Final answer:
The greener north-facing slopes of the San Gabriel Mountains are due to the rain shadow effect and the cooler, moister conditions with thicker soil found on these slopes, which are less exposed to direct sunlight and retain more water, supporting more plant growth.
Step-by-step explanation:
The north-facing side of the San Gabriel Mountain slopes is greener and more heavily grown with plants than the south-facing side facing Los Angeles due to the rain shadow effect and differences in microclimate caused by the relief of the land. The rain shadow effect occurs when moist air masses, often coming from the ocean, encounter mountains. As the air rises over the mountains, it cools and loses its moisture as precipitation on the windward side. This leads to drier conditions on the leeward side, creating a rain shadow area with desert-like conditions.
In the case of the San Gabriel Mountains, the prevailing winds from the Pacific Ocean drop their moisture on the western slopes, leaving less moisture for the south-facing slopes toward Los Angeles, making them drier and less vegetated. Conversely, the north-facing slopes are subject to less direct sunlight and therefore retain more moisture, creating a more suitable environment for plant growth. This is compounded by the fact that north-facing slopes in the northern hemisphere are generally cooler and moister, with thicker soil development and less erosion.