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Introduction to Forestry

Science Concepts:

Part 1. Forestry

  • Forestry is the theory and practice, the science and art, of the study, creation, conservation, use and management of forests and related processes, the organisms that live in it, and all forest-derived resources and products. 

What is sustainable forest management?

  • Sustainable forest management addresses forest degradation and deforestation while increasing direct benefits to people and the environment. At the social level, sustainable forest management contributes to livelihoods, income generation and employment. At the environmental level, it contributes to important services such as carbon sequestration and water, soil and biodiversity conservation.

Sustainable forest management practices:

  • Planning forest management: Periodic assessment of the status and condition of forest resources should be ensured in a permanent and continuous manner. This should take into account both biotic and abiotic factors that can have an impact on the vitality of forest ecosystems (e.g. parasites, overgrazing, fire, climate change and pollution). Management plans should consider all resources, users and ownership rights and should be periodically updated. They should define the resources and methods needed to minimise the risk of forest degradation and should seek to rehabilitate previously degraded ecosystems. They should be based on consultation and exchange of information among all the various stakeholders affected by the plan.
  • Forest management practices: Activities should aim for a quantitative and qualitative balance in growth and extraction by minimising direct and indirect damage to the resource. Regeneration, harvesting and maintenance activities should be programmed in space and time in order not to reduce the site’s productive capacity. Infrastructure should be planned so that it minimises negative impacts on the environment. Silvicultural treatments should promote structural diversity in forest stands and encourage natural regeneration. Afforestation of fallow or deforested land should be considered a priority each time there is a possibility to increase economic, ecological, social and cultural values from such activities. Afforestation should rely on species and silvicultural methods that are appropriate for each site. Appropriate measures should be taken to balance the pressures of livestock herds and grazing on forest regeneration and growth, as well as on biodiversity.
  • Research: Tropical forest research has come in for criticism because it is (or has been perceived in the past as being) remote from reality. Nevertheless, many experimental activities have been pursued in the tropics, particularly in the very practical area of the evolution and dynamics of forest stands subject to human intervention (usually after harvesting). Yet much remains to be done. For example, the growth characteristics of high value species, such as Meliacea,Swietenia macrophylla and Cedrela odorata in tropical South and Central America, are still largely unknown. This makes it difficult to manage them sustainably, because of a lack of information about how to obtain sufficient regeneration.
  • Technical requirements: Certain forest management tools still need to be refined, including: field inventory techniques; tele-detection and geographical information systems; the use of sample plots to monitor forest stands; and activities that increase the quality and value of standing wood. Finally, it is important to have easily useable and updated forest resource databases to enable rational resource management decisions to be taken.

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Part 2. Hydrology

  • A hydrologic system is defined as a structure (surface or subsurface) or volume (atmospheric) in space, surrounded by a boundary, that accepts water and other inputs (such as air or heat energy), operates (physical, chemical, biological) on them internally and produces them as outputs.
  • We treat the hydrologic cycle as a system whose components are precipitation, evapotranspiration, interception, runoff, infiltration, etc.. We give up the quest to know the precise spatiotemporal water flow patterns within the system and settle instead for knowing total water storage, and spatially averaged water fluxes in and out of the control volume.

What is a watershed?

  • A watershed is an area of land that drains all the streams and rainfall to a common outlet such as the outflow of a reservoir, mouth of a bay, or any point along a stream channel. The word watershed is sometimes used interchangeably with drainage basin or catchment. Ridges and hills that separate two watersheds are called the drainage divide. The watershed consists of surface water–lakes, streams, reservoirs, and wetlands–and all the underlying ground water. Larger watersheds contain many smaller watersheds. It all depends on the outflow point; all of the land that drains water to the outflow point is the watershed for that outflow location. Watersheds are important because the streamflow and the water quality of a river are affected by things, human-induced or not, happening in the land area “above” the river-outflow point.



1-min fun video intro to the concept of watershed:

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Part 3. What is the relationship between forest and hydrology?

Forest hydrology combines aspects of two separate disciplines: hydrology and forestry. Hydrology is the science that studies the waters of Earth. Hydrology seeks to understand where water occurs; how water circulates; how and why water distribution changes over time; the chemical and physical properties of water; and the relation of water to living organisms.

Ecologists consider water to be the defining part in an ecosystem, including the forest ecosystem. Water shapes the physical landscape through erosion and deposition. It also shapes the biological parts of the ecosystem by its presence or absence; its quantity and quality; and its occurrence and distribution. The water cycle plays a key role in ecosystem functions and processes.

Forests, in turn, are vital to the water cycle and to water quality. In essence, the forest acts like a giant sponge, filtering and recycling water. Approximately 80 percent of U.S. fresh-water resources are estimated to originate in forests, which cover one-third of the U.S. land area.

