Micro-Irrigation Alternatives for Hybrid Poplar Production Trial of 2007

Hybrid poplar (cultivar OP-367) was planted for sawlog production in April 1997 at the Malheur Experiment Station. Five irrigation treatments were established in 2000 and were continued through 2007. Irrigation treatments consisted of three treatments using microsprinklers and two using drip tape. The three microsprinkler treatments consisted of one adequately irrigated check (irrigations at 25 cb soil water tension and 2 inches of water per irrigation) and two deficit irrigation treatments (irrigations initiated when the check treatment is irrigated, but with of 1.54 and 0.77 inches applied per irrigation). The two drip-irrigated treatments were irrigated separately at 25 cb soil water tension and 1 and 0.5 inches applied per irrigation. Soil water tension was measured at 8-inch depth. Stem volume in the fall of 2007 and stem volume growth from 2000 through 2007 were highest with drip irrigation applying 1 inch of water per irrigation at a soil water tension of 25 cb.

With timber supplies from Pacific Northwest public lands becoming less available, sawmills and timber product companies are searching for alternatives. Hybrid poplar wood has proven to have desirable characteristics for many nonstructural timber products. Growers in Malheur County, Oregon have made experimental plantings of hybrid poplars for saw logs and peeler logs. Clone trials in Malheur County during 1996 demonstrated that the clone OP-367 (hybrid of Populus deltoides xP. nigra) grew well on alkaline soils. Over the last 10 years OP-367 has continued to grow well on alkaline soils. Some other clones have higher productivity on soils with nearly neutral pH.

Hybrid poplars are known to have high growth rates (Larcher 1969) and water transpiration rates (Zelawski 1973), suggesting that irrigation management is a critical cultural practice. Research at the Malheur Experiment Station during 1997-1999 determined optimum microsprinkler irrigation criteria and water application rates for the first 3 years (Shock et al. 2002). These results showed that tree growth was maximized by irrigating at a soil water tension of 25 cb, but 38 irrigations were required for 3-year-old trees, and more were anticipated for larger trees. To reduce the number of irrigations and simplify operations, we decided to use an irrigation criterion of 50 cb for the fourth year (starting in 2000) with an irrigation rate of 2 inches per irrigation for the microsprinkler and 1.54 inches per irrigation for drip. In 2002 we noticed that the rate of increase in annual tree growth started to decline for all treatments. One of the causes probably was the use of an irrigation criterion of 50 cb. Starting in 2003 the irrigation criterion was changed to 25 cb with irrigation rates of 1 inch per irrigation for microsprinkler and drip irrigation. The objectives of this study were to evaluate poplar water requirements and to compare microsprinkler irrigation to drip irrigation.

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 8.1 with 0.8 percent organic matter. The field had been planted to wheat for the 2 years prior to poplar and to alfalfa before wheat. In the spring of 1997 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 by 14-ft spacing. The sprinkler system applied 1.4 inches on the first irrigation immediately after planting. Thereafter the field was irrigated twice weekly at 0.6 inches per irrigation until May 26. A total of 6.3 inches of water was applied in 9 irrigations from April 25 to May 26, 1997.

In late May 1997, a microsprinkler system (R-5, Nelson Irrigation, Walla Walla, WA) was installed with the risers placed between trees along the tree row at 14-ft spacing. The sprinklers delivered water at 0.14 inches/hour at 25 psi with a radius of 14 ft. The poplar field was used for irrigation management research (Shock et al. 2002) and groundcover research (Feibert et al. 2000) from 1997 through 1999.

In March 2000 the field was divided into 20 plots, each of which was 6 tree rows wide and 7 trees long. The plots were allocated to five treatments arranged in a randomized complete block design and replicated four times (Table 1). The microsprinkler-irrigation treatments used the existing irrigation system. For the drip-irrigation treatments, either one or two drip tapes were laid along the tree row in early May 2000 (Nelson Pathfinder, Nelson Irrigation, Walla Walla, WA). The plots with 2 drip tapes per tree row had the drip tapes spread 2 ft apart, centered on the tree row. The drip tape had emitters spaced 12 inches apart and a flow rate of 0.22 gal/min/100 ft at 8 psi. Each plot had a pressure regulator (set to 25 psi for the microsprinkler plots and 8 psi for the drip plots) and ball valve allowing independent irrigation. Water application amounts were recorded daily from the water meters in each plot.

