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Communication

Accuracy and Precision of Commercial Thinning to Achieve Wildlife Management Objectives in Production Forests

by
Kent Keene
1,
William Gulsby
1,*,
Allison Colter
2,
Darren Miller
3,
Kristina Johannsen
4,
Karl Miller
2 and
James Martin
2
1
School of Forestry and Wildlife Sciences, Auburn University, Auburn, AL 36849, USA
2
Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA 30602, USA
3
Weyerhaeuser Company, Columbus, MS 39701, USA
4
Georgia Department of Natural Resources, Wildlife Resources Division, Social Circle, GA 30025, USA
*
Author to whom correspondence should be addressed.
Forests 2021, 12(4), 411; https://doi.org/10.3390/f12040411
Submission received: 6 January 2021 / Revised: 26 January 2021 / Accepted: 26 March 2021 / Published: 30 March 2021
(This article belongs to the Special Issue Using Forest Management to Achieve Wildlife Conservation on Production Forest Landscapes)
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Abstract

:
Tree stocking and the associated canopy closure in production forests is often greater than optimal for wildlife that require an open canopy and the associated understory plant community. Although mid-rotation treatments such as thinning can reduce canopy closure and return sunlight to the forest floor, stimulating understory vegetation, wildlife-focused thinning prescriptions often involve thinning stands to lower tree densities than are typically prescribed for commercial logging operations. Therefore, we quantified the accuracy and precision with which commercial logging crews thinned pre-marked and unmarked mid-rotation loblolly pine (Pinus taeda) stands to residual basal areas of 9 (low), 14 (medium), and 18 (high) m2/ha. Following harvest, observed basal areas were 3.36, 1.58, and 0.6 m2/ha below target basal areas for the high, medium, and low basal area treatments, respectively. Pre-marking stands increased precision, but not accuracy, of thinning operations. We believe the thinning outcomes we observed are sufficient to achieve wildlife objectives in production forests, and that the added expense associated with pre-marking stands to achieve wildlife objectives in production forests depends on focal wildlife species and management objectives.

1. Introduction

The southeastern United States produces more lumber than any other region in the country [1]. Most of that lumber is generated in loblolly pine (Pinus taeda) plantations, and coverage of planted pine is expected to increase from 19% to 24–36% over the next 40 years [2]. Therefore, the extensive coverage of these production forests and their management have significant implications for a variety of wildlife species, especially given that the Southeast is a hotspot for biological diversity [3,4]. However, many endemic wildlife species within the region are in decline due to the loss of plant communities associated with pine (Pinus spp.) woodlands and savannas (commonly referred to as open-pine systems) [5]. Some examples include northern bobwhite (Colinus virginianus), gopher tortoise (Gopherus polyphemus), and red-cockaded woodpecker (Leuconotopicus borealis). Historically, much of the southeastern U.S. was comprised of mature pine savannas maintained with frequent, low-intensity fires that perpetuated a park-like understory comprised of early successional plants important to these species [6,7,8]. Today, most of these savannas have been converted to agriculture or production-focused, short-rotation slash (P. elliottii) or loblolly pine stands [6,9].
The structural characteristics of planted pine stands are much different from the historic stands they have replaced. For instance, intensive management can result in canopy closure as early as 4–7 years post-planting in loblolly pine stands [10]. At canopy closure, understory vegetation is shaded out, save for some shade-tolerate species [11]. Additionally, when planted pine stands are managed to maximize timber volume, or a combination of timber revenue with wildlife considerations, tree stocking may still be greater than optimal for wildlife that require an open canopy and the associated understory plant community [12,13]. Fortunately, these plant and animal species are not inherently precluded from planted loblolly pine forests [14,15], and various mid-rotation treatments, including thinning, enhance coverage of herbaceous and woody understory plants [16,17,18,19].
Thinning recommendations have been developed for a variety of species including gopher tortoises, northern bobwhites, red-cockaded woodpeckers, and others [20,21,22]. For example, pine stands managed for northern bobwhite are generally thinned to basal areas between 9–14 m2/ha, with some recommending basal areas as low as 7 m2/ha [23]. Similarly, [21] recommend maintaining mature pine stands at low to moderate basal areas to maximize habitat quality for red-cockaded woodpeckers. Though there are relatively few mature pine stands remaining, artificial nest boxes allow for occupancy of red-cockaded woodpeckers in younger pine stands, provided that trees are of adequate size [24].
However, many of these wildlife-focused recommendations involve thinning stands to lower tree densities than are typically prescribed for commercial logging operations [25]. If logging crews are unable to consistently achieve these target basal areas, efforts to create, maintain, or enhance conditions for open-pine dependent species could be impeded. In addition, although pre-thin marking operations can improve thinning accuracy and precision, such practices add considerable costs [26]. Therefore, we evaluated the accuracy and precision with which commercial logging crews were able to thin to three target basal areas via two methods within mid-rotation loblolly pine stands. We predicted that accuracy and precision of thinning operations would decrease as the target basal area decreased, and that accuracy and precision would be greater for pre-marked stands versus those harvested via operator-select thinning.

