This is an example of temporal separation of niches. The left photo is of trembling aspen (Populus trembuloides) trees. Aspen is an intolerant, pioneer species and first colonizes a stand following a disturbance. The photo on the right is a hemlock (Tsuga canadensis) forest. Hemlock is a tolerant, 'climax' species that can establish and survive under the canopy of pioneer tree species such as aspen. Eventually, through time, the hemlock replaces the aspen as the dominant canopy species.