Because the vegetation in rainforests is frequently so tall and dense, a variety of microclimates are available to forest organisms.
a. Light: Canopy trees are exposed to an extremely high light regime. They are not protected at all from the rays of the sun, which are very intense because of the low latitude. In addition, the light regime is fairly constant throughout the year. Below the canopy there is considerably less light, as mid- and low-level vegetation is screened from the sun by the crowns of the canopy trees. Here only plants which are somewhat shade-tolerant will be able to survive. This vegetation is always attempting to reach the light, sometimes by climbing up trees or other plants. Only about 1% of available light reaches the forest floor and, consequently, ground vegetation is limited to shade-hardy species. This is why the ground is not heavily vegetated in many tropical forests.
b. Moisture and vapor pressure: Vapor pressure is another important element in forest microclimates. Since 80% of rainfall reaches the forest floor, moisture available to roots is probably not a limiting factor for growth. However, the vapor pressure (the amount of water vapor in the air), which is produced by the evaporation of rainfall and from transpiration (which is the water released during metabolic processes in the plant), depends upon the degree of air saturation, wind, and air temperature, all of which vary from the canopy to the forest floor. Generally vapor pressure decreases from lower to upper strata of the forest. Vapor pressure is much more variable in the canopy (because of high evaporation rates) than in lower, more protected layers of the forest, and at the forest floor, water vapor flux (variation) is only 25% of that in the canopy. This is partially responsible for so-called “microclimate” levels of vegetation growing on the forest trees. Some plants, for example certain epiphytes in the canopy, can tolerate diurnal (daily) changes in water vapor levels; others, living at lower levels, need a relatively constant degree of saturation.
c. Temperature: A third important factor in forest ecology is temperature. Within the lower strata of the forest the temperature will be lower than in the canopy by 7-10o C. Temperature affects the rate of chemical reactions, the oxidation of humus in the soil and other processes. High temperatures will also increase the dessication (drying) rate. In the soil the temperature varies little, and is rarely lower than 23o C. Just above the surface the temperature varies by only 5o C or so (falling between 22o C – 27o C, generally), and therefore activities in the soil and on the forest floor are not usually interrupted by temperature variations.
The interactions among all of these factors, and their influence on important physiological activities, (particularly photosynthesis) determine how and where plants grow.
How are emergent trees able to flourish so luxuriantly under the difficult conditions in the canopy? There is a plethora of mechanisms, such as varying photosynthetic and CO2 exchange rates in the leaves, numerous water-retention devices. A fig tree, Ficus insipida, has the highest known photosynthetic rate, which enables it to capitalize on the high light intensities available in forest gaps and thereby to fill these gaps rapidly. Canopy trees, which live in a highly variable environment, may produce different types of leaves: during the wet and dry seasons. Some produce thick leaves during the dry season, which, although water-retentive, are inefficient at capturing sunlight under the cloudy conditions which prevail during the rainy season, and are therefore replaced by thinner leaves at that time. A canopy tree, Anacardium excelsum, produces whorls of leaves enclosing dead air space. When water vapor is released from the leaves during photosynthesis, this dead air space retains much of it, moisturizing the surrounding leaves and preventing desiccation. Total leaf area is also variable. Rainforest trees probably maintain maximal leaf area for only about one-third of the year, usually prior to flowering. Leaf area may vary by 50%- 300% during the wet season.