Erik B. G. Feibert, Clinton C. Shock, and Lamont D. Saunders
Malheur Experiment Station
Oregon State University
Hybrid poplar (Populus deltoides x P. nigra, cultivar 'OP-367') was planted in April 1997 at the Malheur Experiment Station to test the effects of five groundcover treatments on saw-log growth. Groundcover treatments consisted of 1) bare ground maintained with a preplant herbicide and cultivations, 2) mowed weed cover, 3) alfalfa between tree rows, 4) wheat between tree rows, and 5) squash between the tree rows. The field was irrigated uniformly using microsprinklers along the tree row. Wood volume at the end of September in 1997 and 1998 was highest for the bare ground treatment and lowest for the mowed, alfalfa, and wheat treatments. Tree height increment in 1998 was not significantly different between treatments, but wood volume and DBH increments in 1998 were, as in 1997, highest for the bare ground treatment and lowest for the mowed, alfalfa, and wheat treatments.
With timber supplies from public lands in the Pacific Northwest becoming less available, economic opportunities may exist for alternatives. Hybrid poplar wood has proven to have desirable characteristics for many timber products. Growers in Malheur County have expressed interest in growing hybrid poplars for saw logs. Clone trials in Malheur County have determined that the clone OP-367 (hybrid of Populus deltoides X P. nigra) performs well on alkaline soils for at least two years of growth.
We hypothesized that groundcover management could have a major influence on establishment cost and tree growth during the first few years before full canopy closure. An intercrop could provide income to offset initial costs of plantation establishment. Intercropping with poplars and other tree species shows some success (Beaton, 1987; Ralhan et al., 1992; Williams and Gordon, 1992). Intercrop species can effect tree growth and survival (Williams and Gordon, 1992). However, studies with plantation forests (Nambiar and Sands, 1993), poplars (Burgess, et al., 1996; Kennedy, 1984; Marino and Gross, 1998; McLaughlin et al., 1987), and orchards (Hogue and Neilsen, 1987) report negative effects of cover crops and weeds on tree growth, especially in the first few years. Mclaughlin et al. (1987) report that despite cover crops having reduced tree biomass relative to bare-soil plots in the first 3 years, in the fourth year the cover-crop plots have higher tree biomass than the bare-soil plots. Poplars might be more sensitive to weed competition than other tree species, because of their superficial root system. One-year-old poplar trees can have a horizontal root spread of up to 9 ft (Friend et al., 1991). Poplar root systems can be concentrated in the upper 1.2 ft of soil (Heilman et al., 1994). The objective of this study was to test three intercrops and two weed-control strategies and their effects on hybrid poplar growth.
Materials and Methods
Procedures common to all treatments. The trial was conducted on a Nyssa-Malheur silt loam (bench soil) with 6 percent slope at the Malheur Experiment Station. The soil has a pH of 8.2 and 0.8 percent organic matter. The field was planted to wheat for the two years before poplars and before that to alfalfa. The field was marked for planting by a tractor, and a solid-set sprinkler system was installed before 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 acre-in/acre on the first irrigation immediately after planting. Thereafter the field was irrigated twice weekly at 0.6 acre-in/acre per irrigation until May 26.
On May 27, the solid-set sprinkler system was removed and a microsprinkler system using R-5 nozzles (Nelson Irrigation, Walla Walla, WA) was installed with the risers placed between trees along the tree row at 14 ft spacing. The microsprinklers had a water application rate of 0.12 in per hour and a radius of 14 ft at 25 psi. The microsprinkler system was designed to have a low enough water application rate to avoid runoff and erosion, even with the 6 percent slope.
Soil water potential (SWP) was measured in each plot by two granular matrix sensors (GMS; Watermark Soil Moisture Sensors model 200SS; Irrometer Co., Riverside, CA) at 8-inch depth. The sensors were installed along the middle row in each plot and between the riser and the third tree. The sensors were read at 8 am daily starting on June 19. The field was irrigated when the average reading of all sensors reached -50 kPa. Poplar evapotranspiration (Etc) was estimated in 1998 using an AgriMet (U.S. Department of Interior, Bureau of Reclamation, Boise, ID) weather station at the Malheur Experiment Station and a modified Penman equation (Wright, 1982). Etc was recorded from May 1 to September 30, 1998.
Leaf tissue samples, consisting of the first fully developed leaf in the adjacent poplar irrigation study, were taken in 1997 on July 24, August 11, and September 2 then analyzed for nutrients. Based on the leaf analyses, trees were sprayed in 1997 on July 30 with Fe at 0.2 lb/acre and K at 2 lb/acre; and on August 14 with Fe at 0.2 lb/acre, Mg at 2 lb/acre, and B at 0.1 lb/acre. Magnesium at 10 lb/acre as MgSO4 was applied to the ground around each tree on September 15, 1997, using the same method as described in the previous paragraph. Boron at 0.2 lb/acre was injected into the sprinkler system on September 10, 1997.
