Heaton, W. C., State Wildlife Specialist, Extension Assistant Professor, Clemson Extension Service
Bean, R. C. , Extension Forestry and Natural Resources Agent, Clemson Extension Service

ABSTRACT

Seedling survival rates were evaluated between containerized and bare-root seedlings at 6-months and 1-year after planting in the Sandhill region of South Carolina.  The selected study site was representative of the geographic region and characterized by drought-prone deep sandy soils.  Seedlings were planted in one acre blocks. Blocks were randomly assigned to 1 of 3 common planting densities: 908, 545, or 436 trees per acre (6 ft x 8 ft, 8 ft x 10 ft, and 10 ft x 10 ft, respectively).  Seedling types were alternated from row to row within each block to create pseudo-replications.  Seedling survival was estimated at 6-months and 1-year after planting.  Survival differed significantly between bare-root and containerized seedlings (P < 0.0001).  Bare-root seedling survival was 38.78% at 6-months and 27.78% at 1-year.  Containerized seedling survival was 99.43% at 6-months and 90.00% at 1-year.  Planting density did not influence seedling survival (P = 0.1552 at 6-months, P = 0.1802 at 1-year).  Based on the results of this study, landowners interested in restoring longleaf pine are encouraged to utilize containerized seedlings for sites subject to stressful growing conditions.  Additionally, the lack of influence of planting density on seedling survival suggest landowners can utilize the plant spacing that best matches their management goals.

Introduction

 

The longleaf pine (Pinus palustris) is a species of southern pine found throughout much of the southeastern US.  While it is not uncommon to see other species of southern pines growing in conjunction with longleaf pine (Oswalt et al., 2012), the longleaf is easily distinguished from other species at every growth stage.  Mature longleaf pine forests have a distinct appearance with widely scattered mature longleaf trees dominating a rather open canopy, a frequently absent mid-story, and a diverse ground layer dominated by grasses and herbaceous plants (Franklin 2008; Oswalt et al., 2012).  Longleaf pine forests support significant biological diversity, and are considered among the most diverse ecosystems in the western hemisphere (Walker and Peet 1983; Ramsey et al., 2003; Peet, 2006; Franklin, 2008; Carr et al., 2010; Oswalt et al., 2012).

The longleaf pine and its associated ecosystems played a significant role in the history and ecology of the US, and was once the most widely distributed forest ecosystem in North America (Oswalt et al., 2012).  The original range of the longleaf forest stretched from Virginia south to Florida along the Atlantic coast and west to Texas through the Gulf States (Franklin, 2008).  This vast expanse once represented 90 million acres of longleaf forests (Frost, 2006; Franklin, 2008; Oswalt et al., 2012).  Changes in land use and management practices have significantly reduced the acreage of longleaf forests across the native range with approximately 3.4 million acres of longleaf forests remaining (Franklin, 2008; USDA, 2011; Oswalt et al., 2012).  Large scale removal of longleaf forests from the southern landscape has had serious ecological impacts, and these forests are considered among the most threatened ecosystems in the US (Ramsey et al., 2003).  In recognition of the significance of the longleaf pine ecosystem, the United States Department of Agriculture (USDA) through the Natural Resources Conservation Service (NRCS) initiated the Longleaf Pine Initiative in 2010 with a goal of adding 4.6 million acres of longleaf forests to the current landscape.

Private landowners are becoming increasingly aware of the importance of longleaf forests through outreach efforts of government agencies, non-government organizations (NGOs) and other parties.  Financial and technical assistance from these groups has led to an increasing interest in establishing new longleaf forests and replacing other southern pine plantations with longleaf pine.  Establishment of any forest requires quality seed stock and successful planting measures.  Extensive research has been conducted with respect to longleaf seedling production and post-planting survival (Goodwin,1974; White, 1981; Amidon et al., 1982; Goodwin et al., 1982; Barnett, 1989; Boyer, 1989; Barnett, 1991; Barnett and McGilvray, 1993; Barber and Smith, 1996; McGuire and Williams, 1998; Cram et al., 1999; Rodriguez-Trejo et al., 2003; South et al., 2005).  Research and survival surveys have reported mixed survival rates between containerized and bare-root seedlings (South et al., 2005).  Barnett and McGilvray (1993) reported containerized southern pine seedlings were superior to bare-root seedlings in survival and growth.  The literature further suggests survival rates of containerized seedlings were superior to bare-root seedlings on stress prone sites (Goodwin 1976; Amidon et al., 1982; Barnett, 1984; Boyer, 1989; Barnett and McGulvray, 1997).  Longleaf pine seedling production has exhibited a major shift from bare-root to containerized seedlings over the past 20 years with containerized seedlings representing over 70% of production in 2004 (South et al., 2005).

