Measuring vegetation in quadrats
To make measurements per area, you must somehow delineate areas (with just a few exceptions). The simplest way to delineate an area is with a quadrat frame. You then take your measurements within this quadrat. Measurements from multiple quadrats properly distributed through the community will allow you to extrapolate from your quadrats measurements to the whole community.
Measuring biomass per area by the harvest method
Although simple in principle, biomass measurements are tricky in practice. The basic technique for measuring biomass is the harvest method: Simply remove biomass, dry it in an oven to remove water, and weigh it. (Because the water content of plants can vary dramatically day to day, even hour to hour, drying to get "dry weight" is a more stable measure of biomass than is fresh weight.) An important choice is whether to measure total biomass (aboveground and belowground) or just aboveground biomass. Most biomass studies in vegetation science record aboveground biomass, for the straightforward reason that belowground biomass is very difficult to measure.
When measuring biomass per area, harvest all plant material within the boundaries of the quadrat. That is, harvest the parts of plants within the quadrat, even if the plant is rooted outside the quadrat. Similarly, do not harvest parts of plants outside the quadrats, even if the plant is rooted within the quadrat.
Other folks harvest the biomass of anything rooted within the quadrat, but I find it troubling, for several reasons. (a) Some species trail great distances, and it is hard to collect all of their pieces. (b) Many species are rooted in more than one location, making the "rooting" rule inconsistent. (c) The measurements are supposedly "per area" but because some species are large and more likely to extend beyond the quadrat, there is a confounding of area and individual.
Put the harvested material in a carefully labeled bag. The label should include the quadrat identifier, the collection date, and your name. If you are measuring biomass separately for each species, use a separate bag with the species's name on it. Likewise, if you are measuring biomass by plant parts (like stems, leaves, and reproductive structures), have separate, labeled bags for each. If you use grocery-quality paper bags, make sure the flaps are fully glued and won't leak pieces of your hard-won collection. Paper bags are better than plastic bags because paper is less likely to tear and paper bags can go directly into the drying oven. Metal collection tins also work.
Harvesting plants can be messy. Be sure to save any parts, like leaves, that become dislodged during the process.
For measuring dry-weight biomass, dry your samples in an oven hot enough to evaporate the water but not hot enough to break down the plant. Common drying temperatures are 65 C and 100 C. Dry your samples until all the water is evaporated. All the water is evaporated when the weight of the sample no longer changes. Small samples of herbaceous plants will dry to constant weight overnight. Large samples or woody material will take longer.
Bags from the oven can hydrate rather quickly, causing the weight you measure to be too high. Either place the dried bags in a desiccator, or weigh each bag within a minute or two of removing it from the oven.
After weighing the sample, you must account for the weight of the bag. The easiest way to do this is to weigh each bag before placing the biomass sample in it, making sure that the bag is completely dry. You might even want to put your collection bags in the oven before weighing them. If you weigh bags after drying, be careful to remove all the plant material and re-dry the bag. Weighing bags before collection has the advantage of allowing you to keep all your samples in their labeled bags, in case there is a mix up.
Measuring belowground biomass is difficult. One technique is to leave the plant in place and remove the soil around it. This small-scale version of placer mining is most feasible along road cuts or other places where the soil has already been cut away. Another technique is to remove the plant and its surrounding soil from the whole quadrat. Back in the laboratory, you can then carefully wash the soil away from the plant roots. Be careful to use gentle streams of water to wash away the soil so you minimize the loss of fine roots. It is almost impossible to harvest root hairs, because they bind so well to soil particles.
A machine called a hydropneumatic elutriator provides a mechanized way to separate organic material from inorganic soil (Smucker et al. 1982).
Another difficulty with harvesting belowground biomass is that roots are hard to identify to species. As a result, most studies of belowground biomass focus on the biomass of the entire community and not the biomass of individual species. Likewise, productivity and nutrient turnover rates are almost exclusively studied in terms of ecosystems. (An important exception is the measurement of productivity of agricultural crops (including forest plantations), where monocultures make ecosystem measurements and species measurements nearly synonymous.)
Special techniques are required to harvest bryophytes (like mosses and lichens). See McCune (1990) for details.
Measuring density and frequency
If you think that measuring biomass from field quadrats is difficult and time-consuming, you are correct. That is why vegetation scientists have developed other measures of plant abundance.
Density is the number of individuals per area. You measure density within a quadrat by counting the number of individuals and dividing by the quadrat's area. Density can be measured for all species or separated into the density of individual species or species groups. Usually, individual plants are counted only if they are rooted within a quadrat.
As mentioned in the Ecological Background chapter, density has little meaning when individuals of a species can vary greatly in size. Density is little used in vegetation science.
Frequency is the proportion of quadrats in which a species is present. Usually, a species is counted as present if a plant of that species occurs anywhere within the quadrat, whether or not it is rooted within the quadrat. Frequency values can vary from 0% to 100%. Frequency reflects both a species's abundance and how much it is spread over a community.
That Gleasonian bedrock, the Ecological Background chapter, shows has frequency also depends on quadrat size. As a result, frequency cannot be compared from community to community, from study to study, or from year to year unless the quadrat size is the same.