2. Materials and Methods

Our study sites were in the Piedmont physiographic region of Georgia within 5, 15–20-year-old, unthinned loblolly pine stands (Figure 1). All stands had ≥1 loblolly pine rotation prior to establishment of the current stand and ranged from 36–53 ha in size. Three of the stands were located in Hancock County, Georgia and were owned and managed by Weyerhaeuser Company. The remaining two were located on Oconee Wildlife Management Area (WMA) in Greene County, GA, and managed by the Georgia Department of Natural Resources’ Wildlife Resources Division. The climate in the region was subtropical, with a mean annual temperature of 16.7 °C, and mean annual precipitation of 117 cm [27]. The topography across the region primarily consisted of rolling hills and elevation ranged from a minimum of 134 m to a maximum of 195 m [28].
Two of the Weyerhaeuser stands contained moderately eroded, well-drained soils comprised primarily of Lloyd gravelly loam and Cataula-cecil complex [29]. The third Weyerhaeuser stand was comprised of well- to excessively well-drained soils predominantly consisting of Lakeland sand, Valcluse-Norfolk complex, Fuquay loamy sand, and Ailey-Vaucluse-Lucy complex [29]. Soils in the stand on the northern portion of Oconee WMA were moderately eroded, well-drained, and predominantly consisted of Lloyd gravelly loam and cecil gravelly loam [29]. Soils in the stand on the southern portion of Oconee WMA were moderately to severely eroded, well-drained, and predominantly consisted of Lloyd gravelly lam, Pacolet sandy loam, and cecil-Cataula complex [29].
We divided each stand approximately into thirds, resulting in 3, 11–21 ha plots per stand, and randomly prescribed a thinning treatment (i.e., target basal area) to each plot for 5 replicates per treatment. Thinning treatments included post-thin (residual) basal areas of 9 m2/ha (low), 14 m2/ha (medium), and 18 m2/ha (high). The high residual basal area treatment represented the maximum retained volume while minimizing density-related mortality following a first thinning in loblolly pine stands managed primarily for timber production. In contrast, low and medium basal area treatments represented management alternatives landowners might employ in stands where enhancing understory plant abundance and diversity is a primary or competing objective to timber production. Prior to thinning, all trees that were to be retained within Weyerhaeuser-owned stands (n = 3) were marked by commercial logging crews. In contrast, Oconee WMA stands (n = 2) were harvested via operator-select thinning after loggers thinned pre-marked “model” patches to obtain a visual reference for each target basal area. Thinning operations were implemented by commercial logging crews during February–June 2017.
Following thinning, we conducted a 5% timber inventory within each treatment unit. We systematically distributed 0.04-ha fixed-radius plots at a density of 1/0.8 ha throughout each unit (n = 278). Within each plot, we measured the diameter at breast height (DBH; cm) of all loblolly pine trees with a DBH ≥ 11.94 cm (i.e., merchantable diameter class), and used these data to calculate loblolly pine basal area for each plot. We calculated accuracy of thinning operations by subtracting expected (i.e., target) basal area from observed basal area for each fixed-radius plot. We also calculated the coefficient of variation for basal area in each treatment unit as a measure of thinning operation precision.
We used mixed-effects analysis of variance (ANOVAs) in the R software package “nlme” [30,31] to test for effects of target basal area and thinning method (pre-marked vs. operator-select) on accuracy and precision of thinning operations. We included research block, treatment unit, and fixed-radius plot as random effects in the accuracy analysis to account for the data structure associated with our study design. We set α = 0.05 for all tests.

3. Results

Across stands, mean post-thinning basal areas were 9.50, 13.21, and 16.04 m2/ha for the low, medium, and high residual basal area units, respectively (Figure 2). Thinning accuracy was greater for medium and low basal area units than for high basal area units. Specifically, model results indicated that observed basal areas were 3.36, 1.58, and 0.6 m2/ha below target basal areas for the high, medium, and low basal area units, respectively. Pre-marking stands did not increase thinning accuracy (Table 1).
Precision of thinning operations was relatively low but did not differ statistically across basal areas. Coefficient of variation estimates were 29.24, 29.42, and 38.52% for high, medium, and low basal area units, respectively. Finally, the coefficient of variation for pre-marked stands was about 12% lower (i.e., more precise) than for operator-select harvested stands (Table 2).

