The big old ones have a haunting, numinous beauty. But more than that, left to decay in a forest, they provide critical habitat for a multitude of species, not just the insects and fungi that eat the dead wood. Those organisms become food sources for a diversity of other species at higher trophic levels.
Hollows in dead trees provide shelter, nesting, and roosting places for many birds and mammals. Bats even use the galleries created by wood-boring beetles as hibernation sites, and bat species richness is higher in forests with larger amounts of dead wood.
The decomposition of wood plays a large role in forest nutrient cycles, carbon budgets, soil composition and natural regeneration. “At the time a tree dies, it has only partially fulfilled its potential ecological function,” Jerry Franklin, H.H. Shugart and Mark Harmon noted in a classic 1987 essay in BioScience. “Of course, the impact of the individual tree gradually fades as it is decomposed and its resources dispersed, but the woody structure may remain for centuries and influence habitat conditions for milenia.”
Forestry practices have drastically reduced the amount of dead wood in heavily logged forests worldwide. So I’m thrilled every time I encounter one of these legacy giants that’s been allowed to continue enriching a wild forest or city park.