Sampling efficiencies of new and existing variable radius plot growth estimators
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Variable Radius Plot (VRP) sampling is a unique invention developed by Forest Biometricians. Its development over a half-century ago was revolutionary to forest mensuration in that it allows for the estimation of basal area density—a vital forest management parameter—without the need for even approaching individual trees. Being based on basic geometric properties, it extends naturally to the appraisement of other, closely related values, such as biomass, as a probability proportional to size technique. Even with its vast use as a method of current inventory assessments, its utility for estimation of growth values has been in question. The very attributes that make this method desirable for one-time estimates detracts from its wide-usage as a technique for growth estimation. Several alternatives have been developed that maintain some of the pleasing properties of VRP sampling, each with their relative advantages and disadvantages, but no clear favorite has emerged. We present here methods that increase the sampling efficiency of VRP growth estimation without the disadvantages of past alternatives. We empirically compare the various estimators for VRP growth estimation from permanent sampling positions in a simulation study. We found the gains with the new estimators to be as much as a 90 percent reduction in root mean squared error in some cases.