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Based on the text, which choice best explains why high-elevation P. contorta produce higher concentrations of terpene…

To deter herbivores, many subalpine conifers synthesize terpene resins. Because producing these carbon-rich defenses is metabolically costly, researchers long hypothesized that trees at higher elevations—where shorter growing seasons limit annual carbon assimilation—would maintain lower terpene concentrations than trees in lowland valleys. However, ecologist Elena Vance and colleagues found that high-elevation stands of · Pinus contorta · consistently contained higher terpene concentrations than lowland stands. Vance attributes this pattern to environmental limits on nutrients: high-elevation sites feature shallow soils and scarce moisture that severely restrict nitrogen uptake. Because nitrogen is essential for protein synthesis and new tissue growth, its scarcity suppresses needle and shoot expansion far more than it dampens baseline photosynthetic carbon capture.

Based on the text, which choice best explains why high-elevation · P. contorta · produce higher concentrations of terpenes than lowland trees do?

Answer & explanation

Correct answer: A. A deficit of nitrogen constrains their structural growth, leaving unspent photosynthetic carbon available for defense production.

According to the text, terpenes are carbon-rich defense compounds, and producing them requires carbon assimilated through photosynthesis. Vance and colleagues note that at high elevations, shallow soils and scarce moisture restrict nitrogen uptake. Because nitrogen is required for protein synthesis and tissue expansion (needle and shoot growth), nitrogen scarcity curtails structural growth to a much greater degree than it dampens carbon capture. Consequently, high-elevation trees capture carbon that cannot be invested in tissue expansion due to the nitrogen bottleneck, resulting in surplus carbon that is channeled into synthesizing carbon-rich terpene defenses (Choice A).

Choice B is incorrect because the passage does not claim that harsh soil conditions stimulate carbon capture; rather, it states that nitrogen deficiency dampens photosynthesis far less than it suppresses growth, which is not an active increase above lowland rates.

Choice C is incorrect because the passage describes terpenes as defenses against herbivores, not as agents that accelerate somatic tissue growth.

Choice D is incorrect because terpenes are explicitly described as "carbon-rich defenses" rather than nitrogenous compounds, meaning it is unspent carbon—not diverted nitrogen—that fuels elevated terpene synthesis.

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Based on the text, which choice best explains why high-elevation *P. contorta* produce higher concentrations of terpene…