Nutrient availability affects all levels of biotic organization and substantial shifts in plant community composition and diversity in response to anthropogenic nitrogen (N) deposition are well documented. The effects of N deposition have rarely been investigated in early successional habitats, which tend to be strongly nutrient limited. In addition to N deposition, non-native organisms drive communities change worldwide and in the context of succession, may strongly alter successional trajectories. While many studies investigate effects of N deposition on specific plant-insect interactions, few examine realistic levels of N deposition or investigate these effects on non-native insect herbivores, and none link the two factors in the context of succession. Such investigations are warranted because alterations of host plant quality from nutrient additions may affect successional processes if it changes herbivore load or impact, and we need to evaluate such community responses to realistic rates of N input. To investigate these questions, we simulated N deposition at three levels (0, 8, and 16 kg/ha/yr) in 60 plots distributed across primary and secondary successional sites at Mount St. Helens volcano. We focused on the response by Sitka willow (Salix sitchensis), the first widespread woody plant in upland primary sites. Previous work shows that sitka willow growth and survival is severely curtailed by the invasive Willow Stem-boring Weevil (WSW), Cryptorhynchus lapathi, so we factorially manipulated weevil presence.
Results/Conclusions
Weevil exclusion dramatically increased willow relative growth rate (RGR) in primary but not secondary succession. Nitrogen addition increased RGR primary sites, and its affects were stronger when weevils were excluded. Leaf %N increased non-linearly in primary sites, with highest %N at 8kg/ha/yr and was not affected in secondary sites. Leaf d15N values suggesting a dilution effect of growth on %N at 16kg/ha/yr. Neither treatment affected specific leaf area or condensed or hydrolysable tannins, but in primary sites, weevil attack and N addition both changed the composition of flavonoids and total polyphenols. Weevil attack decreased total flavonoids and quercetin derivatives, and increasing myricetin derivatives, while N addition decreased myricetin derivatives.
We conclude that atmospheric N deposition has the potential to increase willow biomass in early primary succession while partially mitigating the impact of WSW, but not in more developed, less N-limited secondary successional communities. N deposition affects increases host quality by increasing %N and changes the composition of carbon-based defenses. Follow up work examines these effects on plant and arthropod communities.