Soil water tension (SWT) was measured in each plot by 6 granular matrix sensors (GMS; Watermark Soil Moisture Sensors model 200SS; Irrometer Co. Inc., Riverside, CA); 2 at 8-inch depth, 2 at 20-inch depth, and 2 at 32-inch depth. The GMS were installed along the middle row in each plot and between the riser and the third tree. The GMS were previously calibrated (Shock et al. 1998) and were read at 8:00 a.m. daily starting on May 2 with a 30 KTCD-NL meter (Irrometer Co. Inc., Riverside, CA). The daily GMS readings were averaged separately at each depth within each plot and over all plots in a treatment. Irrigation treatments were started on May 2.

In 2007, the irrigation treatments consisted of three microsprinkler and two drip-irrigated treatments (Table 2). The three microsprinkler treatments consisted of one adequately irrigated check (2 inches of water per irrigation) and two deficit irrigation treatments (irrigations initiated when the check treatment is irrigated, but with 1.54 and 0.77 inches applied per irrigation). The two drip-irrigated treatments were irrigated separately, with 1 and 0.5 inches applied per irrigation. From 2000 through 2002, all plots in the 3 microsprinkler-irrigated treatments were irrigated whenever the SWT at 8-inch depth, averaged over all plots in the check treatment, reached 50 cb. The plots in each drip-irrigated treatment were irrigated whenever the SWT at 8-inch depth, averaged over all plots in the respective treatment, reached 50 cb. In 2003, the irrigation criterion was increased from 50 cb to 25 cb. In 2006, due to salt accumulation in the soil, the irrigation rates for the microsprinkler treatments were doubled. Irrigation treatments were terminated on September 30 each year.

The heights and diameter at breast height (DBH, 4.5 ft from ground) of the central three trees in the two middle rows in each plot were measured monthly from May through September. Tree heights were measured with a clinometer (model PM-5, Suunto, Espoo, Finland) and DBH was measured with a diameter tape. Stem volumes (excluding bark and including stump and top) were calculated for each of the central six trees in each plot using an equation developed for poplars that uses tree height and DBH (Browne 1962). Growth increments for height, DBH, and stem volume were calculated as the difference in the respective parameter between October of the current year and October of the previous year. Curves of current annual increment (CAI) and mean annual increment (MAI) of stem volume over the 8 years for the microsprinkler and drip-irrigated treatments at the highest rates were used to assess the growth stage of the plantation. The MAI is the CAI divided by the tree age.

The side branches on the bottom 6 ft of the tree trunk were pruned from all trees in February, 1999. In March of 2000, another 3 ft of trunk were pruned, resulting in 9 ft of pruned trunk. The pruned branches were flailed on the ground and the ground between the tree rows was lightly disked on April 12. On April 24, Prowl^® The microsprinkler-irrigated plots received 0.7 inch of water to incorporate the Prowl. To control the alfalfa and weeds remaining from the previous years' groundcover trial in the top half of the field, Stinger ^® at 0.19 lb ai/acre was broadcast between the tree rows on May 19, and Poast^® at 0.23 lb ai/acre was broadcast between the tree rows on June 1. On June 14, Stinger at 0.19 lb ai/acre and Roundup^® at 3 lb ai/acre were broadcast between the tree rows on the whole field. at 3.3 lb ai/acre was broadcast for weed control.

On May 19 the trees received 50 lb nitrogen (N)/acre as urea-ammonium nitrate solution injected through the microsprinkler system. Due to deficient levels of leaf nutrients in early July, the field had the following nutrients in pounds per acre injected in the irrigation systems: 0.4 lb boron (B), 0.6 lb copper (Cu), 0.4 lb iron (Fe), 5 lb magnesium (Mg), 0.25 lb zinc (Zn), and 3 lb phosphorus (P). The field was sprayed aerially for leafhopper control with Diazinon AG500^® at 1 lb ai/acre on May 27 and with Warrior^® at 0.03 lb ai/acre on July 10.

