Why Be So Concerned about Seasonality?
The seasonal shift from rainy to dry seasons has direct
effects on plants and animals inhabiting rain forests
as well as other tropical ecosystems. One common
misconception about the tropics is that seasonality can
generally be ignored. Far from true. Images of year-
round sunny skies and soft trade winds are the stuff
of myths. During the rainy season, skies are typically
cloudy for most of the day, and showers, some heavy,
are intermittent, often becoming especially torrential
during late afternoon and evening. Such cloud cover,
blocking sunlight from reaching the forest, can be a
limiting factor on total photosynthesis, the overall
productivity of the forest (chapter 5).
Seasonal shifts are normal and often pronounced, and
many ecological patterns, ranging from photosynthesis
to flowering, reflect responses to seasonal changes.
Some shifts are obvious, but many tend to be subtle;
they vary considerably depending on the magnitude of
the seasonality. Unlike an oak forest in Ohio in winter
and summer, a moist forest in Panama will not look
very different in December compared with July. But
there are differences, such as leaf drop by deciduous
trees (plate 2- 1).
Henry Walter Bates, in The Naturalist on the River
Amazons (1863), wrote of seasonal patterns as they
affect life along the Amazon. At the onset of rainy
season, he observed:
All of the countless swarms of turtle of various
species then leave the main river for the inland
pools: sand banks go under water, and the flocks of
wading birds then migrate northerly to the upper
waters of the tributaries which flow from that
direction, or to the Orinoco; which streams during
the wet period when the Amazons are enjoying the
cloudless skies of their dry season.
Many more recent studies have abundantly documented
the compelling drama of the changing seasons of the
tropical forest.
Many species of trees flower more commonly during
the dry season, when less frequent and intense showers
permit insect pollinators to be active for longer periods,Plate 2- 1. The bare trees in this Panamanian forest are Cuipos (Cavanillesia platanifolia). The Cuipo is deciduous, losing its leaves
in dry season, which is when this photo was taken. Photo by John Kricher.
34 chapter 2 why it is hot, humid, and rainy in the tropics