Environmental Engineering Reference
In-Depth Information
Background
In this lab, we will assess a forest stand containing interacting populations of trees,
which form a community, in order to determine how it will change in the future.
This skill is important to many natural resource agencies, which need to predict the
future composition of the land.
A population is a group of individuals of the same species in the same place at
the same time. At any moment in time, a population has the attributes of
population size and spatial distribution.
A community contains interacting species in the same place at the same time.
The species composition of communities can change over time - a process called
succession.
One of the fundamental parameters of interest to ecologists is the density of
organisms in a given area. However, in nature it is either impossible or impractical
to count all organisms, and so we
estimate
density. For relatively small, immobile
organisms, quadrat sampling is used to estimate density. For large, immobile
organisms, remote-sensing, plot-based, or plotless techniques can be used. For
mobile organisms, ecologists use mark-recapture techniques.
Factors controlled by the investigator that can affect the density estimate:
the experience of the observer
method of observation (instrument or chosen sampling technique)
the number of samples taken
Factors beyond the control of the investigator:
organism density
organism spatial arrangement
Plot-based techniques frequently rely upon frames to isolate a sample area.
These frames are called quadrats: arbitrarily-sized and -shaped sampling units.
There are alternative techniques that are especially useful for large plants
(trees). These are commonly called plotless samplingmethods. During this laboratory,
you will use the plotless Point Quarter Method (PQM) to estimate tree density and
basal area
Regular Sampling Scheme
It takes time to establish a random grid and locate plots on it. Although totally
random plot placement is the statistical “gold standard,” it may be infeasible due to
resource constraints. In addition, sometimes you want to ensure an even distribution
of plots across a site, in which case totally random sampling may not be appropriate.
Regular sampling consists of using a set spacing between plots. Like random
sampling, it typically precludes intentional and unintentional observer bias.
