This is really interesting, but they're not carnivorous. The decaying salmon deposit nutrients in the soil. Carnivorous plants actually digest the insects directly for nutrients, usually because the soil is inhospitable.
It depends on how much you look into the forest floor. Many trees in a mature forest actively poison the soil to reduce competition for nutrients. Look at a pacific coastal rain forest (or Star Wars' Endor) and you see big trees and an open forest floor. The big trees do not allow smaller/medium trees to get going.(Some of this is lack of sunlight, but also soil PH.) So a salmon-dependent tree could adapt to prevent other plants from getting ahold of salmon nutrients.
Similar to the nitrogen from salmon on the coasts, I have read about plants getting calcium from deer/elk/moose antlers. You would think it wasn't much but then you realize how depleted a forest can be of calcium. It can put a halt on plant growth. Migrating deer then become a significant source of calcium movement.
Yeah, I think it's the absence of traits evolved to attract, trap, and digest salmon that are all missing here.