In March of 2001, another 3 ft of trunk were pruned, resulting in 12 ft of pruned trunk. The pruned branches were flailed on the ground on April 2. On April 4, Roundup at 1 lb ai/acre was broadcast for weed control. On April 10, 200 lb N/acre, 140 lb P/acre, 490 lb sulfur (S)/acre, and 14 lb Zn/acre (urea, monoammonium phosphate, zinc sulfate, and elemental sulfur) were broadcast. The ground between the tree rows was lightly disked on April 12. On April 13, Prowl at 3.3 lb ai/acre was broadcast for weed control. The microsprinkler-irrigated plots received 0.8 inch of water to incorporate the Prowl.

A leafhopper, willow sharpshooter (Graphocephala confluens, Uhler), was monitored by three yellow sticky traps attached to the lower trunk of selected trees. Traps were checked weekly. From mid-April to early June only adults were observed in the traps. A willow sharpshooter hatch was observed on June 6, as large numbers of nymphs were noted in the traps and on the lower trunk sprouts. The field was sprayed aerially with Warrior at 0.03 lb ai/acre on June 11 for leafhopper control.

In March of 2002, another 3 ft of trunk were pruned, resulting in 15 ft of pruned trunk. The pruned branches were flailed on the ground on April 12. On April 23, 80 lb N/acre, 40 lb potassium (K)/acre, 150 lb S/acre, 20 lb Mg/acre, 6 lb Zn/acre, 1 lb Cu/acre, and 1 lb B/acre (urea, potassium/magnesium sulfate, elemental sulfur, zinc sulfate, copper sulfate, and boric acid) were broadcast and the field was disked. On April 24, Prowl at 3.3 lb ai/acre was broadcast for weed control. The microsprinkler-irrigated plots received 0.7 inch of water to incorporate the Prowl.

The willow sharpshooter was monitored by three yellow sticky traps attached to the lower trunk of selected trees. Traps were checked weekly. The field was sprayed aerially with Warrior at 0.03 lb ai/acre on June 10 for leafhopper control.

In March of 2003, another 3 ft of trunk were pruned, resulting in 18 ft of pruned trunk. The pruned branches were flailed on the ground on March 31. On April 23, 80 lb N/acre as urea and 167 lb S/acre as elemental sulfur were broadcast and the field was disked. On April 16, Prowl at 3.3 lb ai/acre was broadcast for weed control. The microsprinkler-irrigated plots received 0.4 inch of water to incorporate the Prowl.

Starting in 2003 the irrigation criterion was changed to 25 cb and the water applied at each irrigation was reduced accordingly. All plots in the three microsprinkler-irrigated treatments were irrigated whenever the SWT at 8-inch depth, averaged over all plots in treatment 1, reached 25 cb. The plots in each drip-irrigated treatment were irrigated whenever the SWT at 8-inch depth, averaged over all plots in the respective treatment, reached 25 cb. Irrigation treatments were terminated on September 30.

The drip tape needed to be replaced because iron sulfide plugged the emitters. The drip tape was replaced with another brand (T-tape, T-systems International, San Diego, CA) in mid-April because Nelson Irrigation discontinued production of drip tape. The drip tape specifications were the same.

On March 31, 2004, N at 80 lb/acre, S at 250 lb/acre, P at 50 lb/acre, K at 50 lb/acre, Cu at 1 lb/acre, Zn at 4 lb/acre, and B at 1 lb/acre were broadcast. The field was lightly disked on April 1. On April 13, Prowl at 3.3 lb ai/acre was broadcast for weed control. The microsprinkler-irrigated plots received 0.4 inch of water to incorporate the Prowl. On June 12 the field was sprayed with Warrior at 0.03 lb ai/acre for leafhopper control. A leaf tissue sample taken on July 7 showed a P deficiency. On July 9, P at 10 lb/acre as phosphoric acid was injected through the sprinkler and drip systems.

