Forests: Changes in the Land
Part A: Deforestation: Changes in Forest Cover
Examine the image of changes in forest cover on the right. In the past, much of the Earth was once covered by forests. However, the majority of these forests were cleared long ago to make way for a growing human population. This is particularly true in regions with a temperate climate such as Britain and other parts of Europe where agriculture took an early hold. Interestingly, the World Wide Forest Report found that when the Roman Empire was in control of Europe, 90% of the continent was forested. Today, as indicated in the image on the right, Western Europe has now lost over 99% of its primary forest.
Forests and climate:
Forests play an integral role in the Earth's climate. Tropical, temperate and boreal forests each have varying impacts on the climate by both cooling and warming the Earth. The relationship between forests, climate and the carbon cycle is currently an area that scientists are intensively studying.
- Forests help reduce a warming climate by absorbing the greenhouse gas carbon dioxide through photosynthesis and storing that carbon in their leaves, wood and roots. Tropical forests and temperate forests are strong absorbers of CO2 compared to Boreal Forests.
- Forests help cool the atmosphere through evapotranspiration. Evapotranspiration is the combination of two simultaneous processes, evaporation and transpiration, both of which release moisture into the air. During transpiration, water is drawn up by the roots into the plants. Some of this water evaporates from the leaves. Example: A large oak tree is capable of transpiring 40,000 gallons of water into the the air through the process of evapotranspiration Increased evapotranspiration tends to cause clouds to form low in the atmosphere, reflecting the sun's warming rays back out into space. This has a cooling influence on climate.(G. Ban-Weiss et. al, 2011) Tropical and temperate forests both have cooling influences on the climate via their strong evaporative cooling rates.
- Forests also influence climate through albedo Albedo is another name for reflectivity and is a measure of how much light that hits a surface is reflected without being absorbed. Something that appears white reflects most of the light that hits it and has a high albedo, while something that looks dark absorbs most of the light that hits it, indicating a low albedo. The albedo of a surface determines how much sunlight will be absorbed and warm the surface compared to another surface that reflects most of the light and does not change temperature. effect. For example, the Boreal Forest in the northern latitudes is darker than the surrounding terrain and absorbs the Sun's energy more easily. This leads to a warming effect.
Deforestation: Causes and stages
If left to themselves, forests impacted by natural events such as wildfires will regenerate over time. Forests can also be restored by reforestation and afforestation Reforestation and afforestation both refer to planting trees on non-treed land. Reforestation refers to reestablishing forest on land that had recent tree cover, whereas afforestation refers to land that has been without forest for much longer. projects. Deforestation is different. When land is deforested, trees are not replaced and the land is converted to a non-forest use. For this reason, deforestation has long term effects on the carbon cycle, climate and the environment. Watch CNN Explains Deforestation to learn about:
- the causes of deforestation
- the impact of deforestation on the carbon cycle and the environment
Next, click on this link to view the basic Stages of Deforestation. As you view these stages, think about following:
- Undisturbed forests absorb and store more carbon via photosynthesis than they release via cell respiration.
- Trees that have been cut down no longer absorb CO2 from the atmosphere via photosynthesis.
- Trees that are burned release CO2 to the atmosphere via combustion.
- Trees left behind to decompose emit CO2 via the process of soil microbial respiration.
- Agricultural land created from deforested areas emits CO2 to the atmosphere via increased respiration and diffusion from soil that has been turned over (tilled).
Stop and Think
1. Describe how deforestation can impact the the amount of carbon dioxide in the atmosphere.
Trouble in the Amazon?
When we think about the Amazon Rainforest being cut down, we might immediately think of a tragic loss of biodiversity. However, as indicated in the NASA image on the right, there are massive amounts of carbon stored in the biomass of the Amazon Rainforest.
NASA has used satellites to track changes in tropical forest cover. Click on NASA's time series interactive Forest changes in Rondônia, Brazil to examine satellite images of deforestation from 2000 to 2012. In the times series interactive, you can:
- Click on the arrow to start the time series to look at forest changes over time.
- When in the pause mode, click on the "view large" tab on the right. Once the large view comes up, you can click on sections of the image to get a closer look at forest changes.
- Read the text that accompanies this interactive.
- Next, listen to scientist Bruce Pengra in the video Amazon Deforestation describing what the satellite images are revealing.
In terms of the carbon cycle, explain why deforestation in the Amazon is a global concern and not just a regional concern for the Brazilians.
Mini-Case Study: Using Sustainable Farming to Stop a Vicious Feedback Cycle of Slash and Burn Farming in Honduras.
In many parts of the world, small farmers and ranchers use a very old technique called "slash and burn" to clear and prepare the land for agriculture and cattle. In slash-and-burn agriculture, farmers will typically cut forests months ahead of the dry season. During the dry season, the "slashed" trees dry out and are then burned. The ash from the burned trees fertilizes the soil to support crops such as as rice, corn and soybeans and grass for cattle. Unfortunately, soil nutrients such as nitrogen are used up fairly quickly, the land becomes barren and farmers then move on to slash-and-burn other parts of the rainforest. In this way, fire and soil become part of this complex carbon cycle, the economy of the rainforest and the people who live there. So, what can be done to mitigate the impact of slash and burn farming on the carbon cycle? Read about the work of Mike Hand, a British ecologist, and Inga trees to find out!
Activity:With a partner or group, watch this PBS Newshour special: Up in Smoke Film Examines Perils of Slash and Burn on YouTube. You can also access this video on the PBS Newshour website As you watch, take notes that will allow you to compare and contrast the "slash and burn" farming method with the sustainable "crop alley" farming method in terms of:
- Impact on local people and their economies
- Pros and cons of each method
- How the carbon cycle responds to each method.
DiscussionIn what ways does deforestation impact local people and their economies?
Stop and Think
2. Think about the complex story of deforestation, slash and burn agriculture and the carbon cycle in the Central American and Amazon rainforests. Describe how deforestation, combustion and decomposition all impact the amount of carbon dioxide in the atmosphere.
Research the latest research! New research on the carbon cycle, climate and the environment is on-going. You can use ScienceDaily and Phys.org to research recent research on deforestation by using combinations of the following tags: deforestation, afforestation, reforestation, carbon, carbon cycle. As you discover and read more recent research, how does this research inform what you have learned so far about the carbon cycle in this module? Here are two examples: