In the early nineteenth century, Alexander von Humboldt observed that the distribution of plants up a mountainside mirrors the distribution of plants at increasing latitude. Explain the observed shift of the plants to lower elevations by doing each of the following; A number of modern studies have reinforced the idea that as the global climate warms, plant species are becoming redistributed to higher elevations or latitudes.

However, in a separate large-scale study comparing historical (1930–1935) plant distributions with modern (2000–2005) distributions in the western United States, researchers observed that 72% of identified plant species shifted to lower elevations despite an associated increase in average temperature.

Explain the observed shift of the plants to lower elevations by doing each of the following.

1. Propose a hypothesis about an environmental factor other than temperature that could be driving the shift to lower elevations.

2. Describe ONE piece of evidence that a researcher could use to support your hypothesis.

Provide reasoning to connect the evidence with the environmental factor you proposed.