This article appeared in the Spring 2021 issue of Northern Woodlands.
My work as a field ecologist has taken me to some rugged and remote places. I’ve kayaked lakes and rivers, trekked through hot, mosquito-infested wetlands, and spent entire seasons above tree line on the windswept peaks of the Northeast. Ask me what my most useful tool is, and you might be surprised. No, not bug netting, not crampons or trekking poles, not GPS or a hand lens or even a field guide. It’s the familiar bundle of plastic and string strapped to the outside of my pack: a quadrat. It just doesn’t feel like field work without one.
A quadrat is a square measuring device typically used for sampling of plants and plant communities. It consists of a frame structure made of simple materials such as PVC pipe, string, or stakes, with sides of a standard length (usually one meter). Often it has other reference marks or subdivisions to assist the user in making visual estimations.
Quadrats were developed in the late 1890s by the pioneering ecologist Frederic Clements who, among other things, became known for his theory of succession. Vegetation communities, he said, are not static, as previously thought, but rather exist in a continual state of change. Species are constantly being replaced by others in a linear trajectory from simple to complex, from bare ground to grasses, shrubs, immature forest, and finally old growth or “climax” community. Given enough time and barring external disturbance, a patch of earth will proceed through these predictable stages. While we now understand much more about succession and Clements’ theories have been replaced by a more nuanced view of vegetation change, his work influenced an entire generation of ecologists, and the humble quadrat remains an essential tool more than a century after Clements first used one.
Quadrats have proven their worth as highly versatile, rugged, and portable tools, and their use is now widespread among field ecologists. Quadrats rely on the simple scientific principle of randomized sampling: place a quadrat over a portion of forest understory or grassland or alpine tundra, and you have a perfectly framed microcosm of that ecosystem that is easily digestible by a single person. A quadrat forces the researcher to look only within the framed area and ignore all else, reducing bias and increasing reproducibility. Within a quadrat, a researcher may count numbers of individual plants (population density), total plant cover (percent cover), cover of layered strata, numbers of species (diversity), or relative abundance of multiple species (species composition). With enough of those snapshots, researchers may assume the footprint within the quadrat represents the sampled area as a whole, without having to actually sample the entire thing. It is a simple way to quantify the bewildering complexity of mother nature.
Depending on the needs of a given study, quadrats can be placed randomly, proportionally according to herbaceous, shrub, and tree cover layers, regularly along a transect or grid, or permanently fixed, to be returned to on a regular basis. Because quadrats are a standard size, comparisons can easily be made to other studies, other ecosystems, or – in the case of permanent quadrats – previous visits.
Most cash-strapped field ecologists (i.e., graduate students) prefer to make their own field gear whenever possible, and I was certainly no exception. For my study of how snowmelt affects alpine plant phenology and growth on New Hampshire’s Mount Washington, I knew I would be spending a lot of time stooping over a quadrat counting plants, estimating percent cover, and carefully watching and recording the flowering of a few focal species. And since Mount Washington proudly claims to have the “worst weather in the world,” my quadrats would have to be sturdy. I decided to build them with a half-inch PVC frame with three-way adapters at the corners, allowing for legs to be inserted as a stand. I drilled small holes every 10 centimeters along the frame and threaded it with nylon string, creating a grid of 100 squares, each representing one percent of the total area. I made two quadrats, which were collapsible and rolled up neatly into a bundle that I could tie down to the outside of my hiking pack. The two seasons I spent doing that research went off without a hitch, and both quadrats held up superbly. I still have them today.
The fact that one tool is so widely employed is testament to its simplicity, ease of use, and elegance as a scientific instrument. They may not be the flashiest, but unlike more complicated scientific tools, quadrats have virtually no learning curve, and they are nearly impossible to break. Their utility has also been proven far beyond just plants. Scientists have used quadrats in studies of small or slow-moving animals (such as insects), counting artifacts at archaeological digs, and even monitoring changes in coral reefs. Some researchers have also expanded the concept of the quadrat to include “virtual” quadrats – photographs taken downward from a standard distance and focal length, allowing analysis by computer software programs. I wonder what Clements would think of that!
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