MICRO-IRRIGATION ALTERNATIVES FOR HYBRID POPLAR

PRODUCTION, 2005 TRIAL

Clinton C. Shock, Erik B. G. Feibert, and Lamont D. Saunders

Malheur Experiment Station

Oregon State University

Ontario, OR

Summary

Hybrid poplar (cultivar OP-367) was planted for saw log production in April 1997 at the Malheur Experiment Station. Five irrigation treatments were established in 2000 and were continued through 2004. Irrigation treatments consisted of three water application rates using microsprinklers and two water application rates using drip tape. Irrigation scheduling was by soil water potential at 8-inch depth with a threshold for initiating irrigations of -50 kPa in 2000 through 2002 and -25 kPa in 2003 and 2004. Increasing the water application rate increased the annual growth in stem volume for the microsprinkler-irrigated treatments. There was no significant difference between the microsprinkler treatment irrigated at the highest rate and the drip-irrigated treatments in terms of height, DBH, or stem volume growth in 2000 and 2001. In 2002 and 2003, drip irrigation with two tapes per tree row resulted in higher tree growth than microsprinkler irrigation. In 2004, the microsprinkler and the drip-irrigated treatments irrigated at the highest rate had among the highest stem volume growth. In 2005, drip irrigation with two tapes per tree row resulted in the highest stem volume growth.

Introduction

With timber supplies from Pacific Northwest public lands becoming less available, 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, 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 x P. nigra) grew well on alkaline soils. Over the last 8 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 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 -25 kPa, but 38 irrigations were required for 3-year-old trees, and more were anticipated for larger trees. Based on simplicity of operations, we decided to use an irrigation criterion of -50 kPa for the wettest treatments starting in 1998. In 2000 we noticed that the rate of increase in annual tree growth started to decline in the wettest treatment. One of the causes probably was the use of an irrigation criterion of -50 kPa. Starting in 2003 the irrigation criterion was changed to -25 kPa for the wettest treatment. The objectives of this study were to evaluate poplar water requirements and to compare microsprinkler irrigation to drip irrigation.

Materials and Methods

Establishment

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 and 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.

Procedures Common to all Treatments

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 (Nelson Pathfinder, Nelson Irrigation Corp., Walla Walla, WA) were laid along the tree row in early May 2000. 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 a ball valve allowing independent irrigation. Water application amounts were monitored daily by water meters in each plot.

Soil water potential (SWP) 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.

The five irrigation treatments consisted of three water application rates for the microsprinkler-irrigated plots and two water application rates for the drip-irrigated plots (Table 2). From 2000 through 2002, all plots in the 3 microsprinkler-irrigated treatments were irrigated whenever the SWP at 8-inch depth, averaged over all plots in treatment 1, reached -50 kPa. The plots in each drip-irrigated treatment were irrigated whenever the SWP at 8-inch depth, averaged over all plots in the respective treatment, reached -50 kPa. Irrigation treatments were terminated on September 30 each year.

Soil water content was measured with a neutron probe. Two access tubes were installed in each plot along the middle tree row on each side of the fourth tree between the sprinklers and the tree. Soil water content readings were made twice weekly at the same depths as the GMS. The neutron probe was calibrated by taking soil samples and probe readings at 8-inch, 20-inch, and 32-inch depth during installation of the access tubes. The soil water content was determined gravimetrically from the soil samples and regressed against the neutron probe readings, separately for each soil depth. The regression equations were then used to transform the neutron probe readings during the season into volumetric soil water content. Coefficients of determination (r²) for the regression equations were 0.89, 0.88, and 0.81 at P = 0.001 for the 8-inch, 20-inch, and 32-inch depths, respectively.

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 treatment 1 microsprinkler-irrigated trees and for the 2 drip tape configurations were used to assess the growth stage of the plantation. The MAI is CAI divided by the tree age.

2000 Procedures

The side branches on the bottom 6 ft of the tree trunk had been pruned from all trees in February 1999. In March of 2000, another 3 ft of trunk was 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^® 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. 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.

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 lb/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.

2001 Procedures

In March of 2001, another 3 ft of trunk was 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.

2002 Procedures

In March of 2002, another 3 ft of trunk was 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.

2003 Procedures

In March of 2003, another 3 ft of trunk was 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 kPa and the water applied at each irrigation was reduced accordingly (Table 2). All plots in the three microsprinkler-irrigated treatments were irrigated whenever the SWP at 8-inch depth, averaged over all plots in treatment 1, reached -25 kPa. The plots in each drip-irrigated treatment were irrigated whenever the SWP at 8-inch depth, averaged over all plots in the respective treatment, reached -25 kPa. 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.

