Lily Castle TidwellDr. SmithForestry 26221 April 2015

Restoration of oak-hickory dominated forests on the Cumberland Plateau through prescribed burns and

mechanical thinning

IntroductionThroughout the Cumberland Plateau region and other

areas of the eastern United States, oak-hickory dominated forests have been in decline. While mature oaks and hickories remain common in many canopies, a severe dearth of oak-hickory regeneration, especially in the face of increasing pine and maple regeneration, means that forests are changing in composition, a phenomenon known as mesophication (Iverson et al 2007). The relationship between fire regimes and oak regeneration has been studied thoroughly and regular burns have been shown to be vital in maintaining healthy oak-hickory dominated forests (Iverson et al 2007, Blankenship and Arthur 2005, Brose and Lear 1998).

Cumberland Plateau forests are among the most diverse in North America, making the preservation of native species composition of both ecological and scientific importance. Additionally, oak and hickory products are far more valuable than softer pine and sugar maple products, meaning that there are also economic factors motivating the conservation of the oak-hickory matrix. A long history of fire suppression and

planting of eastern white pines, both of which have been based largely on economically-oriented forestry practices, have contributed to the decline of oak regeneration since the 1930s (Iverson et al 2007). However, undesirable species can be suppressed while improving oak growth, regeneration, and stem form through mechanical thinning and regular growing-season fires of mid- to high-intensity (Iverson et al 2007, Blankenship and Arthur 2005, Brose and Lear 1998). Improved regeneration of native shortleaf pine in place of non-native loblolly and eastern white pine is also of concern on the Cumberland Plateau.

Our interest in Sewanee has been to improve the quality of oak-hickory stands and to learn more about the relationship between various silvicultural prescriptions and species composition. Objectives for our study were to: 1) regenerate oak and shortleaf pine, 2) open the canopy to create understory diversity, 3) examine the effects of fire and thinning, 4) remove planted pine and unwanted regeneration, and 5) help students gain management and fire experience.

Methods

Site Descriptions

Compartment 46Compartment 46 is a 30-acre stand located in Sewanee,

Franklin County, Tennessee at 86º58’15” N and 35º12’45” W (Figures 1-3). The soil is classified as sandy loam with sandstone parent material, measuring about 3.4 feet deep. The average temperature at the site is 55ºF with annual precipitation of 55-

60 inches. Eastern white pine (Pinus strobus) and loblolly pine (Pinus taeda) were planted in the compartment in the 1960s. These pines have been impacted by pine beetle outbreaks in 1997 and 2008. Additionally, the site was harvested for hardwood timber in the 1950s and for pulpwood in the 1980s. Compartment 46 was thinned in early 2010 and then lightly burned a few months later with temperatures in the 70s, relative humidity in the 40s, and 1- to 5-foot flame heights with 80% coverage. The compartment was burned moderately in April 2013, temperatures in the 70s, relative humidity in the 40s, and flame heights of 1 to 20 feet. Thousands of loblolly pine seedlings were killed and coverage was 100%. In March 2015, the site was burned lightly once again with 70-degree temperatures, 40% relative humidity, 1- to 10-foot flames, and 95% coverage.

Compartment 20Compartment 20 is a 56-acre stand also located in

Sewanee at 35º13’22”N and 85º12’45”W (Figures 1 and 4). Like Compartment 46, the soil is a sandy loam with sandstone parent material. Soil depth is 1.87 feet. Average temperature at the site is 55ºF with annual precipitation of 55-60 inches. In 2011, Compartment 20 was thinned. In April 2013, the compartment was burned moderately with temperatures in the 70s, relative humidity in the low 40s, and 1- to 15-foot flame heights with about 90% coverage of the site.

