Loblolly pine outperforms slash pine in the southeastern United States – A long-term experimental comparison study

Forest Ecology and Management 450 (2019) 117532

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Loblolly pine outperforms slash pine in the southeastern United States – A long-term experimental comparison study

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Dehai Zhao , Bronson P. Bullock, Cristian R. Montes, Mingliang Wang, Dale Greene, Lori Sutter Warnell School of Forestry and Natural Resources, The University of Georgia, Athens, GA 30602, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Stand basal area carrying capacity Stand density index Biomass allocation Nutrient contents Species comparison

Loblolly pine (Pinus taeda L.) and slash pine (Pinus elliottii Engelm.) are the two most important commercial timber species in the southeastern U.S. A long-term experimental comparison study, in which loblolly and slash pine plots were paired for combinations of planting density (741, 2224, 3706 trees ha−1) and cultural intensity (operational versus intensive), was established in the lower Coastal Plain to investigate species differences in growth dynamics. Using age 2–21-year inventory plot data and destructive biomass/nutrient sampling data, species comparisons were conducted comprehensively, and the reasons of the species differences and management implications were discussed. When compared to slash pine, loblolly pine had higher stand basal area carrying capacity and maximum stand density index, lower fusiform rust infection rate and less wind damage, leading to its lower mortality. With no species difference in average DBH, loblolly pine consistently had greater average height and live crown length and higher level of tree size inequality. Loblolly pine had lower foliar biomass and lower foliar density, and higher crown length/width ratios due primarily to its longer crowns, suggesting perhaps a slightly greater shade tolerance or less dense canopy. As a result, loblolly pine outperformed slash pine in terms of stand basal area, total volume, aboveground biomass and carbon accumulation. Loblolly pine consistently accumulated more potassium, and this species difference increased with increasing stand age; while slash pine had slightly higher nitrogen and phosphorus contents before age 12. Species differences in mortality patterns and in nutrient accumulations in aboveground biomass emphasized the need to develop species-specific management strategies including different thinning and fertilization regimes.

1. Introduction There are 87 million hectares of forestland in the southern US, including about 10 million hectares of natural pine forests and 14 million hectares of pine plantations. These forests produce 57.9% of national roundwood and 11% of global roundwood, more than any other country (Hanson et al., 2010; UN FAO, 2017). Loblolly pine (Pinus taeda L.) and slash pine (P. elliottii Engelm. Var. elliottii) are the two most important commercial timber species in the southeastern US (Smith et al., 1994; Dicus and Dean, 2008). Forest landowners need to make a proper selection between loblolly and slash pine on some sites, based on the relative performance of the two species. Numerous investigations have documented growth differences between loblolly and slash pine plantations (Cole, 1975; Shoulders, 1976; Clason and Cao, 1983; Borders and Harrison, 1989; Shiver et al., 2000). In these previous studies, loblolly and slash pines were either planted at a specific planting density, or managed under relatively low cultural intensity (e.g., no application of fertilization or additional weed control).

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Furthermore, most previous studies compared the performances of loblolly and slash pine at a specific age or in a short period of time. Intensive silvicultural treatments including site preparation, competing vegetation control, and fertilization have been increasingly applied over the past four decades to enhance the production of loblolly and slash pine plantations (Martin and Jokela, 2004; Zhao et al., 2009). Loblolly and slash pine could respond to different culture intensities and planting densities in different ways (response type, magnitude and duration) (Jokela and Martin, 2000; Xiao et al., 2003; Zhao et al., 2008; Zhao et al., 2009). Thus, growth differences between these two species may also be influenced by cultural intensity and planting density. Forest landowners need more information on the long-term growth, structural, and health dynamics of loblolly and slash pine plantations especially under different management regimes to make species-site deployment and management decisions. Using inventory plot data from paired loblolly and slash pine plots in a unique culture/density study, Zhao and Kane (2012) compared growth performance of loblolly and slash pine over 2–15 years of age

Corresponding author. E-mail address: [email protected] (D. Zhao).

https://doi.org/10.1016/j.foreco.2019.117532 Received 13 April 2019; Received in revised form 1 August 2019; Accepted 5 August 2019 Available online 12 August 2019 0378-1127/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Locations of installations of loblolly and slash pine comparison study. Table 1 Silvicultural treatments for the loblolly and slash pine comparison study in southern Georgia and northern Florida. Operational regime

Intensive regime

Bedding in the spring followed by a fall herbicide treatment in 1.52-m bands over the rows (876.9 ml Arsenal + 2338.5 ml Garlon-4 per hectare if competition was waxy-leafed species such as galberry (Ilex glabra) or palmetto (Serenoa repens) or 876.9 ml Arsenal + 2338.5 ml Accord per hectare if the competition consisted mainly of grass or upland hardwood species)

Bedding in the spring followed by a fall broadcast herbicide treatment (1169.2 ml Arsenal + 4677.0 ml + 4677 ml Accord per hectare)

Fertilization: at planting, 561 kg ha−1 of 10–10–10; before 8th , 12th , 16th, and 20th growth season, 224 kg ha-1N + 28 kg ha-1P