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 5 for leafhopper control.

2004 Procedures

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.

2005 Procedures

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.

Results and Discussion

In 2005, the microsprinkler-irrigated treatment with 1 inch of water applied at each irrigation received 51.5 acre-inch/acre of water in 55 irrigations (Table 1). The drip treatment with 1 inch of water applied with 2 tapes received 56.1 acre-inch/acre applied in 38 irrigations. The drip treatment with 0.5 inch of water applied with 1 tape received 34.4 acre-inch/acre in 37 irrigations. The large discrepancies between the number of irrigations applied and the actual amount of water applied can be explained by inefficiencies in the irrigation system, such as leaks caused by rodent damage. The tree squirrel population in an adjacent walnut orchard was inadvertently allowed to increase in the past, resulting in damage to the drip and microsprinkler irrigation systems.

In November 2005 (ninth year), trees in the two-drip-tape configuration had the highest stem volume (Table 2). In November 2005, trees in the wettest sprinkler-irrigated treatment averaged 63 ft in height, 9.5 inch in DBH, and 2,515 ft³/acre in stem volume (Table 2). In November 2005, trees in the drip-irrigated treatment with 2 drip tapes per tree row averaged 76.8 ft in height, 10.3 inch in DBH, and 3372 ft³/acre in stem volume. Trees in the two-drip-tape configuration had the highest tree growth increment in 2005 and had the highest accumulated tree growth from 2000 through 2005.

Although tree growth increased with increasing applied water up to the highest amount tested, tree growth was not maximized in this study (Fig. 1). There were similar linear relationships, with similar slopes, between total water applied and stem volume growth for the drip and microsprinkler systems in 2005 (Y = -61.86 + 11.70X, R² = 0.99, P = 0.05 for the drip and Y = 52.25 + 3.38X, R² = 0.92, P = 0.05 for the sprinkler).

For 2000 through 2005, there were distinctively different linear relationships, with similar slopes, between total water applied and the accumulated stem volume growth for the drip and microsprinkler systems (Fig. 2). The greater stem volume growth for the drip system reflected the higher water use efficiency of the drip system.

The SWP at 8-inch depth was maintained above the criterion of -25 kPa, except for brief periods during the season for microsprinkler irrigation with 1 inch of water applied and for drip irrigation with 2 tapes (Fig. 3). The SWP at 8-inch depth was reduced, as expected, with the reductions in the water application rate in the sprinkler treatments (Fig. 3, Table 3). During irrigations the SWP at 8-inch depth in the drip treatments was greater than in the sprinkler treatments, as expected, since the wetted area was smaller with drip irrigation (Fig. 3). It was difficult to maintain the irrigation criterion with the one-drip-tape configuration because of the smaller amount of water applied at each irrigation. With 1 drip tape, it takes 33 hours to apply 0.5 inch of water at each irrigation and usually about 30 hours later (the second morning after) the SWP would be equal to or considerably drier than -25 kPa.

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. 4). 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 SWP from -25 to -50 kPa in 2000 might be 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 the change to a wetter irrigation threshold, from -50 to -25 kPa. In 2005, tree growth for the drip-irrigated trees was less than in 2003, but higher than in 2004, for unknown reasons. In 2005, tree growth for the microsprinkler-irrigated trees was lower than in 2004 or 2003. The lower tree growth in 2005 compared to 2003 and 2004 could be related to the cooler weather in 2005.

Since the trees were planted in 1997, 2005 had the fewest growing degree days (50-86°F) from April through October (Table 4).

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. Leaf analyses of the chlorotic leaves in the microsprinkler-irrigated plots and the green leaves in the drip-irrigated plots showed only minor differences on July 20. The analysis for the microsprinkler trees showed deficiencies of N, P, and K and for the drip trees deficiencies of P and K. The microsprinkler trees had 2.3 ppm N, 0.18 ppm P, and 1.63 ppm K. The drip trees had 2.5 ppm N, 0.17 ppm P, and 1.2 ppm K. The minimum sufficiency points for poplar are 2.3 ppm N, 0.25 ppm P, and 1.7 ppm K. The deficiencies were inadvertently not corrected. The foliar ratio of K to Mg was substantially below the optimum: it was 1.3 to 1 in the drip plots and 1.6 for the sprinkler plots, the ideal is 8 to 1.

