Plains cottonwood and Russian olive response to elevated CO2, root nodulation and nitrogen fertilization

Background/Question/Methods

The introduced tree, Russian olive (Elaeagnus angustifolia), is common and abundant in western U.S. riparian ecosystems. Because Russian olive is actinorhizal (forms root nodules in association with actinomycetes in the genus Frankia) and therefore has greater access to nitrogen (N), it may benefit more from elevated CO₂ than native riparian competitors. In a previous study, elevated CO₂ did not benefit non-nodulated Russian olive seedlings more than two native species, but effects of elevated CO₂ on nodulated Russian olive seedlings have not been tested. To determine how Russian olive plants with and without Frankia sp. nodules respond to elevated CO₂ relative to a dominant, native riparian tree, plains cottonwood (Populus deltoids ssp. monilifera), we are growing seedlings of both species in greenhouses with either ambient or elevated CO₂ and three N treatments (low, high & Frankia sp. innoculation). We measured plant height throughout the experiment and biomass, stomatal conductance, and tissue C:N and Δ¹³C (integrated water use efficiency) at the end of the experiment.

Results/Conclusions

Preliminary height data indicate that Russian olive and plains cottonwood respond differently to the N treatments. Although both species were significantly taller in the high N treatment than in the low or nodulation treatment (F=27.94, P<0.0001), Russian olives increased more in height in the high N treatment than cottonwood (F=3.81, P=0.025). Russian olives were significantly taller than plains cottonwoods when averaged across all treatments (F=5.15, P=0.025). As the experiment progresses, we expect that elevated CO₂ and nodulation will also influence plant growth and physiology. For example, non-pathogenic, necrotic leaf spots, which occur on the cottonwood seedlings but not on the Russian olives, are significantly more abundant on the cottonwood seedlings under elevated CO₂ than ambient CO₂ (F=42.85, P<0.0001), but are not significantly influenced by N treatment (F=0.68, P=0.5). These preliminary results indicate that Russian olive and cottonwood may respond differently to changing environmental conditions, leading to changes in the competitive landscape of western riparian ecosystems.