The “correct” planting density for pines: depends on your objectives and who you ask

This paper may be cited as:

South, D.B. 2003. “Correct” planting density for loblolly pine depends on your objectives and who you ask.

Forest Landowner Manual: 34:46-51.

“Correct” planting density for loblolly pine depends on your objectives and who you ask

David B. South Professor, School of Forestry and Wildlife Sciences, Auburn University, AL

I met a landowner who was getting out of the cattle business and wanted to plant trees for some income, to encourage wildlife and for recreation. She asked me how many loblolly pine seedlings she should plant on her pastureland. I replied “it depends on your objectives and who you ask.” You see, there are two schools of thought regarding the number of seedlings per acre (SPA). One school recommends high planting densities (>500 SPA) and the other recommends low stocking levels (<400 SPA). Most foresters from the “plant-‘em thick and cut-‘em quick” school recommend planting 650 SPA or more. Since I am from the “plant-‘em thin and you’ll likely win” school, I think 346 SPA (14 feet between rows and 9 feet between trees) would be a better target. When I was asked why there was such a difference, I said the difference could be due to a number of reasons including: (1) holding on to traditional practices; (2) assuming a low price ratio between sawtimber and pulpwood ($S/P); (3) using poor quality seedlings; (4) relying on an unrealistic growth-and-yield program; (5) assuming logging costs do not vary with log size; (6) assuming everyone’s land is close to a mill; and (7) a fear that a low stocking will reduce both wood quality and stumpage values.

Early tree planting recommendations in the US were handed down to us by European foresters. Foresters in Scotland have been planting about 1,000 SPA for over a century (6.6 foot rows and 6.6 feet between trees) and the “Forestry Handbook” (published in 1953 by the Alabama Forestry Council) gave almost the same recommendation (7 foot rows and 6 feet between trees). In the past, the objective of planting “thick” was to “maximize pulpwood production” on lands owned by paper companies. However, if the objective of a private landowner is to “optimize sawlog production” with reduced establishment costs, then most computer models (Figure 1) and most spacing trials point to low planting rates.

When the market results in a small $S/P ratio (e.g. 2), it makes sense to concentrate on volume production instead of sawtimber production. However, stumpage $S/P ratios tend to be higher than ratios based on prices obtained at the mill. For example, a mill $S/P ratio can be 2.8 ($56/ton vs $20/ton) while the stumpage ratio is 7.5 ($45/ton vs $6/ton). Historically, pine sawtimber has increased in real value over time while pulpwood has remained the same. Those of us from the “plant-‘em-thin” school tend to use stumpage $S/P ratios when determining the economic optimum SPA for private landowners. In contrast, members of the “plant-‘em-thick” school often use lower mill $S/P ratios and fixed logging costs (i.e. the same logging cost for all tree sizes) to determine the SPA that company foresters should use. In one case, a mill $S/P ratio of 2 was used to conclude 1,294 SPA would be the optimum stocking level in Georgia.

She asked why the distance to the mill would affect SPA recommendations. The difference relates to how much money it takes to get the wood to the mill. Currently, about two-thirds of the value of pulpwood at the mill is the cost of harvesting and trucking to the mill (Figure 2). In some distant locations, transportation costs can eat up all the value of pulpwood. Therefore, it makes more sense to grow mostly sawtimber and chip-n-saw if your land is far from a chip-mill.

She asked why seedling quality would affect the recommended SPA. In the old days, nursery managers often grew seedlings too thick (30 to 50 seedlings per square foot) in seedbeds and, therefore, tree planters in the South became accustomed to planting small seedlings with small roots. First-year survival of 70% was deemed acceptable during the 1950s but in dry years, or on weedy sites, poor quality seedlings and poor hand-planting practices resulted in low survival. This often resulted in having to conduct a replant (most foresters do not like to interplant). To avoid having to start over, foresters would plant twice as many trees as needed and would plan on fixing any over-stocked stands by removing the extra trees during the first thinning. Today, some nurseries grow “morphologically improved” loblolly pine at around 19 seedlings per square foot and when lifted properly, these seedlings have large roots and a greater capacity for survival. By planting large-diameter seedlings (root-collars 6 to 10 mm) and providing good planting supervision, many farmers in New Zealand hand-plant 330 SPA using planting spades.

Next she wanted to know why I recommend a rectangular instead of square spacing. For an old pasture, I recommend a scalping treatment combined with machine planting. Machine planting usually results in planting trees about 6” deeper than they grew in the nursery [without machine planting, ripping should be conduced prior to hand planting because this allows the crew to plant seedlings deeper]. A rectangular spacing will have economic advantages when using machines during establishment. For large acreages, it will be about 42% quicker to machine-plant 14-foot rows instead of 8-foot rows (tractor speed does not change). Therefore, when renting equipment, it will cost proportionally less to use wide row spacings. Herbicide costs will also be less when applying a banded treatment. The wide row spacings also make it easier to move equipment through the stand after the canopy closes. A 14-foot row spacing allows thinning to be selective while 8-foot rows require a row-thinning (which removes all large, straight trees in the row). Of course when planting 346 SPA, seedling costs will be half that when planting 700 SPA.

