Spatial pattern corrections and sample sizes for forest density estimates of historical tree surveys |
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| Authors: | Brice B Hanberry Shawn Fraver Hong S He Jian Yang Dan C Dey Brian J Palik |
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| Institution: | (1) Department of Forestry, University of Missouri, 203 Natural Resources Building, Columbia, MO 65211, USA;(2) USDA Forest Service, Northern Research Station, Grand Rapids, MN 55744, USA;(3) Institute of Applied Ecology, Chinese Academy of Sciences, 110016 Shenyang, China;(4) USDA Forest Service, Northern Research Station, University of Missouri, 202 Natural Resources Building, Columbia, MO 65211, USA |
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| Abstract: | The U.S. General Land Office land surveys document trees present during European settlement. However, use of these surveys
for calculating historical forest density and other derived metrics is limited by uncertainty about the performance of plotless
density estimators under a range of conditions. Therefore, we tested two plotless density estimators, developed by Morisita
and Pollard, for two, three, and four trees per survey point under simulated ranges of tree densities, non-uniform densities,
and different tree spatial distributions. Based on these results, we developed estimator corrections and determined number
of survey points needed for reliable density estimates. The Morisita estimator was accurate for densities ranging from 5 to
1,000 trees per unit area, non-uniform densities, random and regular spatial distribution, and outperformed the Pollard estimator.
Estimators using points with two or three trees did need a simple correction to account for overestimation. Likewise, for
clustered distributions, depending on the number of trees per survey point and the amount of clustering, there should be adjustment
for a range of under and overestimation. Sample sizes for survey points with three or four trees should be at least 200 survey
points, and 1,000 survey points will have density estimates within ±10% tolerance range of actual density. For survey points
with two trees, the minimum sample size should be 600 survey points, and 2,000 survey points should be the target value. These
results provide guidelines for researchers to improve density estimates of historical forests. |
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