Question 26·Medium·Inferences
During an unusually long drought, an ecologist compared two nearby forests with similar climate and soils. In one forest, dominated by species A, most trees kept their leaves well into the dry season. In the other, dominated by species B, many trees shed their leaves early. After brief rains, the soil under the fallen leaves in the species B forest stayed moist longer, likely because the leaf layer reduced evaporation. A colleague proposed that early leaf drop was not primarily to create mulch but was instead a response to drought-reduced nitrogen released by soil microbes, making it less beneficial for trees to maintain leaves.
If the colleague’s explanation is correct, which outcome is most consistent with it?
For questions asking what outcome is most consistent with a given explanation, first restate the proposed cause-and-effect in simple terms, then ask: “If this explanation is true, what new observation or experiment result would I expect?” Look for the choice that tests or reflects that specific mechanism (often by changing the supposed cause and seeing whether the predicted effect changes) and avoid answers that (1) merely repeat background details, (2) introduce an alternative explanation, or (3) flip or contradict the stated cause-and-effect direction.
Hints
Identify the cause and effect in the colleague’s idea
Look carefully at the final sentence: what does the colleague say drought does to nitrogen levels, and how does that supposedly affect whether trees keep or drop their leaves?
Think about what would happen if nitrogen were not reduced
If the colleague is right that reduced nitrogen is the reason for early leaf drop, what would you expect to happen to leaf timing in trees that somehow get more nitrogen during a drought?
Test each option against the proposed mechanism
For each answer choice, ask: Does this describe what we would observe if early leaf drop is mainly a response to reduced nitrogen from microbes, or does it introduce a different cause or reverse the direction of cause and effect?
Step-by-step Explanation
Restate the colleague’s explanation in your own words
Focus on the colleague’s proposal in the last sentence:
- Drought reduces nitrogen release by soil microbes.
- Because there is less nitrogen available, it becomes less beneficial for trees to keep their leaves.
- Therefore, trees respond by dropping their leaves early.
So the suggested cause-and-effect is: drought → less microbial nitrogen → lower benefit of leaves → early leaf drop.
Translate that explanation into a testable prediction
If early leaf drop really happens because nitrogen is scarce, then changing how much nitrogen is available should change when trees drop their leaves.
Ask: What would we expect to see if nitrogen were not scarce during a drought?
- If nitrogen stays plentiful, then the reason to drop leaves early (low nitrogen) goes away.
- So, during drought, trees with more available nitrogen should be able to keep their leaves longer than similar trees without that extra nitrogen.
Check which choice matches that prediction
Now compare each answer to the prediction that more nitrogen during drought → later leaf drop:
- One choice says that in a drought, trees that get extra nitrogen keep their leaves longer than similar trees that do not get extra nitrogen. This is exactly what we would expect if low nitrogen is what triggers early leaf drop.
- The other choices either focus on mulch and microbial activity under leaf layers, introduce a different cause (root depth), or flip the direction of cause and effect between leaf drop and nitrogen release.
The choice that states that trees receiving supplemental nitrogen delay leaf drop relative to similar trees that do not (choice A) is therefore the most consistent with the colleague’s explanation.