NMSU: Ten Years of Poplar Research at NMSU's Agricultural Science Center at Farmington
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Authors: Respectively, Professor, NMSU Agricultural Science Center at Farmington (P.O. Box 1018, Farmington, NM 87499-1018; moneill@nmsu.edu); Senior Research Specialist, NMSU Mora/John T. Harrington Forestry Research Center; Agricultural Research Scientist, ASC-Farmington; Assistant Professor, ASC-Farmington; College Professor, ASC-Farmington; and Superintendent/Professor, ASC Farmington.

Department of Plant and Environmental Sciences

Abstract

Fast-growing tree plantations are important for timely production of timber, wood products, and biofuels, as well as for providing environmental services such as soil conservation, carbon sequestration, phytoremediation, and wildlife habitat. Under well-managed systems, hybrid poplar (Populus spp.) is recognized as one of the fastest-growing temperate trees, capable of producing marketable products in rotations of three to 15 years (Figure 1). In arid areas with abundant solar radiation and adequate water, improved clones of hybrid poplar can produce 35–40 Mg ha-1 yr-1 of biomass over a five-year rotation. Research on hybrid poplar’s adaptation to the Four Corners region began at the New Mexico State University (NMSU) Agricultural Science Center at Farmington (ASC-Farmington) in 2002 with the establishment of a clonal trial consisting of 10 entries of various poplar species. Further trials were established in 2003, 2005, and 2007. It was determined in early and subsequent trials that P. deltoides × P. nigra crosses (P. × canadensis) demonstrated the greatest level of adaptation to the region. Of particular interest is the P. × canadensis clone OP-367, originally crossed at the Oxford Paper nursery in New York state nearly one hundred years ago. This clone has consistently produced greater biomass than other clones at ASC-Farmington under conditions of high solar radiation, elevated temperatures, and high soil pH and calcium carbonate content. Results from these trials suggest that superior hybrid poplar production is possible in northern New Mexico and the surrounding Four Corners region.

Fig. 1: Photograph of fall foliage in the 2002-planted trial after eight growing seasons.

Figure 1. Fall foliage creates a cathedral of light in the 2002-planted trial after eight growing seasons.

Introduction

Tree plantations in North America provide a wide array of benefits and services, including conservation of natural forests; production of timber, wood products, and biofuels; and key environmental services such as soil conservation, phytoremediation, wildlife habitat, and carbon sequestration. Among the various plantation species, hybrid poplar (Populus spp.) has gained in popularity due to its short rotation cycle (3–15 years) and its adaptability to a range of growing conditions. In addition to timber and wood products, hybrid poplar grown in the Four Corners region could supplement aspen (Populus tremuloides) for use in excelsior production (Figure 2), provide bioenergy feedstock for direct combustion or cellulosic conversion to ethanol and other distillates, and provide poles for traditional Navajo construction. Plantations established perpendicular to the prevailing westerly winds can reduce wind velocity, thereby reducing soil erosion and massive dust transport, especially during the spring when wind velocities are generally at their highest. Environmental pollutants, such as petrochemical products and nitrates left over from oil drilling and uranium ore processing, could be cleaned from surface spills and groundwater via the phytoremediatory action of poplars (Figure 3). The possibility of obtaining tradable credits for sequestered carbon may also create incentives for poplar plantation development around coal-burning power plants located within and neighboring the region.

Fig. 2: Photograph of excelsior fiber made from aspen (Populus tremuloides).

Figure 2. Excelsior fiber made from aspen (Populus tremuloides) can also be made from other poplar species and hybrids.

Fig. 3: Photograph of co-author Sam Allen planting an 18-ft hybrid poplar pole at an abandoned oil refinery site for an NMSU phytoremediation research project.

Figure 3. Co-author Sam Allen plants an 18-ft hybrid poplar pole at an abandoned oil refinery site for an NMSU phytoremediation research project.

North American poplar production is scattered throughout several regions. Canada has approximately 8,100 ha of poplar plantations along its southern border. In the United States, rainfed poplar production is concentrated in the Mississippi Valley (11,350 ha), the North Central region (10,125), the Atlantic Northeast (6,900), and Pacific Northwest (4,000). An additional 14,000 ha of drip-irrigated hybrid poplar are produced in the Pacific Northwest. Warmer temperatures and abundant solar radiation during the growing season in Washington and Oregon east of the Cascades compared to conditions west of the Cascades have enhanced productivity from 25–35 m3 ha-1 yr-1 on eight-yr rotations to over 40 m3 ha-1 yr-1 after six to seven years (Stanton et al., 2002).

