The boreal forest, also known as taiga, stretches across northern Canada and covers approximately 270 million hectares, making it the largest forest biome i...

(a)(i) Two characteristics of the climate in a boreal forest region: [2]

1. Long, cold winters with temperatures well below freezing for several months of the year, often reaching -20 to -40 degrees Celsius [1].
2. Short, cool summers with temperatures typically between 10 and 20 degrees Celsius, providing a brief growing season of only 3 to 5 months [1].

(a)(ii) One coniferous tree species found in boreal forests: [1]

Spruce [1]. Other acceptable answers include pine, larch, or fir. These are coniferous (cone-bearing) trees with needle-shaped leaves adapted to cold conditions and heavy snow loads.

(b) Structure of a boreal forest ecosystem (from Fig. 3): [4]

1. The boreal forest has a canopy layer formed by coniferous trees such as spruce and pine, typically reaching 15 to 25 m in height. The triangular tree shapes in Fig. 3 represent the distinctive conical form of conifers, which allows snow to slide off their branches [1].
2. Below the canopy is a sparse understorey of shade-tolerant shrubs and small bushes (shown by the oval shapes at ground level). Unlike tropical forests, the boreal forest understorey is relatively open due to the dense shade and acidic needle litter [1].
3. The ground layer consists of mosses, lichens, and low-growing vegetation. These organisms thrive in the cool, moist conditions and form a continuous mat across the forest floor [1].
4. Below the ground surface, the soil profile shows a thick litter layer of slowly decomposing needles, twigs, and branches on the surface, above a thin mineral soil layer. Decomposition is slow in the cold climate, so organic matter accumulates rather than being quickly recycled as it is in tropical forests [1].

(c) Comparison of soil properties between Site A (undisturbed) and Site B (logged 2 years ago) from Table 3: [4]

1. Soil organic matter was much lower at Site B (6.8%) than Site A (14.2%), a reduction of more than half. Logging removed the trees that supplied the thick litter layer with continuous needle fall [1].
2. Nitrogen was significantly lower at Site B (120 mg/kg compared to 285 mg/kg at Site A), a decrease of approximately 58%. Without tree roots to recycle nutrients, nitrogen has been leached from the exposed soil [1].
3. The water infiltration rate dropped dramatically from 85 mm/hr to 32 mm/hr, indicating severe soil compaction at the logged site caused by heavy forestry machinery [1].
4. The number of earthworms decreased from 48 to 12 per m\(^2\), a 75% reduction. This shows that logging severely disrupted soil biological activity through physical disturbance, habitat loss, and changes in soil conditions [1].

(d) Why soil organic matter and nitrogen are lower at the recently logged site: [3]

1. The removal of trees means there is no longer a supply of needles and dead branches falling to the ground, so the organic matter in the thick litter layer is not replenished. Existing organic matter decomposes without replacement [1].
2. The remaining organic matter on the exposed soil surface decomposes more rapidly because it is no longer sheltered by the forest canopy. Increased exposure to sunlight and temperature fluctuations accelerates microbial breakdown [1].
3. Nitrogen is lost because the tree roots that previously absorbed and recycled dissolved nutrients are no longer present. Heavy rainfall and snowmelt leach dissolved nitrogen from the exposed soil, carrying it away in surface runoff and through the soil profile [1].

(e) Why soil at Site C (logged 10 years ago) shows partial recovery: [3]

1. Over 10 years, pioneer vegetation such as grasses, shrubs, and young trees has begun to regrow naturally in the cleared area, adding new leaf litter and dead organic matter to the soil surface through their annual growth cycle [1].
2. The roots of regrowing vegetation help to bind the soil particles together and reduce nutrient losses from leaching and surface runoff. The root network also begins to break up compacted soil, improving infiltration [1].
3. Soil organisms such as earthworms and decomposers have begun to recolonise the site (28 per m\(^2\) compared to only 12 at the recently logged site), increasing the rate at which organic matter is broken down and nutrients are recycled back into the developing soil [1].

(f) Two ways heavy machinery damages forest soil: [4]

1. Heavy machinery compacts the soil, crushing the air spaces and pores between soil particles into a dense mass. Table 3 confirms the water infiltration rate dropped from 85 to 32 mm/hr [1]. Compacted soil prevents rainwater from soaking in, so more water runs across the surface as overland flow, dramatically increasing the risk and rate of soil erosion [1].
2. The weight and movement of machinery tears up and mixes the soil layers, destroying the structured profile. The nutrient-rich upper litter and humus layers are mixed with less fertile subsoil [1]. Compacted soil also makes it extremely difficult for new plant roots to penetrate, slowing or preventing the natural regeneration of trees and ground vegetation for years after logging ends [1].

(g) How clear-felling could affect the water cycle in the surrounding region: [3]

1. Trees return large quantities of water to the atmosphere through transpiration. Without trees, much less water vapour enters the atmosphere from the land surface, potentially reducing local cloud formation and rainfall [1].
2. More rainfall flows across the ground surface as overland flow rather than being absorbed by root systems or infiltrating the soil, increasing the volume and speed of water entering nearby rivers and streams. This raises the risk of flooding [1].
3. The water table may rise in logged areas because far less water is being taken up by tree roots through transpiration. This can cause waterlogging of the soil, creating anaerobic conditions that prevent the growth of many plant species and slow natural regeneration [1].

(h)(i) Two measures the logging company could adopt to reduce environmental impact: [2]

1. Replant logged areas with native tree species such as spruce, pine, and larch to speed up forest regeneration, rebuild the litter layer, and restore the soil protection provided by root networks [1].
2. Limit the size of clear-felled areas and leave corridors of standing forest between logged sections to reduce soil erosion, maintain wildlife habitats, and provide seed sources for natural regeneration of cleared patches [1].

(h)(ii) Why selective logging causes less damage to soil than clear-felling: [2]

1. Selective logging removes only certain mature trees, leaving the majority of the canopy and the associated root network intact across the site [1].
2. The remaining trees and their roots continue to protect the soil from compaction (fewer passes by heavy machinery), erosion, and nutrient loss. They also continue to supply needle litter that maintains the thick organic layer essential for boreal soil health [1].

(i) Why release of carbon stored in boreal forest soils is a concern for climate change: [2]

1. As global temperatures rise, the permafrost beneath boreal forests may thaw and the thick litter layers (which store an estimated 208 billion tonnes of carbon globally) may decompose more rapidly, releasing the stored carbon as carbon dioxide and methane into the atmosphere [1].
2. This would increase atmospheric greenhouse gas concentrations and accelerate further warming, creating a positive feedback loop: warming causes carbon release, which causes more warming, which causes more carbon release. This makes climate change progressively harder to control [1].