Mechanisms of species assembly and coexistence in vegetation under a changing climate: Plant traits, contested resources, and ecosystem services

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A fundamental goal for conservation & restoration ecology is to understand the mechanisms that explain why species composition, richness & productivity of natural vegetation vary across space & time. Answers to these questions are essential for making informed predictions and recommendations for mitigation associated with vegetation response to ongoing climate change (precipitation & temperature), atmospheric pollution (nitrogen inputs), success of invasive species, and habitat & species loss from impacts of a burgeoning human population that is destined to continue increasing in size and density in the coming decades. Faced with this, we also need to learn how to anticipate what we can expect from the emergent properties & services of the ecosystems of tomorrow as they are shaped in new ways by powerful, persistent and growing forces of anthropogenic selection. Because reproductive success of resident plants in most natural vegetation is routinely limited by near-neighbour effects, guidance in support of the above goals will require a deep understanding of the selection consequences of species interactions. Accordingly, the research program proposed here-with collaboration involving training for 9 graduate students & 15 undergraduate students-will test recent hypotheses about the key plant traits that affect success in leaving descendants in crowded vegetation (competitive ability), and under heavy impact of browsing from a continuing population explosion of white-tailed deer in eastern Ontario-and will examine the role of water versus nutrients as key objects of soil resource competition in mesic habitats that are predicted to become more nitrogen-enriched plus wetter or more arid as climate change unfolds, and as these effects (modulated by species interactions) impact on community structure, diversity and productivity. Competition between near neighbours is conventionally regarded as one of the most intense forces of natural selection that has shaped the species composition, diversity & evolution of plant traits across the globe. Traditional theory is based on a `size-advantage' (SA) hypothesis-i.e. intense competition between plants generally selects for a strategy that includes capacity to grow to a relatively large body size. My research program will explore a paradox-a disconnect with the SA hypothesis: most resident species are relatively small within virtually all vegetation types. I will develop and investigate a paradigm shift for understanding competitive ability in plants-one that recognizes that fitness (gene transmission) under competition involves more than just growth (plant size) & survival; it also involves, most importantly, producing offspring. Projects will examine how the deployment and allocation of plant meristems/buds affects all three components of fitness under competition-size, survival & fecundity. They will test predictions about interactions and trade-offs between these three fitness components, and how these can be used to define revisions for species coexistence theory that do not depend on weak competition (through traditional niche differentiation or effects of disturbance), and without requiring a heterogeneous physical environment. Since meristem production is tied directly to leaf production, this research will also develop more broadly into a series of studies based on a novel meristem-based approach for defining plant strategies in terms of 'leafing intensity' (e.g. size/number) trade-offs and 'reproductive economy' (ability to deploy reproductive meristems successfully despite severe plant size suppression). This approach will be tested across a wide range of life forms (herbaceous & woody), and at several scales ranging from within-plant to between-taxa, and across habitat types.