The importance of cotyledon functional morphology and patterns of seed reserve utilization for the physiological ecology of neotropical tree seedlings

Seed mass, resource makeup of seed, and functional morphology of cotyledons are three traits that together determine patterns of seed reserve utilization by seedlings during their critical establishment stage. The main objective of this thesis is to examine how these traits differentiate the patterns of seedling growth among tree species found in the tropical moist forest on Barro Colorado Island, Panama. Among epigeal phanerocotylar species, photosynthetic capacity per unit cotyledon mass is linearly related to the reciprocal of cotyledon thickness. The relative balance of the dual functions of cotyledons, export of seed reserves vs. photosynthesis, can be estimated by whether cotyledons are free of the seed coat and how thick they are. The larger the cotyledon photosynthetic capacity of a species, the earlier its seedling growth starts to depend on the external light availability. Overall, species with smaller seeds develop cotyledons with a larger photosynthetic capacity. Concentrations of energy and nitrogen reserves in seeds vary up to three fold among species and are independent of total seed mass or cotyledon functional morphology. Species with higher energy concentration in seeds transform a unit of seed mass into greater seedling mass during the initial development in shade, while species with higher nitrogen concentration in seeds depend exclusively on seed nitrogen reserves for a longer period. In all species, dependency on external sources starts earlier for energy (7-16 d) than for nitrogen (14-40 d). Almost all species exhibit phenotypic plasticity in leaf photosynthetic capacity, leaf thickness, and allocation patterns in response to light environment. In both sun and shade, more light-demanding species have higher relative growth than shade-tolerant species, enabled by their photosynthetic cotyledons, high specific leaf area, and high leaf area ratio. Species with semi-photosynthetic storage cotyledons have as low relative growth rates as those with solely storage cotyledons in shade; however, their growth rate is more responsive to light availability. These seed and seedling traits may be under selective pressure to segregate species regeneration niches in the tropical forest community.U of I OnlyETDs are only available to UIUC Users without author permissio