SEEDLING ESTABLISHMENT AND ENDURANCE IN TROPICAL FORESTS: ECOPHYSIOLOGY OF STRESS DURING EARLY STAGES OF GROWTH

Abstract

Regeneration of natural vegetation requires seasonal production of dispersal units (seeds, propagules) able to germinate and become established under stressful conditions. Establishment depends on several processes (seed germination, root growth and leaf production) constrained by physic-chemical factors such as humidity, light intensity, and soil texture and chemistry. Furthermore, intra- or interspecific competition of established individuals reduces the space for root and canopy development of growing seedlings. In the tropical forests low light and nutrient availability are often the main stresses regulating growth, water relations and photosynthesis. Light intensity requirement decreases from the light-demanding pioneer and early successional tree species to the late successional, shade-tolerant tree species. Root competition and mutualistic symbiosis (mycorrhizae) regulate long-term carbon gain in the forest understory. Higher CO₂ concentrations near the forest floor possibly increase seedling survival by improving their carbon balance. In dry forests and savannas, seedling survival is associated with the length of the dry season and the intensity of root competition. Non-tree species (grasses and forbs) are able to take up most of the water and nutrients available in the upper soil layers. In swamp forests recurrent or permanent flooding interacts with light intensity in determining growth and survival of seedlings and saplings.