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Difficulty: MediumCommand of Evidence: Textual

Ecologist Dr. Keith Sterling and colleagues investigated the relationship between mature Douglas fir trees (*Pseudotsuga menziesii*) and surrounding understory seedlings in a forest ecosystem. Some researchers have proposed that mature trees actively facilitate the growth of conspecific seedlings (seedlings of the same species) over heterospecific seedlings (seedlings of other species) by transferring carbon through shared underground mycorrhizal networks. Which finding, if true, would most directly support the researchers' proposal?

  1. A
    Mycorrhizal networks in the study area were found to be composed of fungal species that thrive in soil containing high levels of organic matter from mature Douglas fir needles.
  2. B
    Both conspecific and heterospecific seedlings grew significantly taller when planted near mature Douglas firs than when planted in open meadows lacking mature trees.
  3. Conspecific seedlings connected to mature Douglas firs via mycorrhizal networks showed significantly higher rates of received carbon and greater survival rates than did heterospecific seedlings connected to the same networks.Answer
  4. D
    Mature Douglas fir trees transferred carbon at identical rates to all connected seedlings, but conspecific seedlings had higher survival rates due to superior light absorption.

Answer

The finding that conspecific seedlings connected to mature Douglas firs via mycorrhizal networks received more carbon and had higher survival rates than heterospecific seedlings connected to the same networks.
The correct finding must directly support the claim that mature trees actively facilitate the growth of conspecific seedlings over heterospecific seedlings by transferring carbon through shared mycorrhizal networks. The finding that conspecific seedlings connected to mature Douglas firs via mycorrhizal networks received more carbon and survived at higher rates than heterospecific seedlings connected to the same networks provides direct empirical support for this claim by establishing both the unequal distribution of carbon and the resulting survival benefit for the same species.

Step-by-Step Solution

1
Identify the researchers' primary claim from the passage.
The claim is that mature Douglas fir trees facilitate the growth of conspecific seedlings over heterospecific seedlings specifically by transferring carbon through underground mycorrhizal networks.
To evaluate potential evidence, we must first establish the precise relationship and mechanism proposed by the researchers.
2
Analyze each option to determine if it shows both the selective facilitation of conspecific seedlings and the mechanism of mycorrhizal carbon transfer.
Only the finding showing that conspecific seedlings received more carbon and survived better than heterospecific seedlings through these networks demonstrates both elements.
Comparing the options allows us to rule out findings that do not isolate the specific mechanism or the specific comparative advantage described in the claim.

Key Concept

Command of Evidence: Textual
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Command of Evidence: Textual

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