Plant Succession (1963)

there is continuous change in the plant life around us most of us are aware of one aspect of this change the constant process of growth and decay but there is another type of change perhaps less apparent the replacement of one type of plant life by another because one kind of plant succeeds another this process is called plant succession to understand this we must first understand that the type of plants that grow depend on the environment that is on the type of soil the amount of sunlight and precipitation and the average temperature most soil is formed in part of rock crumbled into small particles large rocks are often split by the action of freezing water into smaller rocks these are further ground down or weathered by rain and wind over a period of hundreds of years the original rock may be broken down into the small particles we call Sand natural forces cause movement of the sand Wind and Rain remove great quantities of material from one place and deposit it in another the surface of the Earth is constantly changing but sand and rock are not enough decayed organic matter must be added to form soil in which plants can grow how do the first plants start growing how does a forest appear where there was once Rock here is a form of plant that can grow on bare rock these are lyans there are many types of lyans but essentially each consists of two kinds of plants living in close Association the alga which contains chlorophyll and can photosynthesize its own food and the fungus the fungus absorbs and stores the water which the alga uses for photosynthesis and the fungus in turn lives off the food which the alga produces lyans help to form soil suitable for the succeeding stage of plant light lyans do this in two ways first all lyans have a tremendous capacity to absorb water during respiration the lyan gives off carbon dioxide which dissolves in the water absorbed by the ly forming carbonic acid the carbonic acid attacks The Rock on which the lyan grows in order to demonstrate the process we take some chips of limestone a rock on which lyans can grow and place them in a beaker of carbonic acid the bubbles are evidence of the chemical action of the carbonic acid on the marble the carbonic acid formed by lyen attacks the rocks on which the lyen grows in much the same way but extremely slowly as we have seen there are a number of other ways in which rock is broken and crumbled eventually the lyan dies the dead lyan and bits of pulverized Rock are blown by the wind and Washed by the rain to the base of the Rock and into cracks and crevices the organic matter which decays and the powdered Rock together form the first soil lyans are thus partly responsible for preparing the way for the next stage of plant succession quite typically lyans are succeeded by mosses which do require soil the tiny light spores from which mosses grow are blown about by the wind which is one of the reasons Mosses are so widespread mosses May grow along a crack in a rock where there is some soil or in some other depression anywhere there is a deposit of soil often Mosses are found around the base of a rock where most of the soil tends to collect compared with many other plants Mosses are able to thrive in a relatively small amount of soil what holds plant and soil together and makes it possible for the Moss to obtain necessary minerals even when the supply of soil is quite limited is the network of root-like Riso Mosses are Hardy and can go for some time with very little water when Mosses are dried out they often turn brown and become brittle they may appear dead even though they are actually alive this piece of moss readily absorbs a drop of water this ability to absorb water plays an important role in the building of soil the carbon dioxide given off by the Moss during respiration dissolves in the water which the Moss has absorbed forming carbonic acid the carbonic acid breaks up rocks and Stones by dissolving some of the minerals in them m like lyans contribute to the formation of soil by the huge quantities of water they can absorb Moss often covers the lyans which made it possible for the Moss to grow in the first place over a period of many years mosses die and Decay the soil deepens where conditions permit Moss thus prepares for the next stage in the succession grass grasses have intricate Roots which reach down deeper into the Earth for the available water than the riseo of the Mosses grasses also grow taller than mosses eventually grasses went out in the competition for moisture and light grasses replace the Mosses grasses in their turn die and Decay eventually they build up the soil sufficiently for shrubs to grow at first the shrubs are scattered and do not interfere seriously with the grass but soon the competitive struggle Begins the shrubs take hold and grow in denser clumps there are so many leaves that little Light reaches the ground and the grasses can no longer survive but the dim light under the shrubs does suit the early growth of another type of plant the seedlings of the pioneer trees as it matures the sapling no longer needs the protection of the shrubs once they are taller than the shrubs the trees shade out the bushes they now provide the shade required for more tree seedlings to grow thus the forest provides the conditions needed to continue its own growth the deciduous forest dominated by trees which shed their leaves each year is the final stage in many areas the climax vegetation but not all plant communities Reach This climax in regions of less rainfall or poorer soil Evergreens or conifers may be the final level attained the climax in fact evergreens are the climax growth over great areas of the United States and Canada where there is even less rainfall a different climax vegetation grasses have continued to grow for thousands of years in the desert where hardly any rain falls only specialized plants can survive at the lower temperatures further from the equator conifers again are the climax vegetation further north or south where the temperature is lower even conifers grow more sparsely and as we approach closer to the Arctic regions shrubs are the climax vegetation in the very short growing season of the tundra area of the far north many of the shrubs and other plants produce unusual flowers in some parts of the tundra where the soil is poor grass is the climax growth closer still to the poles the temperatures are so low that scarcely any vegetation except moss and lyan can grow temperatures also drop as altitudes increase on the side of the mountain there is a limit above which trees do not grow the tree line higher up where it is even colder no plants can grow over large parts of the land men are changing the natural course of plant succession the environment is changed when men level whole forests it is changed again when they plant the new New Growth will affect the soil by seeding man can often grow the plants he wants these may not be the same as those that would grow here in the natural course of plant succession men may control very directly the plants that grow often the plant growth of whole Landscapes is affected by men's labors when man abandons a place this too has an effect some plants which man protects are replaced by plants he calls weeds even under the most unfavorable conditions a great variety of weeds can grow if it is left abandoned long enough a road will eventually disappear when a field is abandoned it quickly becomes overgrown with weeds annual weeds first these are the first in a succession of plants that may differ from the succession we already considered the annual weeds are succeeded by perennial weeds Golden Rod for example these crowd out the annual weeds plants like Golden Rod are in turn replaced by grasses which have longer roots and win out in the competition for moisture again one kind of plant makes possible the growth of Another Kind which eventually replaces it in this particular plant succession the grass is invaded by vines in the conditions created partly by the grass The Vines flourish and finally replace the grass The Vines are succeeded in their turn by shrubs and the shrubs are succeeded by trees the general changing conditions make possible new cycles of vegetation the succession of plants is directly related to four governing factors soil rainfall temperature and sunlight with enough soil and moisture rootless lyans are replaced by mosses with root like Riso next come grasses which have fully developed roots and are taller each plant in the succession builds up the soil and prepares the way for the next each new plant replaces its predecessors the climax vegetation varies from one region to another but where soil rainfall temperature and sunlight are suitable the final stage in the succession of plants is the Deciduous hardwood forest