A major challenge in ecology is to understand what determines the number and relative abundance of plant species in natural communities. This challenge is especially daunting in wet tropical forests, which harbor the highest species diversity of any ecosystem in the world. The seedling phase of a plant's life cycle is especially sensitive to pathogens and to the availability of light, nutrients, and water. What happens during this critical stage impresses a signature on forest species composition, from the understory to the canopy. Trees can live for much longer than humans and the dynamics of forests play out over even longer time scales; therefore, long-term data collection is necessary to capture important events that would be missed by short-term studies (e.g., seed production may occur every 12 years for some species). This proposal will extend an ~8 year record of seedling surveys in five forest stands in Costa Rica, with the goal of developing forest models of critical aspects of forest regeneration (seedling production, dispersion, growth and mortality) in response to light, soil fertility, and natural enemies.
A better understanding of the factors that govern the relative abundance of tree species will aid in predicting forest responses to human influences, including the deposition of nitrogen, sulfur, and other chemical pollutants from the atmosphere. Furthermore, in tropical areas, exotic tree species often are used for reforestation because the regeneration requirements of native species are not well known, a knowledge gap that this research will help remedy. As part of this project, K-6 students will be growing plants in Michigan to learn about ecological processes that operate in all terrestrial ecosystems (from agricultural fields to wet tropical forest). Research results, data, and on-line identification tools will be shared with the public and other scientists through a project website.
This research has focused on understanding the constraints on the growth and survivorship of seedlings and saplings in the understory of wet tropical forests. Seedlings and saplings will make up the future forest of large trees and this juvenile phase of tree life often is a bottleneck; thus, understanding the juvenile phase is important to predicting what tree species will make up the future forest. We have tracked the survivorship and growth of tree seedlings in 1000 plots (each 1 x 1 meter) since 2000. There are approximately 50,000 seedlings of 400 woody species in the data set. We found that soil-borne diseases are major constraints to the survivorship of tree seedlings in the forest understory and their impact on a species increases as there are more seedlings and mature trees of the same species in the vicinity. This feedback keeps the population of any given species in check and contributes to the persistence of high levels of tropical tree species diversity. Despite the low light in the forest understory (usually 1-2% of full sun), we found that soil nutrients are still important to seedling growth; this result is surprising as we expected heavy shade to be the dominant constraint. We also found, for both seedlings and mature trees, that base cations (calcium, magnesium, and potassium), nitrogen, and phosphorus are associated with tree growth. Previous research has focused mostly on phosphorus. The seedling data set is available to other researchers and soon will be published as a data set. This project also has contributed to an electronic guide on identifying tree seedlings (http://fm1.fieldmuseum.org/seedlings/index.php). Also see the Project Website for additional information and results (www.msu.edu/~kobe/wsd.html). The project has supported the research of four graduate students (two PhD and two MS). Several undergraduate students also have learned about forest ecology and science through their involvement in this project. Having baseline data on seedling dynamics is important for assessing the potential impacts of climate change on future forest composition. It is important to better understand how forests work because of the tremendous benefits they provide to humans, including: wood and paper products, storage of atmospheric carbon to mitigate climate change, influence on rainfall, and the biodiversity that contains a vast storehouse of potentially useful pharmaceuticals and other chemicals.
