There are usually several such plots on one site, some with elevated CO2 and a number of control plots in which CO2 remains at normal background levels. (carbonbrief.org)
They then tracked bird populations in each experimental and control plot using point - count surveys, in which researchers look and listen for birds for a set amount of time. (sciencedaily.com)
«Everything used to look like that over there,» she says, pointing to a distant control plot with visible, thick crusts. (discovermagazine.com)