Bryophyte phytomass in tropical ecosystems

Knowledge about bryophyte production and phytomass in the tropics is poor and has been derived principally from studies of epiphytic bryophytes. Such work has been undertaken in Transvaal, Tanzania, Venezuela, Peru and Borneo. Except for the studies in Peru and Borneo, the vegetation types studied and the methods used have been too variable to allow many comparisons or generalizations to be made. However, in general it can be stated that phytomass of epiphytic bryophytes in equatorial latitudes increases from the tropical lowlands to the foreslline. This striking phenomenon has been hypothetically explained by gradients of factors such as precipitation, humidity, temperature and desiccation, or combinations of these. Recent experiments on the gas exchange of tropical-montane bryophytes have revealed that the rate of net assimilation decreases dramatically above 25^oC and that high temperature combined with low light intensities, as realized in the tropical lowland forest, results in high rates of respiration and insufficient net photosynthesis. Experiments with temperate bryophytes show the same results. If can be assumed that tropical lowland species have special physiological adaptations to survive these unfavourable conditions, such as a specialized phytochrome system or effective storage of photosynthetic products. Additional factors could include the relatively high nutrient supply provided by abundant precipitation in tropical montane forests, and damage to cells by strong desiccation in the tropical lowlands.