A note about distance measures for estimating density. Several measurement techniques use plant-to-plant or plant-to-random-point distances to estimate plant density. The basis for these techniques is that when plants are more crowded, they are closer to each other. Unfortunately, nearly all of these techniques assume that plants are randomly distributed across the study area. Since this assumption is seldom warranted, estimating plant density from distance measures is unreliable.
cover in quadrats
The most common measure of plant abundance is cover. Remember that cover is the proportion of the ground obscured by a species's aboveground leaves and stems (and flowers). (Cover can also be measured for the whole community by not categorizing plants into species.) Cover is popular because it can be measured quickly yet it reflects a plant's structural importance.
A cranky pronouncement about "percent cover"
Cover is often called "percent cover," as in "I measured percent cover." Cover is the attribute, but percent is the unit. Would you say "meter length"? No! So abandon "percent cover" and just say "cover."
There are several approaches to measuring cover in quadrats. One is the photographic method. When you want to measure cover that is essentially in a single stratum, with little overlap in cover between individuals, you can measure cover by taking a photograph and digitally calculating cover. This technique has been used mostly for measuring the cover of forest canopies, using fish-eye photographs. Recently a digital camera device was developed for agricultural settings, or when you do not need to distinguish one species from another, but it has already been discontinued. The photographic method has also been used with intertidal communities. But when the cover of plants overlaps each other, the photographic method won't work.
The most common way to measure cover is the visual estimation method. Visual estimation is popular because it is fast, requires no specialized equipment, and can be adapted to plants of various growth forms. A disadvantage of visual estimation is its subjectivity, making it hard to maintain consistent and accurate measurements.
Estimating cover can be demanding work. Not only do you have to estimate plant cover, you have to be able to identify plants to species (or to whatever category you are using) and find all cover of each species within the quadrat. You will learn not only the process of visually estimating cover, but some techniques for overcoming the drawbacks of subjective estimation. These tricks of the trade make your work easier and more reliable.
The leaves of different species often overlap. This means that the sum of the individual species cover values can exceed 100%. This also means that you have to look beneath other species when recording cover.
A short glossary of cover terms
You will often hear the term "total cover." Unfortunately, just as often it is unclear what it means! In this course I have tried to use three separate terms for different aspects of "cover."
- Total cover is the cover of all plants, ignoring what leaf belongs to which species. Total cover can take values of 0% to 100%.
- Combined cover is the sum of the cover values for different plant species or groups. For example, you might measure the cover of each species, then add them together to get combined cover. In one of the course projects, you will measure the cover of different plant groups, and later add them together to get combined cover. Because different species or groups can have leaves that overlap, combined cover can take values that exceed 100%.
- Overall cover is cover across the entire study area, usually estimated by the average of individual cover values taken from quadrats, lines, etc. For example, total cover in three quadrats might be 60%, 100%, and 50%, leading you to estimate overall total cover as 70%.
Zone of influence
Vegetation scientists recognize that cover in the strict sense is virtually impossible to estimate visually. Leaves and stems of a plant often do not completely obscure the ground. Although these small gaps between leaves technically should not count towards estimates of cover, their large number and small size makes it impractical to account for. Instead, most vegetation scientists apply the "zone of influence" approach. (Warning: Most students, when they first hear of this approach, think it is pretty flaky. But take my word for itwith the proper care, the approach can work very well.)
What is the zone of influence? It is an imaginary boundary around a plant's crown, filling in minute gaps within the crown and smoothing its boundary. This concept is best understood with a diagram. Estimating the cover of the zone of influence is much easier than estimating the cover trying to account for each leaf, stem, and gap. The trick, of course, is being able to define a zone of influence that is both meaningful and consistent. Zones should fill in small gaps in the crown, but exclude large and important gaps. What makes a gap important depends on the vegetation being studied and the study objectives. Similarly, how much the zone of influence smooths the perimeter of a crown depends on how important irregularities of shape are. In practice, vegetation scientists have their own versions of how to define zones of influence, developed on their own or learned from their professors.
Here are two examples of how I apply the zone of influence approach (the red lines show the zones of influence). Notice that the plant at top is relatively simple in outline, with a few simple leaves clustered together. As a result, the zone and the plant border are nearly identical. The larger plant in the middle (Emmenanthe penduliflora ) is an entirely different story. The many nooks and crannies around the plant's strongly dissected leaves and fan-shape rosette would drive any cover-estimator crazy. Besides, the plant really does have a zone of influence beyond the limits to its leaves. If I were to estimate cover for this plant, I would estimate the cover within the red line. Wouldn't you?
(Photograph courtesy of Brother Alfred Brousseau, St. Mary's College)
What the zone of influence is not. The technique of using the zone of influence is not a short-cut method for estimating the true cover of a plant's foliage. Foliar cover and the zone of influence are two separate ways of measuring a plant's abundance. The cover of the zone of influence will always be larger (or equal to) a plant's true foliar cover.
No matter how you develop your own system of zones of influence, it is imperative that you apply the system consistently throughout a study. You will hear more on this shortly.
Building up vs. dividing down
Sometimes it is easiest to estimate cover by thinking directly what proportion of the quadrat is being covered. This works well when cover is consolidated (not scattered over the quadrat) and cover is between 15% and 85%. The figure shows how this system works. In your mind you divide the quadrat into halves or quarters (or sometimes eighths). In this example, you can see that the plant almost covers one quarter, and the cover that overlaps the edges of the quarter is about the size of the uncovered area within the quarter. Therefore, you can estimate the plant's cover to be around 25%. (Remember, you ignore the parts of the plant outside the quadrat.)