Since wildlife habitat is often a stated objective of landowners, the 346 SPA spacing will be more beneficial to some browsers than a spacing of 650 SPA. Wildlife studies find the amount of forage is related to tree stocking. Up until age 25, there should be more sunlight reaching the ground when using the lower stocking.

She said one forester warned her against planting 346 SPA because the faster growth would reduce specific gravity, reduce average tree heights, reduce the number of rings per inch and increase the live-crown ratio. In addition, average branch size would increase and the larger knots would produce weaker 2 by 4s. She asked the forester how much of a decrease in lumber or stumpage value would occur by planting at the lower stocking and he admitted he did not know; that was not taught at his forestry school. He also could not say how much bigger the knots would be when going from 650 to 346 SPA.

First of all, the forester is wrong when he said the lower stocking would result in wood of lower specific gravity. This myth has been in the forestry field for over 50 years. Qualified researchers who compare pine trees of the same age conclude “there is no inherent relationship between growth rate and specific gravity” (Table 1). In fact, sometimes when pines grow fast in wet summers they produce more “late wood” and have higher specific gravity than pines growing in regions with dry summers. However, specific gravity is affected by harvest age (since it affects the amount of juvenile wood percentage). Loblolly trees in the Atlantic Coastal plain may contain 100% juvenile wood when harvested at age 10 (with a specific gravity of 0.47). In contrast, at age 31, the tree may have a specific gravity of 0.54 and contain 35% juvenile wood. Pines harvested at age 20 will have lower specific gravity wood than pines harvested at age 31, but this difference is due to age, not growth-rate.

Table 1. Effect of growth rate on diameter at breast height (DBH), height and weighted specific gravity at breast height of 30-year old loblolly and 35-year old slash pine (Clark and Saucier 1989).

Species	Initial trees per acre	Average DBH (inches)	Average Height (feet)	Specific gravity
Loblolly	1210	8	68	0.46
	650	9	71	0.46
	435	9.7	71	0.47
	303	11.3	73	0.47
Slash	1210	7.3	60	0.51
	650	8.9	71	0.52
	435	10.7	74	0.54
	194	12.9	76	0.54

Planting 346 SPA will not result in shorter trees but it will result in wider growth rings than 650 SPA (Table 2). However, most sawmills typically buy wood based on weight and size, not on rings per inch (there are a few exceptions). The landowner who sells 56-year old 14” sawlogs by weight at the mill-gate will likely not benefit financially from growing the trees slowly. To check this out, I told her to call her local sawmill to see how much difference there would be in the price paid for a ton of 14” sawlogs with 4- versus 8-rings per inch. Even so, some researchers report that Grade 1 logs (12 to 16-inchs in diameter) that are 50-years-old might be worth $2/green ton more to a sawmill owner than 25-year-old Grade 1 logs of the same diameter.

Knot size is related to initial stocking but the effect is slight until stocking drops below 150 SPA (Figure 3). For example, by age 38, the average branch size for 353 SPA may only be 0.12 inch greater than at 714 SPA. I doubt many people can even detect this small difference when walking through a plantation. Many “plant-‘em-thick” foresters imagine the difference will be much greater because they see large branches on pines that are “open-grown.”

Pruning is currently practiced by Weyerhaeuser, Gulf States Paper, and a few other companies. Although knot free wood is valuable to a sawmill owner, it will likely not prove as valuable to a private landowner who has invested in pruning. When a truck of pruned logs is scaled, it can be difficult to tell if the trees have been properly pruned. As a result, the landowner likely will be unable to sell pruned trees for twice that of unpruned trees. In the South, there are no pruning certification programs like those in New Zealand to help farmers obtain higher prices for their pruned logs.

Some studies show a relationship between tree stocking and the strength of 2x4s made from unpruned loblolly and slash pines planted at low stockings. USFS researchers in Georgia reported that for loblolly pine, harvested at age 38, a stand planted at 680 SPA produced wood that was worth 4.5% more PER SAWN BOARD FOOT to the sawmill owner than trees planted at 303 SPA (Figure 4). I seriously doubt this small difference would ever be reflected in either the stumpage price or the gate price. Besides, 10% more sawtimber volume was produced at 303 SPA (23.5 MBF/acre) than at 680 SPA (21.3 MBF/acre). This greater sawtimber volume more than made up for decrease in wood quality. Not only was the 303 SPA stand worth $300 more per acre, but it cost approximately $41 less per acre to plant (@ 5 cents per seedling and 6 cents to plant) than the 680 SPA stand.

