{"id":989,"date":"2021-07-06T18:29:47","date_gmt":"2021-07-06T18:29:47","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-binghamton-envi101&envi201\/chapter\/__unknown__-7\/"},"modified":"2021-08-02T14:49:34","modified_gmt":"2021-08-02T14:49:34","slug":"chapter-22","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-binghamton-envi101&envi201\/chapter\/chapter-22\/","title":{"raw":"Chapter 27 ~ Environmental Effects of Forestry","rendered":"Chapter 27 ~ Environmental Effects of Forestry"},"content":{"raw":"
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Key Concepts<\/h2>\r\n

After completing this chapter, you will be able to:<\/p>\r\n\r\n

    \r\n \t
  1. Explain how forest harvesting removes nutrient capital from the site.<\/li>\r\n \t
  2. Outline how forestry can damage aquatic ecosystems, and how many of those effects can be avoided.<\/span><\/li>\r\n \t
  3. Describe how clear-cutting affects biodiversity.<\/span><\/li>\r\n \t
  4. Explain the special qualities of old-growth forest, and how they are affected by timber harvesting.<\/span><\/li>\r\n \t
  5. Discuss ecological consequences of the conversion of natural forest into plantations.<\/span><\/li>\r\n \t
  6. Explain the concept of integrated forest management.<\/span><\/li>\r\n<\/ol>\r\n

    Introduction<\/h1>\r\n

    Forestry includes both the harvesting of trees and the management of post-harvest succession to foster the regeneration of another forest. Forest science guides these activities by providing an understanding of the environmental factors that affect the productivity of trees. The primary goal of commercial forestry is to provide sustainable harvests of tree biomass that can be used to manufacture lumber, paper, and other industrial products, or as a source of bio-energy. Secondary goals are related to the management of hunted animals (such as deer and fish) on forest landscapes, and sometimes the designation of certain areas as protected (where no timber harvesting occurs).<\/p>\r\n

    Forestry is an important economic sector in many countries. In the U.S., the forest products industry generates over $200 billion a year in sales (U.S. Forest Service, 2021). Similarly, in Canada, a large industrial enterprise depends on a continuous supply of tree biomass for the production of products that have a great economic value ($34 billion in 2013; see Chapter 12 for forestry-related economics).<\/p>\r\n

    Of course, to achieve these economic benefits, trees must be harvested from extensive areas of mature forest. In Canada, clear-cutting is the most commonly used method of forest harvesting \u2013 this method removes all of the economically useful trees on a site at the same time. Clear-cutting accounts for about 90% of the annual timber harvest. Clear-cutting is also heavily utilized in the U.S. as a method for harvesting trees.<\/p>\r\n

    However, clear-cutting is not the same as deforestation. Deforestation involves the permanent conversion of a forest into some other kind of ecosystem, such as an agricultural or urbanized land-use. In the U.S. and most other developed countries, industrial clear-cutting is generally followed by the regeneration of another forest. In fact, it is common practice to manage the post-harvest succession to speed up the rate of regeneration of trees. This allows the next harvest to be made after a relatively short time, so more profit can be made. (This period of time is known as a harvest rotation.) In this sense, forestry as it is usually practiced does not result in a net deforestation, and if appropriately managed the forest resource is not depleted.<\/p>\r\n

    Timber harvesting can be viewed as a disturbance of the forest ecosystem, followed by regeneration. Additional disturbances are associated with silvicultural activities, such as preparing the site for planting, thinning dense stands, and applying herbicide or insecticide to deal with pest problems. Silviculture, the branch of forestry that involves tending and growing forests, is practiced over an extensive area in North America. Some planted areas are managed intensively to develop a plantation, which is an anthropogenic forest, but one that lacks many of the ecological and aesthetic values of natural forest.<\/p>\r\n

    In this chapter we examine some of the ecological effects of timber harvesting and silviculture, with a focus on site quality, hunted animals, and biodiversity. Additional effects of forestry are examined in other chapters: pesticide spraying in Chapter 26, implications for carbon storage in Chapter 21, and tropical deforestation and global biodiversity in Chapter 14.<\/p>\r\n\r\n