ASC-Farmington is located in the San Juan River Basin of northwest New Mexico (lat. 36° 41' 20.86" N, long. 108° 18' 22.25" W; elevation 1,720 m). The center is located on land farmed by Navajo Agricultural Products Industry (NAPI), a large 35,000-ha commercial operation administered by the Navajo Nation. NAPI represents the largest target community for ASC-Farmington to address agricultural improvement and market development issues. An excelsior sawmill in southern Colorado has expressed interest in the production of hybrid poplar as a sustainable substitute for aspen currently harvested from the surrounding National Forests. Challenges in obtaining harvest permits and a rise in the incidence of sudden aspen decline have been the main stimuli for this interest. Several thousand hectares of rectangular fields remained after NAPI’s transition to center pivot irrigation, and rehabilitating these fields using drip-irrigated tree plantations is seen as an additional strategy to demonstrate beneficial water use as well as provide wood products and environmental services.

The center of drip-irrigated hybrid poplar production in the Pacific Northwest is east of the Cascades in the Boardman, OR, area (Figures 4 and 5). With precipitation, temperatures, growing degree days, and edaphic conditions being similar in eastern Oregon and northwestern New Mexico (O’Neill et al., 2010), a series of trials was carried out beginning in 2002 to determine adoption potential of hybrid poplar in northwestern New Mexico. Objectives of these trials were to:

  • Identify hybrid poplar clones suitable for climate and alkaline soils inherent to the region,
  • Determine water use requirements and growth rates of poplar species grown in high-pH soils, and
  • Identify potential post-harvest markets for the material.

Fig. 4: Photograph of large-scale hybrid poplar plantation operated by GreenWood Resources in Boardman, OR.

Figure 4. Large-scale hybrid poplar plantation operated by GreenWood Resources in Boardman, OR.

Fig. 5: Photograph of senior author Mick O'Neill in a 10-year-old hybrid poplar plantation in Boardman, OR.

Figure 5. Senior author Mick O’Neill stands in a 10-year-old hybrid poplar plantation in Boardman, OR.

Methods and Materials

Common Across All Trials

A series of on-station hybrid poplar trials was begun in 2002. Common across all trials, unless otherwise indicated, were soil type, planting method, irrigation scheduling, fertilizer applications, and growth determinations. The soil at the experimental site was originally classified as a Kinnear sandy loam (fine-loamy, mixed, calcareous mesic Typic Camborthid) (Anderson, 1970) and later re-classified as a Doak sandy loam (fine-loamy, mixed, mesic Typic Haplargid) (Keetch, 1980). Water holding capacity (in a 1-m profile) was 138 mm and pH averaged 8.2, resulting in a moderately calcareous soil that might pose challenges to poplar production.

Prior to tree planting, fields were disked, leveled, and spot sprayed with glyphosate for weed control. First-year plantings were made using 23-cm cuttings with at least four buds inserted into a 3-cm diameter hole leaving only the topmost bud exposed above soil level. At elevated soil pH, iron availability is reduced, resulting in leaf chlorosis (Brady and Weil, 1999; Havlin et al., 1999), and therefore an iron and zinc micronutrient blend was applied through the irrigation system during fertilizer injections.

Although poplar consumptive-use estimates were not available for the Farmington area, monthly water-use rates of first-, second-, and third-season poplar growth at Boardman, OR (a site with similar climatic characteristics), were used, based on Gochis and Cuenca (2000), to generate crop coefficients for each year of poplar growth as related to growing degree days (GDD). The crop coefficients were then used to modify the Penman-Monteith reference evapotranspiration value for a given day (ETTALL) (Snyder and Eching, 2003), and the subsequent values were used to program irrigation. Equation 1 is for the first year, Equation 2 for the second, and Equation 3 for the third and subsequent years of hybrid poplar production at Farmington. Equation 4 calculates the ET replacement requirement for a given day in a given season of poplar production. Seasonal irrigations were generally begun in April and terminated at the end of September. Crop coefficients for season 1 (KC1), season 2 (KC2), and season 3 and onward (KC3), along with crop evapotranspiration (ETC) in inches, are as follows:

KC1 = 3.93 × 10−1 − 2.58 × 10−5 (∑GDD) + 5.39 × 108 (∑GDD2) − 8.98 × 10−12 (∑GDD3)                (1)

KC2 = 3.71 × 10−1 + 1.38 × 10−4 (∑GDD) + 2.95 × 10−8 (∑GDD2) − 8.20 × 10−12 (∑GDD3)              (2)

KC3 = 5.18 × 10−1 + 4.57 × 10−5 (∑GDD) + 1.19 × 10−7 (∑GDD2) − 2.40 × 10−11 (∑GDD3)              (3)

ETC = KC(year) × PETTALL                                                                                                                   (4)

where
K = crop coefficient for a given season,
∑GDD = cumulative growing degree days (°F),
ETC= crop evapotranspiration (in.), and
PETTALL = potential evapotranspiration (in.) for a tall crop.

Tree height and diameter at breast height (DBH) were determined on main boles during dormant months of November, December, and January following the growing season (Figure 6). Wood volume (WVol) per tree (ft3 tree-1) was calculated after Browne (1962) using Equation 5 as follows:

WVol = 10(−2.945047 + 1.803973*Log (DBH) + 1.238853*Log(Ht))                                          (5)

where
WVol = bole wood volume expressed without branches (ft3 tree-1),
DBH = diameter at breast height (in.), and
Ht = height (feet).

Fig. 6: Photograph of diameter at breast height (DBH) taken on 3-year-old hybrid poplar trees.

Figure 6. Diameter at breast height (DBH) is taken on 3-year-old hybrid poplar trees.

Total aboveground biomass (TAB) per tree (kg tree-1) was calculated using an allometric function for aspen, cottonwood, and willow, developed by Jenkins et al. (2003), using Equation 6 as follows:

TAB = exp[−2.2094 + 2.3867 ln(DBH)]                                                                  (6)

where
TAB = total aboveground biomass (kg) for trees ≥ 2.5 cm DBH,
DBH = diameter at breast height (cm),
exp = exponential function, and
ln = natural log base “e” (2.718282).

Results from equations 1–5 in English units were converted to SI units. Both WVol and TAB were expressed on a per hectare basis dependent on tree stand density. Statistical analyses were carried out using the ANOVA procedure in the CoStat software package version 6.400 (CoHort Software, 2008). Least significant differences were determined at the 0.05 level of probability.

2002 Trial

The 2002 trial, the oldest hybrid poplar trial at ASC-Farmington, was established for the purpose of identifying suitable crosses for our region from among several well-documented species (Figure 7). During spring 2002, 10 different hybrid clones were obtained from nurseries in Oregon and Washington. These clones were various crosses between P. deltoides, P. maximowiczii, P. nigra, and P. trichocarpa. Prior to planting, the field was disked, leveled, and spot sprayed with Roundup. Netafim Ram pressure-compensating surface drip line (flow rate of 1.59 L hr−1 and with emitters every 0.91 m) was installed with two lines per row of trees. Sixteen cuttings per clone per plot were planted on May 15, 2002, at a 3.05 × 3.05 m spacing. Clone entries were replicated in three blocks for a total of 480 trees for the trial.

Fig. 7: Photograph of former graduate student Renae Pablo planting hybrid poplar cuttings in the first poplar trial at ASC-Farmington.

Figure 7. Former graduate student Renae Pablo plants hybrid poplar cuttings in the first poplar trial at ASC-Farmington.

2003 Trial

The 2003 trial continued the examination of species’ suitability to the region and also examined the effects of closely planted trees on tree survival and growth. In the spring of 2003, 20 different hybrid clones were obtained from nurseries in Oregon and Washington. These clones were various crosses between P. deltoides, P. maximowiczii, P. nigra, and P. trichocarpa. Netafim Ram pressure-compensating surface drip line (1.59 L hr−1 with emitters every 0.91 m) was installed with one line per row of trees. Six cuttings per clone per plot were planted in a 2 × 3 grid on May 15, 2003, at a 1.52 × 1.52 m spacing. Clone entries were replicated in four blocks for a total of 480 trees. An additional tier of six border trees was planted at the front and end of each drip line.

