Impact of Forest Harvest

Timber harvest has short and long-term effects on your farm and the surrounding landscape; the subsequent land use may have even greater impacts. Forest harvest can affect soil, vegetation, watershed characteristics, wildlife habitat, aesthetics and economics. Impacts can include: elimination of species (vegetation or wildlife species), and damage to the soil and water system. Proper harvesting usually reduces the negative impacts. The nature and extent of the effects will depend on the logging practices, subsequent land use and landscape characteristics.

Soil

Forest soil plays an important role in forest ecology. During forest harvesting, it is important to minimize negative impacts on soil. Logging might affect the physical, chemical and biological characteristics of soils which in turn may have long term consequences for the soil’s productivity, nutrient regime and capability. Logging activities such as landings, access roads, and main skid trails may significantly reduce soil productivity.

Some of the negative impacts of harvesting on soil include:
1. Wind erosion
2. Soil compaction
3. Rutting
4. Puddling

Wind erosion
Depending on the subsequent land use, harvesting can also increase the risk of soil erosion by wind. Conditions promoting wind erosion include:

• sparse or absent vegetative cover;
• dry, loose and finely aggregated soil;
• smooth soil surface;
• large fields, and;
• high velocity winds.

Soil erosion by wind
Source by: Ducks Unlimited Canada

Soil compaction
Soil compaction is the process of increasing soil density by packing the particles closer together and reducing pore space. Soil compaction can reduce and disrupt soil porosity, and decrease water and air movement into and through the soil. The result is poor soil aeration, poor root penetration, limited water movement and reduced activity of soil organisms involved in nutrient cycling. Soil compaction can also increase surface water runoff which may lead to soil erosion and increased sedimentation in watershed.

The best way to avoid soil compaction is to log in the winter when soil is frozen or in a dry period when soil moisture is low.

Rutting
Rutting is the creation of depressions by forest equipment. Rutting usually occurs under wet conditions when the soil strength is not sufficient to support pressure from equipment. Rutting can effect the surface hydrology, reduce aeration, reduce water infiltration, reduce root penetration, and dam surface water flows which can increase soil saturation and create soil erosion

Puddling
Puddling is a physical change in soil properties due to shearing forces that alter soil structure and porosity. Puddling occurs when the soil is at or near the liquid limit. Wet, fine-textured soils are more susceptible to puddling than coarse soils. Avoiding logging when the soil is wet is the best way to prevent puddling.

To avoid all these negative impacts, make sure your plan for logging operations includes detailed information on soil texture, moisture content, type of equipment and time of operations.

Soils with fine texture and medium texture are susceptible for soil compaction and rutting. Different types of soils are more or less susceptible for different type of hazards. Brunosolic and Regosolic are more susceptible for wind erosion than soil compaction, rutting and puddling.

Moisture content is most important factor in all these hazards. Poorly drained soils such as organic soils are more susceptible to compaction, rutting and puddling then erosion.

The type of equipment and its activity on the site and landing areas during harvesting may cause soil compaction, rutting and surface water puddling. Avoid operating heavy equipment on the site when adverse impacts are likely and limit all traffic on the site to the smallest area possible.

Timing of logging operations is very important. In spring and summer when soil moisture content is higher than in fall or winter soils are most susceptible to compaction, puddling and rutting.

Watershed

Water and watersheds are defined in law as a public resource. All activities around water or adjacent to water bodies may require approval from government agencies. In Alberta, the Water Act regulates all activities related to water. Please see more information on Water Act

The negative impact of logging on watersheds include: runoff and gully erosion, in-field-water erosion, sedimentation of streams, and changing watertable depth.

Harvesting near streams and lakes requires careful planning and management to reduce these negative effects. If a cleared forest is regenerated promptly, the impact on the watershed can be minimized.

Wetland and woodlot
Source by: Ducks Unlimited

Runoff and gully erosion
Gully erosion and other runoff-related problems such as flooding can occur when surface runoff from rainfall or snowmelt becomes extreme. By clearing forested land, you may increase the risk of runoff problems for you and your neighbours.

On forested land, the potential for such problems is low. The trees intercept rain and dissipate rainfall energy, reducing the volume and energy of rain reaching the forest floor. Water on the forest floor moves within the litter cover layer which slows water movement. This allows more water to infiltrate the soil, leaving less water to contribute to surface runoff.

On cleared land, however, watersheds of only a few hundred acres can generate sufficient runoff to wash out culverts or flood low lying land and to form gullies too large to repair with farm equipment.

