A8 Ecosystem Disturbance

Topic

Ecosystem disturbances—such as fire, logging, habitat fragmentation, the invasion of exotic species, or climate change—simultaneously alter three distinct aspects of the ecosystem: the canopy, plant competition, and the flows of mass and energy moving through the system. Once altered by these disturbances, each of these three aspects produces its own specific consequence for the ecosystem; the speed and completeness of the ecosystem's recovery from these consequences depend, in turn, on its resilience.

When ecosystem disturbances alter the canopy—reducing or completely eliminating it—the system loses the buffering canopy that normally moderates environmental conditions beneath the tree crowns. Without this buffer, the exposed microclimate becomes extreme, characterized by higher daytime temperatures, lower nighttime temperatures, and greater fluctuations in air humidity than would have been maintained by an intact canopy.

When ecosystem disturbances alter mass and energy flows, the balance between carbon capture and release shifts. An ecosystem that previously functioned as a carbon sink—capturing more carbon than it released—may switch to functioning as a carbon source, releasing more carbon than it captures; the more intense the alteration to these flows, the closer the ecosystem moves toward the carbon-source end of the spectrum. When ecosystem disturbances alter competition among plants, they change the resource availability conditions under which each species competes, prompting a reorganization of the plant community: within this competitive dynamic, each species requires a specific minimum resource level to survive against its competitors; when disturbances modify available resources, the dominant species become those with the lowest minimum resource requirements under the new conditions, thereby displacing the species that dominated the community prior to the disturbance.

Ecosystem resilience determines—in the face of these three consequences (extreme microclimate, the ecosystem becoming a carbon source, and a reorganized plant community)—how quickly and completely the ecosystem recovers the canopy, competitive dynamics, and mass and energy flows it possessed before the disturbances occurred: the higher the ecosystem resilience, the faster the canopy resumes buffering the microclimate, the faster the ecosystem returns to functioning as a carbon sink, and the faster the plant community regains its original composition; conversely, the lower the ecosystem resilience, the longer the ecosystem remains characterized by an extreme microclimate, functions as a carbon source, and maintains a plant community reorganized around species different from those that dominated prior to the disturbances.

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