    Forest Harvesting and Site Capability<\/h1>\r\n

    In Chapter 12, we defined site capability (or site quality) as the potential of land to sustain the productivity of agricultural crops. This is also relevant to forestry, in terms of the ability of land to sustain the productivity of trees. Site capability is a complex attribute that involves the amounts of nutrients and organic matter in soil, the availability of moisture, and other factors affecting plant growth. These factors are influenced by soil type, climate, drainage, rate of nutrient cycling, and the kinds of plant and microbial communities that are present.<\/p>\r\n

    The ability of soil to supply plants with nutrients is a critical aspect of site capability. In large part, this ecological function depends on the nutrient capital of a site, which is the amount of nutrients present in the soil, living vegetation, and dead organic matter. When trees are harvested, the nutrients in their biomass are also removed, which can deplete the nutrient capital of the site.<\/p>\r\n

    A stand of forest may be harvested using a variety of methods, which vary in the amount of biomass and nutrients that are removed from the site. A selection harvest is a relatively \u201csoft\u201d method because it involves the harvesting of only some of the trees from a stand, leaving others behind and the structure of the forest substantially intact. The most intensive harvest is a clear-cut, in which all economically useful trees are removed. The smallest clear-cuts, typically involving a hectare or less, are known as a group-selection harvest. More typically, clear-cuts entail the harvesting of trees from larger areas, on the order of 20-100 ha. The largest clear-cuts may extend over hundreds, and even thousands, of hectares. However, such extensive operations are unusual, and are usually associated with the salvaging of trees that have been damaged by a wildfire, windstorm, or insect infestation.<\/p>\r\n

    There are also some less intensive methods of clear-cutting. A shelterwood harvest is a staged clear-cut, in which some larger trees of economically desirable species are left standing during the initial cut. These provide a seed source and a partially shaded environment that encourages natural regeneration. Once the regeneration is well underway, the large \u201cleave\u201d trees are harvested (Image 27.1). A strip-cut is another kind of staged harvest, in which long and narrow clear-cuts are made at intervals, with uncut forest left in between to provide a source of seed to regenerate trees in the cut strips. Once the regeneration is established, another strip-cut is made, again leaving intact forest on one of the sides. This system of progressive strip-cutting continues until all the forest in the management block (the specific area being managed this way) has been harvested. Typically, an area is harvested in three to four strips. To regenerate trees on the final strips, foresters may rely on advanced regeneration \u2013 that is, on small individuals of tree species that existed in the stand prior to harvesting and that survived the disturbance of clear-cutting. Alternatively, they may plant the last strip with seedlings.<\/p>\r\n

    \"image\"<\/p>\r\n

    Image 27.1. Shelterwood Cut. This photo shows a three-year-old shelterwood cut of hardwood forest in Nova Scotia. About 60% of the trees were removed during the harvest, but many of the \u201cbest\u201d trees were left to grow into high-quality sawlogs and to shed seeds to promote regeneration. This treatment produces a complex habitat that supports a mixture of birds and other wildlife typical of both clear-cuts and mature forest. Source: B. Freedman.<\/p>\r\n

    Clear-cutting systems also vary in how intensively the biomass of individual trees is harvested. The usual stem-only harvest involves the removal of tree trunks, leaving the roots, stumps, and logging \u201cslash\u201d (cut branches and foliage) on the site. The harvested logs can then be processed into lumber or pulp for manufacturing paper. A whole-tree harvest is more intensive because it removes all of the above-ground biomass of the trees, including the branches and foliage. This intensive kind of harvest will recover considerably more biomass than a stem-only cut, which is an advantage if the wood is to be used as a source of bio-energy.<\/p>\r\n\r\n

    Nutrient Losses during Harvesting<\/h2>\r\n

    Although intensive harvests such as a whole-tree clear-cut increases the yield of biomass, there is also considerably more removal of nutrients. Some forest scientists have suggested that the nutrient removals from whole-tree harvests could degrade the capability of sites to sustain tree productivity. The problem would be especially severe if the harvests are conducted over a short rotation. This might not allow enough time for the nutrient capital to recover by natural inputs, such as by precipitation, nitrogen fixation, and weathering of minerals (Figure 27.1).<\/p>\r\n

