Final answer:
The hypothesis states that a decrease in rainfall during a drought will result in higher atmospheric carbon dioxide and lower vegetation density; drought resistance will impact species' survival and evolution, especially under the influence of natural selection.
Step-by-step explanation:
During a drought, the hypothesis is that the decrease in rainfall will reduce the amount of vegetation that can grow and increase the occurrence of natural fires. Therefore, an elevation in atmospheric carbon dioxide and a reduction in vegetation density should be found in the cores. Trees that have matured over decades and experience drought stress will likely show a decrease in growth and may die, leading to a positive feedback loop where the loss of trees increases carbon dioxide levels further due to less carbon being sequestered. This is supported by changes in stomatal density; under drought conditions, stomata close to conserve water, but this also means less carbon dioxide is absorbed for photosynthesis.
In the context of natural selection, plant species that are drought-resistant would thrive in warm temperatures (option d), whereas other species would decline. The impact of drought on forest ecosystems is also reflected in wildlife; with small seeds becoming rare and large seeds more common, birds with large and broad beaks will have a selective advantage, potentially leading to evolutionary changes in beak size in the isolated bird population under the influence of natural selection.