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Question

Difficulty: HardWeaken

Ecologist: Subterranean mycorrhizal fungal networks can transport carbon and defensive biochemical compounds between established canopy trees and developing understory saplings. A forestry study monitored Douglas fir saplings planted within five meters of mature birch trees and compared them to saplings planted in open, cleared plots without neighboring trees. The saplings planted near birch trees grew thirty percent faster and exhibited significantly lower rates of foliar blight. The researchers concluded that subterranean nutrient and defense transfers through shared fungal networks from the birch trees directly account for the improved growth and blight resistance of these saplings.

Which of the following statements, if true, most seriously undermine the researchers' conclusion? (Select all that apply.)

  1. Douglas fir saplings whose root zones were isolated from nearby birch trees by subterranean impermeable mesh barriers that prevent fungal connections exhibited the same enhanced growth rates and blight resistance as connected saplings.Answer
  2. B
    Isotopic tracing assays confirmed that substantial concentrations of carbon compounds synthesized by the mature birch trees were absorbed into the root vascular systems of the neighboring Douglas fir saplings.
  3. The cleared plots used in the study had undergone heavy mechanical scraping that severely compacted the subsoil and stripped topsoil moisture, whereas the soil surrounding the mature birch trees remained loose and hydrated.Answer
  4. D
    Douglas fir saplings planted in proximity to mature alder trees demonstrate even higher annual biomass gains than those planted near mature birch trees.
  5. E
    Foliar blight spores invariably kill every infected conifer sapling regardless of host nutritional status or general vigor.

Answer

The statements establishing that saplings isolated by subterranean mesh barriers still experience the same gains and that the cleared comparison plots suffered from severe soil compaction and degradation together undermine the conclusion.
The conclusion asserts a direct causal relationship: shared underground fungal networks transfer nutrients and signals, causing improved sapling growth and disease resistance. The statement showing that saplings separated by physical mesh barriers still exhibit identical benefits severs the causal mechanism by showing the effect occurs in the absence of the proposed cause. Additionally, the statement detailing severe soil compaction and degradation in the cleared plots provides an alternative explanation for the performance gap between the two test groups, undermining the claim that the birch trees were responsible.

Step-by-Step Solution

1
Isolate the argument's premises and main conclusion.
Premises establish that birch trees and fir saplings can share fungal networks, and saplings planted near birch trees grew faster and suffered less blight than saplings in cleared plots. Conclusion claims that underground nutrient and defense transfers through fungal networks directly cause these improvements.
Weaken questions require identifying the specific gap between the observed correlation in the premises and the causal claim in the conclusion.
2
Identify unstated assumptions and vulnerabilities in the causal inference.
The author assumes that the fungal network is the causal agent rather than aboveground microclimate factors (e.g., canopy shade), and that the cleared plots were otherwise identical in growing conditions to the soil near the birch trees.
Causal conclusions in observational field studies are vulnerable to confounding baseline differences and alternative causal pathways.
3
Evaluate each answer choice to identify those that break the causal link or introduce plausible alternative explanations.
The option showing identical benefits when fungal connections are physically blocked removes the proposed cause while the effect remains. The option describing degraded soil in the cleared plots introduces a confounding factor that accounts for the disparity.
Both findings directly invalidate the attribution of the effect to shared mycorrhizal transfers.

Key Concept

Evaluating Causal Arguments and Confounding Variables
Estimated Time:2m 15s
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Topics

Weaken

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