A soil sample taken on April 4, 2005, showed the need for N at 50 lb/acre, and S at 400 lb/acre, which were broadcast on April 7. On April 8, Prowl at 3.3 lb ai/acre was broadcast for weed control. On June 22, the field was fertilized with N at 50 lb/acre as urea ammonium nitrate solution injected through the drip and sprinkler systems. On June 24 the field was sprayed with Warrior at 0.03 lb ai/acre for leafhopper control.

A soil sample taken on October 21, 2005, showed the need for P at 50 lb/acre, S at 400 lb/acre, and Cu at 1 lb/acre, which were broadcast on October 25, 2005. Due to bird damage the drip tape was replaced with drip tubing in May. The drip tubing (Triton X, Netafim, Fresno, CA) had emitters spaced 2 ft apart with 0.2 gal/hour flow rate. On June 12 the field was sprayed with Warrior at 0.03 lb ai/acre for leafhopper control. On June 16, the field had Fe at 1 lb/acre broadcast aerially. Leaf analyses on July 26 showed the need for N and P in the drip trees. A total of 50 lb N/acre, 20 lb P/acre, and 8.7 lb of Fe were applied to the drip plots in 2006 (Table 1). A total of 50 lb N/acre, 10 lb P/acre, and 5.2 lb Fe/acre were applied to the microsprinkler plots in 2006. On April 14, 2006, Prowl at 3.3 lb ai/acre was broadcast for weed control.

A soil sample taken on March 12, 2007 showed the need for N at 50 lb/acre, S at 400 lb/acre, Mn at 1 lb/acre, and B at 1 lb/acre, which were broadcast on April 3, 2007. On April 10, Goal^® at 2 lb a.i./acre was broadcast for weed control.

The increase in the irrigation intensity starting in 2006 for the microsprinkler-irrigated plots resulted in an increase in the total amount of water applied compared to previous years. The total amount of water applied for the season was higher than estimated ET_c (47.3 inches) for all treatments except the driest microsprinkler-irrigated treatment (Table 1). In 2007 and from 2000 through 2007, drip irrigation at 1 inch per irrigation had the highest stem volume growth increment (Table 2). In the fall of 2007 (tenth year), the highest wood volume per acre was achieved with drip irrigation at 1 inch per irrigation. In the fall of 2007, the drip-irrigated treatments and the 2-inch microsprinkler treatment had among the highest DBH.

Although tree growth increased with increasing applied water up to the highest amount tested, tree growth may not have been maximized in this study (Fig. 1). The slope of the regression line between total water applied and stem volume growth from 2000 to 2007 was steeper for the drip-irrigated trees than for the microsprinkler-irrigated trees (Fig. 1). The greater stem volume growth for the drip system reflected the higher water use efficiency of the drip system.

The SWT at 8-inch depth was maintained below the criterion of 25 cb, except for brief periods during the season when microsprinkler irrigation applied 2 inches of water and for drip irrigation (Fig. 2). The average SWT at 8-inch depth reflected the treatments (Table 3).

The rate of increase in annual stem volume growth increased (growth approximately doubled every year) up to 2001, when the stem volume growth for the microsprinkler-irrigated trees started to decline (Table 4, Fig. 3). In 2002 the stem volume growth for the drip-irrigated trees started to decline. The decline in annual growth was not expected until later, when the trees approach harvest size. The reduction of the SWT for irrigation scheduling from 25 to 50 cb in 2000 might have been associated with the decline in annual stem volume growth. Tree growth was substantially greater in 2003 and was approximately double the growth in 2002; this could have been due to changing the irrigation threshold from 50 to 25 cb.

