EFFECT OF PRUNING SEVERITY ON THE ANNUAL GROWTH OF HYBRID POPLAR

Clint Shock, Erik Feibert, and Jake Eaton
Malheur Experiment Station
Oregon State University
Ontario, OR, 2000

Summary

Hybrid poplar (clone OP-367) planted at 14-ft x 14-ft spacing was submitted to five pruning treatments. Pruning treatments consisted of the rate at which the side branches were removed from the tree to achieve an 18-ft branch free stem. Starting with a 6-ft (from ground) pruned stem, the trees will be pruned to 18 ft in either 3, 4, or 5 years. Starting in March 2001, the side branches on the stem were pruned to a height of 9, 12, or 15 ft. In subsequent years the trees will be pruned in 3-ft increments annually. A check treatment where trees will be left pruned to only 6 ft is included. Another treatment compares the effect of pruning during tree dormancy to pruning after leaf out. In 2001 the treatments ranged in pruning severity from 22 to 47 percent of the total tree height. In 2001 none of the treatments reduced tree growth. Pruning after leaf out reduced epicormic sprouting and did not reduce tree growth or cause pest damage at the pruning wound.

Introduction

With reductions in timber supplies from Pacific Northwest public lands, sawmills and timber products companies are searching for alternatives. Hybrid poplar wood has proven to have desirable characteristics for many nonstructural timber products. Growers in Malheur County have made experimental plantings of hybrid poplar and demonstrated that the clone OP-367 (hybrid of Populus deltoides x Populus nigra) performs well on alkaline soils for at least 4 years of growth. Research at the Malheur Experiment Station during 1997-1999 determined optimum irrigation criteria and water application rates for the first 3 years (Shock et al., 2000).

Pruning of the side branches of trees allows the early formation of clear, knot-free wood in the tree trunks and increases their value as saw logs. The amount of live crown removed in 1 year might have an effect on tree growth. More severe pruning might improve the efficiency of the pruning operation (fewer pruning operations to reach the final pruning height), but could reduce growth excessively. The timing of pruning could also affect the amount of epicormic sprouting during the season, wound healing, and insect damage at wound sites. The objective of this study was to evaluate the effect of pruning severity and timing on tree growth.

Materials and Methods

The trial was conducted on a Nyssa-Malheur silt loam (bench soil) with 6 percent slope at the Malheur Experiment Station. The soil had a pH of 8.2 and 0.8 percent organic matter. The field was planted to wheat for the 2 years prior to 1997 and before that to alfalfa. The field was marked using a tractor, and a solid-set sprinkler system was installed prior to planting. Hybrid poplar sticks, cultivar OP-367, were planted on April 25, 1997 on a 14-ft x 14-ft spacing. The field was used for irrigation management research (Shock et al., 2000) and groundcover research (Feibert et al., 2000) from 1997 through 1999. All side branches on the lower 6 ft of all trees had been pruned in February 1999.

In March 2000, the field was divided into 20 plots that were 6 rows wide and 7 trees long. The plots were allocated to five irrigation treatments that consisted of microsprinkler irrigation with three irrigation intensities and drip irrigation. The microsprinkler-irrigated plots used the existing irrigation system. For the drip-irrigated plots, either one or two drip tapes (Nelson Pathfinder, Nelson Irrigation Corp., Walla Walla, WA) were laid along the tree row in early May 2000. The management of the irrigation trial is discussed in the previous report.

For the pruning study, only plots in the two wetter microsprinkler-irrigated treatments and the drip-irrigated treatments were used. The middle two rows in each irrigation plot were assigned to pruning treatment 3 (Table 1). The remaining two pairs of border rows in each plot were randomly assigned to pruning treatments 2, 4, and 5. The pruning treatments were replicated eight times. The trees in treatments 2, 3, and 4 were pruned on March 27, 2000. Trees in treatment 5 were pruned on May 16, 2000. Trees were pruned by cutting all the side branches up to the specified height measured from ground level. The side branches were cut using loppers and pole saws. An additional four plots, in which the trees would remain pruned only to 6 ft, were selected for a check treatment (treatment 1).

