What is the human impact on coniferous forests?
There are numerous, but the two that are likely most significant would be climate change and a century of battling forest fires.
Climate change has resulted in milder winters with a shorter duration - this results in a greater number of insects (ie. tent caterpillars and pine beetle are two prime examples) surviving and subsequent infestations increasing in both severity and area.
In British Columbia ~ 50% of lodgepole pine have been killed due to this beetle directly (and climate change indirectly). You can read more about the pine beetle and the lodgepole pine here.
Battling forest fires interferes with the natural process of forest succession. For example, the lodgepole pine has a lifespan of ~100 years, about every ~100 years there is a forest fire. The lodgepole pine will only germinate following a fire (the pine cones have a thick, spiny exterior that only open up to disperse seeds following a fire).
Years of battling fires has prevented the natural renewal processes that sustain these great forests - there are a greater number of pests and pathogens and a large population of older trees unable to withstand the onslaught.
It is common to see "prescribed burns" where we intentionally set localized forest fires where we have previously interfered - this minimizes the potential for large, catastrophic forest fires in the future.