In 2004-2006, the trees in the microsprinkler-irrigated plots started exhibiting leaf chlorosis around mid-July, whereas the trees in the drip-irrigated plots did not exhibit leaf chlorosis. Foliar analysis has not revealed a clear cause-effect relationship. A small trial was conducted to evaluate two iron fertilizers as a remedy for the leaf chlorosis. Iron sulfate (75 percent) was applied at 2, 1, 0.5, 0.25, and 0.125 lb/tree (7.1, 3.5, 1.7, 0.9, and 0.4 oz Fe/tree, respectively). Iron Hi-Yield^® (11 percent N, 13 percent S, and 16 percent Fe) was applied at 10, 5, 2.5, and 1.25 lb/tree. Each rate of each fertilizer was applied to one chlorotic tree in a microsprinkler-irrigated plot on July 2. The fertilizer was applied in two narrow bands adjacent to the tree trunk. By July 30 the trees receiving the iron sulfate at the 1 lb and 2 lb/tree rates showed reduced leaf chlorosis. On August 3, all trees had SulFeGro G^® (8 percent Fe, Simplot Soilbuilders, Boise, ID) applied at 2 lb/tree (2.6 oz Fe/tree) in two narrow bands adjacent to the tree trunk.

The soil sodium (Na) concentration increased over the years through the fall of 2005, but decreased in 2006 and again in 2007 (Table 5). The increase in Na concentration over the years could be due to the high Na levels in the well water used for irrigation (200 ppm). The recommended maximum Na level in irrigation water is 69 ppm. The higher irrigation rates adopted in 2006 could have reduced the soil Na concentration.

The current annual increment (CAI) and the mean annual increment (MAI) for the drip-irrigated trees increased in 2007 (Fig. 3). The CAI and MAI for the microsprinkler-irrigated trees were similar in 2005, 2006, and 2007, suggesting a decline in tree growth vigor. Typically, both the CAI and MAI initially increase, reach a maximum, and then decline. The CAI peaks before the MAI. The intersection of the two curves is termed the economic rotation and is used in some poplar plantations to determine the harvest timing. The two curves were close in 2005, 2006, and 2007 for the microsprinkler trees, but not for the drip trees. The lower CAI for the microsprinkler trees could be related to the iron deficiency discussed above. The iron fertilization probably occurred too late during the 2007 season to have much of an effect on tree growth in 2007.

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.

Larcher, W. 1969. The effect of environmental and physiological variables on the carbon dioxide exchange of trees. Photosynthetica 3:167-198.

Shock, C.C., J.M. Barnum, and M. Seddigh. 1998. Calibration of Watermark Soil Moisture Sensors for irrigation management. Pages 139-146 in Proceedings of the International Irrigation Show, Irrigation Association, San Diego, CA.

Shock, C.C., E.B.G. Feibert, M. Seddigh, and L.D. Saunders. 2002. Water requirements and growth of irrigated hybrid poplar in a semi-arid environment in eastern Oregon. Western Journal of Applied Forestry 17:46-53.

Zelawski, W. 1973. Gas exchange and water relations. Pages 149-165 in S. Bialobok (ed.) The poplars-Populus L. Vol. 12. U.S. Dept. of Commerce National Technical Information Service, Springfield, VA.

Table 1. Irrigation rates, amounts, and water use efficiency for hybrid poplar submitted to five irrigation regimes in 2007, Malheur Experiment Station, Oregon State University, Ontario, OR.

Treatment	Irrigation threshold	Water application	Irrigation system	Total water applied^b	Water use efficiency
	kPa^a	inch		acre-inch/acre	ft³ of wood/acre-inch of water
1	25	2	Microsprinkler	83.1	3.7
2	coincide with trt #1	1.54	Microsprinkler	54.3	6.5
3	coincide with trt #1	0.77	Microsprinkler	36.4	11.2
4	25	1	Drip, 2 tapes	83.7	12.0
5	25	0.5	Drip, 1 tape	58.1	9.2
LSD (0.05)				12.6	NS

Table 2. Height, diameter at breast height (DBH), and stem volume in early November 2007, and 2007 growth in height, DBH, and stem volume for hybrid poplar submitted to five irrigation treatments, Malheur Experiment Station, Oregon State University, Ontario, OR.

Table 3. Average soil water tension for hybrid poplar submitted to five irrigation treatments in 2007, Malheur Experiment Station, Oregon State University, Ontario, OR.

