Forest-Based Biomass Basic Computations - Mathematics and Statistics for the Environment - page 325

13.1.1.1 Necessary Terms and Concepts
Inventory —The systematic acquisition and analysis of information necessary to describe,
characterize, or quantify vegetation. As might be expected, data for many different
vegetation attributes can be collected. Inventories can be used not only for mapping
and describing ecological sites but also for determining ecological status, assessing the
distribution and abundance of species, and establishing baseline data for monitoring
studies.
Population —A population (used here in the structural, not biological, sense) is a complete
collection of objects (usually called units ) about which one wishes to make statistical infer-
ences. Population units can be individual plants, points, plots, quadrats, or transects.
Sample —A set of units selected from a population used to estimate something about the
population (a procedure that statisticians refer to as making inferences about the popula-
tion). In order to properly make inferences about a population, the units must be selected
using some random procedure. The units selected are called sampling units .
Sampling —A means by which inferences about a plant community can be made based on
information from an examination of a small proportion of that community. The most com-
plete way to determine the characteristics of a population is to conduct a complete enumer-
ation (or census). In a census, each individual unit in the population is sampled to provide
the data for the aggregate. This process is both time consuming and costly, and it may
also result in inaccurate values when individual sampling units are difficult to identify;
therefore, the best way to collect vegetation data is to sample a small subset of the popula-
tion. If the population is uniform, sampling can be conducted anywhere in the population,
but most vegetation populations are not uniform. It is important that data be collected that
can ensure that the sample represents the entire population. Sample design is an important
consideration in collected representative data.
Sampling unit —One of a set of objects in a sample that is drawn to make inferences about
a population of those same objects. A collection of sampling units is a sample. Sampling
units can be individual plants, points, plot, quadrats, or transects.
Shrub characterization —This topic is addressed here because it is not covered in most of
the techniques in this text. Shrub characterization is the collection of data on the shrub
and tree component of a vegetation community. Attributes that could be important for
shrub characterization are height, volume, foliage density, crown diameter, form class, age
class, and total number of plants by species (density). Another important feature of shrub
characterization is the collection of data on a vertical as well as horizontal plane. Canopy
layering is almost as important. The occurrence of individual species and the extent of
canopy cover of each species are recorded in layers. The number of layers chosen should
represent the herbaceous layer, the shrub layer, and the tree layers, though additional layers
can be added if needed.
Tren d —Refers to the direction of change. Vegetation data are collected at different points
in time on the same site, and the results are then compared to detect a change. Trend can
be described as moving toward meeting objectives , moving away from meeting objec-
tives , not apparent , or state . Trend data are important in determining the effectiveness
of on-the-ground management actions. Trend data indicate whether rangeland is moving
toward or away from specific objectives. The trend of a rangeland area may be judged
by noting changes in vegetation attributes such as species composition, density, cover,
production, and frequency. Trend data, along with actual use, authorized use, estimated
use, utilization, climate, and other relevant data, are considered when evaluating activity
plans.