Parts of the Amazon grow more leaves when the rain stops
In the tropics, dry months are not a fixed calendar fact but the shadow of a wandering band of rain. As that belt swings between hemispheres, rivers shrink, herds walk, fires spread, and measles cases climb in Africa. Yet undisturbed Amazon forest answers the drought by sprouting roughly a quarter more foliage.
Tropical weather is steered by a rain belt that drifts north and south over the year. From about November to March it sits south of the equator, so the northern tropics get sparse rain and mostly sunny days. Between May and September the pattern flips and the southern tropics dry out. The Köppen climate scheme counts a tropical month as dry when its average rainfall falls below 60 millimetres.
How many dry spells a place gets depends on its latitude. Close to the Tropic of Cancer or the Tropic of Capricorn, the belt arrives once, giving one wet and one dry season. On the equator it passes overhead twice, once heading north and once heading south, so the year holds two of each. Places in between may see a short and a long version of either season, and local terrain can bend the pattern further.
Low humidity drains watering holes and streams, and grazing animals follow the remaining water and grass. Zebras, elephants and wildebeest make these moves in Africa, as do rheas in South America and kangaroos in Australia. Parched vegetation also makes bushfires common. People move too: with farming nearly impossible without irrigation, some African farmers head for cities, and researchers link that crowding to a rise in measles at the start of the dry season.
The Amazon offers a twist. In its seasonal zones, foliage grows faster and covers about 25 percent more in the dry months. Researchers think this extra greenery evaporates more water and may help bring on the wet season. The effect shows up only where the forest is undisturbed, perhaps because roots there reach deep stores of rainwater. Ozone levels over the basin are also markedly higher in the dry season.
Source: Dry season
