CAREER: Functional Bases for the Trade-Off Between Growth and Survival of Tree Seedlings

University Of Florida, Gainesville FL

Investigators

Abstract

How limited resources are allocated to different functions has strong consequences to growth rates and survival of organisms at both individual and population levels. Fast growth is inherently incompatible with high survival potential, a trade-off strongly suggested in recent studies of regeneration of trees in tropical and temperate forests. Shade tolerant species recruit saplings through maintenance of slow-growing seedlings that tolerate various biotic and abiotic stresses in the understory. In contrast, light-gap dependent species colonize with abundant and well-dispersed seeds and fast-growing seedlings having low tolerance of biotic and abiotic stress. The simplest yet most general hypothesis for such a trade-off relationship is that species allocating more resources to storage and defense survive better, but exhibit slower growth (at least in a short term). In plants, higher allocation of carbon (i.e., photosynthetically captured energy) to construction of new leaves leads to a compound return of carbon income and fast growth rates. However, this allocation pattern must come at the cost of relatively less carbon allocated to storage (i.e., energy savings as non-structural carbohydrates) and defense (e.g., tough support tissue, and chemical toxins that resist against herbivory and disease). Consequently, species that employ allocation strategies for fast growth should suffer from high mortality rates because these species are more vulnerable to herbivory, pathogen attack and physical disturbance. Kitajima will test this overall hypothesis through a series of comparative studies of carbon storage and carbon-based defenses in seedlings of tropical tree species in Panama. Seven family pairs of species exhibiting contrasting growth rates and survival will be raised under standardized conditions in a greenhouse for comparison of allocation to energy storage in stems and roots, and carbon-based defenses (lignin and other phenolic compounds). In a second experiment, this researcher will test the ecological role of storage allocation in the field by examining growth and survival of seedlings of species displaying contrasting degrees of storage allocation as they respond to shading and simulated herbivory treatments. In the third study, patterns of allocation to storage and defense will be examined to elucidate their relationship to longer-term growth and survival in naturally recruited seedlings in the forest understory. Whole-plant carbon budgets (total gross and net carbon gain, proportion of carbon gain to respiration, tissue construction, storage, and root exudate) will be quantified for two species of plants in the wild-coffee genus (Psychotria) in the final study. This research will be integrated with educational activities. The philosophical implications of the physiological constraints in ecological interactions go beyond purely biological interactions. Ecological models based on allocation-based trade-offs are important in training not only those students anticipating professional careers as ecologists, but also for those students pursuing studies in non-scientific disciplines that affect the societal responses to ecological and environmental issues. Allocation trade-offs, thus, will be emphasized in Kitajima's teaching in both undergraduate- and graduate-level courses. Her research and teaching will be most directly integrated in the course "Summer Research Experience in Tropical Ecology," in which undergraduate students will have an outstanding educational experience as hands-on participants in this research in Panama.