Every year, around a million acres of loblolly pine forests are planted by tree farmers in the southeastern United States (McKeand et al. 2021). When purchasing seedlings, it is critical to choose families with genetics that have the appropriate amount of cold hardiness. From the Coastal Plain to the Piedmont, North Carolina has several growing environments, so understanding the right genetics for your land is paramount.

Loblolly pine families with ancestry from more coastal and southerly latitudes tend to be less cold hardy. Cold hardiness is affected by several biological factors. One important factor is the timing of bud break in the spring and the timing of bud set in the fall. Families with more Coastal/southern ancestry tend to break bud earlier in the spring and set bud later in the fall, which can expose succulent tissue to harsh cold conditions in more northern/inland regions. Cold injury can cause shoot die back, leading to loss of growth or stem defects. Injury is more likely when extreme cold weather is preceded by several days of warm weather.

Cold hardiness also determines how well a pine family will fare in ice or snowstorms. Families with warmer ancestry will have more broken branches and stems, which cause defects like forks and crooks. These defects can downgrade trees from high-value products (like sawtimber) to low-value products (like pulpwood). In severe storms, no genetics will be immune to damage, but families with more cold hardiness will incur less damage.

Tip dieback can occur when loblolly pine families with Coastal-source ancestry break bud too early in the spring and are exposed to sudden temperature drops.

Photo credit Austin Heine.

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Tip dieback can occur when loblolly pine families with Coastal-source ancestry break bud too early in the spring and are exposed to sudden temperature drops.

Photo credit Austin Heine.

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Ice and snow storms can cause crooks that downgrade the tree from a potential solid-wood product, like sawtimber or poles, to a low value product like pulpwood.

Photo credit Austin Quate.

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Ice and snow storms can cause crooks that downgrade the tree from a potential solid-wood product, like sawtimber or poles, to a low value product like pulpwood.

Photo credit Austin Quate.

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Families with Coastal or more southerly ancestry tend to grow faster, so there is a temptation to plant genetics with warmer ancestry. Research pioneered by the US Forest Service indicates that genetics can be moved up to 5° F colder (in terms of average minimum winter temperature) than their ancestry with little to no increased risk of cold damage, whereas movement of 10° F or more has a high risk of cold damage (Schmidtling 2001).

For example, the maps in Figure 1 show the risk of cold damage for two genetic options, one with ancestry from the South Carolina Coastal Plain, and another from the South Carolina Piedmont. The average minimum temperature for these genetic options is located on the isotherm lines labeled as “0”. Each isotherm line to the north/inland is 5° F colder that the genotype’s ancestry. There is no increased risk of cold damage within the -5° F isotherm. In between the “-5” and “-10” isotherm, the risk of cold damage increases quickly. The family on the left would be appropriate for planting in the NC Coastal Plain, but not the NC Piedmont. The family on the right could be planted in both regions, but will tend to have slower growth than the Coastal family if planted in the NC Coastal Plain.

Figure 1. Transfer distance maps for two loblolly pine families, one with ancestry from the SC Coastal Plain (left) and another with ancestry from the SC Piedmont (right). The isotherm line at 0 indicates the ancestry origins of the family. There is no little increased risk of cold damage if planting the family within the -5 isotherm, but risk increases rapi

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Figure 1. Transfer distance maps for two loblolly pine families, one with ancestry from the SC Coastal Plain (left) and another with ancestry from the SC Piedmont (right). The isotherm line at 0 indicates the ancestry origins of the family. There is no little increased risk of cold damage if planting the family within the -5 isotherm, but risk increases rapi

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Once you have identified your cold hardiness needs, seedling family options can be compared based on their growth rate, disease resistance, and stem form, as well as price. Increasingly, controlled-pollinated seedlings have become popular due to their elite genetic scores. However, good open-pollinated seedlings can be more cost-effective for some landowners.