The leaf analyses also showed that both the microsprinkler and drip trees had excesses of both Ca and B, with the excesses being slightly higher for the microsprinkler trees. The upper sufficiency point for Ca is 2.4 ppm and for B is 65 ppm. The microsprinkler trees had 3.6 ppm Ca and 144 ppm B. The drip trees had 2.9 ppm Ca and 132 ppm B. These excessively high leaf B concentrations could result in B toxicity. Boron toxicity symptoms include leaf chlorosis. The leaf chlorosis in the microsprinkler trees could be related to the N and K deficiencies and/or to the B excess, and could be related to the decline in growth in 2005. As yet unexplained is the lack of chlorosis in the drip trees, despite the similar nutrient profile to the microsprinkler trees. The excessive B in the leaf tissue could be related to the increase in B concentration in the soil over the years. The soil in the top foot initially had 0.5 ppm B in 1997, increasing to 0.9 ppm in 2001, 2.7 ppm in the spring of 2005, and 4 ppm in the fall of 2005. The increase in B levels could be due to high B in the irrigation water. Up until 2005 all plots were irrigated with the same well. In 2005, the drip plots started being irrigated with a different well, possibly reducing the B uptake by the drip-irrigated trees. Analyses of the irrigation waters as well as the soil and leaf tissue in 2006 might determine causes of these nutrient problems.

Both the CAI and the MAI continue to increase over time for the trees in the two-drip-tape configuration (Fig. 4). However, the microsprinkler trees showed a substantial decline in CAI in 2005, which could be related to the nutrition problems discussed in the previous paragraph. Typically, both the CAI and MAI initially increase, reach a culmination point and then decline. The CAI will culminate before the MAI. The intersection of the two curves is termed the economic rotation and indicates the harvest stage of the plantation.

References

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 J. 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 Comm., Nat. Tech. Info. Serv., Springfield, VA.

Table 1. Irrigation rates, amounts, and water use efficiency for hybrid poplar submitted to five irrigation regimes in 2005, Malheur Experiment Station, Oregon State University, Ontario, OR.

Treatment	Irrigation threshold	Water application	Irrigation system	Total number of irrigations	Total water applied*	Water use efficiency
	kPa†	inch			acre-inch/acre	ft3 of wood/acre-inch of water
1	-25	1	Microsprinkler	55	51.5	6.0
2	coincide with trt 1	0.77	Microsprinkler	55	39.7	5.9
3	coincide with trt 1	0.39	Microsprinkler	55	22.6	5.3
4	-25	1	Drip, 2 tapes	38	56.1	13.1
5	-25	0.5	Drip, 1 tape	37	34.4	9.9
LSD (0.05)				1	14.9	NS

*Includes 2.39 inches of precipitation from May through September.

^†Soil water potential at 8-inch depth.

Table 2. Height, diameter at breast height (DBH), and stem volume in early November 2005, and 2005 growth in height, DBH, and stem volume for hybrid poplar submitted to five irrigation treatments, Malheur Experiment Station, Oregon State University, Ontario, OR.

	November 2005 measurements				2005 growth increment				2000-2005 growth increment
Treatment	Height	DBH	Stem volume		Height	DBH	Stem volume		Stem volume
	ft	inch	ft3/acre		ft	inch	ft3/acre		ft3/acre
1	63.0	9.5	2,515.5		2.0	0.45	306.4		2,284.3
2	52.7	8.3	1,612.6		2.5	0.43	231.5		1,412.5
3	41.2	5.8	614.1		3.2	0.37	120.6		537.3
4	76.8	10.3	3,371.8		6.8	0.71	719.3		3,199.0
5	58.2	8.8	2,035.6		3.0	0.52	339.7		1,849.8
LSD (0.05)	NS	0.8	767.2		NS	NS	178.4		793.1

Table 3. Average soil water potential and volumetric soil water content for hybrid poplar submitted to five irrigation treatments, Malheur Experiment Station, Oregon State University, Ontario, OR.

*significant at P = 0.10.

Table 4. Annual stem volume growth, seasonal average soil water potential at 8-inch depth, and growing degree days for the drip and microsprinkler treatments receiving the most water, Malheur Experiment Station, Oregon State University, Ontario, OR.

Figure 1. Response of hybrid poplar stem volume growth to water applied in 2005 for the drip and microsprinkler systems combined, Malheur Experiment Station, Oregon State University, Ontario, OR.

Figure 2. Response of hybrid poplar stem volume growth to water applied from March 2000 through November 2005 for the drip and microsprinkler systems, Malheur Experiment Station, Oregon State University, Ontario, OR.

Figure 3. Soil water potential at three depths using granular matrix sensors in a poplar stand submitted to five irrigation regimes, Malheur Experiment Station, Oregon State University, Ontario, OR.

  

Figure 4. 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 ninth year for hybrid poplar irrigated with two drip tapes per tree row and with microsprinklers. Data are from plots receiving the highest irrigation rates, Malheur Experiment Station, Oregon State University, Ontario, OR.