In explaining his opposition to efforts aimed at stabilizing moose populations and at suppressing fires in northern forests, Pastor points out that moose, by feeding on hardwood trees, contribute to a characteristic process whereby hardwood trees are eventually overtaken and shaded out as conifers such as spruces block sunlight A decline in hardwood trees leads to fewer nitrogen-rich, quick-rotting leaves on the forest floor which is increasingly carpeted with slower-rotting conifer needles. Hardwoods require more nitrogen for growth than do conifers, so the decline in soil fertility in areas where moose feed reinforces the advance of conifers. But conifers cannot dominate the forest for long, because mature conifer forests are more combustible than are hardwood forests. Fire combined with high winds, attacks of spruce budworm, or even a greatly diminished nitrogen supply will destroy expanses of spruces, allowing hardwoods to recolonize in the nutrient-rich ash. Such oscillations in forest composition and consequently in moose population are, Pastor believes, essential to the integrity and functioning of northern forest ecosystems.