Increasing awareness of the importance of longleaf restoration has led to increasing questions on the subject being posed to Extension Agents at land-grant universities within the native range of longleaf pine.  In order to address these questions at the local level, we implemented a long-term demonstration of longleaf pine production.  The demonstration will provide important biological, production and economic data to address questions of stakeholders.  This report focuses on seedling survival with regards to seedling types and planting densities.  The primary objectives of this portion of the study were to: 1) compare survival rates of bare-root and containerized longleaf seedlings at 6 mo and 1 yr after planting, 2) compare survival rates of seedlings with regards to planting density, and to 3) determine if there were interactions between seedling type and planting density in respect to survival.

 

Materials and Methods

 

Study Site

This study was conducted at Clemson University’s Sandhill Research and Education Center (Sandhill REC) located in Richland County, South Carolina.  Sandhill REC was originally established to serve as an agriculture research facility for crop production in drought prone soils.  Soils of the Lakeland soil series are the dominate soil type on the property.  Both planted and natural forest stands exist on the property.  Natural upland forests on the property are dominated by longleaf pine and turkey oak (Quercus laevis).  The location matches the xeric sand barrens and upland longleaf forest community type as described by Oswalt et al. (2012).  Planted forests are monocultures consisting of either longleaf pine or slash pine (Pinus elliottii).  The site selected for this study was an established pecan (Carya illinoinensis) orchard immediately prior to the initiation of this study.

 

Site Preparation

Pecan trees were cut with chainsaws (Husqvarna 455 Rancher, Husqvarna Professional Products, Charlotte, NC USA) and removed from the study site in the fall prior to planting.  A prescribed burn was conducted to clear vegetation (predominately bahiagrass (Paspalum notatum)) for ease of laying out plots.  A bulldozer (Dressta TD8, Liugong Dressta Machinery, Stalowa Wola, Poland) and scalper (Fesco P3HJR Pull Type Fire Plow, Oliver and Dahlman Equipment Co., Inc., Hastings, FL USA) were utilized to lay out rows within each plot.  Plots were not ripped.  Containerized and bare-root seedlings were planted by hand with a dibble (Model 69241 Jim-Gem® Containerized Dibble Bar and Model 69042 Jim-Gem® OST Dibble Bar, Forestry Suppliers, Jackson, MS USA).  Trees were planted in January of 2017.  This study was established to represent common practices of longleaf pine plantation establishment, as such no irrigation was used.

 

Competition Control

No additional control measures were taken to reduce herbaceous vegetation within the study area following planting.  Spot spray foliar applications of triclopyr were utilized to control callery pear (Pyrus calleryana) within the study plots.  Spot spray applications were conducted two times during the growing season.

 

Longleaf Seedlings

Bare-root seedlings were produced by the Georgia Forestry Commission at their Flint River Nursery located in Byromville, GA.  Bare-root seedlings were lifted on January 19, 2017.  Seedlings were stored in a walk in cooler until planting on January 24, 2017.  Containerized seedlings were grown in IP110 IPL Rigi-pots (IPL, Inc., Saint-Damien-de-Buckland, Quebec, Canada) by the South Carolina Forestry Commission at their Taylor Nursery in Trenton, SC.  The IP110 containers have 45 cavities per tray.  Cavities had a diameter of 1.61 inches (4.1 cm) and a depth of 4.72 inches (12.1 cm).  Containerized seedlings were obtained from Taylor Nursery on January 30, 2017 and planted the same day.