Tree farmers with access to strong pulpwood markets may select seedling based on growth rather than stem form. Landowners with poor pulpwood markets might consider wider planting spacings, in which case stem form traits (like straightness and forking) will need more emphasis to ensure an adequate number of trees are free of defects. Disease hazard maps for fusiform rust should be consulted to determine the appropriate emphasis to place on resistance (Walker and McKeand 2018).

When comparing genetic options, foresters/landowners should request Performance Rating System (PRS^TM) sheets from their seedling supplier. These sheets are certified by the Cooperative Tree Improvement Program at NC State University. The scores are developed using measurements from thousands of trees planted in hundreds of genetic test trials across the southeastern United States.

A study installed at the Umstead Research Station near Butner, NC evaluated 20 families with ancestry having mean winter temperatures that ranged from 0° F to 10° F warmer (Walker et al. 2024). At age 11, the total biomass yield (weight of trees in green tons per acre) was generally greater for trees with warmer ancestry up until around 7° F. Trees with ancestry warmer than 7° F had less biomass due to impaired growth from cold damage (Figure 2, left). Trees with warmer ancestry tended to have a lower percentage of defect-free trees (Figure 2, right). Trees from the local source (0° F transfer distance) had around 55% of trees free of defects (i.e., the potential to grow into sawtimber). Only 40% of the trees from families with 10° F warmer ancestry had sawtimber potential. The stand experienced several severe winter storms, which caused defects such as forks, crooks, and broken stems. However, there was a large amount of variability in these trends due to different genetic scores for growth, straightness, and disease resistance among the families.

Figure 2. The effect of transfer distance on biomass yield (left panel) and percent of trees without defect (right panel). Trees from warmer ancestry tended to produce more biomass, unless their ancestry was from more than 7° F warmer. Trees from warmer sources tended to have more defects and produced fewer trees with the potential to grow into sawtimber.

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Figure 2. The effect of transfer distance on biomass yield (left panel) and percent of trees without defect (right panel). Trees from warmer ancestry tended to produce more biomass, unless their ancestry was from more than 7° F warmer. Trees from warmer sources tended to have more defects and produced fewer trees with the potential to grow into sawtimber.

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Some seedling vendors offer families that are a controlled-pollinated cross between Coastal and Piedmont parents. Research has indicated that these “hybrids” tend to be intermediate for both growth and cold hardiness, and can outperform the pure Piedmont families on many sites, as long as they are not too cold for the source ancestry (Shalizi et al. 2022). The cold-hardiness maps described above still apply for these hybrids.

McKeand, S.E., K.G. Payne, A.J. Heine, R.C. Abt. 2021. Economic Significance of Continued Improvement of Loblolly Pine Genetics and Its Efficient Deployment to Landowners in the Southern United States. Journal of Forestry. 119(1):62–72. ↲

Schmidtling, R. C. 2001. Southern pine seed sources. Gen. Tech. Rep. SRS-44. Asheville, NC: US Department of Agriculture, Forest Service, Southern Research Station. 25 p. ↲

Shalizi, M. N., K. G. Payn, T. D. Walker, F. Isik, A. J. Heine, and S. E. McKeand. 2022. Long-term evaluation of intra-and inter-provenance hybrids of loblolly pine in the Piedmont region of the southeastern United States. Forest Ecology and Management. 522:120469. ↲

Walker, T. D., J. A. Maynor, F. Isik, A. J. Heine, R. W. Whetten, K. G. Payn, T. A. Quate, and S. E. McKeand. 2024. Stem Defect Rates and Ice Storm Damage for Families of Pinus taeda from Coastal and Piedmont Provenances Planted on a North Carolina Piedmont Site. Forest Science. :fxae016. ↲

Walker, T. D., and S. E. McKeand. 2018. Fusiform rust hazard mapping for loblolly pine in the southeastern United States using progeny test data. Journal of Forestry. 116(2):117–122. ↲