Responses to light in tropical Moraceae of different successional stages.

摘要

The objective of this dissertation was to determine whether tropical tree species of different habitat breadths differ in their responses to light in predictable ways. Seedlings of Cecropia obtusifolia (Bert.), Ficus insipida (Willd.), Poulsenia armata (Miq.), Brosimum alicastrum (Swartz.) and Pseudolmedia oxyphyllaria (Donnell-Smith), all neotropical Moraceae, were chosen for this study. The plants were grown under three different light levels (100% full sun, 37% and 8%). A sub-set of plants within each species was transferred to the high light treatments after six months, simulating the creation of forest gaps. A distinction was thus made between plasticity and acclimation response mechanisms. Leaf-based physiological (e.g. gas-exchange), biochemical (chlorophyll and nitrogen content), and anatomical (internal leaf structure) responses were assessed in conjunction with whole plant morphological characters (growth and carbon allocation).; The early successionals Cecropia obtusifolia and Ficus insipida were highly plastic in all characters measured and were characterized by a high acclimation potential. The highest photosynthetic rates were consistently obtained by these species in high light. The late successional, canopy emergents Poulsenia armata and Brosimum alicastrum, increased photosynthetic capacity only at high light levels, while the sub-canopy Pseudolmedia oxyphyllaria exhibited no difference in performance between medium and high light grown plants. Overall, the late successional species were characterized by low photosynthetic rates and conductance values. Brosimum alicastrum was the only late successional species fully acclimating from a physiological standpoint upon being switched to higher irradiances.; While all species produced thicker leaves with increasing irradiance, blade and palisade thickness and plasticities were independent of successional class, contrasting sharply with temperate-based responses. Leaf anatomical variables correlated weakly with photosynthetic performance. Whole plant growth was consistently enhanced with increasing light levels, the most plastic morphologies and most massive plants being exhibited by the early successionals. Interestingly, all species unexpectedly deployed significantly larger leaves with increasing irradiance. Among late successionals, this effectively overcame the effects of an intrinsically restricted physiological flexibility, maximizing plant growth.