ARE SYMBIOTIC NITROGEN FIXATION STRATEGIES TIED TO INVASIVENESS FOR NON-NATIVE WOODY LEGUMES IN HAWAIʻI?

Date
2020-05
Authors
Kirby, Angalee
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Ostertag, Rebecca
Department
Tropical Conservation Biology & Environmental Science
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Abstract
A diversity of strategies is used by symbiotic nitrogen-fixing plants, each well suited for specific environmental conditions. Little is known about whether fixation strategies are related to invasiveness when these species are introduced to new environments. Weed risk assessment scores were used as an index for invasiveness for eight non-native N-fixing tree species in Hawaiʻi. In a shade house experiment using an isotopic tracer, I show that these eight species (four high risk and four low risk for invasiveness in Hawaiʻi) varied in their growth, allocation, and N-fixing traits, in response to three levels of nitrogen fertilization and could be grouped into three distinct fixation strategies: one obligate, four facultative, and three over-regulating facultative. Strategies are associated with the trait plasticity of each species, but do not appear related to risk assessments for invasiveness in Hawaiʻi. Over-regulating facultative fixers had the highest trait plasticity and were able to regulate symbiotic nitrogen fixation with the greatest magnitude, while the obligate fixer had low trait plasticity and did not regulate fixation. This implies that species identity is a more likely predictor of N fixation strategy, and thus how a species will respond to varying nutrient conditions, than weed risk assessment scores.
Description
A diversity of strategies is used by symbiotic nitrogen-fixing plants, each well suited for specific environmental conditions. Little is known about whether fixation strategies are related to invasiveness when these species are introduced to new environments. Weed risk assessment scores were used as an index for invasiveness for eight non-native N-fixing tree species in Hawaiʻi. In a shade house experiment using an isotopic tracer, I show that these eight species (four high risk and four low risk for invasiveness in Hawaiʻi) varied in their growth, allocation, and N-fixing traits, in response to three levels of nitrogen fertilization and could be grouped into three distinct fixation strategies: one obligate, four facultative, and three over-regulating facultative. Strategies are associated with the trait plasticity of each species, but do not appear related to risk assessments for invasiveness in Hawaiʻi. Over-regulating facultative fixers had the highest trait plasticity and were able to regulate symbiotic nitrogen fixation with the greatest magnitude, while the obligate fixer had low trait plasticity and did not regulate fixation. This implies that species identity is a more likely predictor of N fixation strategy, and thus how a species will respond to varying nutrient conditions, than weed risk assessment scores.
Keywords
Conservation biology, Environmental science, Ecology, 15N, facultative strategy, HPWRA, isotopic tracer, N-fixation regulation, obligate strategy
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47 pages
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