Question

When all the trees were felled in a Hubbard Brook catchment, there were dramatic differences in the chemistry of the stream water draining the catchment. How do you think stream chemistry would change in subsequent years as plants begin to grow again in the catchment area?

Short answer

Stream chemistry initially worsens but stabilizes as plants regrow, decreasing nutrient concentrations.

Step-by-step solution

Understand the Initial Change

When all the trees are felled, the nutrients that were previously used by the trees can accumulate in the soil and stream water. Initially, there will be an increase in the nutrient concentration (like nitrates) in the stream water because the plants are not there to uptake these nutrients, and soil erosion could increase due to lack of root structures.

Identify the Role of Progressive Plant Growth

As plants begin to grow back, they gradually take up the available nutrients for their growth process. This results in a decrease in the nutrient concentration of the stream water. The plant roots also help in stabilizing the soil, reducing erosion, and therefore lessening the runoff of nutrients into the stream.

Predict Long-term Stabilization

Over time, as the vegetation becomes fully reestablished, the system will likely approach its prior equilibrium state. The nutrient levels in the stream will stabilize as the plants continuously take up nutrients and help maintain soil integrity, resulting in reduced leaching of nutrients into the stream.

Key concepts

Nutrient Cycles

The nutrient cycle is a fundamental concept in ecology, describing the movement and exchange of essential elements within an ecosystem. In a healthy forest ecosystem, trees and other plants absorb nutrients from the soil to grow. This is part of the natural nutrient cycle. Elements like nitrogen, phosphorus, and potassium play vital roles in plant growth.

• Plants take up nutrients through their roots.
• These nutrients are used for growth and energy.
• When plants and animals die, their matter returns to the soil, replenishing the nutrient cycle.

When trees in a forest are removed, as seen in the Hubbard Brook experiment, this cycle is disrupted. Without plants, there are fewer organisms to absorb the excess nutrients from the soil. Consequently, these nutrients, particularly nitrogen, accumulate and leach into nearby streams, altering the stream's chemistry. Over time, as vegetation begins to regrow, the nutrient cycle gradually restores balance by stabilizing nutrient uptake and preventing excessive runoff into the streams.

Deforestation Effects

Deforestation has significant ecological consequences, impacting everything from climate to biodiversity. When trees are cut down, such as in the Hubbard Brook catchment, multiple ecological impacts follow.

• Soil becomes vulnerable to erosion due to the loss of root systems.
• There is a marked increase in nutrients leaching into water bodies.
• The local climate may change, with shifts in temperature and moisture levels.

The loss of trees removes an essential buffer that intercepts rainfall and reduces runoff. Consequently, streams can become enriched with substances they ordinarily wouldn't have. Over time, as new plants establish themselves, some of these effects may diminish. For example, increased plant cover can gradually help restore hydrological balance by controlling soil erosion and improving water saturation efficiency.

Ecological Succession

Ecological succession is a natural process through which an ecosystem recovers and develops through different stages after a disturbance. It involves a series of progressive changes in the species composition of an ecosystem. In the context of the Hubbard Brook scenario, once the trees were removed, the ecosystem starts with low complexity, dominated by pioneer species known for rapid growth and reproduction.

• Pioneer plants first colonize the cleared area.
• These plants improve soil conditions, making them favorable for other species to colonize.
• Over time, a more stable and biodiverse community develops.

This transition results in a gradual decrease in nutrient levels in streams as plant biomass in the catchment area increases. Mature trees eventually reestablish, stabilizing the soil and normalizing nutrient uptake, leading to the ecosystem regaining a balance similar to its pre-disturbance state.

Soil Erosion Control

Soil erosion occurs when the top layer of soil is displaced, which can severely affect an ecosystem's health. Deforestation, as occurred at Hubbard Brook, removes the protective canopy and root systems safeguarding the soil, making it vulnerable to erosion.

• Roots anchor soil in place, preventing it from washing away during rainfalls.
• Plant cover helps to slow down the impact of rain on the soil surface.
• Organic matter from plant debris enriches soil structure, enhancing its resilience.

As vegetation reestablishes, it acts as a natural erosion control agent. New plant growth stabilizes the soil, reducing the sediment load entering streams. Living roots, especially those of mature trees, help bind the soil effectively, minimizing erosion and promoting water infiltration, ultimately supporting the recovery of the area's original ecological function.