Land on the planet is frequently disturbed. In a radical disturbance, various ecological factors will have been drastically changed. Trees may have been removed, changing the temperature at the ground level and the access to sunlight. Flooding may have deposited soil over the previous ground area. Bulldozing may have removed much of the topsoil, or plowing may have turned the soil layers upside down. Your challenge is to determine roughly when the disturbance occurred and just what items in the environment were altered and to what extent.
I hope you will also try to determine what plants survived and, according to information presented below, what life styles these plants exhibit.
A recurring disturbance is more likely to be disturbing more to the plants themselves than to the general environment. The plants may be regularly trampled on by human feet or the sharp hooves of grazing animals. The leaves of the plants may be regularly chewed off by grazers or other growing plants may shut off the light supply as they expand seasonally. In other words their growth and development is frequently altered.
Radical disturbance generally brings about relatively rapid successional changes of plant species and communities. Recurring disturbance is more likely to bring about establishment of plant species that have become adapted to one or more subsets of the four major patterns of stress and disturbance
Four Conditions for Plants
Plants generally face two major problems in their world stress and disturbance. Stress is generally caused by shortages or excesses of essential materials such as water, minerals, or sunlight.
Disturbance is generally physical, such as being chopped up, burned over, turned over, or eaten down. There are essentially four major situations that plants must face:
low stress and low disturbance
· high stress and low disturbance
· low stress and high disturbance
· high stress and high disturbance.
For the area you are investigating you will want to determine which of these major challenges to plant life prevails. You will of course find that these represent the extremes; that your area may, for instance, exhibit moderate stress and low disturbance. Such in-between situations exist for each factor. There are plant life styles to cope with each type. Only the last, high stress and high disturbance, essentially excludes all higher plant life.
There are several life styles that help plants survive disturbed earth.
1. First life style: plants grow slowly and have long-lived organs. Many are evergreen. They may flower infrequently. They store and hold on to the material they make through photosynthesis (photosynthates). Because of their slow growth, loss of leaves through disturbances can be fatal and, at best, cause considerable setback. Consequently many plants of this ife style have toxic materials in their leaves or have protective spines. This life style does well in situations of high stress and low disturbance.
2. A second life style involves a rapidly completed life cycle. This usually is an annual cycle or a short-lived perennial status. Seed ripening is very fast and a flower cluster may contain both blossoms and ripe seeds. In the cycle, this style invest photosynthates into flower and seed production rather than plant growth. This style, the ruderal style, is well adapted to low stress, high disturbance situations.
Some plants, such as grasses, are adapted to similar situations structurally. Whereas most plants grow outward from the tip of the branch, grasses push upward with growth at the base of the leaf. Thus if the leaf is grazed or mown it can continue growing.
3. A third major life style involves putting the bulk of the energy into plant growth. This involves both leaf growth and root expansion. The plants have good root storage to supply the new growth and may have branching rhizomes or tillers to rapidly occupy vacant areas. These plants take rapid advantage of the resources of their environment and try to overwhelm competing plants. They are exploiters of environment rather than conservers. They are competitors and prefer areas of low stress and low disturbance.
Just as there are intermediate sets of environmental conditions there are intermediate life styles among the plants. For example, ragweed (Ambrosia artemesiifolia), that bane of hayfever sufferers, has a long vegetative phase and puts up tall shoots. Where many ragweed plants are growing close together, the broad spread of their leaves will form something of a dense canopy characteristics of competitors. Yet the plant comes in on disturbed ground and is an annual that produces many seeds and dies a mark of the ruderal life style.
SPECIES At Your Study Site
To gain some insight into the happenings on your disturbed site you may wish to create a chart something like the following to determine the life style of the local plants on your disturbed area.
Selected Species Number __________
annual _______
biennial _______
perennial _______
massive seeding _______
limited seeding _______
heavy leaf and shoot growth _______
growth
fast growth _______
slow growth _______
tillers or rhizomes _______
storage roots protective spines or chemicals _______
In areas that have experienced random radical disturbance, such as from hurricanes or tornadoes, you will probably discover a range of plant life styles remaining. For the first few years chances are that the ruderal life style will prevail, but if the soil was initially good, stress tolerators and competitors will begin to prevail and eventually to dominate in a normal successional pattern. In areas that have recurring disturbance on a frequent basis you will probably discover a different pattern of surviving life styles.
To gain some information about such things it will be useful to establish some quadrats or transects that can be examined in detail year after year. Try to take photos of each area several times a year from the same spot. This will be a good source of continuing data.
You will want to determine not only the species of plants and their abundance, but also information on their life styles. Students may wish to make predictions about what types of life style will be found in each environmental situation. Which can students predict more accurately, the species of plants they will find or the life styles of the plants to be found in that situation? Although the species will differ in different regions of the country or around the globe, do students in different areas find that the life styles of the plants in similar situations are predominantly the same? You may wish to find companion schools in different regions to explore this issue.
Reference: Plant Strategies & Vegetative Processes by J.P.Grime. New York: John Wiley & Sons, 1979