People, in general, don’t like change. Foresters are no different. Over the years, I’ve seen a lot of forest management done for no other reason than “that’s the way we have always done it.” One area that I’ve found this to be especially true is loblolly pine planting density. I’ve looked at a mind-numbing number of planted loblolly pine stands in the Piedmont of the Carolinas over the past 25 years. The one thing almost all of the loblolly stands I’ve seen have in common is that they were planted with too many trees per acre (TPA) for the final objective the landowner had in mind. Planting at too high a density results in overcrowding before a thinning harvest is economically viable. Trees that develop in overcrowded conditions have poor root and crown development, which in turn, results in compromised health, diameter growth stagnation, and increased risk for damage by storms and insects.
Planting at too high a density results in overcrowding before a thinning harvest is economically viable.
Considerations Before Planting
There is no one-size-fits-all planting density. Your initial tree spacing should be determined by:
Your management objectives. Are you focused on timber production, wildlife, or both?
Markets. Are there markets available for the forest products you intend to grow?
Terrain and Acreage. Is a future thinning harvest physically or economically feasible?
How a Stand Gains Density and Volume
Productivity (fertility) limits the amount of biomass a given area will produce. Therefore, a tree plantation will only grow a given amount of forest product tonnage per acre before it peaks, stagnates, then declines.
A forester checks a stand’s density by measuring its basal area (See Figure 1 below). Basal area correlates very closely to stand volume and health. As a stand grows, the basal area increases. In the early stages of development, the more trees you have per acre, the faster the basal area (and volume) levels rise. While fast volume accumulation is a plus, the downside is that the volume is distributed among many trees, so the diameters will be smaller on average than a stand that has fewer trees per acre. The key to reaching your plantation forestry goals is establishing a stand with the right number of trees per acre for your management goals.
Given enough time, the basal area (and volume) in a stand with fewer trees per acre (TPA) will catch up with that of a higher TPA stand but will be comprised of trees with larger diameters on average. As a general rule, timber stands begin to show the adverse effects of overcrowding (slowed diameter growth, thinning crowns) when basal area surpasses 120 ft² per acre.
Planting Density Recommendations
For the following TPA planting recommendations, I’ve assumed proper site preparation is in place. A couple of extra feet of spacing between trees may not seem like much but will make a dramatic difference in the future health, form, size, and volume gain.
Pine Pulpwood Production; 680 TPA (8’x8’ spacing; 64ft² growing space per tree ).
If your land is in an area where pulpwood markets are strong, you may want to consider this high-density spacing, which will reach an optimal harvest volume at a relatively early age, at which point it should be clearcut. By the time this stand reaches the projected 15 to 20-year-old harvest age, overcrowding results in trees with poor live crown development, stagnating diameter growth, and weak fiber strength. Thinning is not recommended, especially if you live in an area prone to winter ice storms. If thinned, the diameter growth response will be lackluster, and the stand will be highly susceptible to wind and ice damage.
In a loblolly pine spacing study (Amateis and Burkhart) that compared planting densities from 2,272 TPA to 303 TPA, this density was shown to have the highest total volume of all spacings by age 20. You’ll gain no advantage by planting more trees per acre—you’ll spend more for poorer results.
Pulpwood, Chip-n-Saw, and Sawtimber Production; 454 TPA (8’x12’ spacing; 96ft² growing space per tree).
A study (Adams and Clason) found this spacing to have the best combination of growth characteristics for growth, form, natural limb pruning, and ice damage resistance. Many stands in the Piedmont region get planted at an 8’x10’ spacing (545 TPA), but I prefer the slightly less dense 8’x12’ (454 TPA) or 10’x10’ (436 TPA) spacings for the following reasons:
Diameter growth stagnation is less likely to occur before the first thinning harvest.
Heights will be greater and average diameters larger at the time of the first thinning, making the thinning harvest more attractive to buyers.
The lower density spacing results in wind-hardened trees with slightly more taper, making a stand more resistant to ice and storm damage after thinning.
Sawtimber Production, Wildlife Habitat, and Sites with Limitations; 303 TPA (12’x12’ spacing; 144ft² growing space per tree).
At this spacing, a substantial amount of sawtimber can be grown by age 25 without a thinning harvest.
Consider this low-density planting spacing in the following situations:
Small acreages (less than 20 acres), where a thinning harvest may not be economically feasible due to low harvest removal volumes.
Sites with terrain limitations. Tracts with steep or eroded hillsides are impossible to thin effectively.
On tracts in areas with poor or no pulpwood markets.
Areas prone to ice storms—trees will be more resistant to ice and wind damage than high-density plantings.
In cases where wildlife habitat is a priority—this wider spacing will retain quality early successional wildlife habitat longer than high-density plantings.
Summary
Before going with a one-size-fits-all planting regime, carefully consider your objectives and investment timeline. Contact me and I'll be glad to discuss your land, consider your goals, and make recommendations for your existing woodland and future reforestation projects.
