Forestry operations invariably affect the environment in one way or another. Some of these effects can be beneficial to the environment while others can be adverse. Obviously, it is the latter that is regarded with concern by both regulatory authorities and the public.
When we speak of the environment, we often think of the physical and biological components of the environment: that is, the soil, the existing vegetation and wildlife and the waterways. Increasingly, the cultural, historic and amenity values associated with these more fundamental components are being considered part of the environment. Considering the impact of forest operations and management at the landscape level, not only on physical and biological objectives but also on the social values, has resulted in the evolution of concepts such as ecosystem management and forest stewardship. Therefore, this discussion of environmental health also draws on some of the social impacts.
Not All Bad News
Understandably, regulation and public concern regarding forestry throughout the world have focused on, and will continue to focus on, the negative impacts on environmental health. Despite this focus, forestry has the potential to benefit the environment. Table 1 highlights some of the potential benefits of both planting commercial tree species, and harvesting both natural and plantation forests. These benefits can be used to help establish the net effect (sum of positive and negative impacts) of forest management on environmental health. Whether such benefits accrue, and to what extent, often depends on the practices adopted (e.g., biodiversity depends on species mix, extent of tree mono-cultures and treatment of remnants of natural vegetation).
Table 1. Potential benefits to environmental health.
Increased carbon absorption (sequestration)
Increased slope stability
Increased recreational opportunity (amenity forests)
Increased landscape biodiversity
Flood control management
Increased public access
Reduced wildfire and disease risk
Promotion of secessional development of natural forests
Environmental Health Issues
Despite there being major differences in forest resources, environmental regulations and concerns, as well as in forest practices throughout the world, many of the existing environmental health issues are generic across the forest industry. This overview focuses on the following issues:
- decline in soil quality
- soil erosion
- changes in water quality and quantity (including sedimentation)
- impacts on biodiversity
- adverse public perception of forestry
- discharge of chemicals (oil and pesticides) into the environment.
The degrees to which these general issues are a concern in a particular area will be largely dependent on the sensitivity of the forested area, and the nature of the water resources and water users downstream or offsite from the forest.
Activities within forested areas can affect other areas. These impacts can be direct, such as visual impacts, or they may be indirect, such as the effects of increased suspended sediment on marine farming activities. Therefore, it is important to recognize the pathways linking different parts of the environment. For example: skidder logging --- streamside soils --- stream water quality --- downstream recreational water users.
Decline in soil quality
Forest management can affect soil quality (Powers et al. 1990; FAO/ECE/ILO 1989, 1994). Where forests have been planted to rehabilitate degraded soils, such as eroded soils or mining overburden, this net impact may be an increase in quality by improving soil fertility and structural development. Conversely, forest activities on high-quality soil have the potential to reduce soil quality. Activities causing nutrient depletion, organic matter loss and structural loss through compaction are particularly important.
Soil nutrients are used by vegetation during the growing cycle. Some of these nutrients may be recycled back to the soil through litter fall, death or by residual logging waste. Where all the vegetative material is removed during harvest (i.e., whole tree harvest) these nutrients are removed from the onsite nutrient cycle. With successive growing and harvesting cycles, the store of available nutrients within the soil may decline to levels where growth rates and tree nutrient status cannot be sustained.
Burning of logging wastes has in the past been a preferred means of promoting regeneration or preparing a site for planting. However, research has shown that intensely hot burns can result in the loss of soil nutrients (carbon, nitrogen, sulphur and some phosphorus, potassium and calcium). The consequences of depleting the store of soil nutrients can be reduced tree growth and changes in species composition. The practice of replacing lost nutrients through inorganic fertilizers may address some of the nutrient depletion. However, this will not mitigate the effects of the loss of organic matter which is an important medium for soil fauna.
The use of heavy machinery for harvesting and preparation for planting can result in soil compaction. Compaction can cause reduced air and water movement in a soil and increase the strength of the soil to the extent that tree roots can no longer penetrate. Consequently, compaction of forest soils can reduce tree survival and growth and increase rainfall runoff and soil erosion. Importantly, without cultivation, compaction of subsoils may persist for 20 to 30 years after logging. Increasingly, logging methods that reduce the areas and degree of compaction are being used to reduce decline in soil quality. The codes of forest practices adopted in a growing number of countries and discussed in the article “Rules, legislation, regulations and codes of forest practices” in this chapter provide guidance on such methods.
