Growth and resistance to herbivory in N2-fixing alders

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Abstract
Fixation of atmospheric N2 may provide an advantage to woody plants in N-limited environments, but may also alter their resistance to herbivory. Studies in adjacent plantings of three North American shrub alders (Alnus spp.) and three Eurasian tree alders showed significant species differences in susceptibility to a leaf-mining sawfly (Fenusa dohrnii) and in response to inoculation with a N2-fixing actinomycete (Frankia). During the first 5 years, woody biomass production ranged from 0.3 t∙ha−1∙year−1 in Alnus viridis ssp. sinuata (Sitka alder) to 8.3 t∙ha−1∙year−1 in Alnus incana ssp. incana (grey alder). Grey alder and another tree species (Alnus japonica) were attacked infrequently and suffered little sawfly damage except in plots with poor growth. The tree species Alnus glutinosa (black alder) was attacked frequently, and sawfly damage was greatest in plots with the best growth. The shrub species Alnus viridis ssp. crispa (green alder) was also attacked frequently but was highly resistant to larval feeding. Frequently attacked species showed greater damage in the lower portion of the crown. Frankia inoculation increased green alder biomass by 87% but had no significant effect on grey alder or black alder. The concept of a growth-defense trade-off does not fully explain the interactions between Alnus spp. and F. dohrnii. Key words: Alnus, Fenusa dohrnii, Frankia, biomass production, plant–herbivore interactions, leaf miner.

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