5.2 - Clearcutting

Learning Objectives

Definition and Economic Advantage

• Purpose: Usually done to harvest lumber or to clear the land for another use, like agriculture.

• Economic Advantage: It’s a really efficient way to harvest lumber, which tends to maximize economic profits in the short term.

Short-Term Ecological Impacts

• Loss of Stabilization: Without the stabilizing effect of tree roots, the topsoil, which holds all the nutrients and organic matter, is much more prone to erosion.

• Mechanism: The soil gets easily blown away by wind or washed away by rain.

• Water Pollution (Turbidity): The eroded soil ends up deposited in nearby bodies of water, which increases turbidity, a measure of suspended solids, or how cloudy the water is.

• Consequences of Turbidity:
• It makes it harder for fish to get oxygen and can clog their gills.
• It makes it harder for aquatic plants to photosynthesize because sunlight can’t penetrate the water.
• Note: In selective cutting, a strip of vegetation left next to the stream (a riparian buffer) could absorb the sediments, but clear-cutting removes that barrier entirely.

• Soil Warming: Without the shade from tree leaves, sunlight that the canopy used to absorb now hits the soil directly. Trees also used some of that energy to evaporate water through transpiration, which kept the forest floor cool. With both gone, soil temperature goes up.

• Water Warming: Loss of shade also warms the river. On top of that, the muddy, darker color of the now-turbid river makes it absorb even more heat from the sun.

• Effects of Heat: Direct sunlight and drying heat can kill microorganisms in the soil and makes the soil more prone to erosion.

• Compaction and Water Loss: Logging machinery compacts the soil, and the increased sunlight has dried it out. Together, this decreases the water holding capacity of the soil.

• Runoff: When it rains, the soil holds a lot less water, so there’s a lot more runoff.

• Consequences: The lack of stabilizing roots plus the soil’s inability to absorb water can lead to flash floods and landslides.

• Real example (Hubbard Brook): At the Hubbard Brook Experimental Forest in New Hampshire, scientists cut every tree in one small watershed in late 1965 and used herbicides to keep plants from growing back. In the first year, the stream draining it carried about 39% more water than expected, and its nitrate concentration was about 41 times higher than normal, because there were no roots left to take up water and nutrients.

Consequences of Replacing Mature Forests with Tree Plantations

• Low Biodiversity and Resilience: Plantations usually consist of just one species of fast-growing tree, which greatly lowers biodiversity.
• Less diversity means fewer habitats for other organisms and lower ecosystem resilience.
• The plantation ends up far more vulnerable to total ecosystem collapse from natural disasters or drought.

• Lack of Dead Wood: All the trees are planted at the same age and size, so there are no dead trees. Dead trees are needed by many organisms like woodpeckers, insects, and decomposers, and they’re part of a mature forest’s natural nutrient progression.

Longer-Term Impacts (Loss of Ecosystem Services)

• Forests absorb pollutants from the air.

• Tree leaves use their stomata (leaf pores) to take in pollutants like volatile organic compounds, nitrogen dioxide, and particulate matter.

• The trees convert these into less harmful substances and store them in the tree or the soil, literally scrubbing the air clean. Deforestation reduces the ability of forests to do this.

• Forests store, or sequester, carbon dioxide ($\text{CO}_2$) from the atmosphere, which reduces the effects of climate change.

• During photosynthesis, trees take in $\text{CO}_2$ and convert the carbon into tissues like the wood of the trunk.

• Clear-cutting contributes to climate change because cutting down and burning trees releases that stored $\text{CO}_2$ back into the atmosphere.

• Even when tree scraps like stumps and branches decompose, they release their carbon dioxide.

• Slash-and-burn agriculture releases stored $\text{CO}_2$ as well as nitrous oxide ($\text{N}_2\text{O}$), which is an incredibly potent greenhouse gas.

• Forests provide important habitat for many organisms and serve as a great reservoir for biodiversity.

• High biodiversity leads to more ecosystem resilience.

• Diverse ecosystems offer financial benefits to humans through ecotourism.

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