The development of forest communities is shaped by both environmental factors and interactions between organisms. Recent evidence from both tropical and temperate forests suggests that host-specific pathogens play a major role in shaping forest community development at seedling stage, especially for less abundant species. The seedling to sapling transition is a critical stage of the tree life cycle that will determine forest community composition for years to come. The goal of this project is to determine, using experiments in natural forests and the greenhouse, the role of pathogens in determining the outcome of the seedling to sapling transition in relation to the plants' proximity to adult trees of the same species. Seedlings and saplings will be sampled in three different forests where all trees greater than 1 centimeter diameter have been mapped and tagged. These forests in Virginia, Indiana and Wisconsin will provide a broad geographic sampling of eastern temperate forests. The survival and growth of seedlings and saplings of several species will be tracked for two years to determine their rates of mortality and growth around randomly selected adult trees. The results will inform species selection for greenhouse studies to determine the factors behind seedling success or failure in the forests. The researchers predict that rare species in the community will be more affected by pathogens in the early life stages than more common species.

Forests are vital to a sustainable human society and provide an essential renewable resource. It is therefore important to understand how forests grow and develop. This research will inform conservation and management efforts of forest systems by highlighting the role that pathogen-driven mortality has in structuring forest communities. Additionally, it will provide evidence on how forest regeneration may be affected by climate change by examining the regional differences in species performance across a latitudinal gradient. Finally, the researchers will continue their outreach partnerships with local and national land conservancies to educate the public on the results of this research and the importance of forest in general.

Project Report

A central goal of ecology is to understand the factors that affect the distribution and abundance of species. Understanding the distribution and abundance of species requires an understanding of the factors controlling species diversity. Species diversity is important because more diverse systems are more resilient to changes and tend to be more productive than less diverse systems. The dissertation work of Daniel Johnson funded by this project aimed to help understand the factors important for determining the diversity of temperate forest tree species. The fact that many plant species can co-exist in a given area has historically driven this line of questioning. Plants need a few basic things to survive, grow and reproduce: light, water, carbon dioxide and a small suite of soil nutrients. Given these limited requirements, why does a single plant species not out-compete and displace every other species for those requirements? There are likely trade-offs in how plants compete for their basic requirements. For example, being good at acquiring light might make a plant less tolerant to drought. In recent years, the concept those species-specific enemies play an important role in maintaining species diversity has gained increasing support, especially in tropical forests. The presence of a given species may attract enemies of that species like insects and pathogens. These enemies can limit the ability of a species’ offspring to survive in the same area as their parents. This project sought to investigate whether the signatures of species-specific enemies were evident in the spatial distribution of adult and juvenile trees in the forests of the eastern USA. We examined to comparative distribution of adults and seedlings from multiple perspectives and across different spatial scales. By examining the US Forest Service Forest Inventory and Analysis database from the eastern USA, we found that many tree species produce fewer successful seedlings when there are increasing densities of adult trees of that species nearby. Further, the strength of the response of seedlings to increasing adult density was related to the abundance of the species. Less abundant species tended to have stronger negative responses to increasing adult densities while abundant species suffer lower reductions in seedling densities near adults. The strong negative reaction in rare species could be one reason why they are rare. We also looked at this issue at a finer scale by evaluating mortality patterns of tree seedlings at three eastern U.S. forests where all trees larger than a centimeter in diameter had been mapped and identified across a 62 acre (25 ha) area. We sampled the seedling layer and related seedling survival to the density of adult trees of that species in the area. We found that species tended to have greater seedling mortality when they are close to higher densities of adult trees of that species, especially at small sizes when they are most susceptible to attack by natural enemies. Finally, we wanted to test individual species by growing seedlings in soil that was collected around adults of that species versus soil from other species. The goal of this experiment was to determine if there were natural enemies living in the soils for example, fungi or other pathogens that might kill young seedlings. We collected soil from around given tree species in the forest and then grew seedlings of that species in that soil in the greenhouse. One of the five species tested had significantly reduced growth in soil collected from around adults of that species. The fact that four of five species did not exhibit any effects is not strong evidence of soil-borne natural enemies affect all species but is consistent with what we expected to find. Many species remain to be examined individually for effects of soil-borne pathogens, which represent the most likely mechanism for the patterns we observed in the forests of the eastern USA. These broader findings have direct bearing on forest management issues of regeneration and species composition. Much like we rotate crops in agricultural fields to avoid depletion of soil resources and build-up of pathogens, forest managers should consider a mix of species and rotating species in plantation-style management to avoid build-up of natural enemies. The research funded by this award fostered collaborations between Indiana University and University of Wisconsin and the Smithsonian Institution. During the course of this research over a dozen undergraduates and several masters students were involved at various stages with the research. We have also led several tours of the mapped forest site in southern Indiana that was part of this project including a group of international students from Indiana University, a high school environmental science club, and a group of forest ecologists visiting during an international meeting in the area. To date, data collected here have been reported in four presentations at professional meetings and two research articles published in peer-reviewed journals.

National Science Foundation (NSF)
Division of Environmental Biology (DEB)
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Alan James Tessier
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Indiana University
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