Final answer:
The color of lichens changes from south to north due to environmental factors and they serve as indicators for direction and as a food source in the tundra. Their abundance and color are affected by climate change, which in turn affects caribou populations.
Step-by-step explanation:
The general shift in the color of lichens from south to north can be attributed to various biological and environmental factors. Lichens are hardy organisms that can survive in harsh conditions, and their presence on the north side of trees is often used as a directional guide because they thrive in shaded conditions, which is less likely on the sunnier south side. In the tundra, lichens serve as a winter food source for caribou, but as climate change affects average temperatures, shrubs and trees are beginning to replace them in southern regions, which impacts the survival of these herds. The color and abundance of lichens are influenced by such environmental stresses, leading to observable changes over geographical gradients.