Plant Inventory

Part of the Biodiversity Counts Curriculum Collection.

Is doing a plant inventory like looking at the trees, not the forest? Is it a matter of counting blades of grass? Or is it more like looking for a needle in a haystack? How, when it comes down to it, do working scientists actually inventory a site? We asked a botanist who specializes in rainforest trees and an ecologist whose work often involves looking at entire habitats how they go about the task.

Brian Boom, the tree specialist, who is now Vice President for Botanical Science and the Pfizer Curator of Botany at the New York Botanical Garden, told us, “I usually study trees in tropical lowland forest areas, but most of the principles apply anywhere.” Brian works in hectares—10,000-square-meter units—and you will be working in 4-square-meter units, but in either case, the first step is to mark out the plot. “Your plot can be an absolute square or a longer, narrower shape,” he said. The shape of the plot might be determined by what the scientist wants to find out or by what the terrain requires. For example, Brian said, “You will pick up more diversity with a long, narrow plot because you are going through more microhabitats.” On the other hand, in some areas, such as a mountain summit, “I'd fall off the edge if I didn’t measure my plot with an eye toward the topography.” 

After you have measured the plot, mark the perimeter with colored flagging. You can write on the flags with a permanent marker. Brian said that biodegradable flagging is available from forestry supply houses. If you use flagging that is not biodegradable, be sure to remove it when your study is over. “Then I start systematically going through the site, dividing it into subplots and working my way through, measuring, collecting, and taking observations of each plant that meets my size class criteria.” 

Dividing a plot into smaller subplots is an excellent strategy, especially if the plants you are inventorying are smaller than trees. Liz Johnson, Manager of Metropolitan Biodiversity Programs at the American Museum of Natural History’s Center for Biodiversity and Conservation, typically inventories 300-acre plots, so subdividing is absolutely necessary. 

She told us about trying to count individual plants of a species of ground cover that grew in a twisted mat throughout a site. “It was so dense and had so many offshoots that it was really impossible to do a count. Instead, we made a grid within the plot and tried to count the number of individual plants within a single square.” She suggested getting down on your hands and knees and looking for the place where the plant comes out of the ground. Then multiply the number of plants within a single square by the number of squares in the entire plot that are filled by the plant. ”Another way is to estimate percent of coverage. This works for grasses, some ferns, and other ground cover plants.” 

The grid also helps you move through your plot in a systematic way, Liz said. “You just have to start at a particular point and count.” Each team member could take one or several subplots, or each team member could take certain types of plant or plants of a particular height, and work through the entire plot counting only those plants. “Concentrate on what you’re doing and try not to lose track,” she advised.

Liz agrees that counting plants can seem impossible, particularly if you have limited time. The best thing, she said, is to break the job into parts and then set priorities. “Think about how much time you have for each session and how frequently you can come back, and then decide what to do first, second, and so on. You may have to work very fast if you have only one or two sessions, or if the season is changing and you have to do the inventory before certain things disappear.” Otherwise, start with the most important things and then, if you have time or can make another visit, do as much of the rest as you can manage.