2005 Trial

In the spring of 2005, an evaluation of 64 hybrid clones was initiated as part of a multi-site study funded by the Western Sun Grant Initiative under the project leadership of Washington State University. The purpose of the study was to compare growth rates of the clones at eight sites in the western United States. In addition to the 64 clones (supplied by GreenWood Resources of Portland, OR), two locally collected clones were added (O’Neill et al., 2008). Netafim Ram pressure-compensating surface drip line (3.48 L hr−1 with emitters every 0.91 m) was installed with one line per row of trees. Four cuttings per clone per plot were planted on April 28 and May 2 and 3, 2005, at a 1.83 × 1.83 m spacing. Clone entries were replicated in four blocks for a total of 1,056 trees. An additional 344 cuttings were planted around the perimeter of the trial for border purposes, resulting in a field containing 1,400 trees total. TAB was determined from measurements of all tree stems after Jenkins et al. (2003), using Equation 6. In the case of trees with multiple stems, TAB was calculated using the sum of the DBH from all stems per tree. At the end of the fifth season, TAB was also determined via destructive sampling after Emerson et al. (2010), and used as a comparison with TAB from the other seven sites participating in the study.

2007 Trial

In this most recent trial, four top-producing hybrid clones were selected from the 2005 trial for further study of growth performance under conditions of low- and high-watering regimes in order to gain greater knowledge of tree plantation performance in the region’s semi-arid environment. Four watering regimes were used: low-watering at 70% and 80% of replacement tree evapotranspiration (ET) and high-watering at 120% and 130% of replacement ET. These clones (clones 433, 544, 910, and 911) were the leading producers in the biomass study established in 2005 in cooperation with GreenWood Resources. Netafim Ram pressure-compensating surface drip line with four emitter sizes (2.01, 2.35, 3.48, and 3.79 L hr−1 with emitters every 0.91 m) was installed with one line per row of trees. A split-plot design was established with whole plots set up as four 117-m-long by 3.66-m-wide rows of a single emitter size (irrigation level) across which four split plots (comprising the four clones, randomly assigned) were superimposed. Thirty-two cuttings of a single clone per plot were planted in a 4 × 8 grid on April 27, 2007, at a 3.66 × 3.66 m spacing. Irrigation treatments and clone entries were replicated in four blocks for a total of 2,048 trees across a total area of 2.75 ha.

Results

Four trials were initiated at ASC-Farmington between 2002 and 2007 to investigate adaptation potential and growth characteristics of hybrid poplar in the Four Corners region of northwest New Mexico. Given the need for testing a variety of clones for their potential suitability to the region, suggestions for entries were requested from poplar breeders and researchers in the western United States. Table 1 identifies the entries used in the four trials. Five taxa were included in the initial 2002 trial and repeated during 2003. The main taxon for these two seasons was the cross P. trichocarpa × P. deltoides, with six entries planted during 2002 and 14 planted in the 2003 season. The number of entries was greatly expanded in 2005 with the inclusion of 64 different hybrid clones obtained from GreenWood Resources, plus two locally obtained entries. There were eight different taxa, with the P. deltoides × P. nigra cross (P. × canadensis) numbering 19 entries, followed by the P. trichocarpa × P. deltoides cross with 17 entries. Four of the top P. × canadensis hybrids from the 2005 trial were used in the 2007 differential water application trial.

Table 1. Number of Taxa Studied and Parental Species for Hybrid Poplar Clones Grown in Four Drip Irrigation Trials at the NMSU Agricultural Science Center at Farmington, NM, 2002–2011.

Trial Number
Studied
Taxon Female Parent Male Parent
2002 1 DxM P. deltoides P. maximowiczii
2002 1 DxN P. deltoides P. nigra
2002 1 NxM P. nigra P. maximowiczii
2002 6 TxD P. trichocarpa P. deltoides
2002 1 TxN P. trichocarpa P. nigra
2003 1 DxM P. deltoides P. maximowiczii
2003 2 DxN P. deltoides P. nigra
2003 1 NxM P. nigra P. maximowiczii
2003 14 TxD P. trichocarpa P. deltoides
2003 2 TxN P. trichocarpa P. nigra
2005 1 DxB P. deltoides P. balsamifera
2005 7 DxD P. deltoides P. deltoides
2005 9 DxM P. deltoides P. maximowiczii
2005 19 DxN P. deltoides P. nigra
2005 6 DxT P. deltoides P. trichocarpa
2005 2 NxM P. nigra P. maximowiczii
2005 17 TxD P. trichocarpa P. deltoides
2005 5 TxN P. trichocarpa P. nigra
2007 4 DxN P. deltoides P. nigra