Compartment 8

Compartment 8 in Sewanee is a 40-acre stand at 35º19’N and 85º90’W (Figures 1 and 5-6). In January 2010, the stand was damaged by an F1 tornado. It was subsequently salvage logged in 2011. In 2013, eastern white pine, loblolly pine, and small diameter hardwoods were removed using a 411B hydro-ax and grapple skidded. The compartment was burned lightly in April 2014 in 70 degree temperatures, 40% relative humidity, and 1-5mph winds. Around 80% of the stand was burned. In the winter of 2015, eastern white pines were removed from the understory and midstory using hand tools.

Data CollectionIn each compartment, predetermined plots were studied by

several groups. Each plot was 1/20 acre in size and circular. All trees within the plot of breast height or greater were measured for diameter at breast height with their species recorded. Once trees were measured, 75-foot transects were measured due east and due west. At 15 through 21 feet on the transects, 1-hour, 10-hour, 100-hour, and 1000-hour fuels on the forest floor were counted. At 21 through 27 feet, 10-hour, 100-hour, and 1000-hour fuels were counted. At 27 through 75 feet, only 1000 hour fuels were counted. The diameters of all 1000-hour fuels were measured. At 45 and 75 feet on the transects, depth of litter and duff were measured.

Data Analysis

The basal area in square meters per hectare of each plot

was calculated using the formula . The number of stems per acre was calculated by simply multiplying the number of stems in the plot by 20. The total basal areas and stems per acre of each plot were averaged together for overall basal area of each compartment. Basal area and stems per acre of oaks and pines were calculated by processing data from these species separately from the rest of the data in addition to total calculations.

Fuel calculations were more complex. The tons per acre in fuels from 1-hour fuels, 10-hour fuels, and 100-hour fuels were calculated using the formulas ,

, and ,

respectively. Sound 1000-hour fuels were processed using the formula

and rotten 1000-hour fuels were processed using the

formula . Fuels in litter were calculated using the

formula and duff fuels were calculated using the formula

. The total fuels in tons per acre were added up for each transect in each plot, then averaged between the two transects in each plot. These averages were then averaged from across all plots.

Results

Compartment 46

In Compartment 46, total basal area in 2015 was measured to be 11.4 m²/ha, less than half of that measured in 2010. Oak basal area decreased by about half from 17.7 m²/ha to 9.6 m²/ha, but pine basal area dropped even more dramatically from 5.8 m²/ha to nearly zero. Trees per hectare decreased from 1237 in 2010 to 214 in 2015, once again with much more dramatic decrease in pines than in oaks. Additionally, it is interesting to note that in 2010, all measured trees had a diameter at breast height greater than 4 centimeters, whereas less than one third of measured trees in 2015 had a diameter at breast height greater than 4 centimeters (Table 1). Fuel data for Compartment 46 from 2010 is limited, though total tons of fuel per acre is shown to have increased from 10.5 to 12.9. In 2015, there were 83 snags per hectare with an average diameter at breast height of 6.4 inches (Table 2).

Compartment 20

Total basal area in 2015 for Compartment 20 was measured to be 15.1 m²/ha, once again about half of the basal area measured previously in 2011, which was 29.8 m²/ha. Oak basal area decreased by about two thirds from 18.1 m²/ha in 2011 to 4.9 m²/ha in 2015, and pines decreased by about one third from 7.3 m²/ha in 2011 to 5.8 m²/ha in 2015. Trees per hectare greater than 4cm DBH decreased by around three quarters from 711 to 196, and total trees per hectare plummeted from 1136 in 2011 to 396 in 2015. Oaks and pines per hectare decreased from 319 to 148 and 139 to 104, respectively (Table 3). Snags per acre in Compartment 20 nearly doubled from 22.5

to 48, with average snag DBH jumped from 4.43 inches to 23 inches. Tons of fuel per acre increased slightly from 10.19 in 2011 to 12.9 in 2015 (Table 4).