Tree leaves intercept water from rain, snow, and fog; the leaves also release water back to the atmosphere by evapotranspiration . Tree roots extract water from the soil while helping hold the soil in place. Forested land reduces the surface impact of falling rain through interception and delay of water reaching the surface. Forestland also decreases the amount and velocity of storm runoff over the land surface. This in turn increases the amount of water that soaks into the ground, a portion of which can ultimately recharge underlying aquifers . Conversely, water from hydraulically connected surficial aquifers may enter streams and wetlands , helping to maintain their water levels during dry periods.

Forests and the Hydrologic Cycle.

The surface water in a stream, lake, or wetland is most commonly precipitation that has run off the land or flowed through topsoils to subsequently enter the waterbody. If a surficial aquifer is present and hydraulically connected to a surface-water body, the aquifer can sustain surface flow by releasing water to it.

In general, a heavy rainfall causes a temporary and relatively rapid increase in streamflow due to surface runoff. This increased flow is followed by a relatively slow decline back to baseflow, which is the amount of streamflow derived largely or entirely from groundwater. During long dry spells, streams with a baseflow component will keep flowing, whereas streams relying totally on precipitation will cease flowing.

Generally speaking, a natural, expansive forest environment can enhance and sustain relationships in the water cycle because there are less human modifications to interfere with its components. A forested watershed helps moderate storm flows by increasing infiltration and reducing overland runoff. Further, a forest helps sustain streamflow by reducing evaporation (e.g., owing to slightly lower temperatures in shaded areas). Forests can help increase recharge to aquifers by allowing more precipitation to infiltrate the soil, as opposed to rapidly running off the land to a downslope area.

Riparian Areas.

The riparian zone is broadly defined as the area between a body of water and the upland parts of the landscape that are rarely flooded except under the most extreme conditions. But the term also can refer more specifically to the immediate streamside area.

Riparian areas represent less than 10 percent of most forest ecosystems, yet these areas often are the most productive portions. Compared to upland regions, riparian areas have more water available; the vegetation is more robust; the soils are deeper; the timber often is of higher quality; and the waterbodies have more shade. The riparian zone also may include wetlands bordering streams and lakes. This combination of factors makes riparian areas among the most heavily used portions of a forest. Riparian and wetland areas provide abundant and reliable forage for wildlife, as well as transportation corridors. They also may receive heavy human use for recreation.

Riparian zones also are attractive destinations for logging and for livestock grazing; as a result, riparian areas in forests are sometimes heavily damaged, especially in the forests of the arid American Southwest. Fortunately, riparian areas respond well to good management practices.

Aquatic Biodiversity.

Forest lands and waters are vitally important in maintaining biodiversity and providing habitat for fish and wildlife, including threatened or endangered aquatic species. In the United States, over one-third of national forest lands are critical for maintaining aquatic biodiversity and protection of listed species.

For aquatic species, watersheds provide the basic unit of any conservation strategy. Many watersheds also contain isolated habitats with unique characteristics producing a high potential for rare species. Some species occur only near a single spring or in a single stream within a given watershed. Lands set aside to protect these unique habitats also benefit the entire watershed and its ecosystem.

Forest Management and Watershed Quality

Wind, fire, insects, and disease are all part of properly functioning, healthy ecosystems in watersheds. For example, natural fires, although temporarily devastating, periodically restore the balance between vegetation types, and release nutrients from the vegetation and soil. In contrast, widespread clear-cut logging and excessive or improper road-building can degrade watersheds, as can land uses such as ski runs and housing projects. Many human activities can increase overland runoff, resulting in more erosion of the land surface and concurrently reducing the amount of water that soaks in the ground to potentially reach nearby streams or recharge underlying aquifers.

Moreover, fire prevention and suppression have created “imbalanced” forests with excessive amounts of undergrowth and dead vegetative matter that serve as fuels when fire does occur. Hence, these forests are at increased risk of high-intensity, destructive fires.

Watershed management and restoration may include controlled thinning, prescribed burning, and other management practices to restore the proper balance of timber, undergrowth, and grassy meadows in the watershed. Restoration also may include planting of appropriate native plants.



Forest and water 1 min intro:

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Part 4. Relationship with Climate Change

Different forest management strategies could potentially mitigate or exacerbate effects associated with climate change. Forest management affects the vegetation structure and function of the watershed. Streamflow responses depended on the management treatment, and they could be used to mitigate climate change effects. Looking purely at water quantity shows forest management can mitigate for extreme precipitation events in a changing climate. However, these changes should be taken in context with other factors such as carbon sequestration, local climate, and water quality.