Growth characteristics for eight clones in the 2002 trial are presented in Table 2. Two clones were eliminated from the trial after the first season due to poor survival (O’Neill et al., 2006). The P. × canadensis clone OP-367 has been the greatest producer throughout the trial. It is followed by the clone 311-93, a TxN cross, and then by 58-280, a TxD hybrid. An initial growth target was to have an entry or entries with a 25-cm DBH after 10 seasons. OP-367 obtained this target after eight seasons and reached a DBH of 28.0 cm by the end of the 2011 growing season. The clone Eridano (DxM) had the smallest DBH after 10 seasons, measuring 15.5 cm.

Table 2. Diameter at Breast Height (cm), Height (m), and Wood Volume (m3 ha-1) for Eight Hybrid Poplar Clones Grown Under Drip Irrigation at the NMSU Agricultural Science Center at Farmington, NM, 2002–2011.

  ------------------- DBH† (cm) -------------------
Clone (Taxon) 2003 2004 2005 2006 2007 2008 2009 2010 2011
OP-367 (DxN) 4.6 10.2 15.8 19.9 22.1 24.0 25.5 26.9 28.0
311-93 (TxN) 3.0 6.6 12.0 15.2 17.3 19.4 20.4 21.6 23.0
58-280 (TxD) 3.3 7.1 11.9 15.5 17.6 19.0 20.6 21.8 23.0
49-177 (TxD) 2.5 6.6 10.9 13.7 15.2 16.5 18.3 20.1 21.1
195-529 (TxD) 2.0 5.3 9.4 11.9 13.0 10.9 17.0 17.5 18.5
52-225 (TxD) 2.0 4.8 8.6 11.2 12.4 14.5 15.5 16.8 18.0
NM-6 (DxM) 0.1 3.8 8.6 12.2 13.5 14.7 16.0 17.0 17.9
Eridano (DxM) 1.5 4.6 8.4 10.7 12.2 13.5 14.0 14.7 15.5
Mean‡ 2.5 6.1 10.7 14.0 15.5 16.8 18.8 19.8 20.9
p > F <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
CV (%) 46.1 35.4 23.9 21.3 19.9 25.7 17.8 17.1 17.5
LSD (0.05) 0.5 1.1 1.1 1.2 1.3 2.3 1.8 1.8 1.9
  ------------------- Height (m) -------------------
OP-367 (DxN) 4.1 8.0 11.8 15.6 16.2 17.5 18.6 19.2 19.9
311-93 (TxN) 3.5 6.5 9.9 12.6 13.2 13.8 15.0 16.0 16.6
58-280 (TxD) 3.3 6.1 9.2 11.6 12.5 13.6 14.3 14.9 15.4
49-177 (TxD) 3.1 5.5 9.5 11.7 12.1 13.5 14.8 16.1 16.9
195-529 (TxD) 3.0 5.6 9.0 10.7 10.8 12.7 13.6 14.5 15.1
52-225 (TxD) 2.7 4.7 7.7 9.8 9.8 9.9 11.5 12.2 13.1
NM-6 (DxM) 2.5 4.6 8.5 10.9 11.1 12.0 12.7 14.1 14.9
Eridano (DxM) 2.3 4.0 6.8 8.7 9.1 9.8 10.3 12.0 12.5
Mean‡ 3.1 5.7 9.1 11.5 11.9 13.0 14.0 15.1 15.7
p > F <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
CV (%) 24.8 23.1 16.8 16.2 15.9 16.4 15.0 13.8 13.3
LSD (0.05) 0.3 0.7 0.6 0.8 0.8 1.1 1.1 1.1 1.1
  ------------------- Wood Volume (m3 ha-1) -------------------
OP-367 (DxN) 2.9 25.8 88.5 187.5 235.0 303.1 366.6 417.6 472.9
311-93 (TxN) 1.1 10.8 45.2 91.3 121.3 151.4 190.1 230.0 268.0
58-280 (TxD) 1.2 10.9 40.2 85.0 117.7 155.4 183.1 217.4 241.8
49-177 (TxD) 0.9 8.5 37.0 72.6 88.9 123.7 162.1 205.9 238.6
195-529 (TxD) 0.6 6.8 29.2 54.8 66.2 101.6 129.7 155.3 177.7
52-225 (TxD) 0.6 4.5 19.6 41.0 47.9 64.0 88.1 110.6 134.1
NM-6 (DxM) 0.3 2.9 21.3 52.8 67.6 85.7 103.7 128.4 155.3
Eridano (DxM) 0.6 4.1 18.4 36.7 46.5 60.6 68.0 88.7 101.4
Mean‡ 1.1 9.4 37.8 78.4 100.2 136.5 168.5 201.5 232.0
p > F <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
CV (%) 84.6 77.9 48.1 44.1 40.1 40.1 37.0 36.0 35.5
LSD (0.05) 0.4 3.5 7.5 14.7 17.0 27.8 32.3 37.4 42.5
†DBH = Diameter at breast height (~1.37 m).
‡Mean is calculated from eight clonal entries, with each entry consisting of three replications of 16 trees for each plot.

