). Opposed to this view, is the view that such a change would have very little impact on forest decline. This view is supported very strongly by private forest owners that want to continue their commercial forestry operations. They reject any suggestion of drastic changes in overall forest species composition and highlight the enormous costs2 and poor profitability that such a move would bring about, with little guarantee of the certainty of success from such a change.
A somewhat surprising fact is that, overall, the forests in Central Europe are now growing faster than ever before, despite the present problems with air pollution in some areas. The explanation for this is complex, but one reason is that air pollutants typically contain nitrogen that can act as a fertiliser. The soil conditions in many forests have also simultaneously deteriorated due to air pollution and have shown sharp decreases in pH-levels and losses of mineral nutrients. It is suggested that, to help maintain the long-term fertility of the soil in such areas, fertiliser should be applied to compensate for the loss of mineral nutrients through leaching and lime should be applied to improve soil pH.
2 For example, it is estimated that it would cost about US$ 15,000/ha to create one hectare of oak plantation in place of a pine or spruce stand in Germany.