Insecticides (usually Pounce) designed to control tip moths were applied as often as necessary to maintain tip moth control through the first two growing seasons Repeated herbicide application to achieve complete vegetation control (sprayed with 292.3 ml Oust per hectare along with directed sprays of Accord) Fertilization: at planting, 561 kg ha−1 of 10–10–10; spring 3rd grow season, 673 kg ha−1 10–10–10 + micronutrients + 131 kg ha-1NH4NO3; spring 4th grow season 131 kg ha-1NH4NO3; spring 6th grow season 336 kg ha-1NH4NO3; spring 8th, 10th , 12th , 14th, 16th, 18th and 20th grow season, 224 kg ha-1 N + 28 kg ha-1 P

concentrations were significantly greater for loblolly than for slash pine, while foliar K concentration was significantly greater for slash than for loblolly pine. Slash pine had significantly higher stem N and K concentrations than loblolly pine, while stem wood P concentrations were not affected by species, but stem bark P concentrations were significantly greater for loblolly than for slash pine. They also found that amount of foliage, rather than its nutrient content, was a better estimator of growth. The species differences in carbon and nutrient acquisitions and their change over time may help us understand the species difference in growth dynamics. Therefore, as the culture/density study is ongoing, it is worth checking to see if loblolly and slash pines still maintain the reported trends beyond the age of 15 years. It is also necessary to make further comparisons between these two species in terms of biomass production and allocation, carbon and nutrient accumulation, and stand or crown structures for developing species-specific management regimes. With new inventory plot data and destructive carbon/nutrient sampling data, the objectives of this study were to: (1) update the comparison of loblolly and slash pines through age 21 years, in terms of growth and yield of stand attributes such as average DBH and height, survival, stand basal area and volume; (2) compare the two species with respect to biomass accumulation and allocation, stand and crown structural variables such as tree size inequality, foliage/branch biomass ratio, foliage density; (3) explore species differences in carbon and nutrient concentrations and contents in aboveground biomass; and finally (4) discuss the management implications.

Table 2 CRIFF soil groups used in the study. Soil group

Drainage class

Diagnostic horizons

B1

Very poor Very poor Poor Poor

poor to somewhat

No spodic, argillic 51 – 102 cm

poor to somewhat

No spodic, argillic > 102 cm or absent Spodic with argillic Spodic, no argillic

B2 C D

to somewhat poor to somewhat poor

and assessed the effects of cultural intensity and planting density on species differences in basic mensurational variables. Loblolly pine outperformed slash pine in terms of stand basal area and total volume, partially due to the higher fusiform rust (Cronartium quercuum f. sp. fusiforme) infection and mortality of slash pine (Zhao and Kane, 2012). Why loblolly and slash pine differ in self-thinning behavior, however, is not fully understood. The possible reasons may include species differences in stand basal area carrying capacity, maximum stand density index (SDI), nutrient demands, and other relative density such as relative spacing (RS) or stand structure measures such as tree size inequality. Zhao and Kane (2012) found that there were no significant species difference in average DBH. The species difference in tree size inequality, however, may contribute species difference in productivity, because Bourdier et al. (2016) found that tree size inequality reduces forest productivity. Several studies have shown large variations in carbon and nutrient concentrations among different species and among different tree issues for a given species (Barron-Gafford et al., 2003; Zhang et al., 2009; Hellsten et al., 2013; Zhao et al., 2014). In 4-year-old loblolly and slash pine plantations, Barron-Gafford et al. (2003) found that foliar N and P

2

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Fig. 2. Average DBH and average height of loblolly pine (LOB) and slash pine (SLA) affected by cultural intensity (Int: intensive; Op: operational) and planting density (741, 2224, 3706 trees ha−1).

2. Materials and methods

12, 14, 18 and 21 years, respectively, due to landowner harvesting or thinning. Data from all un-thinned plots of 7 viable installations through age 21 were used for the analysis reported here. Three installations were on CRIFF soil group B1, two installations on soil group B2, one installation on soil group C, and one installation on soil group D. A description of these CRIFF soil groups is presented in Table 2.

2.1. Study descriptions The lower Coastal Plain culture/density study was established by the Plantation Management Research Cooperative (PMRC) at the University of Georgia during the 1995/1996 dormant season. On each of eleven installations of this study located in northern Florida and southern Georgia (Fig. 1), loblolly and slash pine plots were paired for combinations of three levels of planting density and two levels of cultural intensity. Site preparation and subsequent silvicultural treatments were designed to represent two levels of cultural intensity: operational and intensive regime (Table 1). The intensive regime included frequent fertilization and complete sustained competition control. The operational regime included less frequent fertilization and only early competition control. It should be noted that the operational regime in this study is more intensive than the one used in practice. Within each level of cultural intensity, three loblolly pine subplots and three slash pine subplots were paired and planted with densities of 741, 2224, and 3706 trees ha−1. At each installation (site), there was a random allocation of cultural intensities to main plots, and paired loblolly and slash pine density subplots were randomly assigned. This arrangement results in a split-plot design with one replication at each installation. The planting stock used on all installations was first generation, open-pollinated family (loblolly pine 7–56 and slash pine 5–61) that was an aboveaverage grower with significant fusiform rust resistance. All seedlings were grown in one nursery. To ensure the targeted initial density, each planting spot was double-planted and reduced to a single surviving seedling after the first growing season. One installation was lost at age

2.2. Measurements Treatment plots were comprised of an interior measurement plot ranging from 80, 96 and 160 trees per plot, for the 741, 2224 and 3706 trees ha−1 densities, respectively, and a surrounding 7.3 m wide buffer area. Dormant-season tree measurements were taken at ages 2, 4, 6, 8, 10, 12, 15, 18 and 21 years. At each measurement all surviving trees that were at least 1.4 m tall were measured for DBH. After the fourth growing season, every other tree was tagged and measured for total height (H) and height to live crown (Hc). The live crown length (CL) or the height of the crown is calculated as CL = H − Hc , and live crown ratio (CR) is defined as the live crown length divided by the total height of the tree: CR = CL/ H . The average CL and average CR of a stand is calculated based on the height measured trees. Each tree was inspected for fusiform rust infection (infected or uninfected). Total height of trees not measured for height was estimated from ln(H ) = b0 + b1 DBH−1 fitted separately for height measured trees in each plot at each measurement age. The average dominant height (HD) is defined as the average height of trees with diameter (DBH) larger than the average DBH of the stand. SDI was calculated for each plot using SDI = N (Dq /25)1.6 , where N is trees per hectare surviving at the SDI age and Dq is quadratic mean DBH 3

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Fig. 3. Average crown length and crown ratio of loblolly pine (LOB) and slash pine (SLA) affected by cultural intensity (Int: intensive; Op: operational) and planting density (741, 2224, 3706 trees ha−1).