    \"image\"<\/p>\r\n

    Figure 27.1. Effects of Harvest Intensity and Length of Rotation on Nutrient Capital. Diagram (a) suggests that a relatively long rotation can allow harvested nutrients to be replenished by natural inputs through rainfall, weathering of minerals, nitrogen fixation, and other means. An adequate post-harvest recovery means that harvesting is sustainable with respect to the nutrient capital of the site. Under the shorter rotation (dashed line), the harvested nutrients are not totally replenished between successive clear-cuts, resulting in a degradation of nutrient capital. Site capability can also be degraded by an increase in the intensity of the harvest. This is illustrated in diagram (b), in which a whole-tree clear-cut removes twice as much nutrient as a stem-only harvest. Diagram (c) indicates that fertile sites (solid line) are less likely to be degraded by intensive harvests over short rotations, compared with less fertile sites (dashed line). Source: Modified from Kimmins (2003).<\/p>\r\n

    Site impoverishment caused by intensive cropping is a well-known problem in farming, in which severely degraded land may have to be abandoned for some or all agricultural purposes. However, usually this problem can be managed, to a degree, by applying fertilizer or composted organic matter to the land. Sometimes, however, the degradation of site capability, especially of tilth, is too severe, and this simple mitigation is not successful. Of course, the harvest rotation in agriculture is usually annual, whereas in forestry it ranges from about 20 to 100 years. However, each timber harvest involves the removal of a huge quantity of biomass, and thus of nutrients.<\/p>\r\n

    Compare, for example, the amounts of biomass and nutrients removed by clear-cuts of a conifer forest in Nova Scotia (Table 27.1). In this case, a whole-tree clear-cut yielded 30% more biomass than a stem-only harvest. The increased yield may be an advantage, particularly if the harvest is to be used for energy production. The increased harvest of biomass is, however, due to the removal of nutrient-rich tissues such as foliage and small branches. Consequently, the whole-tree harvest removed up to twice as many nutrients as did the stem-only clear-cut. In effect, a 30% increase in biomass yield by the whole-tree method was \u201cpurchased\u201d at the ecological \u201cexpense\u201d of 54-99% increases in the removal of nutrients.<\/p>\r\n

    Table 27.1. Removal of Biomass and Nutrients by Clear-Cuts of a Conifer Forest in Nova Scotia. This study involved weighing the biomass harvested by (a) a stem-only clear-cut and (b) a whole-tree clear-cut (each 0.5 ha). Nutrient concentrations were determined in subsamples of the biomass and were used to calculate the amounts of nutrients removed. \u201cPercentage increase\u201d refers to the whole-tree removals, compared with stem-only removals. Source: Data from Freedman et al. (1981). \"image\"<\/p>\r\n

    Unfortunately, there are few studies that allow foresters to compare the productivity of subsequent harvest rotations on the same site. Such studies would take more than 50-100 years, requiring several generations of foresters! Therefore, it is difficult to evaluate the implications of nutrient removal by clear-cutting. Overall, however, it appears that a degradation of site nutrient capital is a less severe problem in forest harvesting than in agriculture. Consequently, nutrient removals by timber harvesting should be viewed as a potential long-term problem. Because forestry is an economically important activity, and the maintenance of site capability is critical to the sustainability of the enterprise, scientists should continue to study the effects of harvesting on nutrient capital. In the short term, however, forestry causes more immediate kinds of damage to site capability and biodiversity that deserve our attention.<\/p>\r\n\r\n

    Leaching of Nutrients<\/h2>\r\n

    The disturbance of forested land can increase the rate at which dissolved nutrients are transported downward into the soil with percolating rainwater (a process known as leaching). If the nutrients leach deeper into the soil than tree roots can penetrate, they are effectively lost from the \u201cworking\u201d nutrient capital of the site. Eventually, leached nutrients can find their way into groundwater and surface waters.<\/p>\r\n

    The nutrients with the greatest tendency to leach are nitrate and potassium, both of which are highly soluble in water. However, calcium, magnesium, and sulfate may also leach in significant amounts. Of course, following a clear-cut, any nutrient losses by leaching are in addition to that removed with tree biomass.<\/p>\r\n

    A well-known study of nutrient leaching caused by forest disturbance was done at Hubbard Brook, New Hampshire. This large-scale experiment involved felling all of the trees on a 16-ha watershed, but without removing any biomass \u2013 the cut trees were left on the ground. The watershed was then treated with herbicide for three years to suppress regeneration. This experiment was designed to examine the effects of intense disturbance, by de-vegetation, on biological control of watershed functions such as nutrient cycling and hydrology. The research was not intended to examine the effects of a typical forestry practice.<\/p>\r\n

