One of the most visible features on the earth's surface, perhaps even more visible than landforms themselves, is vegetation. With the exception of urgs or glaciers, every part of the earth is vegetated to one extent or another. Accordingly, in almost any type of geographical investigation, fieldworkers will need to know something about measuring and describing vegetation communities and associations. This is particularly crucial for comprehensive ecological assessments and the preparation of environment impact statements [example].
Collecting specimens. Where a high degree of expertise in needed, but unavailable, it is important to collect plant samples which can be later identified by a "systematist" and curated in a herbarium. Collecting such samples involves the use of a plant press. As much of the plant as possible should be dried in a plant press. At the very least a segment containing fruit, flower, and leaves--all attached--should be collected. Each sample should include a lable noting details of the plant's location, date of collection, and name of collector. Each sample should be laid out on newspaper between two driers (things that look like large old-fashioned ink blotters) and separated by two spacers (two sheets of corrogated cardboard). It should then be placed between the frames of the press and squeezed tight with the straps. In some cases where a press is not available or a sample has to be taken quickly and there is not ample time for pressing, plants can be placed in a vasculum, a belt-mounted canister which keeps the sample from wilting. Any botanist worth his or her salt knows that a garbage bag with its inside moistened works just as well as a vasculum.
Keying specimens. There are, of course, those cases where plant specimens need not be collected, but can be identified in the field. If the field worker is familiar with the vegetation in the particular area under investigation, she or he should have little difficulty. If one is not intimately familiar with the region's vegetation, individual plants can be identifed according to common name, as well as by genus and species with the use of one or plant keys, books which are designed specifically for identifying plants. Keys can be very large, detailed, and scientific (e.g., Kearny and Peebles, Arizona Flora) or small, compact, and simple guides (e.g., Dodge and Janish, Flowers of the Southwest Deserts).
In other cases, however, the vegetation might appear to be homogeneous when in fact it is not. For example, an area might appear to be juniper woodland, but in reality there could well be differences in various factors from one part of the area to another. Factors to be considered include among other things frequency, density, dominance or cover, height, and vigor. All of these can be measured in absolute or relative terms. The former involves all plants, the later only one species in relation to all others.
Frequency is the number of times a particular species is recorded in a sample area or transect.
Density is the total number of plants divided by the size of sample area or the length of the sample transect (to be discussed later).
Dominance or cover can be measured in one of two ways--the crown or canopy [example], or the shoot or basal area [example]. Regardless, it is always measured as a percent of the sample area or transect.
Height can be measured directly in the case of plants less than 2 meters tall, estimated for those that are somewhat higher, and calculated by means of a clinometer for those that are very tall [example]. Height of course reveals something about the age of a plant. It can also say something about disturbance and recolonization. For example, a stand comprised of a single species of tree, all of the same height [example] could indicate vegetation removal and subsequent invasion [example]. And, one should not forget to measure the height of the canopy [example] as it informs of the types of plants growing under the trees.
Health or vigor is a qualitative estimation about the nature of a plant's, a specie's, or a community's growth. Sick or weak plants look different than healthy plants, and dead ones are certainly obvious. Patterns of young, old, and dead plants can reveal if a community is expanding, shrinking, or migrating. For example, picture an isolated stand of mature trees in an otherwise grassy plain. If the stand has several younger trees along one side [example] and a number scattered saplings further away [example], one should conclude that the stand is either expanding or migrating, depending on the condition of trees on the opposite side of the stand. If a number of dead trees are found on one edge, then one should conclude that the stand is either shrinking or migrating, again depending on the condition of trees on the other side of the stand.
Releve analysis involves evaluating the vegetation of sample areas. The sizes of these area are critical and variable, depending principally on the nature of the vegetation. For temperate-zone vegetation, the following values can be used as guidelines.
| Forests | 200-500 square meters | ||
| Dry-grassland | 50-100 square meters | ||
| Dwarf-shrub | 10-25 square meters | ||
| Grass | 1-10 square meters [example] |
The best way for determining the sizes of the areas to be sampled is by means of the phenomena-area (or species-area) curve as discussed in the lecture on Sampling Procedures. Sample areas can be selected randomly [example] or systematically [example].
Studies have shown that rectangular sample areas are more accurate than square or circular ones because of the general tendency of vegetation to "clump." Rectangles should be laid-out up and down slope rather than across slope [example].
Once the minimum size and shape of sample plots has been determined, it is necessary to determine how many need to be used. Typically, 5 % of the area is sampled. Any number of techniques, as previously discussed, can be used in selecting the sample areas.
Line Intersect analysis is tantamount to evaluting plants that are located on, or overlap, a baseline. A number of transect lines are laid out, usually with compass and measuring tape, in some predetermined design [example]. Plants of which any part of lie over the transect line are recorded in reference to increments of the line. However, a plant is not tallied if the center of its trunk is past the end of the transect line [example].
The length of transects can vary, depending on stand conditions. For stands of uniform height vegetation, 30 meters is usually adequate [example]. In dense stands, 20 meters or less may be more convenient and adequate [example]. In more open and mixed (trees and grass) stands, it is typically best to use 50 meters or more [example].
One problem is the recognition of individual plants. Some plants have more than one stem while some single-stem plants clump together [example]. Are eight trunks in a live oak motte, one tree or eight trees? A second problem involves crowns that intersect. This is especially problematical in dense stands. How does one determine relative dominance or cover in such situations? A third problem involves individuals that lie on the margins or edges of sample areas. If a plant lies on the line, is it included or excluded?
These problems, and others, require professional judgement in order to be resolved. For example, in the case of marginal plants, one might choose to include those on two connected sides (e.g., the north and east), and exclude those on the opposite sides (e.g., south and west). Whatever strategies are used in making such decisions, they must be applied consistently.
Created by William E. Doolittle. Last revised 19 April 2017, wed