Zhao and Kane (2016):

(Reineke, 1933). RS was calculated for each plot asRS = 10, 000/ N / HD , where HD is the average dominant height (m) (Wilson, 1946). Gini index was used to characterize tree size inequality through the distribution of tree diameter, tree basal area, tree volume or biomass. Gini index was calculated for each plot at each measurement using the following equation (Dixon et al., 1987):

DWtotal

n

G=

∑ (n + 1 − i) yi ⎞ 1 ⎛ n + 1 − 2 i=1 n ⎟ n − 1 ⎜⎝ ∑i = 1 yi ⎠

Vob =

DWtotal

DWwood = 0.0121DBH 2.0545H1.0398 DWbark = 0.0435DBH1.8014H 0.2732 DWbranch = 0.0015DBH 3.0256 DWfoliage = 0.0306DBH 2.8935H−1.1482 = DWwood + DWbark + DWbranch + DWfoliage

(5)

Stand-level stem wood, bark, branch and foliage biomass, total aboveground biomass, the ratio of foliage biomass to branch biomass (RFB), the ratio of stem biomass (stem wood + bark) to crown biomass (branch + foliage) (RSCB), and foliage density (FD, Mg ha−1 m−1) defined as stand foliage biomass divided by average crown length, were estimated for each plot at each measurement age. Four trees per plot (one below average DBH, one average tree, and two dominant and codominant trees) on four selected installations were chosen from the treated buffer rows for destructive sampling in the dormant seasons of 2012 and 2015, and their crown width was

(2)

and for slash pine trees with the formula of Zhao et al. (2019a):

Vob = 0.000049917DBH1.8728H1.0496

(4)

and for slash pine trees with biomass equations of Zhao et al. (2019b):

(1)

where tree sizes yi, i = 1 to n, are indexed in non-decreasing order ( yi ⩽ yi + 1). Tree sizes yi could be tree basal area, tree volume or biomass. A higher Gini index tends to indicate a higher level of tree size inequality for the variable considered. Total height (H, m) and DBH (cm) were used to calculate outside bark stem volumes (Vob, m3) for loblolly pine trees with the formula of Zhao et al. (2016):

0.000045901DBH1.8837H1.0379

DWwood = 0.0057DBH1.9249H1.3674 DWbark = 0.0051DBH1.9221H 0.6258 DWbranch = 0.0066DBH 3.2565H−0.6546 DWfoliage = 0.0399DBH 2.9052H−1.3742 = DWwood + DWbark + DWbranch + DWfoliage

(3)

Stem wood biomass (DWwood, kg), stem bark biomass (DWbark, kg), branch biomass (DWbranch, kg), foliage biomass (DWfoliage, kg), and total aboveground biomass (DWtotal, kg) were estimated for loblolly pine trees using biomass equations of Zhao et al. (2015) that were updated in 4

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Fig. 4. Average fusiform rust infection and stand density index of loblolly pine (LOB) and slash pine (SLA) affected by cultural intensity (Int: intensive; Op: operational) and planting density (741, 2224, 3706 trees ha−1).

Perkin Elmer AAS (Perkin Elmer, MA) for K, Ca, and Mg. Stand-level C, and nutrient contents in biomass components were calculated as the products of C and nutrient concentrations and stand-level component biomass. There was no information about tree crown width (CW) in the inventory plot data. With data from destructively sampled trees, the relationship between tree CW and CL was explored for loblolly and slash pine, respectively, using the scatter plots and regression (CW = a 0 + a1 CL ). The relationships between tree DBH and CW and between DBH and CL were also explored using the scatter plots.

measured on two axes perpendicular to one another before cutting. For each felled tree, the live crown length was measured, and the crown was divided into three equal length sections. Two branches from each crown section were randomly selected for nutrient analyses. An 8-cm section from the middle of each sampled branch was removed, resulting in 24 branch samples (2 branches × 3 crown sections × 4 trees) per plot that were placed in a paper bag. Twenty fascicles from each sampled branch were randomly selected, and all fascicles regardless of crown position were combined to make a bulk sample for each plot. A 2.5-cm thick disk was cut at 2.44 m height above butt from each of four felled trees, and the four disks were placed in one sealed plastic bag per plot and transported to the laboratory for nutrient analyses. Nutrient samples of stem, branch and foliage were oven-dried at 60 °C to constant weight. For stem disk samples, the bark was separated from the wood, and a pie-shaped section was cut from the wood disk. For branches, the wood and bark were not separated. Nutrient sample material from four trees was combined by component, and then ground and mixed resulting in one composite sample of each component for each plot for analyzing carbon (C), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) concentrations. C and N concentrations were determined after ball mill grounding with a SPEX 8000D (SPEX SamplePrep, NJ) on a CE Elantech Flash 2000 (CE Elantech, NJ). P, K, Ca, and Mg were determined after block digestion with nitric acid and hydrogen peroxide following EPA Method 3050B on an OI Analytical Alpkem Flow 3000 (OI Analytical, TX) for P or a

2.3. Statistical analysis Loblolly and slash pine comparison was carried out using a linear mixed-effects model for a split-plot experimental design with repeated measurements (Zhao and Kane, 2012). Differences in tree and stand characteristics of loblolly and slash pine were calculated from the paired plots (the value of an attribute of loblolly minus the value of that attribute of slash at the same level of cultural intensity, the same level of planting density, and the same installation). The species differences were treated as response variables. The installation factor and all factors containing installation were considered as random; the effects of planting density, cultural intensity, and time, and their interaction were treated as fixed. For more detailed information about the statistical analysis, including a description of the linear mixed-effects model, 5

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Fig. 5. Differences in stand survival and relative spacing between loblolly pine (LOB) and slash pine (SLA) by cultural intensity (Int: intensive; Op: operational) and planting density (741, 2224, 3706 trees ha−1).

