During prolonged dry periods the foliage is dropped to
conserve water and prevent death from drought. When the leaves are shed, loos
of water through evapotranspiration is reduced hence allow more rain to seep
into the soil (rather than into roots and through the leaves into the air) and
eventually flow downstream. In contrast, evergreen trees (like pines) retain
their leaves year-round, and have been shown to have higher annual
evapotranspiration and therefore reduce streamflows. Leaf drop is not
seasonally dependent as it is in temperate climates, and can occur any time of
year and varies by region of the world.
