Abstract
In the tropics human induced forest disturbance, i.e. timber extraction or forest slash and burn for agriculture is leading to an increase of secondary forest area. Therefore, people in the tropics, especially the poor, will rely on secondary forests for good and services. Pioneer trees (short-and long lived) and lianas
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are able to regenerate in forest fallows providing a variety of timber and non-timber products and environmental services. Information about the performance of pioneer trees and lianas under natural competition is crucial to understand the mechanism of canopy structure recovery and function and the patterns of species replacement during succession. Short-lived pioneers (SLPs) are able to rapidly dominate the canopy attaining an optimum in abundance during the first 4 years of succession. Long-lived pioneers (LLPs) remain in the understory and replace the SLPs later on. Lianas which are also abundant early in the succession, may play an important role in the regeneration process since they are thought to alter tree communities by suppressing slow-growing species, and thus indirectly promoting pioneer trees. Relatively few studies have addressed the characteristics that: enable SLPs to achieve early dominance, allow LLPs to persist below the short-lived ones early in succession, and enable lianas to successfully compete with trees. I studied the relationship of crown structure development and aboveground mass allocation to light interception and carbon gain of the ten most frequent species of trees and lianas found in naturally regenerating secondary tropical forest in Northern Bolivia. I applied a model approach to quantitatively assess the light requirements and use for species growth. The results showed that the disproportionate increase in size of SLPs and their consequent domination of the top of the canopy is associated to higher rates of carbon gain relative to the other species. These species showed also a high leaf turnover, thus as they get taller they may not be able to compensate their high biomass expenditure in leaves by their total carbon gain. The LLPs, due to their long leaf longevity had an advantage in terms of long term efficiency in light capture and carbon gain with respect to SLPs. This may enable them to persist for long periods in the understory and ultimately replace the SLPs as they die-off. Lianas showed similar characteristics to LLPs during their self-supporting juvenile phase, but once climbing, they had the advantage to convert mass more efficiently to height increment and leaf area production. This improved their chances to obtain light at lower mass investments in stems than LLPs but not SLPs trees. This suggests that lianas having similar leaf traits to LLPs are able to persist at lower light irradiances, but once they start climbing, they can successfully compete with the SLPs for a position in the higher layers of the canopy. The approach presented here can be extended to secondary forest management and restoration. The model makes it possible to match species light requirements with the light environment in which they are planted and this knowledge may increase the success of enrichment planting projects.
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