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Clinton C. Shock, Erik B. G. Feibert, and Lamont D. Saunders
Malheur Experiment Station
Oregon State University
Ontario, OR
Summary
Hybrid poplar (clone OP-367), planted for sawlog production in April 1997 at the Malheur Experiment Station, received five irrigation treatments in 2000, 2001, and 2002. 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 of -50 kPa for initiating irrigations. Reducing the water application rate from 2 inches to 1.54 or 0.77 inches reduced the annual growth in diameter at breast height (DBH) and stem volume for the microsprinkler-irrigated treatments. There was no significant difference between the microsprinkler-irrigated treatment with a water application rate of 2 inches and the drip-irrigated treatments with rates of 1.54 and 0.77 inches in terms of height, DBH, or stem volume growth in 2000 and 2001. In 2002, drip irrigation with two tapes per tree row resulted in greater tree growth than microsprinkler irrigation.
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 have made experimental plantings of hybrid poplars for saw logs and peeler logs. Clone trials in Malheur County have demonstrated that the clone OP-367 (hybrid of Populus deltoides x P. nigra) performs well on alkaline soils for at least 7 years. 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). The objectives of this study were to evaluate poplar water requirements in the sixth year and to compare microsprinkler irrigation to drip irrigation.
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 8.1 and 0.8 percent organic matter. The field had been planted to wheat for the 2 years prior to 1997 and to alfalfa before 1995. 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 nine 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 a rate of 0.14 inches/hour at 25 psi and 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 (Nelson Pathfinder, Nelson Irrigation Corp., Walla Walla, WA) were laid along the tree row in early May 2000. In plots with two drip tapes per tree row, the drip tapes were laid 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/minute/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 monitored daily by water meters in each plot.
Soil water potential (SWP) was measured in each plot by six granular matrix sensors (GMS; Watermark Soil Moisture Sensors model 200SS; Irrometer Co., Riverside, CA); two at 8-inch depth, two at 20-inch depth, and two 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.). 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). All plots in the three microsprinkler-irrigated treatments were irrigated whenever the SWP at 8-inch depth for treatment one reached -50 kPa. The plots in each drip-irrigated treatment were irrigated whenever the SWP at 8-inch depth for 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 depths 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.
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 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 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 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, 0.6 lb copper, 0.4 lb iron, 5 lb magnesium, 0.25 lb zinc, and 3 lb phosphorus. 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 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 S/acre, and 14 lb Zn/acre (urea, monoammonium phosphate, zinc sulfate, and elemental sulfur) were broadcast. The ground between the tree rows was 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 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 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.
The heights and 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 for 2002 were calculated as the difference in the respective parameter between October 2002 and October 2001.
Results and Discussion
The microsprinkler-irrigated treatment with 2 inches of water applied at each irrigation consumed 31 acre-inch/acre of water in 17 irrigations (Table 1). The drip treatment with 1.54 inches of water applied with two tapes consumed 38 acre-inch/acre applied in 24 irrigations. The drip treatment with 0.77 inches of water applied with one tape consumed only 23 acre-inch/acre in 24 irrigations.
In November 2002 (sixth year), trees in the wettest sprinkler-irrigated treatment averaged 48 ft in height, 7.5 inches DBH, and 1,166 ft3 of stem volume (Table 2). In November 2002 (sixth year), trees in the treatment drip-irrigated with 2 drip tapes per tree row averaged 50 ft in height, 7.7 inches DBH, and 1,313 ft3 of stem volume.
Drip irrigation with two tapes per tree row (water application rate of 1.54 inches) resulted in the highest DBH growth and stem volume growth (Table 2). Using one drip tape instead of two per tree row resulted in a reduction in DBH growth and stem volume growth. For the microsprinkler-irrigated treatments, the highest growth in DBH and stem volume were achieved with a water application rate of 2 inches. The growth in tree height in 2002 was not significantly different between any of the treatments.
There were positive linear relationships, with similar slopes, between total water applied and stem volume growth for both the drip and microsprinkler systems (Fig. 1). However, the line for the drip system was above the line for the microsprinkler system, reflecting the higher water use efficiency of the drip system (Table 1).
The soil water potential at 8-inch depth was reduced, as expected, with the reductions in water application rate in the sprinkler treatments (Fig. 2, Table 3). There was no significant difference in 8-inch average soil water potential among the two drip treatments and the sprinkler treatment with 2 inches of water application rate. The soil water potential at 8-inch depth in the drip treatments oscillated with a higher amplitude (became wetter) than in the sprinkler plots, as expected, since the wetted area was smaller with drip irrigation. The soil water potential at 32-inch depth in the wettest sprinkler treatment remained drier than in the first foot during the season, suggesting that applied irrigation water was not lost to deep percolation.
The volumetric soil water content at 8-inch depth averaged over the season (Table 3) was highest for the drip plots and decreased with the reductions in water application rate in the sprinkler treatments. At 18-inch depth the soil water content was highest for the drip treatments. At 30-inch depth, the differences between treatments were smaller and only sprinkler treatment two had a lower soil water content than the drip treatments.
Conclusions
Tree growth in 2000 and 2001 was highest with either microsprinkler irrigation at -50 kPa with 2 inches of water applied or drip irrigation with two tapes per tree row and 1.54 inches of water applied. In 2002, drip irrigation with two tapes per tree row (water application rate of 1.54 inches) resulted in the highest DBH growth and stem volume growth.
The response of tree growth to water applied was not maximized in this study, suggesting that maximum tree growth could be achieved by higher water applications at each irrigation or a higher soil water potential for initiating irrigations. This is in accordance with research done previously with the same trees showing that the optimum seasonal average soil water potential was -20 kPa at 8-inch depth (Shock et al. 2002).