4. Discussion

Although thinning accuracy differed among treatments, the greatest difference between observed and expected residual basal areas was for the high basal area treatment, where the observed basal area was, on average, 3.36 m2/ha below the target (18 m2/ha). Using the equation provided by [32], this would equate to a relatively small (12%) increase in light intensity than expected at the target basal area. Using the same equation, light intensity was 5% and 3% greater than that associated with the target basal areas in the medium and low basal area treatments, respectively. We consider these differences acceptable given the variability in basal area recommendations (and corresponding light intensities) for open pine focal species. For example, common basal area recommendations for northern bobwhite range from 7–14 m2/ha [23]. Similarly, red-cockaded woodpecker management guidelines typically recommend basal areas ranging from 9–18 m2/ha, depending on whether the areas are designated for clusters or foraging [33,34]. Basal area recommendations for gopher tortoise are generally ≤7 m2/ha [20]. The Integrated Science Agenda (ISA) developed by the Gulf Coastal Plain and Ozarks Landscape Conservation Cooperative (GCPO LCC) proposed a pine basal area of 9.2–16.1 m2/ha to support indicator species in mature open pine systems [35]. However, [36] surveyed conditions associated with open pine indicator species in southwest Georgia and found that species presence was robust to minor deviation from basal area recommendations proposed in the ISA. Further, [37] found that mid-rotation thinning can provide ephemeral conditions appropriate for open canopy species, even in stands managed for economic return (i.e., thinned less intensively). Therefore, wildlife-focused thinning prescriptions appear to be robust to minor variation in accuracy.
Although we observed an increase in basal area precision within plots pre-marked for thinning, an important caveat is that ownership differed between harvest methods, so we cannot be certain whether this factor confounded our results. Nonetheless, the implications of this finding depend on management objectives. Pre-marking cost $105/ha in the current study, and averages USD 83/ha across Alabama, a major timber producing state in the region [26]. Therefore, marking timber prior to thinning adds considerable costs. However, precise spacing among trees maintains consistent sunlight availability and ensures more even coverage of understory plants, which is desirable when even coverage of plant species with similar light requirements is an objective.
Conversely, decreased precision increases variation in light availability and plant species composition along the forest floor, which may affect wildlife diversity. Such variation in light availability may also benefit ectothermic reptiles and amphibians by providing access to solar radiation during cool seasons, and shade during warm seasons [38]. Some have even developed silvicultural prescriptions intentionally focused on increasing heterogeneity in tree density throughout the stand to recreate conditions thought to be present in fire-adapted ecosystems pre-Euro-American settlement [39]. However, precise, accurate harvest would still be desirable within each patch of the stand, although stand-level tree density would be more variable.
Regardless of the desired outcome, our results suggest that commercial thinning can be used to achieve wildlife objectives in production forests, including when thinning prescriptions call for atypical reductions in basal area. This is especially important given that recreating effects of large-scale fires is unrealistic within the pine forests of the southeastern United States, or other fire-adapted forest types [40]. Instead, efforts to restore desired understory communities should evaluate alternative management techniques [15,41,42,43]. Commercial logging crews, in particular, represent a readily available, and in some cases profitable, means of decreasing canopy cover to levels that maximize understory development and habitat quality for many southern pine focal species of conservation concern.

Author Contributions

Conceptualization, methodology, and writing—review and editing, K.K., W.G., A.C., D.M., K.J., K.M. and J.M.; formal analysis, K.K., W.G. and J.M.; investigation, K.K. and A.C.; writing—original draft preparation, K.K.; supervision, W.G.; project administration, W.G. and J.M.; funding acquisition, W.G. and J.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Wildlife Section of the Alabama Division of Wildlife and Freshwater Fisheries and the Georgia Wildlife Resources Division through the Federal Aid in Wildlife Restoration Program, which is an excise tax on sporting arms and ammunition paid by hunters and recreational shooters. Weyerhaeuser Company provided additional support.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to ongoing analysis and publication efforts.

Acknowledgments

We thank D. Greene for logistical support, and R. Barlow and S. Ditchkoff for their reviews of earlier drafts of this manuscript.

Conflicts of Interest

The founding sponsors provided input on the design of the study and the writing of the manuscript, but not on the collection, analyses, or interpretation of data, or in the decision to publish the results.