The five central trees in the middle two rows and the five central trees in each inside row of each border pair in each plot were measured monthly for diameter at breast height (DBH) and height. Wood volumes were calculated for each of the measured trees in each plot using an equation developed for poplars that uses tree height and DBH (Browne, 1962). The trees in treatment 5 were observed for insect damage at pruning cuts. Sprouts (epicormic branches) formed during the season on the pruned length of the stem of trees in treatments 3 and 5 were removed and weighed in the winter of 2000-2001. The amount of time needed to remove the sprouts in each plot of treatments 3 and 5 was recorded. The length of live crown (length of unpruned trunk) at pruning was calculated by subtracting the 6 ft pruned in February of 1999 from the total height. The percentage of total height in live crown after pruning was calculated by subtracting the pruning height from the total height at pruning. Growth increments for height, DBH, and wood volume for 2000 were calculated as the difference in the respective parameter between October 1999 and October 2000.

Results and Discussion

The percentage of the live crown pruned in the 2000 season ranged from 0 percent, for treatments 1 and 2, to 28 percent for treatment 4 (Table 2). The percentage of the total height pruned ranged from 22 percent for treatment 2 to 47 percent for treatment 4. The percentage of the total height pruned includes the 6 ft pruned in February 1999. The pruning treatments did not have any significant effect on tree height or wood volume increment in 2000 (Table 2). The check treatment had a significantly higher DBH increment than the other treatments. This could be due to the trees in the check treatment being released from past unfavorable experimental treatments. A regression analysis found a negative correlation between percent of total height pruned and DBH increment (Fig. 1). Most of the variability in growth can be explained by factors other than pruning.

The lack of significant effects of pruning severity on tree growth is consistent with the Oregon State University Extension recommendation to limit pruning to 50 percent of total height (Hibbs, 1996).

It took significantly longer to prune the sprouts on trees that were pruned when dormant compared to trees that were pruned in mid-May (Table 2). Trees that were pruned when dormant also had a higher weight of sprouts. There were no insect damage or unhealed wounds at the pruning cuts in either pruning timing treatment.

Literature Cited

Browne, J.E. 1962. Standard cubic-foot volume tables for the commercial tree species of British Columbia. British Columbia Forest Service, Forest Surveys and Inventory Division, Victoria, B.C.

Feibert, E.B.G., C.C. Shock, and L.D. Saunders. 2000. Groundcovers for hybrid poplar establishment, 1997-1999. Oregon State University Agricultural Experiment Station Special Report 1015:94-103.

Hibbs, D.E. 1996. Managing hardwood stands for timber production. The Woodland Workbook, Oregon State University Extension Service, Corvallis.

Shock, C.C., E.B.G. Feibert, and L.D. Saunders. 2000. Irrigation management for hybrid poplar production, 1997-1999. Oregon State University Agricultural Experiment Station Special Report 1015:80-93.

Table 1. Current and intended future poplar pruning trial treatments. Trees were planted in April 1997. Malheur Experiment Station, Oregon State University, Ontario, OR.

Treatment Pruning height* (ft from ground)

2000 2001 2002 2003 2004

1 Check 6^† 6 6 6 6

2 6^† 9 12 15 18

3 9 12 15 18 18

4 12 15 18 18 18

5^‡ 9 12 15 18 18

*Stem height to which all side branches were removed.†pruned in February 1999.‡pruned in May, 2000. All others pruned when trees were dormant.

Table 2. Percentage of live crown pruned in 2000, percentage of total height pruned, and growth increment for poplar pruning trial in 2000. Within columns, means followed by the same letter are not significantly different according to Fisher's LSD test. Malheur Experiment Station, Oregon State University, Ontario, OR.

TreatmentTreatment	Percentage of live crown pruned	Percentage of total height pruned*	Annual increment			Time to prune sprouts	Sprout weight
TreatmentTreatment	Percentage of live crown pruned	Percentage of total height pruned*	Height	DBH	Wood volume	Time to prune sprouts	Sprout weight
	-------------- % -------------		ft	inch	ft³/acre	hours/acre	lb/acre
Check 1	0.0a	24.3a	11.2	1.61a	270.1
2	0.0a	22.2a	8.1	1.34b	270.9
3	14.7b	33.7b	8.3	1.36b	283.9	2.1	254.2
4	28.0c	47.3c	9.0	1.26b	231.9
5^†	14.7b	33.7b	8.6	1.25b	267.1	1.2	82.8
LSD (0.05)	5.0	2.7	NS	0.21	NS	0.5	69.0

*Check treatment and treatment two were pruned in February 1999.^†pruned in May, 2000. All others pruned when trees were dormant (March 27, 2000).

<>Figure 1. Response of diameter at breast height (DBH) increment to pruning severity. Malheur Experiment Station, Oregon State University, Ontario, OR.