Annual stem volume growth shows increases up to 2000, when the stem volume growth for the microsprinkler-irrigated trees started to decline (Table 4). In 2002 the stem volume growth for the drip-irrigated trees started to decline. The decline in annual growth would not be expected until later, when the trees are approaching harvest size. The reduction of the soil water potential for irrigation scheduling from -25 to -50 kPa in 2000 might be associated with the decline in annual stem volume growth. The seasonal average soil water potential was lower than the optimum of -20 kPa starting in 2000 (Table 4). Higher tree growth rates might be achieved by either higher water applications at each irrigation or a higher soil water potential for irrigation scheduling. Increasing the water applications with the drip irrigation system might reduce the water use efficiency because of losses to deep percolation. For the drip system, reducing the water applications and the soil water potential used to schedule irrigations might be more feasible. For the microsprinkler system, reducing the soil water potential used to schedule irrigations or increasing the water applications might increase tree growth.
The pruning initiated in 2000 might be another factor associated with the declining tree growth. According to pruning research conducted on the same field, tree growth will be reduced when pruning exceeds 25 percent of the total height. The trees used in the present irrigation trial were pruned to 34, 29, and 32 percent of total height in 2000, 2001, and 2002, respectively.
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. Techn. Info. Serv., Springfield, VA.
Table 1. Irrigation rates, amounts, and water use efficiency for hybrid poplar submitted to five irrigation regimes, Malheur Experiment Station, Oregon State University, Ontario, OR, 2002.
|
Treatment |
Irrigation threshold |
Water application depth |
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 | -50 | 2 | Microsprinkler | 17 | 30.6 | 4.3 |
| 2 | coincide with trt #1 | 1.54 | Microsprinkler | 17 | 25.2 | 4.4 |
| 3 | coincide with trt #1 | 0.77 | Microsprinkler | 17 | 14.9 | 1.8 |
| 4 | -50 | 1.54 | Drip, 2 tubes | 24 | 38.1 | 6.5 |
| 5 | -50 | 0.77 | Drip, 1 tube | 24 | 23.3 | 6.8 |
|
LSD (0.05) |
|
|
1 | 2.4 | 3.6 | |
*Soil water potential at 8-inch depth.
†Includes 1.43 inches of precipitation from May through
September.
Table 2. Height, diameter at breast height (DBH), and stem volume in early November 2002, and 2002 growth in height, DBH, and stem volume for hybrid poplar submitted to five irrigation treatments, Malheur Experiment Station, Oregon State University, Ontario, OR.
|
Treatment |
November 2002 measurements | |
2002 growth increment | ||||
|
Height |
DBH |
Stem volume | |
Height |
DBH |
Stem volume | |
| |
ft | inch | ft3/acre | |
ft | inch | ft3/acre |
| 1 | 47.9 | 7.5 | 1166.1 | 2.7 | 0.58 | 202.5 | |
| 2 | 40.0 | 6.4 | 714.9 | 2.3 | 0.43 | 110.2 | |
| 3 | 29.5 | 4.6 | 274.6 | 1.3 | 0.26 | 36.6 | |
| 4 | 50.3 | 7.7 | 1313.2 | 4.0 | 0.90 | 309.4 | |
| 5 | 44.8 | 7.1 | 969.1 | 3.8 | 0.59 | 185.2 | |
| LSD (0.05) | 6.6 | 0.6 | 194.4 | NS | 0.12 | 89.9 | |
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, 2002.
|
Treatment |
Average soil water potential |
|
Average volumetric
soil
water content |
||||
| 1st ft | 2nd ft | 3rd ft | |
1st ft | 2nd ft | 3rd ft | |
| |
--------------- kPa ---------------- | |
--------------- % -------------- | ||||
| 1 | 35.8 | 40.8 | 61.5 | |
13.7 | 12.6 | 13.0 |
| 2 | 72.4 | 80.5 | 87 | |
11.4 | 10.7 | 10.8 |
| 3 | 83.1 | 68 | 85.4 | |
7.5 | 13.2 | 13.2 |
| 4 | 31.3 | 26.2 | 41.4 | |
17.1 | 16.0 | 16.0 |
| 5 | 32.6 | 40.7 | 43.5 | |
19.4 | 15.8 | 15.5 |
| LSD (0.05) | 28.8 | 28.2 | 31.2 | |
3.4 | 3.1 | 4.9 |
Table 4. Annual stem volume growth and seasonal average soil water potential at 8-inch depth for hybrid poplar under drip and microsprinkler irrigation at highest irrigation intensities, Malheur Experiment Station, Oregon State University, Ontario, OR.
|
Stem volume growth |
Seasonal
average soil water potential
at 8-inch depth |
||||
| Year | Drip | Microsprinkler | Drip | Microsprinkler | |
| ---- ft3/acre ---- | ---- kPa ---- | ||||
| 1997 | 1.3 | 1.3 | -21.4 | -21.4 | |
| 1998 | 78.5 | 78.5 | -20.0 | -20.0 | |
| 1999 | 177.7 | 177.7 | -22.2 | -22.2 | |
| 2000 | 361.9 | 401.5 | -24.2 | -37.9 | |
| 2001 | 448.7 | 354.7 | -26.4 | -33.9 | |
| 2002 | 413.1 | 256.8 | -31.3 | -35.8 | |
Figure 1. Response of stem volume growth to water applied in 2002 for hybrid poplar using microsprinkler and drip irrigation, Malheur Experiment Station, Oregon State University, Ontario, OR.
Figure 2. 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.
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