FIELD NOTE
Efficiency of Sampling Tree Regeneration with Two Plot Sizes in Northern Arizona ABSTRACT
Joshua J. Puhlick Two circular plot sizes, 1/300 and 1/100 ac, were compared to determine which plot size was the most efficient for use in ponderosa pine forests on the Coconino National Forest, Arizona. Eighteen harvested stands were selected for sampling, and regeneration plots were installed in each stand. The two plot sizes were overlaid on each plot center and data were recorded separately for each plot size. For each stand the expected total sampling time was calculated and used to determine which plot size required the least sampling effort while providing the same precision of estimated seedling densities. We found that the 1/300-ac plot size was the most efficient plot size for sampling tree regeneration. Keywords: coefficient of variation, sample size, seedling density, Coconino National Forest
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n forest inventory, the most efficient plot size is one that requires the least total sampling effort or cost while providing the same precision of the variable being estimated. The most efficient plot size depends on the average time required to sample a plot and the number of plots required to meet a specified level of precision. In the southwestern United States, 1/300- and 1/100-ac circular plots are commonly used for sampling tree regeneration. These two plot sizes were compared to determine the most efficient plot size for use in the southwestern ponderosa pine (Pinus ponderosa var. scopulorum) forest type. Plot sizes were also compared for use on basalt versus sedimentary parent material.
Methods A stratified random sample of 18 harvested stands on the Coconino National Forest in northern Arizona was selected for sampling. Stands were stratified by parent material. The stands were harvested 12–20 years prior to sampling using the shelterwood with reserves method. Regeneration plots were installed in 4 stands on basalt parent material and 14 stands on sedimentary parent material. Two plot sizes, 1/300 and 1/100 ac, were overlaid on each plot center, and data were recorded for each plot size (Table 1). All tree species were measured on plots according to Forest Inventory and Analysis standards (US Forest Service 2005) (e.g., ponderosa pine seedlings were defined as being at least 6 in. in height and less than 1 in. in dbh). The most efficient plot size was determined as follows (Shiver and Borders 1996): 1.
2.
For seedling densities, we calculated the coefficient of variation (CV) associated with each stand using the seedling means and standard deviations shown in Table 1. Next, we calculated the number of sample plots required to meet an allowable error of 50% using each stand’s respective
Manuscript received November 19, 2010; accepted August 3, 2011.
3.
CV. Our error percentage was based on the variance found in similar stands using a sample size of thirty 1/100-ac plots. To derive the expected total sampling time (TT), each stand’s sample size required to meet an allowable error of 50% was multiplied by the average time required to sample a plot. Stand values were averaged, and the most efficient plot size was the one with the lowest TT. The time required to sample a plot included searching the plot for seedlings and determining each seedling’s diameter at rootcollar, height, and estimated age (age was estimated for ponderosa pine seedlings only). Time to sample the plot was rounded up to the nearest minute.
Results When using data from all stands, the plot size requiring the least amount of sampling time was the 1/300-ac plot size. TT was 129 minutes, compared with 236 minutes for the 1/100-ac plot size. For stands on basalt parent material, the TT for the two plot sizes was nearly identical (219 minutes for the 1/300-ac plot size and 197 minutes for the 1/100-ac plot size). On sedimentary parent material, the 1/300-ac plot size was the most efficient. TT was 117 minutes, compared with 244 minutes for the 1/100-ac plot size. On basalt parent material, the average CV was 210% for the 1/300-ac plot size and 154% for the 1/100-ac plot size. On sedimentary parent material, the average CV was 148% for the 1/300-ac plot size. For stands on sedimentary parent material with ⬍320 seedlings ac⫺1, the CV was 193%, compared with 130% for stands with ⱖ 320 seedlings ac⫺1. Stands with ⬍320 seedlings ac⫺1 are likely to have a nonstocked or poor stocking class rating (Reynolds at al. 1953). A one-way analysis of variance test indicated that there were significant differences between the CV for stands with ⬍320 seedlings ac⫺1 and ⱖ320 seedlings ac⫺1 (P ⫽ 0.001).
http://dx.doi.org/10.5849/wjaf.10-039.
Joshua J. Puhlick (
[email protected]), School of Forestry, Northern Arizona University, P.O. Box 15018, Flagstaff, AZ 86011-5018; present address: School of Forest Resources, University of Maine, 5755 Nutting Hall, Orono, ME 04469. Funding for this project was provided by the USDA National Research Initiative, grant number 2008-35101-19046. I thank Prof. M. Moore (School of Forestry, Northern Arizona University, Flagstaff, AZ) for her support and advice. I also thank the staff and students of the Ecological Restoration Institute, Northern Arizona University, for field and laboratory assistance. Copyright © 2012 by the Society of American Foresters.
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Table 1.
Stand values by parent material and plot size. Sampling time plot⫺1
Stand type
Plots
Mean
SD
Min.