( Study from the Coweeta Hydrologic Laboratory

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Part 5. The Coweeta Basin and Research Site

The Coweeta basin, which contains dozens of separate watersheds, is ideal for hydrologic research. The site was strategically selected due to its topography and aspect and the unusually high rainfall (70 to 90 inches per year). The solid bedrock underlying the soils permits the hydrologist to account for most of the rainfall that enters the basin. Many of the watersheds in the basin are very similar in terms of size, climate, soils, and vegetation. The relationship between rainfall and streamflow before disturbance has been charted for many of the watersheds since 1934. To test a theory or evaluate a management practice, scientists can manipulate conditions on a watershed and then compare results with those from a similar undisturbed watershed that serves as a reference.

Since the 1930s, 32 weirs, or stream gauging stations, have been installed on streams in the Coweeta basin; 16 of these weirs are currently operational. Streamflow data has been collected from the weirs since the 1930s using automatic recorders that continuously monitor the height of the water in the weirs which is later translated into streamflow using a mathematical formula based on the dimensions of the weir blade. Because the weirs were precisely constructed, streamflow can be calculated day and night, through storm and sunshine, throughout the year. Sediment that accumulates in the ponding basin constructed behind each weir can also be measured and streamwater chemistry data has been monitored since 1972.

Water quality in the South has been shaped by three centuries of intensive land use, with clearing for agriculture starting in the 1700s, and unregulated logging beginning shortly after the Civil War and lasting through the 1920s. The period between 1860 and 1920 was the most destructive known, with widespread clearing of southern forests without any few erosion control measures. Logging peaked in 1909 and stayed high until 1920, when only a few stands of virgin forest remained. Rivers were filled with sediment from mountain slopes; many still run muddy from those times.

Like most of the Southern Appalachians, the Coweeta basin was heavily harvested in the 1920s. At the time, very little scientific information was available about the impacts of unregulated logging on water quality, but it was clear to the naked eye that large amounts of sediment had reached the streams when mountain watersheds were logged.


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Part 6. Invasive Species

An “invasive” is defined as “a species that is nonnative (or alien) to the ecosystem” (htttp:// where they are found and whose “introduction are likely to cause economic or environmental harm or harms to human health” (Executive Order 13112, App. 1). Nonnatives are organisms that have been moved from their natural habitat to a new environment. However, many nonnative species do not pose danger to man, plants, and animals.

Alien invasive species generate substantial costs to the forest sector in lost revenues, in expenses for their control and in lost conservation values and ecosystem services. Alien invasive species, in particular insect pests and diseases, can damage trees in all stages of development and affect the ability of both natural and planted forests to meet their management objectives (FAO, 2001b).

The most direct economic impact of alien invasive species on the forest sector is related to the loss or reduced efficiency of production. Approximately US$4.2 billion in forest products are lost each year to alien insect pests and pathogens in the United States (Pimentel et al., 2000). In addition to these direct production and trade costs, the associated control costs, including the costs of inspections, monitoring, prevention, and response, of even just a few species can be enormous.

The full economic costs of invasions include not only the direct damage and control costs but also the effects on the ecosystems themselves. The ecological and environmental impacts of alien invasive species can be felt by all levels of organization including the gene, species, habitat and ecosystem level.

The full costs of invasions also include the social and health impacts of alien invasive species on humans, in particular to the rural communities depending on forests. As a result of the negative impacts of alien invasive species on native forest biodiversity, a loss of food sources and traditional medicines may be experienced thereby compromising not only the health of local people but also the livelihoods of those dependent on the collection and sale of such items for income. For small-scale landowners, alien invasive species can also decrease the value of their land. People living in and around invaded forest areas may also suffer allergic or other negative reactions to the alien invasive species themselves or to the measures used to control them such as chemical and biological pesticides.

A few attempts at estimating the costs of alien invasive species have been made.

  • Pimentel, Zuniga and Morrison (2005) estimates that the 50 000 alien species in the United States cost almost US$120 billion in environmental damages and losses yearly. Pimentel et al. (2000) gave an estimate of US$137 billion per year.
  • Pimentel et al. (2001) looked at over 120 000 alien species of plants, animal and microbes that have invaded Australia, Brazil, India, South Africa, the United Kingdom and the United States causing significant economic losses in the agriculture and forest sectors and negatively affecting ecosystems. They estimated that the total cost in the six countries was US$314 billion in damages per year – Australia ($13 billion), Brazil ($50 billion), India ($116 billion), South Africa ($7 billion), the United Kingdom ($12 billion) and the United States ($116 billion).
  • OTA (1993) concluded that about 4 500 exotic species occur in the United States and that about 20 percent of them have caused serious economic and environmental harm. The cumulative loss caused by 79 of these species was estimated at almost US$97 billion for the period 1906 to 1991.
  • While these estimates do not take all components into account, they nonetheless illustrate the enormity of the costs of alien invasive species.


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