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Recruitment dynamics of tree species

Project Abstract: 
The higher temperatures and carbon dioxide concentrations associated with climate change will alter growing conditions for plants. Although a variety of species and community responses have been predicted, one of the most widely expected results is a shift in species ranges as plants expand their ranges poleward to track changing temperatures. The future composition of temperate forests and the ecosystem services they provide will depend on whether migrant tree species can successfully establish before climate change disrupts populations of existing tree species. Most predictions of tree range shifts are based on correlations between environmental conditions and the current range of a species (i.e. climate envelopes; Iverson and Prasad 1998). However, climate envelope projections do not incorporate the biotic factors individuals will experience when they invade already established communities (e.g. interactions with herbivores, pathogens, symbiotic organisms, mutualists and local plants). These interactions with existing communities will play a large role in determining the migratory potential of each species. This knowledge gap highlights the need for experimental work to complement climate envelope approaches. Tree recruitment in established communities is very sensitive to natural enemies (e.g. Hewitt and Kellman 2004); support for the Janzen-Connell hypothesis (Janzen 1970) has shown that conspecific seedling recruitment can be strongly reduced by the accumulation of specialist herbivores and pathogens around host trees (e.g. Packer and Clay 2000). However, I hypothesize that specialist herbivores and pathogens are unlikely to be common in areas beyond the current range of a plant species, potentially releasing pioneer populations of migratory plants from herbivory and pathogens. This could result in a competitive advantage for migrant plants, as decreased herbivory increases growth rates and fecundity while lowering the age of first reproduction (Engelkes et al. 2008, Funk and Throop 2010). This would also increase the rate at which plants can spread. However, it is also possible that migrant plants will be more susceptible to generalist herbivores because of the additional stress of adapting to a new environment; this might prevent their establishment in the new region. In order to elucidate the mechanisms influencing tree species distributional shifts, I will carry out experimental work to quantify the interactions between migrant plants and existing communities. Specifically, I will address the following questions: Do migrant seedlings experience less herbivory and disease in their new ranges than native seedlings? Are migrant species exposed to less herbivory and disease than in their native range? How important is this to plant growth and survival? In order to answer these questions I will plant tree seedlings within and beyond their current distributional ranges and monitor these plants for foliar damage, pathogen activity, growth, and survival.
Status of Research Project: 
Years Active: 
2017 to 2021
To quantify the full array of potential outcomes I have established six study sites along a latitudinal gradient in Michigan, at the University of Michigan Biological Station (UMBS), in Manistee National Forest, and at the Edwin. S. George Reserve (ESGR). Next year I hope to add two additional sites in the UP. Michigan is an ideal location for studying shifts in plant ranges as it encompasses the northern range limits of many common temperate forest species. Furthermore, my advisor, Dr. Inés Ibáñez, is conducting related experiments at some of these sites, giving me access to baseline data and established research infrastructure. EXPERIMENTAL DESIGN: First, I will germinate and raise seedlings in the research greenhouses at the Matthaei Botanical Gardens. At each site I will establish multiple new plots along light and moisture gradients to complement existing plots from my last two summers of fieldwork. At each plot I will plant 5 species of plants under three treatments. These include: herbivore exclosures, opened exclosures which allow herbivore access but control for the effects of the exclosures upon the plants, and lastly seedlings without exclosures. I will plant 5 replicates of each treatment per plot, for a total of 1650 seedlings. During the growing season I will regularly monitor seedling mortality, herbivory on each individual leaf, and pathogen activity. I will also survey the insect community at each site and take samples of the pathogens which are affecting the seedlings. At each site I will also monitor light conditions, temperature, soil chemistry, and soil moisture. The spatial and temporal variability of the environmental data will allow me to evaluate the response of each species to a temperature and soil moisture gradient that includes conditions predicted for future decades. I will analyze the data with hierarchical Bayesian modeling. This will allow me to incorporate several sources of information, including seedling data, herbivory, pathogen activity, environmental variables and site characteristics, and will allow me to determine how important each factor is to seedling recruitment in current and future climate conditions. Furthermore, it will allow me to take in to account the uncertainty associated with the data and the recruitment process (Ibáñez et al. 2009). Seeds and seedlings will also be planted at each site for an additional summer in order to sample the inherent annual stochastic variation in seed germination and seedling recruitment.
Funding agency: 
National Science Foundation