Soil erosion is a major concern to all land users, as it can result in irreversible loss of productive soils, adversely impact visual and amenity values, and may impact water quality (Brown 1985). Forests can protect soils from erosion by:
- intercepting rainfall
- regulating ground water levels
- increasing slope stability because of root growth
- protecting soil from wind and frost action.
However, when an area of forest is harvested, the level of soil protection is significantly reduced, increasing the potential for soil erosion.
It is recognized worldwide that forest operations associated with the following activities are major contributors to increased soil erosion during the forest management cycle:
- road work
Road work activities, particularly in steep terrain where cut and fill construction is used, produce significant areas of loose unconsolidated soil material that are exposed to rainfall and runoff. If drainage control on roads and tracks is not maintained, they can channel rainfall runoff, increasing the potential for soil erosion on lower slopes and on the road edges.
Harvesting of forest trees can increase soil erosion in four main ways:
- exposing surface soils to rainfall
- reducing stand water usage, thereby increasing soil water contents and groundwater levels
- causing gradual decline in slope stability as the root system decomposes
- disturbance of soils during wood extraction.
Burning and cultivation are two techniques often used to prepare a site for regeneration or planting. These practices can increase the potential for surface erosion by exposing surface soil to the erosive effects of rainfall.
The degree of increased soil erosion, by either surface erosion or mass wasting, will depend on many factors including the size of the area logged, the slope angles, the strength of slope materials and the time since the harvesting occurred. Large clear cuts (i.e., total removal of almost all trees) can be a cause of severe erosion.
The potential for soil erosion can be very high during the first year after harvest relative to before road construction and harvesting. As the re-established or regenerating crop begins to grow, the risk of increased soil erosion decreases as water interception (protection of surface soils) and transpiration increase. Usually, the potential for increased erosion declines to pre-harvest levels once the forest canopy masks the ground surface (canopy closure).
Forest managers aim to reduce the period of vulnerability or the area of a catchment vulnerable at any one time. Staging the harvesting to spread harvesting over several catchments and reducing the size of individual harvest areas are two alternatives.
Changes in water quality and quantity
The quality of water discharged from undisturbed forest catchments is often very high, relative to agricultural and horticultural catchments. Certain forest activities can reduce the quality of water discharged by increasing nutrient and sediment contents, increasing water temperatures and decreasing dissolved oxygen levels.
Increased nutrient concentrations and exports from forest areas that have been burnt, undergone soil disturbance (scarification) or had fertilizer applied, can adversely effect water weed growth and cause pollution of downstream waters. In particular, nitrogen and phosphorus are important because of their association with toxic algae growth. Similarly, increased sediment input into waterways can adversely affect freshwater and marine life, flooding potential and water utilization for drinking or industrial uses.
The removal of streamside vegetation and the introduction of green and woody material into waterways during thinning or harvesting operations can adversely affect the aquatic ecosystem by increasing water temperatures and levels of dissolved oxygen in the water, respectively.
Forestry can also have an impact on the seasonal volume of water leaving a forest catchment (water yield) and peak discharges during storm events. Planting of trees (afforestation) in catchments previously under a pastoral farming regime can reduce water yields. This issue can be of particular importance where the water resource below an afforested area is utilized for irrigation.
Conversely harvesting within an existing forest can increase water yields because of the loss of water transpiration and interception, increasing the potential for flooding and erosion in the waterways. The size of a catchment and the proportion harvested at any one time will influence the extent of any water yield increase. Where only small proportions of a catchment are harvested, such as patch cuts, the effects on yield may be minimal.
Impacts on biodiversity
Biodiversity of plants and animals within forest areas has become an important issue for the forest industry worldwide. Diversity is a complex concept, not being confined to different plant and animal species alone. Biodiversity also refers to functional diversity (the role of a particular species in the ecosystem), structural diversity (layering within the forest canopy) and genetic diversity (Kimmins 1992). Forest operations have the potential to impact species diversity as well as the structural and functional diversity.