(Fig. 3a and b). The more intensive cultural treatment generally increased CL for both loblolly and slash pine, but cultural intensity did not significantly affect species differences in CL (Fig. 3a). Planting density significantly influenced CL for both species, and affected species difference in CL (Fig. 3b). When planted at the lower density (741 trees ha−1), loblolly pine had significantly longer CL than slash pine through age 21 years. With higher planting densities (2224, 3706 trees ha−1), however, species difference in CL was no longer significant after age 15 years. Cultural intensity did not affect CR for both species (Fig. 3c). When planted at a lower density (741 trees ha−1), both species had larger CR (Fig. 3d). There was no significant species difference in CR, regardless of levels of cultural intensity or planting density. The CR of both loblolly and slash pine stands had dropped below 0.4 at ages 8, 11, and 18 years, when planted at 3706, 2224, and 741 trees ha−1, respectively.

please refer to Zhao and Kane (2012). All statistical tests were conducted at an α = 0.05 significance level.

3. Results 3.1. Differences in average DBH, height, live crown length and crown ratio More intensive treatment increased average DBH for both loblolly and slash pine (Fig. 2a). Across all spacings, however, species differences in average DBH under either intensive or operational treatments were not significant at each measurement age. After age 4 years, average DBH of both species generally decreased with increasing planting density (Fig. 2b). Species difference in average DBH was not significant at higher planting densities (2224, 3706 trees ha−1). At the lower planting density (741 trees ha−1), loblolly pine had significantly larger average DBH than slash pine from age 10 years, and the difference increased with age. Loblolly pine had consistently greater average height than slash pine over time (Fig. 2c and d). More intensive treatment increased average height for both species, but loblolly pine had a greater height response to the intensive treatment than slash pine (Fig. 2c). Planting density had a significant effect on average height for loblolly pine but not for slash pine (Fig. 2d). After age 10 years, the average height of loblolly pine decreased with increasing planting density. Loblolly pine also had consistently greater CL than slash pine

3.2. Differences in fusiform rust infection and stand density index The incidence of fusiform rust infection tended to increase with intensive management and decrease with increasing planting density (Fig. 4a and b) for both loblolly and slash pine. Slash pine had higher fusiform rust infection incidence than loblolly pine, and this species difference was increased by more intensive management or higher planting density. Slash pine stands under more intensive management or with low planting density had 20–34% fusiform rust infection rate 6

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Fig. 6. Differences in stand basal area and volume between loblolly pine (LOB) and slash pine (SLA) by cultural intensity (Int: intensive; Op: operational) and planting density (741, 2224, 3706 trees ha−1).

Largely due to lower initial survival (Fig. 5a and b) along with lower height growth (Fig. 2c and d), slash pine had higher RS than loblolly pine in the early ages (2–4 years), regardless of cultural intensity (Fig. 5c) or given a planting density (Fig. 5d). When planted at higher densities (2224 and 3706 trees ha−1), both loblolly and slash pine stands tended to approach a common minimum RS with time regardless of cultural intensity. When planted at low density (741 trees ha−1), both species stands still had a higher RS at age 21 years.

after age 6, and loblolly pine stands with low planting density also had larger than 20% of infection rate after age 15 years. There was no significant species difference in SDI until age 8 years, although higher SDI was associated with higher level of cultural intensity and planting density (Fig. 4c and d). After age 10 years, species difference in SDI was significant and increased in magnitude with age. When planted at higher densities or under more intensive treatments, both slash and loblolly pine stands reached a maximum SDI at earlier ages, then began to plateau or decreased. Slash pine approached a smaller value of maximum SDI (880) at about age 10 years and loblolly pine approached a larger value of maximum SDI (1050) at about age 15 years, when planted at 3706 trees ha−1. When planted at 741 trees ha−1, however, both slash and loblolly pine stands had not reached their maximum SDI yet. The observed maximum SDI was less than the expected maximum SDI 1110 for loblolly and 990 for slash pine, respectively.

3.4. Differences in stand basal area and total volume Given cultural intensity or planting density, there was no significant species differences in stand basal area and volume before age 8 years, thereafter loblolly pine had more stand basal area and volume than slash pine (Fig. 6). Across all planting densities, more intensive treatment increased stand basal area until age 12 for slash pine and until age 18 for loblolly pine, thereafter cultural intensity effect on stand basal area was no longer significant for loblolly pine and operational slash pine stands maintained higher stand basal area than intensive stands. Higher fusiform rust infection incidence and higher mortality from fusiform rust infection in intensive slash pine stands could partially explain why the operational slash pine stands can finally achieve higher stand basal area than intensive stands. Across cultural intensities, stand basal area and volume increased with increasing initial density. When planted at higher densities (2224

3.3. Differences in survival and relative spacing For both loblolly and slash pine, more intensive treatment or higher planting density plots had higher mortality than operational or lower planting density plots (Fig. 5a and b). Given a level of cultural intensity or planting density, loblolly pine survived better than slash pine. Although the species difference in survival changed over time, it was not significantly affected by either cultural intensity or planting density. 7

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Fig. 7. Differences in stem wood biomass and total aboveground biomass between loblolly pine (LOB) and slash pine (SLA) by cultural intensity (Int: intensive; Op: operational) and planting density (741, 2224, 3706 trees ha−1).

and 3706 trees ha−1), planting density effect on stand basal area and volume was not significant after age 18 years for both loblolly and slash pine. For loblolly pine there was no significant difference in total volume at age 21 years among all three levels of planting density. Loblolly and slash pine appear to attain different levels of stand basal area carrying capacity (approximately 45 m2 ha−1 for loblolly and 35 m2 ha−1 for slash), while total volume was still increasing at age 21 years for both species with no clear asymptote based on these data.

and slash pines yielded less total and stem biomass than high planting density stands. At age 21, the effect of cultural intensity or planting density was still significant for slash pine, but not for loblolly pine. Loblolly pine produced significantly more total aboveground than slash pine from age 8 when planted at 741 trees ha−1, and from age 10 when planted at 2224 and 3706 trees ha−1; and produced more stem biomass than slash pine from age 8 when planted at any initial density. Planting density significantly affected species differences in total aboveground and stem biomass.