 

Study Design

The study area was divided into three one acre plots.  Each plot was randomly assigned to a planting density.  Planting densities were as follows: 908, 545, or 436 trees per acre (tpa; 2,240, 1,343, and 1,074 trees per hectare[tpha]), with respective plant spacings of 6 ft x 8 ft, 8ft x 10 ft, and 10ft x 10 ft (1.8 m x 2.4 m, 2.4 m x 3.1 m, and 3.1 m x 3.1 m, respectively).  Seedling type (containerized or bare-root) was alternated between rows within each plot.  These pseudo-replications allowed for side by side comparison of the two seedling types within each respective planting density.  Seedling type was replicated 10 times in the 908 tpa (2,240 tpha) plot, 12 replicates in the 545 tpa (1,343 tpha) plot, and 12 replicates in the 436 tpa (1,074 tpha) plot. Seedling survival was estimated for each row within every plot.  Survival estimates were obtained by taking the average of the first, middle and last 10 trees in each row to create a composite average for each individual row. In this study, rows (N=68) were the experimental units (N= 20 (6 ft x 8 ft), N=24 (6 ft x 8 ft), and N= 24 (10 ft x 10 ft)).  Survival estimates were collected at 6 mo and 1 yr after planting.  Data for 6 mo and 1 yr survival estimates were analyzed separately.  Analysis of variance was utilized to identify differences between seedling type and densities.   Interactions were evaluated using a two way factorial analysis of variance.  Statistical analysis were conducted using JMP 14 Statistical Software (SAS Institute, Inc., Cary, NC USA).  All tests were evaluated with an alpha value of 0.05.

 

Results

 

6-Month Survival

Survival estimates differed significantly between bare-root (m = 38.78%) and containerized (m = 99.43%) seedlings at 6 mo after planting (P < 0.0001). Mean survival estimates for seedling densities at 6 mo after planting were as follows 908 tpa (2,240 tpha) = 73.30%, 545 tpa (1,343 tpha) = 68.00%, and 436 tpa (1,074 tpha) = 65.77%.  Survival was not statistically different among seedling spacing (P = 0.1552).   Maximum 6 mo survival (100%) was observed within all three spacing blocks, but only in containerized seedlings.  Minimum observed 6 mo seedling survival was 20%, and was found in 545 tpa (1,343 tpha) and 436 tpa (1,074 tpha) plots for bare-root seedlings. There was no interaction between seedling type and stocking density on seedling survival at 6 mo after planting (P = 0.2974). 

 

1-Year Survival

Seedling survival continued to decrease for both seedling types throughout the growing season.  Seedling type significantly impacted survival at 1 yr after planting with mean survival at 27.78% for bare-root seedlings and 90.00% for containerized seedlings (P < 0.0001).  Survival estimates within planting densities were as follows: 908 tpa (2,240 tpha) = 64.00%, 545 tpa (1,343 tpha) = 55.31%, and 436 tpa (1,074 tpha) = 55.20%.  Seedling survival was not significantly different among plant densities (P = 0.1802).  Maximum observed 1 yr survival (100%) was observed within all three spacing blocks, but only in containerized seedlings.  Bare-root seedling 1 yr survival was 0% in two plots (1 – 545 tpa (1,343 tpha) and 1 – 436 tpa (1,074 tpha)).  The lowest observed survival in containerized seedlings was 70%, and was observed once in each of the three density plots.  The interaction of seedling type and planting density with regards to seedling survival at 1 yr after planting was not statistically significant (P = 0.0739).  Seedling survival data are compiled in Table 1.

 

 

Table 1. Longleaf seedling survival in the Sandhill region of South Carolina.  A comparison of seedling type and planting density.  Survival analysis for 6 mo and 1 yr after planting were conducted separately.  

	6-Month Survival			1-Year Survival
	Mean Survival	P-Value		Mean Survival	P-Value
Seedling Type		< 0.0001			< 0.0001
Bare-root	38.78%			27.78%
Containerized	99.43%			90.00%
Plant Spacing		0.1552			0.1802
6 ft x 8 ft	73.30%			64.00%
8 ft x 10 ft	68.00%			55.31%
10 ft x 10 ft	65.77%			55.20%
Interaction of Seedling Type and Planting Density		0.2974			0.0739

 

 

Discussion

 

Seedling survival observations in this study were within the range of other survival studies reported by South et al. (2005).  However, our findings indicated a greater difference (62.22%) in survival between containerized and bare-root seedlings at 1 yr after planting than previously reported survival studies (Table 2).  McGuire and Williams (1998) observed a 54% difference between seedling types planted in lignite mine spoils in east Texas.  Ram et al. (2006) described lignite mine spoils as problematic due to their fine particle size, poor porosity, nutrient deficiency, and inability to support biological activity.  While these mine spoils are different from the sands of the Lakeland soil series in this study, they are similar with regards to their tendencies to provide stressful media for plant growth.  South et al. (2005) reported that containerized longleaf seedlings typically exhibit higher survival than bare-root seedlings, and this was more noticeable on adverse sites.  Likewise, several other studies report that survival rates of containerized seedlings were greater than bare-root seedlings on drought prone or marginal soils (Goodwin, 1976; Amidon et al., 1982; Barnett, 1984; Boyer, 1989; Barnett and McGilvray 1997). 

