VEGETATION DEVELOPMENT

Introduction

This is the first of 3 classes dealing with the processes creating the vegetation cover of the Earth's surface:

1 - The factors affecting the development of individual plants

2 - Factors affecting plants as they grow together in communities

3 - The spatial distribution of vegetation

In each case, we shall be primarily concerned with the factors influences the spatial variatoin, and in linking distribution to the other elements of the physical environment.

Plant Development

Plants require:

- nutrients

- light

- carbon dioxide

- water

Without these, plant growth is not possible - they are all associated with photosynthesis. Our major concerns are (a) whether their absence, or abundance, makes growth easy, difficult, or impossible, and (b) how the variations in availability lead to spatial vegetation differences.

Nutrients, extracted from the soil, depend on rock type - and on the residues from the breakdown of previous vegetation at the site. Most rocks give a range of nutrients and allow most common vegetation types to grow, although they may have a profound influence on the actual species composition. Other rocks, such as limestone (which is mainly calcium carbonate) give poor soils and are often deficient in nutrients so that distinct vegetation (such as the 'Blue Grass' of Kentucky) dominates

Some plants do not extract nutrients directly form the soil, but rely on other plants

Low, shade-loving plants on a mountain forest floor

Light, directly from sunlight, varies tremendously in amount with geographical location. In addition, there are sunny spots and shady ones. Shade may come about 'externally' (because a plant is in the bottom of a narrow valley, for example), or 'internally' because it is overtopped by another type of vegetation growing in the same area - a major cause of species competition.

Little grows directly on the beach - shifting sands, lack of nutrients, intense sunlight. Specially adapted grasses begin to colonize the more stable dunes behing the beach itself.

Carbon dioxide is normally extracted from the air, or is incorporated into plants through roots. Thus many plants will thrive as the amount of atmospheric carbon dioxide increases. Some, however, experience stress if they become waterlogged so that no soil air can get to the roots.

Swamp vegetation adapted to waterlogged conditions.

Water, primarily the balance between the precipitation and the evapotranspiration, often determines the kind of vegetation. In dry areas grasses are likely to dominate, while in wet ones, digger life-forms, especially trees, can survive.

A dry area because of soil type, not climate - most of North Carolina has plenty of precipitation so that trees have no difficulty in surviving.

In addition to these four required things, there are two which are 'automatically' present, but which can have a major influence. There are (a) temperature and (b) structural supportt0

Temperature. It is impossible not to have temperature, but most plants are best adapted to a fairly narrow range of temperatures. Often those which are best adapted will outgrow and overtop those which are attempting to thrive outside their optimum range. These latter may be starved of light, and the species will not survive in that area.

Structural support on or in the ground is required if a plant is to stay upright. In general there is about as much of the plant's biomass below the ground as there is above. Trees thus need large root systems. This usually means that they require deep soils (as well as lots of water), although many conifers have adapted to shallow dry conditions by developing a single deep root and an extended web of shallow roots covering a large area around the tree.

Support is needed not only in 'normal' conditions, but also in times of stress. When we have a hurricane, for example, more trees are likely to be uprooted if the soil is already wet than if it is dry. A dry soil leads to more snapping damage as the tops are removed by the force of the hurricane winds. the

Swampy soil with trees adapted to survival with capsizing.

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