3.5. Differences in total aboveground biomass and stem wood biomass 3.6. Differences in stand foliage biomass (FB) and foliage density (FD) Across all planting densities, more intensive treatment increased loblolly pine total aboveground biomass and stem wood biomass over time, and the responses increased until age 12 years, then decreased (Fig. 7a and c). For slash pine, however, more intensive treatment increased total aboveground biomass and stem wood biomass until age 15, thereafter operational stands had more total and stem wood biomass. Loblolly pine produced significantly more total aboveground and stem biomass than slash pine from age 8 under intensive treatment, and from age 10 with operational treatment. There were significant cultural intensity effects on species differences in total aboveground and stem biomass. Across cultural intensities, there were no differences in total aboveground and stem wood biomass between higher planting densities (2224 versus 3706 trees ha−1) for both loblolly and slash pines (Fig. 7b and d). When planted at the lower density (741 trees ha−1), loblolly

Given cultural intensity or planting density, slash pine had more FB and higher FD than loblolly pine over time (Fig. 8). With higher planting densities or under more intensive management, loblolly pine approached a stable foliage biomass of 5 Mg ha−1 at age of 6 or 8 years and maintained it thereafter, while slash pine peaked at 7 Mg ha−1 at age of 6 years followed by a decline over ages 6–14 years, and thereafter remained a constant foliage biomass of 5.6 Mg ha−1. The range in stand foliage biomass between cultural intensities or among planting densities tended to become narrower for both species after age 10 years. Generally, loblolly and slash pine stands maintained constant levels of foliage biomass production (4.8 vs. 5.6 Mg ha−1) after age 15 years. Following the same trends as FB, FD of intensive or high planting density slash pine stands rose to a bigger maximum, then decreased over a period, and then maintained constant levels. Loblolly stand FD 8

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Fig. 8. Differences in foliage biomass and foliage density between loblolly pine (LOB) and slash pine (SLA) by cultural intensity (Int: intensive; Op: operational) and planting density (741, 2224, 3706 trees ha−1).

rose to a lower maximum and then flattened. Intensive loblolly and slash pine stands reached the maximum FB and FD at an earlier age than operational stands. Planting density had more influence on slash than loblolly FB and FD. Regardless of planting densities, stand foliage biomass was relative constant for loblolly pine after age 10, and for slash pine after age 12. Stand foliage density, however, still increased as increasing planting density. After age 18, difference in foliage density was not significant among stands planted at higher initial densities (2224 and 3706 trees ha−1), but significant between stands planted at higher and lower densities (741 vs. 2224, 3706 trees ha−1). There was more effect of planting density on slash than loblolly and on foliage density than on foliage biomass.

3.8. Concentrations of C, N, P, K, Ca, and Mg in aboveground biomass components Given a species, nether cultural intensity nor planting density produced a significant difference in C and nutrient concentrations in any aboveground biomass component (stem wood, stem bark, branch, or foliage), but the concentrations of C and nutrients varied significantly among biomass components (Fig. 10). Slash pine had significantly higher C concentration and lower Mg concentration than loblolly pine in all biomass components. Slash pine had significantly higher N concentration than loblolly pine in stem wood component. Slash pine had higher P and Ca concentrations than loblolly pine in stem wood and branch components but had lower P and Ca concentrations than loblolly pine in stem bark and foliage components. In stem wood and stem bark components, slash pine had significantly lower K concentration than loblolly pine; in branch and foliage components, there was no significant difference in K concentration between loblolly and slash pines.

3.7. Differences in stem to crown biomass ratio (RSCB) and foliage to branch biomass ratio (RFB) RSCB was significantly affected by planting density, but not by cultural intensity for both loblolly and slash pine (Fig. 9a and b). For both species, RSCB increased with increasing planting density. Given a planting density, slash pine stands had higher RSCB than loblolly pine stands. There was no significant effect of cultural intensity or planting density on RFB for both species (Fig. 9c and d). Slash pine generally had higher RFB than loblolly pine. RFB tended to decrease with age for both species and approach a common minimum ratio value.

3.9. Differences in C, N, P and K accumulations in aboveground biomass Across planting densities, slash pine accumulated higher C content than loblolly pine before age 8 with intensive treatment and before age 10 with operational treatment (Fig. 11a). After age 12 with intensive treatment or after age 18 in the operational intensity, loblolly pine accumulated significantly more C in aboveground biomass than slash 9

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Fig. 9. Differences in stem to crown biomass ratio and foliage to branch biomass ratio between loblolly pine (LOB) and slash pine (SLA) by cultural intensity (Int: intensive; Op: operational) and planting density (741, 2224, 3706 trees ha−1).

pine. When planted at higher densities (2224 and 3706 trees ha−1), slash pine accumulated modestly more C than loblolly pine at earlier ages; however, after age 12 loblolly pine had higher C content than slash pine (Fig. 11b). When planted at lower density (741 trees ha−1), there was no difference in C accumulation between loblolly and slash pines before age 12, thereafter loblolly pine had significantly higher C content than slash pine, and the difference increased over time. As a result, there was a significant difference in C accumulation among planting densities for slash pine. For loblolly pine, however, this difference was no longer significant after age 18. Across planting densities, slash pine stands generally accumulated more N in aboveground biomass than loblolly pine stands during ages 4–10 years under either cultural intensity; after age 12, loblolly pine accumulated more N than slash pine with the intensive treatment, but no species difference in N accumulation was found with the operational treatment (Fig. 11c). When planted at higher densities (2224 and 3706 trees ha−1), slash pine had significantly higher N content than loblolly pine before age 12 years, and thereafter this difference was no longer significant. When planted at 741 trees ha−1, no species difference in N accumulation was found before age 10, and thereafter loblolly pine accumulated significantly more N than slash pine. P accumulation in total aboveground biomass was impacted by species, cultural intensity, and planting density (Fig. 12a and b). In general, P content in aboveground biomass increased with increasing level of cultural intensity or planting density for both species. Species difference in P content was significantly affected by cultural intensity.