 

 

Table 2. Reported 1 year survival estimates for bare-root and containerized longleaf pine seedlings.*

Containerized Survival (%)	Bare-root Survival (%)	Difference in Survival (%)	Location	Reference
36	14	+ 22	Louisiana & Texas	Amidon et al. (1982)
79	79	0	Louisiana	Barnett (1991)
84	79	+ 5	Louisiana	Barnett (1991)
85	70	+ 15	North Carolina	Goodwin (1980)
56	2	+ 54	Texas	McGuire and Williams (1998)
88.3	55	+ 33.3	Georgia	Rodriguez-Trejo and Duryea (2003)
24.6	10.3	+ 14.3	Georgia	Rodriguez-Trejo et al. (2003)
50	55	- 5	Georgia	Rodriguez-Trejo et al. (2003)
90	27.8	+ 62.2	South Carolina	Heaton and Bean (2019)

*Table modified from South et al. (2005) Table 1: Survival of container-grown and bare-root seedlings of P. palustris in the southern United States.

 

Seedlings that did not survive were hand pulled to inspect for potential causes of mortality.  We readily discovered that expired bare-root seedlings frequently exhibited J-shaped tap roots.  The agricultural history of the site likely led to the development of a hard pan.  Bare-root seedlings appeared to have experienced inability to penetrate this hard pan and the root system bent upwards, nearly reaching the soil surface in several instances.  We were unable to identify this J-root condition in expired containerized seedlings.  Unfortunately, biological data (tap root length, diameter, etc.) that would provide insight into the difference between containerized and bare-root seedlings with regards to root growth were not collected.  Establishment practices described by Franklin (2008) indicate the need for sub-soiling/ripping to break up hard pans in former agriculture fields before planting.  Our observations support this practice for successful stand establishment.

 

 

Conclusions

 

Based on the results of this study and others, landowners should thoroughly evaluate soil and environmental conditions of the sites they intend to plant with longleaf pines.  Sites with drought prone soils should be planted with containerized seedlings.  Additional costs associated with containerized seedlings can be justified considering the initial planting will likely supply a sufficient density for commercial timber stand establishment.  Bare-root survivals on marginal sites are likely to require one or more re-plantings to reach sufficient stand densities, thus discounting the initial planting cost savings

Planting densities analyzed in this study are common stocking densities for commercial softwood timber stands in the southeastern US (SC Forestry Commission). We were unable to identify differences in seedling survival with regards to planting density.  Based on this information, landowners should be able to plant at densities that best match their management goals without concern for impacting seedling survival. 

Efforts to identify interactions of seedling type and plant spacing were not statistically significant.  However, the low test value (P = 0.0739) observed for 1 yr survival data suggests the possibility that a mild interaction of the two factors was present.  Further interpretation of this potential interaction may become more apparent with continued collection of survival data within the stand over the grow-out period. We also anticipate the interaction was influenced by the small sample size (N= 68).  Future work with expanded sample sizes may provide clarity to the interaction of seedling type and plant spacing on seedling survival. It is important to note this study consisted of data from a single year at a single location.  The results of this study provide useful information, but they are limited with regards to location and local climate factors.  

 

Literature Cited

 

Amidon. T.E., Barnett, J.P., Gallagher, H.P., and McGilvray, J.M. (1982).  A field test of containerized seedlings under drought conditions.  Proceedings of the Southern Containerized Forest Tree Seedlings Conference.  USDA Forest Service Gen. Tech. Re SO-37, pp. 139 – 144.

Barber, B.S., and Smith, P.  (1996).  Comparison of first year survival between container-grown and bare-root longleaf pine seedlings outplanted on a site in southeast texas.  Proceedings of the First Longleaf Alliance Conference on Longleaf Pine.  The Longleaf Alliance.  Alabama, USA.  pp. 50 -51.

Barnett, J.P.  (1984).  Relating seedling physiology to survival and growth in container-grown southern pines.  In: Duryea, M.L., Brown, G.N. (Eds.), Seedling Physiology and Reforestation Success.  Martinus Nijhoff/Dr. W. Junk Publishers, Dordrecht, The Netherlands, pp. 157 – 176.

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