THE EFFECTS OF FIXED-AREA PLOT WIDTH ON FOREST CANOPY HEIGHT SIMULATION

Mapped-stand data which includes tree location and mensuration data ma y be used to simulate the height characteristics of a forest canopy in a computer. The dimensions of the rectangular, fixed-area ground plot which is used to collect the tree location and mensuration data can p rofound ly affect the accuracy of the resultant computer reconstructio n of forest cane py heights. Thin fixed-area plots result in forest ca nopy height constructs (i.e., canopy height models, or CHMs) which are shorter and less dense than the actual forest stand. Ninety-nine 0.40 5 ha forest stands in the southeastern and south central United States were analyzed. For each stand, a top-of-canopy height model was gener ated for the entire stand on a 0.25 m x 0.25 m grid. The mean canopy h eight and the canopy density of the simulated stand was calculated. A Monte Carlo simulator established randomly located sample plots of var ying plot width (1 m to 20 m) at three different sampling intensities (1%, 5%, and 10% of stand area) in the mapped stand. The trees selecte d on any particular plot were used to generate a CHM, and the sample m ean canopy height and canopy density were compared with the correspond ing stand values. Samples collected using 1 m wide plots resulted in h eight models which underestimated average canopy height and canopy clo sure with deviations greater than 5% of the stand value, 95-100% of th e time. The underestimation results from the construction of top-of-ca nopy architecture based on measurements of crowns belonging on ly to t rees whose boles are with in the plot. This construction technique omi ts crowns extending over the plot belonging to tree boles located off the plot. Thinner plots tend to miss more of the trees which support t he canopy over the plot. For the 99 stands considered in this study, g round plot widths on the order of 6 to 8 m were required in order to a cquire ground data which could be used to accurately model forest cano py height characteristics. The minimum ground plot width required to p roduce an accurate height model decreased with increasing stem density ; a plot width of 4 m, for instance, may be used at stem densities of similar to 2000 stems ha(-1).