Seedling plot⫺1 Max.
Mean
SD
Min.
Max.
. . . . . . . . . . . . . . .(seconds) . . . . . . . . . . . . . . . Basalt 1/300-ac
1/100-ac
Sedimentary 1/300-ac
1/100-ac
31 37 30 30 31 37 30 30
200 174 60 60 588 519 120 120
165 201 0 0 457 487 39 0
60 60 60 60 120 120 120 120
720 1,200 60 60 1,800 2,820 180 120
0.7 0.6 0.0 0.0 2.1 1.9 0.2 0.0
1.3 1.4 0.0 0.0 2.2 2.6 0.5 0.0
0 0 0 0 0 0 0 0
4 6 0 0 8 10 2 0
28 30 27 12 30 30 30 30 30 30 30 22 30 22 28 30 27 12 30 30 30 30 30 30 30 22 30 22
336 296 258 95 110 680 72 86 176 316 198 155 168 71 842 780 856 200 232 2,087 106 162 440 868 476 415 406 142
569 263 238 74 88 862 29 54 158 377 165 124 139 24 1,061 562 1,043 121 203 2,793 75 127 452 925 343 302 293 97
60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 180 60 120 60 60
3,000 1,140 900 300 240 3,720 180 240 600 1,920 720 600 660 120 5,400 2,220 4,200 480 480 11,280 300 540 1,740 4,680 1,440 1,320 1,320 420
3.5 6.1 6.6 0.5 0.9 18.9 0.4 2.4 3.0 8.0 3.8 2.1 2.6 0.9 9.8 16.4 22.2 1.4 2.2 60.2 1.0 9.8 8.8 23.8 9.4 7.8 10.0 2.8
4.8 6.7 6.8 0.9 2.2 28.3 0.7 3.1 4.1 8.8 6.2 3.1 3.0 1.5 13.5 14.4 21.2 1.7 4.8 87.8 1.6 10.9 12.7 24.5 10.5 9.2 8.2 3.6
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 2 0 0 0 0
21 27 21 3 11 119 2 11 17 31 29 10 14 6 57 48 75 4 24 329 6 34 46 104 50 38 38 10
Min., minimum; max., maximum
Discussion Harvested Stands on Basalt Parent Material Because the total sampling time for the two plot sizes was similar on basalt sites, using either plot size may be appropriate. Complete regeneration failures or low seedling densities were common on basalt parent material. This is related to the greater tendency of basalt-derived soils to frost heave and to retain water from plant roots during drought conditions (Pearson 1950, Heidmann et al. 1982). Frost heaving is a considerable cause of ponderosa pine seedling mortality (Pearson 1950, Heidmann et al. 1982). Complete regeneration failures or low seedling densities contributed to the high variance values computed for basalt sites. Harvested Stands on Sedimentary Parent Material On sedimentary parent material, the lower variance and total sampling time for the 1/300-ac plot size may be due to the high regeneration densities on these sites. In fact, average regeneration densities for all species were greater on sedimentary sites compared with basalt sites. Gambel oak (Quercus gambelii) and New Mexico locust (Robinia neomexicana) occurred on plots with ponderosa pine or in areas where pine was not aggregated. When analysis was conducted with ponderosa pine regeneration only, the CV and TT were still lower for 1/300-ac plots on sedimentary than for 1/300-ac plots on basalt.
Coefficient of Variation and Sample Size In harvest areas with similar attributes, our CV values can be used to determine sample size requirements for measuring regeneration densities. On sedimentary parent material, stands with sparse regeneration (⬍320 seedlings ac⫺1) will require more plots because of the higher CV values associated with these stands.
Literature Cited HEIDMANN, L.J., T.N. JOHNSON JR., Q.W. COLE, AND G. CULLUM. 1982. Establishing natural regeneration of ponderosa pine in central Arizona. J. For. 80:77–79. PEARSON, G.A. 1950. Management of ponderosa pine in the Southwest as developed by research and experimental practice. US For. Serv. Agricultural Monograph No. 6. US Forest Service, Washington, DC. 218 p. REYNOLDS, C., N. JEFFERS, V. BOUSQUET, AND R. STIER. 1953. Regeneration surveys. P. 61– 69 in Reports of the Pacific Northwest Seedling and Planting Committee on various recommended reforestation practices and techniques. Western Forestry and Conservation Association, Portland, OR. SHIVER, B.D., AND B.E. BORDERS. 1996. Sample plot size. P. 60 – 64 in Sampling techniques for forest resource inventory. John Wiley & Sons, Inc., New York. 356 p. US FOREST SERVICE. 2005. Forest Inventory and Analysis national core field guide; Volume 1: Field data collection procedures for phase 2 plots, version 3.0. Available online at www.fia.fs.fed.us/library/field-guides-methods-proc/docs/2006/core_ ver_3-0_10_2005.pdf; last accessed Nov. 11, 2010.
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