Identifying what is the optimum mix of species, ages, structures and functions is subjective. There is a general belief that a low level of species and structural diversity predisposes a forest to increased risk of disturbance with a pathogen or pest attack. To some extent this may be true; however, individual species in a mixed natural forest may suffer exclusively from a particular pest. A low level of biodiversity does not imply that a low level of diversity is an unnatural and unwanted outcome of forest management. For instance, many mixed species natural forests which are naturally subject to wildfire and pest attack go through stages of low species and structural diversity.
Adverse public perception of forestry
The public perception and acceptance of forest practice are two increasingly important issues for the forest industry. Many forest areas provide considerable recreational and amenity value to the resident and travelling public. The public often associates pleasurable outdoors experiences with mature managed and natural forested landscapes. Through insensitive harvesting, particularly large clearcuts, the forest industry has the potential to dramatically modify the landscape, the effects of which are often evident for many years. This contrasts with other land uses such as agriculture or horticulture, where the cycles of change are less evident.
Part of the negative public response to such activities stems from a poor understanding of forest management regimes, practices and outcomes. This clearly puts the onus on the forest industry to educate the public while at the same time modifying their own practices to increase public acceptance. Large clearcuts and the retention of logging residues (branch materials and standing dead wood) are two issues often causing public reaction because of the association of these practices with a perceived decline in ecosystem sustainability. However, this association may not be based in fact, as what is valued in terms of visual quality does not imply benefit for the environment. Retention of residues, although looking ugly, does provide habitat and food for animal life, and provides for some cycling of nutrients and organic matter.
Oil in the environment
Oil can be discharged in the forest environment through the dumping of machine oil and filters, the use of oil to control dust on unpaved roads and from chain-saws. Because of concerns about contamination of soil and water by mineral oil, oil dumping and its application on roads are becoming unacceptable practices.
However, the use of mineral oil to lubricate chain-saw bars is still common practice in much of the world. About 2 litres of oil are used by a single chain-saw per day, which adds up to considerable volumes of oil over a year. For example, it has been estimated that chain-saw oil usage was approximately 8 to 11.5 million litres/year in Germany, approximately 4 million litres/year in Sweden and approximately 2 million litres/year in New Zealand.
Mineral oil has been linked with skin disorders (Lejhancova 1968) and respiratory problems (Skyberg et al. 1992) in workers in contact with the oil. Furthermore, the discharge of mineral oil into the environment can result in soil and water contamination. Skoupy and Ulrich (1994) quantified the fate of chain-saw bar lubricant and found that between 50 and 85% was incorporated in the sawdust, 3 to 15% remained on trees, less than 33% was discharged onto the forest floor and 0.5% sprayed onto the operator.
Concerns primarily for the environment have led to biodegradable oils being compulsory in Swedish and German forests. Based on rapeseed or synthetic-based oils, these oils are more friendly to the environmentally and worker, and can also out-perform mineral-based lubricants by offering better chain life and reduced oil and fuel consumption.
Use of herbicides and insecticides
Herbicides (chemicals that kill plants) are employed by the forest industry to reduce weed competition for water, light and nutrients with young planted or regenerating trees. Often herbicides offer a cost-effective alternative to mechanical or manual weed control.
Despite there being a general mistrust of herbicides, possibly as a result of the use of Agent Orange during the Vietnam war, there have been no real documented adverse impacts on soils, wildlife and humans from herbicide use in forestry (Kimmins 1992). Some studies have found decreases in mammal numbers following herbicide treatment. However, by also studying the effects of manual or mechanical weed control, it has been shown that these decreases are coincidental with the loss of vegetation rather than the herbicide itself. Herbicides sprayed near waterways can potentially enter and be transported in the water, although herbicide concentrations are usually low and short term as dilution takes effect (Brown 1985).
Prior to the 1960s, the use of insecticides (chemicals that kill insects) by the agricultural, horticultural and public health sectors was widespread, with lesser amounts being used in forestry. Perhaps one of the more commonly used insecticides used during this time was DDT. Public reaction to health issues has largely curbed the indiscriminate use of insecticides, leading to the development of alternative practices. Since the 1970s, there have been moves towards the use of insect disease organisms, the introduction of insect pests and predators and modification of silvicultural regimes to reduce the risk of insect attack.