Visualizing Environmental Science

Community Responses to Changing Conditions over Time: Succession 153

PROCESS DIAGRAM

trees, to lilies and other herbs, and—by ten years after

the fires—to a young forest of lodgepole pines, with some

Douglas fir seedlings.

Biologists have studied secondary succession on aban-

doned farmland extensively (Figure 6.21). Although it

takes more than 100 years for secondary succession to oc-

cur at a single site, a single researcher can study old-field

succession in its entirety by observing different sites under-

going succession in the same general area. The biologist

may examine county tax records to determine when each

field was abandoned. Secondary succession on abandoned

farmland in the southeastern United States proceeds in

this sequence: crabgrass m horseweed, broomsedge, and

other weeds m pine trees m hardwood trees.

1. What is ecological succession?


2. How does primary succession differ from
   secondary succession?

the surface of a dune, their roots hold it in place, helping
to stabilize the dune surface. Mat-forming shrubs then
invade to further stabilize the dune, followed by a suc-
cession of tree species over the course of many years. Pri-
mary succession on sand dunes around the Great Lakes
might proceed in this sequence: grasses m shrubs m pop-
lars (cottonwoods) m pine trees m oak trees.

Secondary Succession

Secondary succession is the change in species composi-
tion that takes place after some disturbance destroys the
existing vegetation; soil is already present. A clear-cut for-
est, open areas caused by a forest fire, and abandoned
farmland are common examples of sites where secondary
succession occurs. During the summer of 1988, wildfires
burned approximately one-third of Yellowstone National
Park, a disaster that provided a chance for biologists to
study secondary succession in areas that were once for-
ests. Secondary succession in Yellowstone has occurred
rapidly, moving from ash-covered forest floor and charred

Why do you think

that weeds can initially establish sites

undergoing secondary succession,

rather than the lichens that must initially

colonize environments undergoing

primary succession?

Think Critically

Secondary succession on an abandoned field

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Annual and

perennial weeds

(horseweed,

broomsedge,

ragweed, and

aster)

Pine seedlings

and saplings

(shortleaf pine

and loblolly

pine)

Young pine

forest and

developing

understory of

hardwoods

Mature hardwood

forest (oaks,

hickory)

1

Years after cultivation

2–4 5–15 25–50 150

Annual weeds

(crabgrass)

12 34 5