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Forests 12 00411 g001 550
Figure 1. Location of study areas in Greene and Hancock Counties, Georgia, USA where we evaluated the response of thinning accuracy and precision to three residual basal area treatments and harvest method in loblolly pine (Pinus taeda) stands during 2017.
Figure 1. Location of study areas in Greene and Hancock Counties, Georgia, USA where we evaluated the response of thinning accuracy and precision to three residual basal area treatments and harvest method in loblolly pine (Pinus taeda) stands during 2017.
Forests 12 00411 g001
Forests 12 00411 g002 550
Figure 2. Observed and target basal areas for 5 commercially thinned loblolly pine (Pinus taeda) stands in Greene and Hancock Counties, Georgia, USA. We thinned stands in 2017 and systematically cruised each stand immediately after thinning using 0.04 ha fixed radius plots to sample 5% of the total area. The bottom and top of each box represent the lower and upper quartiles, the middle band represents the median, and the whiskers extend to 1.5 times the interquartile range.
Figure 2. Observed and target basal areas for 5 commercially thinned loblolly pine (Pinus taeda) stands in Greene and Hancock Counties, Georgia, USA. We thinned stands in 2017 and systematically cruised each stand immediately after thinning using 0.04 ha fixed radius plots to sample 5% of the total area. The bottom and top of each box represent the lower and upper quartiles, the middle band represents the median, and the whiskers extend to 1.5 times the interquartile range.
Forests 12 00411 g002
Table
Table 1. Model parameter estimates (β), standard errors (SE), 95% confidence limits (LCL and UCL), and p-values predicting the effects of target thinning basal area (m2/ha) and harvest method (pre-marked vs. operator select) on the accuracy (observed-expected basal area) of the harvest. Basal area was estimated using fixed-radius plots within loblolly pine (Pinus taeda) stands in the Piedmont physiographic region of Georgia during 2017.
Table 1. Model parameter estimates (β), standard errors (SE), 95% confidence limits (LCL and UCL), and p-values predicting the effects of target thinning basal area (m2/ha) and harvest method (pre-marked vs. operator select) on the accuracy (observed-expected basal area) of the harvest. Basal area was estimated using fixed-radius plots within loblolly pine (Pinus taeda) stands in the Piedmont physiographic region of Georgia during 2017.
ModelβSELCLUCLp-Value
Intercept 1−3.361.53−6.36−0.360.03
Medium 21.780.620.363.200.02
Low 32.760.621.334.190.002
Pre-marked 41.391.92−4.697.460.52
1 High residual basal area (18 m2/ha) treatment in operator-select stands; 2 Medium residual basal area (14 m2/ha) treatment in operator-select stands; 3 Low residual basal area (9 m2/ha) treatment in operator-select stands; 4 Pre-marked stands.
Table
Table 2. Model parameter estimates (β), standard errors (SE), 95% confidence limits (LCL and UCL), and p-values predicting the effects of target thinning basal area (m2/ha) and harvest method (pre-marked vs. operator select) on the coefficient of variation (%) of basal area following harvest. Data were collected in fixed-radius plots in loblolly pine (Pinus taeda) stands within the Piedmont physiographic region of Georgia during 2017.
Table 2. Model parameter estimates (β), standard errors (SE), 95% confidence limits (LCL and UCL), and p-values predicting the effects of target thinning basal area (m2/ha) and harvest method (pre-marked vs. operator select) on the coefficient of variation (%) of basal area following harvest. Data were collected in fixed-radius plots in loblolly pine (Pinus taeda) stands within the Piedmont physiographic region of Georgia during 2017.
ModelβSELCLUCLp-Value
Intercept 129.244.7718.7539.73<0.001
Medium 20.185.50−11.9312.290.97
Low 39.285.50−2.8321.390.12
Pre-marked 4−12.444.590.288.920.02
1 High residual basal area (18 m2/ha) treatment in operator-select stands; 2 Medium residual basal area (14 m2/ha) treatment in operator-select stands; 3 Low residual basal area (9 m2/ha) treatment in operator-select stands; 4 Pre-marked stands.
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Keene, K.; Gulsby, W.; Colter, A.; Miller, D.; Johannsen, K.; Miller, K.; Martin, J. Accuracy and Precision of Commercial Thinning to Achieve Wildlife Management Objectives in Production Forests. Forests 2021, 12, 411. https://doi.org/10.3390/f12040411

AMA Style

Keene K, Gulsby W, Colter A, Miller D, Johannsen K, Miller K, Martin J. Accuracy and Precision of Commercial Thinning to Achieve Wildlife Management Objectives in Production Forests. Forests. 2021; 12(4):411. https://doi.org/10.3390/f12040411

Chicago/Turabian Style

Keene, Kent, William Gulsby, Allison Colter, Darren Miller, Kristina Johannsen, Karl Miller, and James Martin. 2021. "Accuracy and Precision of Commercial Thinning to Achieve Wildlife Management Objectives in Production Forests" Forests 12, no. 4: 411. https://doi.org/10.3390/f12040411

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