Farm watersheds, because of their small size, can respond more quickly to runoff events than large, regional watersheds. Even local showers can produce rapid runoff, so the risk of gully erosion and flooding can be high. In addition, as more small watersheds within a regional watershed are cleared, the whole regional watershed becomes more sensitive to runoff. This increases the potential for flooding, erosion, sedimentation and water quality degradation throughout the area.

Water erosion

In-Field Water Erosion

In-field water erosion refers to soil removal by raindrops hitting the ground and runoff flowing as sheet flow or in small rills. Characteristics that increase the risk of in-field erosion include:

• steep slopes or long, uninterrupted slopes;
• sandy or silty soils;
• soils without a protective vegetative cover;
• reduced infiltration resulting from low permeability at or near the soil surface (due to soil crusting, frozen soil, fine textured soil, shallow soil or other characteristics); and
• soils low in organic matter.

The litter and tree residue remaining immediately after logging usually protect the soil from in-field erosion, unless logging practices result in excessive surface disturbance. Land developed to pasture is also reasonably well protected if the pasture is properly managed to maintain good ground cover. On cultivated land, however, slopes greater than 5 per cent are susceptible to in-field erosion. Long slopes (greater than 800 m (2,600 ft) in length) are susceptible to erosion at slopes as low as 2 per cent.

Streams

Tree roots help stabilize stream banks, and tree shade helps reduce algae growth in streams in some cases. Streamside vegetation also traps sediments before they reach the stream and absorbs nitrates from groundwater. Clearing trees removes these benefits.

Stream banks can also be damaged by equipment or livestock trampling, resulting in reduced water quality and increased sedimentation. In extreme cases, stream banks may be destroyed, and the diverted stream flow can cause flooding and sedimentation in new locations.

Trees left standing after selective harvesting are susceptible to windthrow. Excessive windthrow next to streams can reduce bank stability and increase sedimentation.

Impact of a poplar channel buffer on nutrient and sediment export from a field catchment.

Watertables

Trees act as living pumps that draw moisture out of the soil and release it into the atmosphere. At the same time, tree shade and shelter may prevent excessive evaporation from dry sites. Depending on topography, soil and availability of water, clearing trees can have one or more of the following effects on watertables and associated site conditions:

• waterlogged soils that are difficult to reforest or crop,
• reduced soil moisture and drying of existing wetlands,
• fluctuating watertables causing increased soil salinity or changes in soil pH, or
• problems with water quantity or quality in existing dugouts, springs or wells.

Wildlife needs food, water, shelter and cover to hide in. Adequate wildlife habitat must contain all these components. Although clearing may benefit some species, generally forested land provides habitat for more species than pastureland, and pastureland provides habitat for more species than cultivated land.

Impacts on wildlife habitat from timber harvesting depend on the habitat needs , characteristics and population of wildlife species in the area. It One type of habitat may be excellent for one species but not for another. Many wildlife species are very adaptable to forest changes while others have great difficulty adapting to changes.

To prevent damage to wildlife habitat, collect information on the habitat needs of the wildlife species in your woodlot. Based on that information, some logging activities might reduce the level of damage. Some options are:

• Avoiding logging in the riparian/wetland zone. Riparian and wetland areas provide important habitat for many species of fish and wildlife and help to protect water quality.
• Leaving snag trees (dead standing trees). Snag trees are important for supplying food, roosting and nesting for many bird species such as woodpeckers, some songbirds and bats.
• Leaving fallen trees and woody debris on forest floor. Woody debris is important for many small and medium sized fur bearing animals.

Aesthetics

Removing forest cover changes the aesthetics of the local landscape. In some areas, public opinion may result in pressure to modify timber harvesting practices to reduce the visual effect. Many surveys show that scenic beauty is a very common reason to manage woodlots.

Economics for Harvesting

Harvesting and any subsequent land development can have significant economic consequences. You will need to carefully estimate the costs and benefits to decide if timber harvesting is the best economic choice for you. For example:

• The merchantable value of timber varies greatly from site to site. What is the value of the timber at your site?
• Costs for slash disposal and reclamation or development of agricultural land can be large; they may be higher than the value of the harvested trees.
• Removing tree cover removes the environmental benefits associated with trees. This can have economic impacts. For instance, crop yields on open fields are typically less than those on sheltered fields. As well, unsheltered farmsteads have greater heating costs, and unsheltered livestock operations have greater feed and bedding costs.
• Clearing may affect the real estate value of your property. Are wooded areas worth more than cleared land? Are they likely to be worth more in the future?