Effects of Large-Scale Nitrogen Fertilization on Insect–Plant Interactions in the Canopy of Tall Alder Trees with N2-Fixing Traits in a Cool Temperate Forest

Nitrogen (N) deposition is expected to influence forests. The effects of large-scale N fertilization on canopy layer insect–plant interactions in stands tall, atmospheric nitrogen (N2)-fixing tree species have never been assessed. We conducted a experiment (100 kg ha−1 year−1 applied approximately 9 ha) over three years (2012–2014) cool temperate forest northern Japan. Our goal was evaluate relational responses between alder (Alnus hirsuta [Turcz.]) and their insect herbivores deposition. Specifically, we assessed leaf traits (N concentration, C:N ratio, condensed tannin mass per unit area (LMA)) herbivory by feeding guilds (leaf damage chewers the densities gallers miners) fertilized site an unfertilized control. Fertilization led increased galler density spring 2013 late summer 2014. For traits, LMA decreased 2014, ratio 2013. concentrations remained unchanged throughout study period. There negative correlation chewers, but not correlated with density. These results show that had positive plant-mediated (i.e., indirect) effect via decrease layer. Changes physical defenses leaves may be mechanism which affects tall N2-fixing trees.