When averaged across planting densities, slash pine stands had higher P content than loblolly pine over ages 4–15 years under the intensive management and had consistently higher P content than loblolly pine under the operational management. When averaged across cultural intensities, slash pine accumulated more P than loblolly pine after age 4 years when planted at higher densities (2224 and 3706 trees ha−1), while the species difference in P content was not significant when planted at low density (741 trees ha−1). Species difference in K content had very different trends than N or P content. Loblolly pine stands had significantly higher K content than slash pine stands after age 4 years, regardless of cultural intensity or planting density (Fig. 12c and d). More intensive treatment increased K amount in loblolly stands before age 18 and in slash stands before age 12, and thereafter cultural intensity effect on K accumulation was no longer significant for both species. Planting density significantly affected K accumulation in both loblolly and slash pine stands before age 18 years, with higher planting-density (2224 and 3706 trees ha−1) stands accumulating more K than 741 trees ha−1 density stands and without significant difference between 2224 and 3706 trees ha−1 density stands. The species difference in K accumulation increased with increasing stand age. By age 21 years, loblolly and slash pine stands accumulated 137–147 and 54–57 Kg ha−1, respectively, of K in aboveground biomass.

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Fig. 10. Concentrations of carbon (C), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) in aboveground biomass components of loblolly pine (LOB) and slash pine (SLA).

area, total volume and total aboveground biomass. Loblolly pine was more responsive to more intensive silvicultural treatments (e.g., fertilizer additions) than slash pine, as previously reported in several studies (e.g., Clason and Cao, 1983; Jokela and Martin, 2000; Shiver et al., 2000; Xiao et al., 2003; Zhao et al., 2009). Thus, species differences in most attributes analyzed here were affected by cultural intensity and initial density. Loblolly pine consistently had greater average height and crown length than slash pine. Average DBH and crown ratio did not differ between these two species. Loblolly pine consistently had a higher level of tree size inequality than slash pine, but this difference could not be used to explain the species difference in productivity. Bourdier et al. (2016) reported that tree size inequality reduces forest productivity. However, our observed relationship between forest productivity and tree size inequality did not support their statement. In our study, for example, higher planting densities generally increased productivity and increased the level of tree size inequality; the more intensive treatment enhanced the productivity but was associated with a lower level of tree size inequality. In addition to loblolly pine’s height growth advantage, loblolly pine’s higher survival also contributed to the species difference in productivity. Lower fusiform rust infection and higher observed values of stand basal area and SDI in loblolly pine resulted in its lower mortality than slash pine. Trees infected by fusiform rust had higher mortality rate than uninfected trees (Zhao et al., 2006), and loblolly pine survives fusiform rust better than slash pine (Shiver et al., 2000). Slash pine was infected more frequently than loblolly, thus it sustained higher mortality from fusiform rust. The more intensive management resulted in greater fusiform rust infection incidence, especially in slash pine

3.10. Species difference in tree size inequality (Gini index) Higher planting-density stands had higher tree size inequality, and more intensive cultural treatment decreased tree size inequality for both loblolly and slash pine (Fig. 13). With either intensive or operational treatment or when planted at higher initial densities (2224 and 3706 trees ha−1), loblolly pine stands had higher tree size inequality than slash pine stands. When planted at lower initial density (741 trees ha−1), there was no species difference in tree size inequality after age 10. The values of Gini indexes shown in Fig. 13 were calculated from the distribution of individual tree basal area. Gini indexes could be also estimated based on the distributions of individual tree DBH, volume, or biomass, resulting in different values but similar patterns (data not shown). 3.11. Species difference in crown length and crown width relationship The data points and line regressions (Fig. 14a) indicated that loblolly pine generally had higher crown length/width ratios than slash pine. Given the same tree DBH, loblolly pine trees generally had longer crown than slash pine trees (Fig. 14c) but their difference in crown width was very small (Fig. 14b). Thus, higher crown length/width ratios in loblolly pine were due primarily to its longer crowns. 4. Discussion and conclusions After age 15 years, growth dynamics of loblolly and slash pine plantations continued to follow the trends reported in Zhao and Kane (2012). Loblolly pine outperformed slash pine in terms of stand basal 11

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Fig. 11. Total C and N accumulation in aboveground biomass in loblolly pine (LOB) and slash pine (SLA) plantations by cultural intensity (Int: intensive; Op: operational) and planting density (741, 2224, 3706 trees ha−1).

stands. This finding is consistent with previous reports of several studies such as Boggess and Stahelin (1948), Lopez-Zamora et al. (2001), Miller (1970). More fertilization should be avoided in stands with greater than 25% fusiform rust infection incidence, as suggested by Ogden and Morris (2004). Loblolly pine had higher stand basal area carrying capacity than slash pine (45 versus 35 m2 ha−1). This difference implies that loblolly pine should achieve a higher stand basal area than slash pine when in need of a thinning treatment. Loblolly pine had higher maximum SDI than slash pine (1050 versus 880 in the metric system). In our study, many loblolly and slash pine plots with high planting densities already reached the maximum SDI after age 15 years. Stand density is often expressed as a percentage of the maximum SDI, denoted as % SDI. After crown closure at about 25% SDI, stands begin to undergo self-thinning at 50% to 55% SDI. Harrington (2001) recommends thinning at 45% of maximum SDI to manage for conditions of full stocking, absence of selfthinning. If loblolly and slash pine stands were thinned at 45% of our reported maximum SDIs (about 470 and 400, respectively), stand age at the first thinning would be somewhere between 3 and 11 years. In practice, managers rarely thin that early. If based on the same upper limit of %SDI, the first thinning of loblolly pine would likely be slightly later than that of slash pine because of loblolly pine’s higher maximum stand basal area and higher maximum SDI. In addition to species difference in fusiform rust infection rate, species-specific maximum observed BA, and species-specific maximum observed SDI, species difference in wind damage might account for at