Table 4. Irrigation criterion, irrigation rate, annual stem volume growth, seasonal average soil water tension at 8-inch depth, and evapotranspiration (ET_c) for the drip and microsprinkler treatments receiving the most water (Treatments 1 and 4), hybrid poplar study, Malheur Experiment Station, Oregon State University, Ontario, OR.

Table 5. Soil pH, soluble salts, sodium, and boron over time in field used for poplar research, Malheur Experiment Station, Oregon State University, Ontario, OR.

Year	pH	Soluble salts	Na (ppm)	B (ppm)
April 1999	8.2	0.8	159	0.5
March 2001	8.3	0.8	132	0.9
March 2003	8.4	0.7	132	0.8
April 2005	8.0	0.6	265	2.7
October 2005	8.4	0.3	200	4
April 2006	8.2	0.2	139	1.7
March 2007	8.8	0.2	53	0.6
Critical levels		<1.5	<225	0.7-1.5

Figure 1. Response of stem volume growth to water applied from March 2000 through November 2007 for the drip and microsprinkler systems, Malheur Experiment Station, Oregon State University, Ontario, OR.

Figure 2. Soil water tension at 8-inch depth in a poplar stand submitted to five irrigation regimes, Malheur Experiment Station, Oregon State University, Ontario, OR.

Figure 3. Current annual increment (CAI, annual stem volume growth) and mean annual increment (MAI, mean annual stem volume growth) starting at planting in 1997 through the eleventh year for drip- and microsprinkler-irrigated hybrid poplar. Data are from the check microsprinkler treatment and the treatment drip irrigated with 1 inch per irrigation. Data for 1997, 1998, and 1999 for the drip irrigation graph are the same as for the microsprinkler graph. Malheur Experiment Station, Oregon State University, Ontario, OR.

	November 2007 measurements			2007 growth increment			2000-2007 growth increment
Treatment	Height	DBH	Stem volume	Height	DBH	Stem volume	Stem volume
	ft	inch	ft³/acre	ft	inch	ft³/acre	ft³/acre
1	68.0	10.2	3126.7	3.6	0.21	308.0	2895.6
2	58.5	9.3	2240.5	3.8	0.39	362.3	2040.4
3	55.0	7.2	1257.6	9.7	0.59	408.6	1180.8
4	84.8	11.8	5313.1	5.9	0.60	983.4	5140.3
5	64.5	10.3	3004.2	3.1	0.74	514.9	2818.4
LSD (0.05)	NS	1.7	1417.8	NS	NS	299.9	1442.2

Treatment	Average soil water tension
	1st foot	2nd foot	3rd foot
	------------------- cb --------------------
1	27.7	27.5	26.5
2	57.2	30.6	30.9
3	61	49.6	48.3
4	27.1	24.9	27.7
5	31.9	33	24.6
LSD (0.10)	20.3	NS	9.8

	Irrigation criterion^a	Irrigation rate (inches per irrigation)		Annual stem volume growth		Seasonal average soil water tension at 8-inch depth		Water applied plus precipitation		ET_c
Year		Drip	Microspr.	Drip	Microspr.	Drip	Microspr.	Drip	Microspr.
	cb			---- ft³/acre ----		---- kPa ----		----- inches ------
1997	25		0.8		1.3		21.4		27.2
1998	25		1.2		78.5		20.0		45.0	37.1
1999	25		1.54		177.7		22.2		51.0	45.5
2000	50	1.54	2	387.9	401.5	24.2	37.9	35.2	42.1	47.1
2001	50	1.54	2	479.9	354.7	26.4	33.9	35.8	34.3	44.7
2002	50	1.54	2	440.1	256.8	31.3	35.8	30.6	38.1	44.4
2003	25	1	1	737.9	450.7	21.8	26.9	54.8	47.1	45.9
2004	25	1	1	679.4	512.3	20.2	22.2	56.3	51.7	44.1
2005	25	1	1	719.3	306.4	24.0	25.9	56.1	51.5	45.3
2006	25	1	2	629.8	303.2	26.5	26.8	74.3	89.1	49.3
2007	25	1	2	983.4	308.0	27.1	27.7	83.7	83.1	47.3