Compartment 8

Basal area in Compartment 8 changed much less dramatically from 26.4 m²/ha in 2013 to 20.41 m²/ha in 2015. Oaks per acre decreased somewhat from 10.8 m²/ha in 2013 to 8.67 m²/ha in 2015. Pines also showed a small drop from 6.4 m²/ha in 2013 to 5.49 m²/ha in 2015. Data was not available for trees per acre in Compartment 8 from the 2013 study, though it can be seen that about two thirds of measured trees were at least 4 centimeters in diameter at breast height. Additionally, there were about twice as many pines per acre as oaks per acre in 2015 (Table 5). Fuel data for Compartment 8 was not recorded in 2013. In 2015, however, there were 117.14 snags per hectare with an average diameter at breast height of 7.4 inches. Fuels were measured to be 18.59 tons per acre (Table 6).

Comparison of Compartments

Comparing the three Compartments, basal area and trees per acre had decreased in all three places, though much more dramatically in Compartments 46 and 20 than in Compartment 8. Oak basal area and oaks per hectare was similar in all three Compartments, while pine data and changes in pine data varied widely among the Compartments. The basal area and stems per acre of pines, however, varied widely in both number and degree of change. All three of the Compartments were shown to have

many trees smaller than 4 centimeters DBH in 2015, though the portion of trees less than 4 centimeters DBH varies in past measurements (Tables 1, 3, 5). Fuels and snags were shown to increase notably in Compartments 46 and 20, though there was no past data for these measurements in Compartment 8. Increase in average snag DBH in Compartments 46 and 20 was especially dramatic (Tables 2, 4, 6).

Discussion and ConclusionIn examining the changes that have occurred in

Compartments 46, 20, and 8, a spectrum of treatments and results becomes clear. When the recent history of the compartments is viewed, we remember that Compartment 46 was thinned once and burned three times, which is the highest number of burns out of the three compartments. Compartment 20 was also thinned once, but only burned once. Compartment 8 was damaged by a tornado, salvaged logged, and thinned, then burned once, which amounts to the most cutting of all the compartments but relatively little fire exposure. Investigating the results of our study, it can be seen that Compartment 46 experienced nearly complete elimination of pines but only around 50% reduction in oaks (Fig. 7, Tables 1-2). Compartment 20 experienced about a 60% reduction in oaks but only around a 30% reduction in pines (Fig. 8, Tables 3-4). Compartment 8 showed very little reduction of oaks or pines, but notably contained about twice as many pines per acre as oaks per acre. Compartment 8 also had 50% more fuels per acre than the other

two compartments, a result of thinnings and harvests (Fig. 9, Tables 5-6).

The trends of burns, thinning, and changes in oak and pine in our studied compartments support the theory of a positive relationship between these treatments and the successful regeneration of oaks. Specifically, the data suggest that repeated burns are vital in maintaining an oak-dominated structure. Additionally, it seems that thinning without burning does nothing to help oak regeneration in the face of pine regeneration, or that it may even explicitly favor the regeneration of pines. The higher fuel loads in Compartment 8, likely a result of many thinnings but few fires, may have resulted in poor burn conditions that did not positively impact oak regeneration.

These data are consistent with the findings of other studies, meaning that oak-hickory forests on the Cumberland Plateau presumably behave in ways typical of their species composition throughout the eastern United States (Iverson et al 2007, Blankenship and Arthur 2005, Brose and Lear 1998). By extension, the combination of thinning with consistent use of fire may be the best method to support the regeneration of fire-adapted oak-hickory forests and of native shortleaf pine in the Cumberland Plateau.

Given that the most useful data in this study have come from the differences in burning and thinning regimes across the studied compartments, it is interesting to note that more burns were originally planned for Compartments 26 and 8. Continued differential burning and thinning treatments in these

compartments, perhaps with the future inclusion of other compartments to provide a greater sample size, will likely result in a more thorough understanding of how to restore the fire-adapted ecosystems of the Cumberland Plateau.