In 1998 leaf tissue samples from the adjacent poplar irrigation study were taken on June 23, August 4, and August 20. Based on the leaf analyses, trees in both experiments were fertilized with magnesium sulfate at 10 lb Mg/acre. on July 2, 1998. On September 16, 1998 another 10 lb Mg/acre as magnesium sulfate was injected through the sprinkler system.
The heights of the central five trees in the middle row in each plot were measured at the end of June, August, and September, 1997. Diameter at breast height (DBH, 4.5 ft from ground) was measured at the end of August and September, 1997. Tree heights and DBH were measured at the end of May, June, July, August, and September, 1998. After a severe hail storm the evening of July 4, 1998, an additional measurement of tree height was taken on July 14. Wood volumes were calculated for each of the central five trees in the middle row in each plot using an equation developed for poplars that uses tree height and DBH (Browne, 1962). The width of the groundcover strips in each plot was measured in late July to calculate the percentage of weed-free ground.
The experimental design was a randomized complete block with four replicates. The plots were three rows wide and seven trees long. The five groundcovers were established and maintained as follows:
Bare soil. The area between trees was maintained as weed-free as practical. Treflan at 1 lb ai/acre was broadcast and incorporated on April 22, 1997. The plots were kept weed-free by three rototilling operations and five hand-weeding operations in 1997. Each hand weeding took approximately one worker-day/acre. On April 8, 1998, the plots were disked and Goal at 2 lb ai/acre was broadcast between the tree rows with a field sprayer and along the tree rows with a backpack sprayer. In 1998, the plots were kept weed-free by one spot-spray with Roundup and one hand-weeding operation. The spot-spray with Roundup was applied with a backpack sprayer with a cone-shaped drift guard.
Mowed. Weeds were allowed to grow spontaneously and were mowed periodically. Cut weeds remained on the soil for cover. The ground between tree rows was mowed with a sickle bar mower 7 times in 1997 and 1998 to keep weeds below 6-in tall. The ground along the tree row was hand-weeded 5 times in 1997 and once in 1998 to maintain a 2- to 3-ft wide weed-free strip.
Alfalfa. An 11-ft wide band of alfalfa was planted and maintained between the tree rows. Alfalfa seed (cv. Vernema) was broadcast at 20 lb/acre and incorporated with a bed harrow and roller on April 22, 1997. Goal at 2 lb ai/acre was applied in a 2- to 3-ft wide band along the tree row immediately after planting. The alfalfa was harvested as forage three times in 1997 and four times in 1998. The ground along the tree row was hand-weeded 3 times in 1997 and once in 1998 to maintain a 2- to 3- ft wide weed-free strip.
Wheat. Wheat seed (cv. Penawawa) was drilled in an 11-ft wide strip between the tree rows on April 22, 1997 and on April 3, 1998. Goal at 2 lb ai/acre was applied in a 2- to 3-ft wide band along the tree row immediately after planting in 1997. In 1997, the wheat was mowed with a sickle bar mower at the heading stage on June 17, and in 1998 the wheat was harvested with a small plot combine on July 22. Thereafter the ground between tree rows was mowed five times in 1997 and twice in 1998 to keep weed growth lower than 6 in. The ground along the tree row was hand-weeded 3 times in 1997 and once in 1998 to maintain a 2- to 3-ft wide weed-free strip.
Squash. Ten winter squash seeds (cv. Honey Boat) were planted every 2.5 ft along the tree row on May 28, 1997 and every 2.5 ft between the tree rows on May 7, 1998. The ground between tree rows was rototilled twice in 1997 to maintain a weed-free condition using a PTO driven rototiller before vine growth prevented traffic between the tree rows. By July 11, 1997, the squash vines had started to grow up the tree trunks and had to be pulled away. The ground along the tree row was hand weeded once. The squash was harvested on October 7, 1997 and on October 5, 1998.
Results and Discussion
Tree Growth Response to Irrigation. Wood volume at the end of September in both 1997 and 1998 was highest for the bare-soil plots and lowest for the mowed, alfalfa, and wheat plots (Table 1). Tree height at the end of September in both 1997 and 1998 was highest for the bare soil or the squash cover crop (Table1, Fig. 1). Tree-height increment in 1998 was not significantly different between treatments, but DBH and wood volume increment in 1998 were, as in 1997, highest for bare ground and lowest for the mowed, alfalfa, and wheat treatments. Consequently, most of the difference between treatments in wood volume at the end of 1998 was a result of differential DBH increments in 1998 (Table 1).
The amount of bare ground decreased from 100 percent in the cultivated plots to 40 percent in the squash plots, 32 percent in the alfalfa plots, and 22 percent in the mowed and wheat plots. The wood volume at the end of September each year increased with the increase in bare soil, in accordance with research in orchards (Hogue and Neilsen, 1987) and plantation forests (Nambiar and Sands, 1993).
