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Difficulty: Very hardLogical Inferences

In many temperate forest ecosystems, the understory fern *Dryopteris carthusiana* accumulates high concentrations of manganese (MnMn), a metal toxic to most competing angiosperms, in its senescent fronds. Researchers hypothesized that this accumulation acts as an 'allelochemical filter'—when the fronds decay, the released MnMn increases soil toxicity, suppressing the germination of neighboring seedlings. However, in soils rich in organic matter, MnMn readily binds to humic acids, forming stable complexes that render the metal biologically unavailable to plants. In contrast, in sandy, organic-poor soils, MnMn remains highly mobile and soluble. Which choice most logically completes the text?

  1. A
    be the primary factor determining the overall distribution and diversity of all understory plant species across temperate forest ecosystems.
  2. result in a more pronounced suppression of neighboring angiosperm seedlings than would occur in forest stands with organic-rich soils, provided the researchers' hypothesis is correct.Answer
  3. C
    prompt a rapid evolutionary adaptation in neighboring angiosperm seedlings, allowing them to tolerate elevated levels of mobile manganese.
  4. D
    prevent the decomposition of senescent fronds due to the absence of humic acids to break down the accumulated manganese.

Answer

The presence of the fern is likely to result in a more pronounced suppression of neighboring angiosperm seedlings in sandy, organic-poor soils than in organic-rich soils, assuming the hypothesis is correct.
The correct answer is correct because it directly connects the mechanism of the proposed allelochemical filter to the soil conditions described. The hypothesis states that the fern suppresses competitors through the release of toxic manganese. Because the passage establishes that this manganese is bound and rendered harmless in organic-rich soils but remains mobile and active in sandy, organic-poor soils, it logically follows that the suppression of competing seedlings will be more pronounced in sandy, organic-poor environments.

Step-by-Step Solution

1
Identify the core hypothesis and the mechanism of the allelochemical filter.
The hypothesis posits that *Dryopteris carthusiana* suppresses competing angiosperm seedlings by releasing toxic manganese (MnMn) when its fronds decay.
Establishing the premise of the filter effect helps define the condition under which suppression will occur.
2
Analyze the role of soil composition on manganese availability.
In organic-rich soils, humic acids bind MnMn, rendering it biologically unavailable (non-toxic). In sandy, organic-poor soils, MnMn remains mobile and soluble (biologically available).
This contrasts how the two soil environments affect the chemical agent responsible for the filter effect.
3
Draw a logical inference combining the hypothesis and the soil environments.
Since MnMn is only biologically active (available) in sandy, organic-poor soils and neutralized in organic-rich soils, the fern's inhibitory effect on competitors must be stronger in the sandy, organic-poor environment.
This completes the logical chain to determine where the allelochemical filter will successfully function.

Key Concept

Logical Inferences
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Logical Inferences

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