She was curious why the term “cut-‘em-quick” was part of the “plant-‘em-thick” school. This is in part due to the southern pine beetle. Stressed pine stands are susceptible to beetle attack and the risk of attack increases with both stocking rate and age (Figure 5). Precommercial thinning reduces the stress caused by overstocking but most landowners are reluctant to pay $75 per acre for a treatment that receives no income. In today’s market, landowners might receive no income from the first pulpwood thinning. Therefore, many stands owned by private landowners are not thinned and in dry years this can lead to beetle losses. Some landowners clear-cut stands a year or two quicker than normal to reduce beetle hazard. In some “hot-spots,” some companies clear-cut their thick plantations as quick as 17-years in order to reduce the risk of beetle infestations.

The effect of compounding is another economic reason to “cut-‘em-quick.” Forest economists often use discount rates of 6% or more, and this explains why the final harvest is often less than age 26, regardless of which school is involved. Although lumber from a 50-year-old, 12” diameter tree is worth more to a sawmill than lumber from a 25-year-old, 12” diameter tree, the effect of discounting makes a 25-year-rotation more attractive to a landowner who plants old-cropland with pines (Figure 6). Landowners who are willing to accept a lower discount rate (e.g. 2% to 4%) can justify longer rotations.

I showed her some of the differences that might occur when following the recommendations of the two schools (Table 2). The risk from beetle infestation is almost never taken into account in economic analyses of planting density, but it was included in the 650 SPA example (i.e. the stand is harvested two years quicker than normal). I asked her to think about the differences on this table before deciding on which planting density to use. I gave her my business card and she said she would let me know her final decision. About a month later she called to say she decided to plant 109 seedlings per acre (40 foot rows and 10 feet between trees). I was taken back and said “nobody plants pines that low… the lowest pine stocking I know is Carter Holt Harvey in New Zealand who currently plants 220 pine seedlings per acre!” She then said, “I should have told you I decided to keep my cows and plant longleaf pine for silvopasture.” I replied “Oh, of course landowner objectives do affect planting density.”

David South is a Professor in the School of Forestry and Wildlife Sciences at Auburn University. He received his Ph.D. in Forestry from Auburn in 1983. He is the author of over 100 articles on nursery management and reforestation. He is a member of AU’s Southern Forest Nursery Management Cooperative. Dr. South can be reached at southdb@auburn.edu or at 334-844-1022. For more information and references, visit www.forestry.auburn.edu/sfnmc/class/density.htm

Table 2. A hypothetical comparison of wood properties, stand characterizes and economics of two unthinned loblolly pine stands that vary in initial stocking.

	School of Thought
	Plant-‘em thick	Plant-‘em thin
WOOD PROPERTIES
Average rings per inch	4.8	3.9
Specific gravity (at DBH)	0.45	0.45
Average branch size (inch)	1.02	1.14
Basal area in juvenile wood (12 rings)	58%	54%
Modulus of elasticity (lbs/in²)	1,300,000	1,300,000
Moisture content	120%	120%
Pulp yield (sulfate process)	48%	48%
STAND CHARACTERISTICS
Planted trees per acre	650	346
Row spacing (feet)	8	14
Initial survival	85%	85%
Final survival at harvest	69%	76%
Average DBH - age 12 (inches)	5.6	6.2
Average height - age 12 –(feet)	34.2	35.1
Average DBH at harvest (inches)	8.8 (age 21)	10.7 (age 23)
Average height at harvest	62.4	69.8
Live crown ratio at harvest	32%	33%
Beetle hazard index (age 21)	116	102
Harvest age (years)	21	23
Sawtimber per acre (tons)	10	47
Chip-n-saw per acre (tons)	33	10
Pulpwood per acre (tons)	74	60
Total merchantable (tons)	117	117
Tons/acre/year	5.6	5.1
ECONOMICS
Seedling cost per acre	$33	$18
Machine planting cost per acre	$50	$45
Herbicide cost (4’ band) per acre	$50	$45
Total establishment costs per acre	$133	$108
Mill value – Sawtimber ($46/ton)	460	2162
Mill value - Chip-n-saw ($41/ton)	1353	410
Mill value - Pulpwood ($20/ton)	1480	1200
Harvesting efficiency (trees per ton)	3.85	2.25
Harvest cost - sawtimber ($9/ton)	-90	-423
Harvest cost - chip-n-saw ($10/ton)	-330	-100
Harvest cost – pulpwood ($14/ton)	-1036	-840
Average harvesting cost per ton	$12.44	$11.65
Net revenue at harvest	$1837	$2409
Net Present Value	$407	$523
Bare Land Value	$576	$709

6% interest rate. $5/acre annual tax; $5/acre hunting lease.

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