Soil Water Potential and Water Use. Soil water potential at 8-in depth oscillated more in 1998 than in 1997, reflecting the higher rate of water use in 1998 (Fig. 2 and 3). Soil water potential at 8-in depth in 1997 was higher during the season in the plots with bare soil than in the other treatments. The 1997 average SWP at 8-in depth was -21, -36, -27, -27, and - 54 kPa for the bare-soil, mowed, alfalfa, wheat and squash treatments, respectively. The SWP data suggest a lower evapotranspiration for the bare soil treatment than for the other treatments in 1997. The spray pattern of the microsprinklers in the squash plots was partly blocked by the vines in 1997, resulting in disuniform wetting of the soil surface. In 1998, soil water potential at 8-inch depth for the bare soil, wheat, and squash plots was similar and higher than for the mowed and alfalfa plots. The 1998 average soil water potential was -41, -48, -46, -37, and -34 kPa for the bare soil, mowed, alfalfa, wheat and squash plots, respectively.
Precipitation totaled 3.61 in during the irrigation season in 1997. A total of 22 acre-in/acre of irrigation and precipitation was applied in 1997. Precipitation for the 1998 irrigation season totaled 6.97 in. A total of 27 acre-in/acre of irrigation and precipitation was applied in 1998.
Intercrop Performance. Alfalfa yield of variety Vernema totaled 4.0 tons/acre from the three cuttings in 1997, and in 1998 alfalfa yield from the four cuttings totaled 4.7 tons/acre (88 percent dry matter based on the intercropped strip between tree rows, 9.3-ft wide). Compared to the average yields of Vernema (9.9 tons/acre) over four years (1993-1996) in plots at the Malheur Experiment Station, the alfalfa yields in the poplar plots were reduced by 60 percent in 1997. Compared to the average yield in the alfalfa variety trial at the Malheur Experiment Station in 1998 (8.9 tons/acre), yield of alfalfa in the poplar plots was reduced by 47 percent in 1998.
Wheat yields (cv. Penawawa) in 1998 averaged 33 bu/acre based on a 12-ft wide strip between tree rows. Compared to yields of the same variety in plots at the Malheur Experiment Station in 1998 (57 bu/ac), the wheat yield in the poplar plots was reduced by 42 percent. All wheat yields were reduced in 1998 due to hail.
Squash yield of variety Honey Boat averaged 9.1 tons/acre in 1997 and 4 tons/acre in 1998, based on a 64-in bed between tree rows. Compared to yields of the same variety in plots at the Malheur Experiment Station in 1991 (11.8 tons/acre), the squash yield in the poplar plots in 1997 was reduced by 23 percent. This reduction in yield could have been less with seeding of the squash between the tree rows rather than along the tree rows. Squash yields in 1998 in the poplar plots were reduced by 66 percent compared with the past yield of 11.8 tons/acre. When compared to past performance, the reduction of squash yield in the poplar plots in 1998 is higher than in 1997, due to the shading of the trees and severe hail on July 4.
These results are in agreement with those studies showing reductions in poplar growth because of intercropping or weed competition (Beaton, 1987; Burgess et al., 1996; Kennedy, 1984; Marino and Gross; 1998; McLaughlin et al., 1987). Beaton (1987) compared poplar growth in intercropped plots with different amounts of inter-row bare ground: 100 percent, 62 percent, and 29 percent. The tree rows were 21 ft apart and Beaton reports that 62 percent of bare ground resulted in an "acceptable" reduction in tree growth. The 62 percent of bare ground in Beaton's report would be equivalent to a 13 ft row spacing, close to the 14 ft spacing in this study. During soil sampling in the fall of 1998, extreme difficulty was encountered in penetrating below 1-in depth, exemplifying the dense and extensive nature of poplar roots.
However, reductions in growth from intercropping in the first few years before canopy closure could be overcome in the later years, as reported by McLaughlin et al. (1987). If this early reduction in growth is overcome later, intercropping would be a beneficial practice. Future tree measurements in this trial will determine the effects that initial weed control and intercrop treatments have on harvest wood volume.
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Table 1. Hybrid poplar height, diameter at breast height (DBH) and wood volume on September 30 each year, and current annual increment in 1998 in response
to five groundcovers, Malheur Experiment Station, Oregon State University, Ontario, OR, 1998.
|Tree height||DBH||Wood volume||Current annual increment*|
|---- feet ----||---- in ----||-- feet3/acre --||feet||in||feet3/acre|
*from September 30, 1997 to September 30, 1998
Figure 1. Poplar tree height over time with five groundcovers,
Malheur Experiment Station, Oregon State University, Ontario, OR, 1998.
Figure 3. Soil water potential at 8-in depth for poplar trees with five groundcovers, Malheur Experiment Station, Oregon State University, Ontario, OR, 1998.