Photosynthetic Characteristics of Wet Tropical Forest Plants

Abstract

The leaves of forest trees of the wet tropics are generally large and live an average of about one year. They, however, vary greatly, dependent on site, in their average leaf specific weights and nitrogen contents, encompassing values found for leaves of plants inhabiting arid climates. Those factors which affect photosynthesis, either directly or indirectly, light, humidity, and CO₂ concentration, vary greatly from the top of the forest canopy to the forest floor. Those species which inhabit the understory, where the radiation level is only a few percent of that received in the open, utilize brief sunflecks to fix a large fraction of their daily carbon gain. They are able to respond quickly to an abrupt increase in radiation since stomata remain open even at very low light intensities. There is little information available on the photosynthetic responses of tropical trees to CO₂, humdity, temperature, and water potential.

There are intrinsic differences in the photosynthetic capacities of the various growth forms which inhabit humid tropical forests. Fast-growing gap species have the highest capacities and understory plants the lowest. In-progress studies, examining the photosynthetic characteristics of related species occupying different microhabitats will define those physiological properties which are site specific. Other studies comparing closely related species from a variety of climates, including the humid tropics, have shown that these latter species generally have lower photosynthetic capacities and lower nitrogen-use efficiencies than plants from drier climates.

Reports of similar net productivities of arid and tropical wet forests and of nutrient-rich and nutrient-poor tropical wet forests indicates that there may be large compensatory variation among communities in those components, both physical and biological, which determine net productivity. Detailed information is not yet available to assess this possibility.