The love between salmon and riparian vegetation is so whole that for whatever one giveth, the other returneth as much, if not more. The plants aid the salmon in their spawning by providing shade and shelter. And for their part, the salmon, when they die, offer the plants nutrients. That is, their dead bodies provide chemicals like nitrogen that are beneficial to the plants' survival. In fact, trees in salmon spawning sites may grow a good deal taller than they do elsewhere.
Now, putting the love relationship aside, you might say that salmon die for more selfish reasons, to feed their own young and thus perpetuate their own genes. Besides feeding trees, these same nutrients may give rise to more little critters in the water for the baby salmon feed on. All organisms die at some point, and whey they die, their bodies happen to give nutrients back to the earth.