Implications of Within-Species Root and Shoot Trait Variation and Plasticity for Species Coexistence

Foxx, Alicia J

doi:https://doi.org/10.21985/n2-svnk-6c19

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Implications of Within-Species Root and Shoot Trait Variation and Plasticity for Species Coexistence

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Understanding the outcomes of interactions from assessing shoot traits has practical applications and has elucidated major ecological patterns. Roots and shoots differ in their functions and can differ in responses to abiotic and biotic stimuli. And while roots are more difficult to characterize because they tangle and are embedded in soil, they are likely key drivers of interaction outcomes and influence survival. Generally, drawing conclusions only on one part of the plant masks crucial ecological information. In this dissertation, I use four studies to assess four factors that influence plant traits and interaction outcomes. The factors are 1) water availability, 2) population variation due to seed source, 3) plasticity, and 4) neighbor identity and traits. First, I use a meta-analysis on competition studies with varying water availability that partition roots or shoots to assess the contribution of plant parts to competition. I found that root-root interactions and shoot-shoot interactions had opposing responses to water availability: root-root interactions were intense under low water availability whereas shoot-shoot interactions were more intense under high water availability. The competitive relationships denote that root interactions in resource limited environments have a greater impact and should be considered in understanding the dynamics of interacting plants. Second, using a native perennial grass Pseudoroegneria spicata, I assessed the traits and variability in root and shoot traits of three wild and three cultivar accessions. Wild collections had higher survival, and six of eight measured traits showed greater variability in wild compared to cultivar collections. One key root trait, root tip count, was 51% greater on average in wild compared cultivar collections. While cultivars are important to meet seed need for restoration, cultivation practices should limit trait variation loss that could hinder survival and long-term persistence. Next, I assess how plasticity and neighbor identity affect interactions important for species coexistence. The Modern Coexistence Theory denotes that species coexistence is hinged upon stabilizing mechanisms, where intraspecific competition is more intense than interspecific competition, and equalizing mechanisms should function to keep traits important to competition equal between interacting species to avoid competitive exclusion. In the third study, I used a transplant study to show that induced plasticity affected subsequent intraspecific interactions important to stabilizing niche mechanisms, and thus coexistence. I induced plasticity through either intraspecific or interspecific interactions, then moved these induced plants to new intraspecific neighbors to assess if induced plasticity affected subsequent intraspecific interactions. Plasticity induced from interspecific competition lead to subsequent intraspecific interactions that were more negative than plants induced by intraspecific competition. Roots showed stronger responses than shoots to the outcomes of induced plasticity and the interaction outcomes show that interspecific interactions may have a role in population dynamics important to species coexistence. Lastly, I tested if population variation affected stabilizing niche mechanisms and found that populations varied in whether they competed more intensely with intraspecific compared to interspecific neighbors, and this was influenced by neighbor identity. This indicates that certain intraspecific and interspecific population combinations may promote or hinder stabilizing mechanisms and species coexistence. I also tested whether neighbors demonstrated equalizing mechanisms in which competition becomes more intense with increasing trait distance. The interaction outcomes show variation in whether interacting pairs showed equalizing mechanisms by population identity of both neighbors, whether the interactions were intraspecific or interspecific, and whether traits were root- or shoot-derived. Restoration practitioners should consider interactions of populations and species selected to be mixed in restorations which may influence long-term coexistence. In all, the outcomes of these four studies have implications for ecological theory in which the impact of root traits, population trait variation, and plasticity have been understudied. The study outcomes also inform restoration practices in systems where belowground interactions strongly influence community dynamics.

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