Monoculture cash crops impoverish the soil causing eventual soil erosion. Monoculture forestry causes fires by failing to add diversity. alternating the material that is deposited in the soil by including trees that drop the leaves alongside trees that remain evergreen allows the continuation of farming without depleting the soil further. This is why indigenous people who live in the rainforest can do so for thousands of years without damaging the environment. The forest quickly reclaims the small openings they clear and the villagers move on when the soil becomes unproductive in the clearings.
Clear felling severely damages the soils but with careful introduction of indigenous trees alongside the cash crops we can continue to harvest without causing the soil to be washed away in floods.
The potato famine in Ireland gives another insight into cash crops. When they fail it is often not without disastrous consequences.
The picture below is not unique and shows clearly how moisture rolls along a coastline due to thermals rising from the hot sands and black tarmac roads. At night when temperatures drop the clouds cross onto the land and rain falls as a result. Observed many times here in Devon.
Fog has Declined in Past Century along California's Redwood Coast
February 11, 2010
California's coastal fog has decreased significantly over the past 100 years, potentially endangering coast redwood trees dependent on cool, humid summers, according to a new study by University of California, Berkeley, scientists.
It is unclear whether this is part of a natural cycle of the result of human activity, but the change could affect not only the redwoods, but the entire redwood ecosystem, the scientists say.
"Since 1901, the average number of hours of fog along the coast in summer has dropped from 56 percent to 42 percent, which is a loss of about three hours per day," said study leader James A. Johnstone, who recently received his Ph.D. from UC Berkeley's Department of Geography before becoming a postdoctoral scholar in the campus's Department of Environmental Science, Policy and Management (ESPM). "A cool coast and warm interior is one of the defining characteristics of California's coastal climate, but the temperature difference between the coast and interior has declined substantially in the last century, in step with the decline in summer fog."
The loss of fog and increased temperature mean that "coast redwood and other ecosystems along the U.S. West Coast may be increasingly drought-stressed, with a summer climate of reduced fog frequency and greater evaporative demand," said coauthor Todd E. Dawson, UC Berkeley professor of integrative biology and of ESPM. "Fog prevents water loss from redwoods in summer, and is really important for both the tree and the forest. If the fog is gone, we might not have the redwood forests we do now."