The massive amount of precipitation experienced in rainforest is not only a problem on the forest floor but also in the canopy. Plants need to photosynthesize: water sitting on the leaf surface inhibits gas exchange and light capture. Besides waxy leaf coatings or pubescent surfaces that repel water, one of the most distinct features of rainforest plants are their drip tips. At the apex of the leaf, or where the network of veins leads to the leaf margin, the leaf narrows substantially, and is often recurved. Water that falls on the leaf (as either rain or mist) coalesces in the veins and is driven toward the exterior of the leaf by gravity. The mass of water builds at the edge of the leaf where the surface area is drastically decreased and eventually not able to support the weight of the water, causing it to drop off. Depending on the species, the leaf margin may be entire with a single drip tip or, serrated with each tooth having the ability to shed water. Interestingly, the size of teeth on serrated leaves tends to be larger in warmer rainforest, and decrease in size with decreasing temperature. Although leaves are adapted to rid themselves of water as soon as possible, they have an important role in capturing rain and breaking its fall in the canopy, further reducing erosion of the forest floor.
There is generally sufficient precipitation in rainforests to support all plant life, what they instead compete for is sunlight. Leaves in rainforest canopies can be arranged so well that less than 5% of insolation actually reaches the rainforest floor1
. Not only does this mean that there are actually very few plants on the forest floor, but those that exist in the lower parts of the canopy need to get creative in how they capture light. Firstly and most obvious is that many tropical and subtropical rainforest plants have particularly large leaves, which are often oriented towards the sun. By being directed toward the sun and by having a larger surface area, sunlight capture can be maximised. As the temperature of the rainforest decreases, leaf size typically does also. When compared to vegetation from drier climates, rainforest plants tend to have darker leaves (although foliage colour tends to be less vibrant in subtropical and temperate rainforest and rainforest growing at altitude). Leaf colour is directly related to the amount of light an object can absorb: black (or dark) objects absorb all light while white (or lightly coloured) object reflect all light, and therefore darker leaves can absorb more light. Further to this, rainforest plants growing in the lower parts of the canopy may have red or purple undersides which allows them to absorb reflected light, or longer wavelengths of light that infiltrate the upper layers of the canopy. And finally, many rainforest plants, particularly palm species, have concertinaed, zig-zag, or crimped, leaves meaning that two times a day the leaf surface is perpendicular with the sun instead of one, as would be the case for a single-plane leaf.
- Richards, P (1996) The Tropical Rain Forest: An Ecological Study. (2nd) Cambridge University Press, UK