    Overall, during a 10-year period following the cutting, the de-vegetated watershed lost 50 kg\/ha-year of NO3<\/sub>-N (i.e., nitrogen in the form of nitrate), 45 kg\/ha-y of Ca, and 17 kg\/ha-y of K in streamflow (Bormann and Likens, 1994; Figure 27.2 shows data for the first three years, which were the most dramatic). The losses were much larger than from an undisturbed reference watershed: 4.3 kg\/ha-y of NO3<\/sub>-N, 13 kg\/ha-y of Ca, and 2.2 kg\/ha-y of K.<\/p>\r\n

    In part, the increased losses of nutrients were due to a 31% increase in the yield of water from the de-vegetated watershed (during the first three years after cutting). The increased streamflow was caused by the disruption of transpiration from plant foliage. However, increases in nutrient concentration in the streamwater were more important: during the first three years, NO3<\/sub>\u00a0increased by an average factor of 40; K, by 11; Ca, by 5.2; and Mg, by 3.9. The losses of N, Ca, and Mg in the streamwater were similar to their amounts in the above-ground biomass of the forest.<\/p>\r\n

    \"image\"<\/p>\r\n

    Figure 27.2. Nutrient Losses in Streamflow after Deforestation. The experimental watersheds are located at Hubbard Brook, New Hampshire. The data represent \u201cnet flux\u201d, or the difference between inputs from precipitation and outputs due to leaching and streamflow. The data are for the first three years following deforestation of a 16-ha watershed, compared with an uncut reference watershed of similar size. Losses are stated in kilograms per hectare per three-year period. TDS = total dissolved solids. Source: Modified from Bormann and Likens (1979).<\/p>\r\n

    Because this experiment in de-vegetation did not involve a typical forestry practice, the measured effects are unrealistically large. However, watershed-level studies of clear-cutting have also found an increase in nutrient leaching, although to a lesser degree than that caused by the de-vegetation at Hubbard Brook. For example, in the first three years after clear-cutting a 391 ha watershed in New Brunswick, there was an increased loss of nitrate in streamwater of 7 kg NO3<\/sub>-N\/ha-yr (Krause, 1982). A study of nine clear-cut watersheds in New Hampshire found an average nitrate loss of 18 kg NO3<\/sub>-N\/ha-y during the first four years, compared with 3.5 kg\/ha-y for five uncut watersheds (Martin et al., 1986). In addition, calcium losses from the clear-cuts averaged 28 kg\/ha-y, compared with 13 kg\/ha-y for reference watersheds, while potassium losses were 6 kg\/ha-y compared with 2 kg\/ha-y. However, other studies have found smaller effects of clear-cutting on nutrient losses with streamflow, especially if only a portion of the watershed was cut.<\/p>\r\n

    Nitrate and other highly soluble ions are leached from watersheds after clear-cutting (and after other disturbances, such as wildfire) for several reasons. First, disturbance stimulates the activity of microbes involved in the decomposition of organic matter. This occurs because removal of the tree canopy results in warming of the forest floor and surface soil, and decreased uptake by plants leads to an increase of soil nutrients and moisture. Second, disturbance often stimulates the microbial processes of ammonification and nitrification (see Chapter 5), resulting in increased rates of production of nitrate, which is extremely soluble and readily lost from soil.<\/p>\r\n\r\n

    Forestry and Erosion<\/h2>\r\n

    Forestry activities can cause severe losses of soil, or erosion, particularly in terrain with steep slopes. In most cases, erosion is triggered by improperly constructing logging roads, using streams as trails to haul logs, running log-removal trails down slopes instead of along them, and harvesting trees from steep slopes that are extremely vulnerable to soil loss. In general, however, road building is the most important cause of erosion on forestry lands, especially where culverts (channeled stream crossings) are not sufficiently large or numerous, or are poorly installed.<\/p>\r\n

    Severe erosion causes many environmental damages. In extreme cases, the loss of soil may expose bedrock, making forest regeneration impossible. Soil loss also represents a depletion of site nutrient capital. Erosion also causes secondary damage to aquatic habitats, including the deposition of silt (or siltation), which covers gravel substrates that are important to spawning fish. Also, the shallower water increases the risk of flooding.<\/p>\r\n

    However, in many cases erosion is a largely avoidable environmental effect of forestry. The irresponsible practices that can cause erosion are restricted by provincial regulations and occur much less frequently now than in the past. Practices that help to reduce erosion include the following:<\/p>\r\n\r\n