Agenda - Environmental Policy Board - 07/19/2021

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FOREST SAMPLING METHODS 53 moderate stem densities, and the FRP-HA produced accurate (x2 = 0.80, P < 0.05) results in the low density stand (Fig. 2). The QUAD technique produced estimates within 26 stems of the actual, across all stands (Fig. 2). The PCQ and VRP methods under- estimated stem densities in all three stands, but the difference was proportional and constant across all stand densities (Fig. 2). Sampling Time: The FRP-AC was the fastest method applied, closely followed by the FRP-HA and QUAD methods for small diameter stems (Table 1). The BT was much slower because of setup time needed to apply this method. The PCQ method was not directly comparable to other methods because we used it for estimating larger diameter trees. It performed more time efficiently in stands with low density, but was much slower in stands with high densities (Table 1). The VRP method was the most rapid of all techniques to sample all stem size -classes. The BT method was the slowest technique, taking more than twice the time of other methods (Table 1). DISCUSSION Sampling Accuracy: Vegetation is often clumped or patchy in distribution (5). The high standard deviation we found as- sociated with mean estimates derived from these sampling methods reflects the clump- ed and patchy distribution of trees within these stands. Four of the seven accurate estimates were from sampling techniques applied in the high density stand. Density estimates from the various sampling tech- niques in the high density stand were generally more accurate because our ob- servations were that stems tended to be more evenly distributed. Stems in the moderate and low density stands tended to be clumped and patchy in distribution in violation of the statistical assumption that stems were randomly distributed. In situations with nonrandom distributions, sampling intensity must be increased until the variance is lowered rather than going with a set 10% minimum level sample. VRPs and FRPs are unbiased for all distributions but distance measures are not (6,15,16). Any biases found in estimates may be related to field technique or may be attributable to stand conditions such as poor visibility in dense stands and bias arising when determining if a tree should be tallied when using the VRP method. This is particularly problematic when making determinations with a prism for small diameter stems close to the sampling point in dense stands (6). This was apparent in widely varying estimates of stems <11.43 cm derived by the VRP method in different density stands. However, we found that the VRP consistently underestimated density for larger stems. FRPs (circular) were indeed accurate and unbiased at high stem densities, but variation increased between samples when applied to the lower density stands. Under these conditions sampling intensity must be increased and pre -sampling with attention to variance estimates should increase confidence in the estimates. Sampling and Time Efficiency: The techniques that performed best for sampling stems <11.43 cm across different density stands included the FRP methods of dif- ferent size. The FRP-AC was relatively time - efficient and produced excellent results regardless of stem size in the high density stand. However, it produced estimates of poor accuracy in our sparse stand. An advantage of this method is its ability to be applied to all stems regardless of size. The only problem with the method is that it requires more samples to obtain the same sampling intensity as other methods. The FRP-HA has the same advantages and produced similar results to those of the FRP- AC method, but has the additional ad- vantage of sampling a larger area, therefore, requiring fewer samples than the FRP-AC to obtain the same sampling intensity. All methods used to sample stems >11.43 cm produced relatively accurate estimates in the high density stand and may be attributed to the relatively high stem density and even distribution of stems in this stand. The QUAD method consistently produced the most efficient results for sampling larger stems (>11.43 cm), except for the low density stand. The QUAD Proc. Okla. Acad. Sci. 82:49-56(2002)