Figure 1: Map of Studied Areas on Domain

Figure 2: Study Points in Compartment 46

Figure 3: Past Treatments in Compartment 46

Figure 4: Study Points and Past Treatments in Compartment 20

Figure 5: Study Points in Compartment 8

Figure 6: Fire Lines in Compartment 8

Figure 7: Compartment 46 in Jan 2010 and Jan 2015

Figure 8: Compartment 26 in Fall 2011 and Jan 2015

Figure 9: Compartment 8 in Jul 2013 and Jan 2015

Table 1: Compartment 46 Basal Area and Trees per Hectare, 2010 vs 2015Measurement 2010 2015Basal Area (m²/ha) ± SD 28.8 ± 7.3 11.4 ± 6.5Oak Basal Area (m²/ha) ± SD 17.7 ± 9.9 9.6 ± 6.4Pine Basal Area (m²/ha) ± SD

5.8 ± 6.7 .0005 ± .0004

Trees per Hectare >4cm DBH ± SD

1237 ± 316 214 ± 160

Trees per Hectare ± SD 1237 ± 316 748 ± 627Oaks per Hectare ± SD 421 ± 183 176 ± 108Pines per Hectare ± SD 111 ± 129 17 ± 51

Table 2: Compartment 46 Snags and Fuels, 2010 vs 2015Measurement 2010 2015

Snags per hectare ± SD

--- 83 ± 104

Average snag DBH (in) ± SD

--- 6.4 ± 7.1

Fuels (tons/acre) ± SD 10.5 ± 8.7 12.9 ± 0.9

Table 3: Compartment 20 Basal Area and Trees per Hectare, 2011 vs 2015Measurement 2011 2015Basal Area (m²/ha) ± SD 29.8 ± 9.7 15.1 ± 8.4Oak Basal Area (m²/ha) ± SD 18.1 ± 10.2 4.9 ± 6.8Pine Basal Area (m²/ha) ± SD

7.3 ± 14.3 5.8 ± 9.5

Trees per Hectare >4cm DBH ± SD

711 196 ± 185

Trees per Hectare ± SD 1136 396 ± 195Oaks per Hectare ± SD 319 148 ± 165Pines per Hectare ± SD 139 104 ± 199

Table 4: Compartment 20 Snags and Fuels, 2011 vs 2015Measurement 2011 2015Snags per hectare ± SD

22.5 48 ± 30

Average snag DBH (in) ± SD

4.43 ± 7.85 23 ± 11

Fuels (tons/acre) ± SD 10.19 12.9 ± 3.1

Table 5: Compartment 8 Basal Area and Trees per Hectare, 2013 vs 2015Measurement 2013 2015Basal Area (m²/ha) ± SD 26.4 20.41 ± 8.43Oak Basal Area (m²/ha) ± SD 10.8 8.67 ± 7.28Pine Basal Area (m²/ha) ± SD

6.4 5.49 ± 7.06

Trees per Hectare >4cm DBH ± SD

-- 520 ± 324.3

Trees per Hectare ± SD -- 891.43 ± 610.41

Oaks per Hectare ± SD -- 150 ± 95.9Pines per Hectare ± SD -- 317.14 ±

346.49

Table 6: Compartment 8 Snags and Fuels, 2013 vs 2015Measurement 2013 2015Snags per hectare ± SD

-- 117.14 ± 83.71

Average snag DBH (in) ± SD

-- 7.4 ± 4.78

Fuels (tons/acre) ± SD

-- 18.59 ± 8.87

Works Cited

Blankenship, B.A.; Arthur, Mary A. 2006. Stand structure over 9 years in burned and fire-excluded oak stands on the Cumberland Plateau, Kentucky. Forest Ecology and Management 225(1-3):134-145.

Brose, Patrick H.; Van Lear, David H. 1998. Responses of hardwood advance regeneration to seasonal prescribed fires in oak-dominated shelterwood stands. Canadian Journal of Forest Research. 28: 331-339.

Iverson, L. R., T. F. Hutchinson, A. M. Prasad, and M. P. Peters. 2008. Thinning, fire, and oak regeneration across a heterogeneous landscape in the eastern US: 7-year results. Forest Ecology and Management, v. 255, no. 7, p. 3035-3050. 10.1016/j.foreco.2007.09.088.