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Difficulty: MediumLogical Inferences

Forest soil compaction from heavy machinery reduces soil macroporosity, which limits oxygen diffusion to tree roots. Some tree species, such as the black alder (*Alnus glutinosa*), adapt to oxygen-poor soils by developing hypertrophied lenticels—porous tissue structures on their trunks that facilitate gas exchange with the atmosphere. Ecologist Dr. Marcus Vance found that when *A. glutinosa* specimens grew in compacted soils, their root respiration rates remained stable despite low soil oxygen. However, when Vance artificially wrapped the trunks of these specimens with airtight barriers, their root respiration rates fell drastically.

Which choice most logically completes the text?

  1. A
    soil compaction is less likely to occur in areas where *A. glutinosa* has already established a dense network of roots.
  2. B
    the rate of oxygen diffusion through compacted soil increases when the soil is exposed directly to the atmosphere.
  3. the trunk lenticels of *A. glutinosa* facilitate the transport of atmospheric oxygen to the roots to sustain respiration in compacted soils.Answer
  4. D
    *A. glutinosa* specimens grown in compacted soils eventually suffer a decline in root respiration regardless of lenticel exposure.

Answer

the trunk lenticels of *A. glutinosa* facilitate the transport of atmospheric oxygen to the roots to sustain respiration in compacted soils.
The correct answer logically completes the text because the passage states that root respiration remained stable in compacted (low oxygen) soils, but fell drastically when the trunks (which have lenticels for gas exchange) were wrapped with airtight barriers. This directly implies that the roots rely on oxygen taken in through the trunk lenticels.

Step-by-Step Solution

1
Identify the core premises of the passage.
The passage establishes that soil compaction reduces oxygen to roots; black alder has trunk lenticels for atmospheric gas exchange; root respiration remains stable in compacted soil; wrapping trunks with airtight barriers causes root respiration to fall.
To build a chain of logical reasoning, we must compile the given facts.
2
Connect the effect of wrapping the trunk to the function of the lenticels.
Wrapping the trunk blocks the lenticels. Since blocking the lenticels severely reduces root respiration in oxygen-poor soil (where it was previously stable), the roots must depend on the oxygen coming from the lenticels.
This links the independent variable (trunk wrapping/blocking lenticels) with the dependent variable (root respiration).
3
Synthesize the findings to draw a logical conclusion.
The logical conclusion is that the trunk lenticels supply the roots with the atmospheric oxygen needed for respiration when the soil is compacted.
This provides a direct, non-speculative inference based solely on the provided premises.

Key Concept

Logical Inferences
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Topics

Logical Inferences

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