least part of the species difference in survival. Wind (hurricane and storm) damage to loblolly and slash pine stands were not quantitatively assessed in this study, but we observed that winds damaged loblolly pine stands less than slash pine stands during field data collection efforts. Live crown ratio and RS can also aid in determining when a stand should be thinned (Wilson, 1946, 1979; Zhao et al., 2010). For example, thinning treatment is often recommended with CR falls between two given values 0.5 and 0.4 (Harrington, 2001). When planted at density of 3706, 2224, and 741 trees ha−1, both loblolly and slash pine stands in our study reached average crown ratio of 0.4 at about age 8, 12 and 18 years, respectively. For loblolly and slash pine plantations, the upper and lower bounds of RS for thinning treatment may be set at 0.3 and 0.2, respectively (Zhao et al., 2010). Practically, the thinning criteria (appropriate upper and lower limits) proposed based on RS or other density measures such as basal area and SDI should be justified with management objectives. In general, more intensively managed stands or stands planted at higher densities began self-thinning earlier and met the thinning criteria earlier, thus should be thinned earlier than those with lower planting density or operational management. The distinctive species differences found in crown structure and biomass allocation also contribute to species difference in growth performance. Higher crown length/width ratios in loblolly pine due primarily to its longer crowns might suggest a slightly greater shade tolerance. Slash pine stands generally accumulated less stem and total 12

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Fig. 12. Total P and K accumulation in aboveground biomass in loblolly pine (LOB) and slash pine (SLA) plantations by cultural intensity (Int: intensive; Op: operational) and planting density (741, 2224, 3706 trees ha−1). Fig. 13. Tree size inequality (Gini Index) change over time for loblolly pine (LOB) and slash pine (SLA) plantations, affected by cultural intensity (Int: intensive; Op: operational) (a) and planting density (741, 2224, 3706 trees ha−1) (b). The values of Gini indexes were estimated based on the distribution of individual tree basal area.

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shooting the maximum sustainable foliar biomass and then falling to the more sustainable level of foliage (Vose et al., 1994). Loblolly pine did not follow this pattern, although it did to an extent in the highest density plots. Loblolly and slash pine stands maintained a constant level of foliage biomass production of 4.8 and 5.6 Mg ha−1 after age 15 years. When they were converted to the all-sided leaf area (LAI) using the specific leaf area for loblolly pine (0.0115 m2 g−1) and slash pine (0.0100 m2 g−1) (Dalla-Tea and Jokela, 1991; Jokela and Martin, 2000), loblolly and slash pine stands had almost the same stable LAI (5.52 versus 5.60 m2 m−2). Due to its longer crowns and a more open canopy (lower foliar density), however, loblolly pine could intercept more radiation than slash pine (Emhart et al., 2007). Plus, its higher light use efficiency (Dalla-Tea and Jokela, 1991) and higher leaf area efficiency, loblolly pine continued its outperformance over slash pine. Our results also showed that the species difference in biomass allocation between leaves and branches had diminished as stand foliage biomass reached a stable level. Slash pine had consistently higher C concentration in each biomass component than loblolly pine. Therefore, relative species difference in C content tended to be smaller than that in total aboveground biomass, although they exhibited similar trends. Due to significant species differences in nutrient concentrations in some biomass components and species differences in biomass allocation, nutrient contents and total aboveground biomass showed different trends. Slash pine stands accumulated little more N and P in above ground biomass than loblolly pine stands until mid-rotation. Loblolly pine stands consistently accumulated much more K than slash pine stands. In our current study both the operational and intensive plots repeatedly received amounts of N and P, while the operational plots received 56.1 kg ha−1 of K only at planting and the intensive plots received 56.1 kg ha−1 and 67.3 kg ha−1 of K at planning and in 3rd growing season, respectively. Foliar critical concentrations for N, P, and K are set to be 1.20%, 0.10%, and 0.40%, respectively in loblolly pine (Jokela et al., 1991) and 1.00%, 0.09%, and 0.30%, respectively in slash pine (Fisher and Binkley, 2000, p. 294). Foliar N concentration in both species remained above the critical concentration. Foliar P concentration remained above the critical concentration in loblolly pine and was below in some slash pine plots. Foliar K concentration was below the critical concentration in all loblolly pine plots and in some slash pine plots in mid-rotation. This information indicated that K might limit growth in mid-rotation loblolly pine stands and that K and P might limit growth in some mid-rotation slash pine stands. Jokela (2004) suggested that K may limit southern pine growth once N and P demands have been met on certain soil types, and Carlson et al. (2014) found that further growth improvements in response to the addition of K with the N and P was consistent on sites located on the Pleistocene terraces in the southeastern United States, but was not present on other sites. Although our finding is not obviously linked to the growth difference between loblolly and slash pine, it can help us develop and implement forest fertilization in southern pine plantations. Of course, replicated fertilizer trials would be necessary. In summary, the different performance of loblolly pine and slash pine might be explained by the following traits: (1) loblolly pine had a higher maximum SDI, which allows it to carry greater stand basal area, and had lower fusiform rust infection rate, and thereby had higher survival than slash pine; (2) loblolly pine stands consistently had greater average height and live crown length than slash pine stands; (3) loblolly pine stands had longer crowns and a less dense canopy, allowing for a more efficient capturing process. The current study focused on the species differences in aboveground parts. The differences in root systems or water use efficiency may also play important roles on the differences in the growth and yield of loblolly and slash pine plantations and remain to be investigated. Only the data from un-thinned plots were analyzed in this report with the intention of looking for species differences in stand development such as maximum basal area and maximum SDI. The existence of species-specific maximum stand basal area and species-specific maximum SDI, and the species

Fig. 14. Relationships between crown length and crown width (a), between DBH and crown width (b) and between DBH and crown length (c) for loblolly and slash pine trees destructively sampled for biomass and nutrient analyses.

