Selecting plot sizes when quantifying growing conditions in understories

Abstract

While the importance of selecting a neighborhood size for competition studies has been documented, the choice of plot sizes has received little attention when measuring the influence of overstory trees on growing conditions of understory seedlings. Based on data from four sites, we show how plot sizes for measures of overstory cover (using a "cone" approach) and basal area (using an angle gauge) were related. The relationship was a function of the height/diameter ratio of the overstory trees, with taller trees increasing the plot sizes for "cone" plots and trees with larger diameters increasing the plot sizes for basal area plots. Further, we point out that data derived from measurements of diffuse noninterceptance (DIFN) using the "cone" approach varied with the choice of cone opening. Smaller cone openings led to a greater range in DIFN values. Linear correlations between these data and 2 yr seedling height pointed out that the optimal plot size, i.e., cone opening, differs among the sites. The lack of any obvious stand characteristics that would explain these differences indicated the need for further investigations.