aboveground biomass, but their greater stem/crown biomass ratio indicated that they allocated relatively more biomass to stems than loblolly pine stands. Loblolly pine stands maintained less foliage biomass, but their smaller foliage/branch biomass ratio suggested that they allocated relatively more crown biomass to branches than slash pine stands. These findings agree with results of Jokela and Martin (2000) that loblolly pine had more branch biomass but less foliage biomass, and thereby had a significantly lower foliage/branch biomass ratio than slash pine. The negative correlation between foliage/branch biomass ratio and volume growth (Xiao et al., 2003) suggested a higher leaf area efficiency for loblolly pine, which could be one mechanistic explanation for the species difference in productivity. In our study, stand foliage biomass (or canopy foliar biomass) was defined as the dry weight of needles that attached to the branches. It could be estimated using the previously developed foliage biomass equations as we did, or it could be estimated from some destructively harvested trees. Species difference in stand foliage biomass might not correspond to the difference in the annual litterfall mass. For example, Jokela and Martin (2000) reported that slash pine had more stand foliage biomass that was estimated from the destructively harvested trees, but less annual litterfall production than loblolly pine at age 13 years. They also reported that intensively managed loblolly and slash pine reached a stable foliage production of 7 Mg ha−1 and 6 Mg ha−1, respectively, at stand age of 10 years. Our results showed that slash pine had higher levels of foliar biomass, although it had lower productivity than loblolly pine. Slash pine followed the classic pattern of over14

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differences in mortality patterns and in nutrient accumulations emphasize the need and help us develop species-specific management strategies including different thinning (timing, intensity and frequency) and fertilization (type, amount and timing of fertilizer application) regimes. The conclusions about species performance in this comparison study were based on only one family of each species, the best stock available at the time of study installation. Over the past more than two decades, plantation productivity has been substantially enhanced by the implementation of improved genetics and intensive management and the genetic gains have been greater in loblolly pine than in slash pine. Therefore, some findings in this report should be interpreted cautiously with other families or genetically improved pine varieties, and new genetics and silviculture field trials are needed.

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Acknowledgements The culture/density study was established and maintained by the Plantation Management Research Cooperative (PMRC) at the University of Georgia’s Warnell School of Forestry and Natural Resources. Destructive biomass sampling and nutrient analysis work was supported in part by the USDA Forest Service (Agreement 16-JV11330145-075). The main research findings of the paper have been presented at the 103rd Annual Meeting of the Ecological Society of America (New Orleans, LA, USA, 2018), the 4th International Congress on Planted Forests (Beijing, China, 2018), and the 20th Biennial Southern Silvicultural Research Conference (Shreveport, LA, USA, 2019). References Barron-Gafford, G.A., Will, R.E., Burkes, E.C., Shiver, B., Teskey, R.O., 2003. Nutrient concentrations and contents, and their relation to stem growth, of intensively managed Pinus taeda and Pinus elliottii stands of different planting densities. For. Sci. 49 (2), 291–300. Borders, B.E., Harrison, W.M., 1989. Comparison of slash and loblolly pine performance on cutover site-prepared sites in the Coastal plain of Georgia and Florida. South. J. Appl. For. 13 (4), 204–207. Boggess, W.R., Stahelin, R., 1948. The incidence of fusiform rust in slash pine plantations receiving cultural treatments. J. For. 46 (9), 683–685. Bourdier, T., Cordonnier, T., Kunstler, G., Piedallu, C., Lagarrigues, G., Courbaud, B., 2016. Tree size inequality reduces forest productivity: an analysis combining inventory data for ten European species and a light competition model. PLoS ONE 11 (3), e0151852. https://doi.org/10.1371/journal.pone.0151852. Carlson, C.A., Fox, T.R., Allen, H.L., Albaugh, T.J., Rubilar, R.A., Stope, J.L., 2014. Growth response of loblolly pine in the southeast United States to midrotation applications of nitrogen, phosphorus, potassium, and micronutrients. For. Sci. 60 (1), 157–169. Clason, T.R., Cao, Q.V., 1983. Comparing growth and yield between 31-year-old slash and loblolly pine plantations. In: Jones, E.P. Jr. (Ed.), Proc. 2nd Bienn. South. Silv. Res. Conf. USDA For. Serv. Gen. Tech. Rep. SE-24, pp. 291–297. Cole, D.E., 1975. Comparisons within and between populations of slash and loblolly pine. Georgia For. Res. Counc. Pap. No. 81. Macon, GA. 13p. Dalla-Tea, F., Jokela, E.J., 1991. Needlefall, canopy light interception, and productivity of young intensively managed slash and loblolly pine stands. For. Sci. 37 (5), 1298–1313. Dicus, C.A., Dean, T.J., 2008. Tree-soil interactions affect production of loblolly and slash pine. For. Sci. 54 (2), 134–139. Dixon, P.M., Weiner, J., Mitchell-Olds, T., Woodley, R., 1987. Boot-strapping the Gini coefficient of inequality. Ecology 68 (5), 1548–1551. Emhart, V.T., Martin, T.A., White, T.L., Huber, D.A., 2007. Clonal variation in crown structure, absorbed photosynthetically active radiation and growth of loblolly pine and slash pine. Tree Physiol. 27, 421–430. Fisher, R.F., Binkley, D., 2000. Ecology and management of forest soils. Wiley, New York, pp. 489. Hanson, C., Yonavjak, L., Clarke, C., Minnemeyer, S., Boisrobert, L., Leach, A., Schleeweis, K., 2010. Southern forests for the future. World Resources Institute, Washington, DC, pp. 88. Harrington, T.B., 2001. Silvicultural approaches for thinning southern pines: method, intensity, and timing. Georgia Forestry Commission 17 Publication #FSP002. Hellsten, S., Helmisaari, H.-S., Melin, Y., Skovsgaard, J.P., Kaalinen, S., Kukkola, M.,

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