Height determinations (Table 2) were obtained during the dormant period following each growing season using a clinometer (Suunto PM-5; Figure 8). OP-367 demonstrated the greatest growth in height beginning in the first season. By the end of the second season, OP-367 had reached a height of 4.1 m and had a height of 19.9 m at the end of 2011, representing an average height growth rate of 2.0 m per year. As with DBH, height of the clones 311-93 and 58-280 followed that of OP-367, with a final height of 16.6 and 15.4 m, respectively. The shortest clone after 10 years of growth was Eridano, with a final height of 12.5 m.

Fig. 8: Photograph of co-author Rob Heyduck determining hybrid poplar tree height using a clinometer.

Figure 8. Co-author Rob Heyduck determines hybrid poplar tree height using a clinometer.

Wood volume (Table 2) was calculated after Browne (1962) on a per tree basis and scaled up to a hectare basis. Since WVol is a function of both DBH and height, results for WVol demonstrated a response similar to these two variables. OP-367 had the greatest WVol after the second season (2.9 m3 ha-1) and continued to perform well throughout the experimental period. After 10 years, OP-367 achieved a WVol of 473 m3 ha-1 (47 m3 ha-1 yr-1), followed by 311-93 and 58-280 at 268 and 242 m3 ha-1, respectively.

The 2003 trial was designed as a short-rotation biomass production test with a tree density of 4,328 trees ha-1. Twenty clones were obtained from nurseries in the Pacific Northwest based on suggestions from poplar researchers working in the western United States. Five-year growth characteristics are presented in Table 3. Once again, OP-367 was the clone that grew the most, although its survival rate was only 71% in this trial as opposed to 100% in the 2002 trial (O’Neill et al., 2006). Final DBH, height, WVol, and TAB for OP-367 were 14.7 cm, 12.1 m, 319 m3 ha-1, and 208 Mg ha-1, respectively. Calculated over the five-year trial, incremental TAB for OP-367 was 42 Mg ha-1 yr-1. The next closest clone that had also been in the 2002 trial was 58-280, with a final TAB of 133 Mg ha-1 (27 Mg ha-1 yr-1). DN-34, the other P. × canadensis entry in this trial, had a TAB of 108 Mg ha-1 (22 Mg ha-1 yr-1), not significantly different than the trial mean TAB of 100 Mg ha-1 (20 Mg ha-1 yr-1) (Table 3).

Table 3. Final Growth and Survival of 20 Hybrid Poplar Clones Grown Under Drip Irrigation at the NMSU Agricultural Science Center at Farmington, NM, 2003–2007.

Clone (Taxon) Survival (%) DBH† (cm) Height (m) Wood
Volume
(m3 ha-1)
Total
Above-Ground
Biomass
(Mg ha-1)
OP-367 (DxN) 70.8 14.7 12.1 319.1 208.2
184-411 (TxD) 54.2 13.2 11.3 260.3 168.3
184-401 (TxD) 70.8 12.2 10.3 213.4 146.5
15-29 (TxD) 70.8 12.2 10.7 222.7 143.5
59-289 (TxD) 75.0 12.2 11.5 220.6 133.0
58-280 (TxD) 70.8 11.9 10.6 207.8 132.9
52-225 (TxD) 87.5 11.2 11.0 185.9 108.7
49-177 (TxD) 75.0 11.2 10.2 167.9 107.8
DN-34 (DxN) 83.3 11.2 9.4 146.4 107.8
50-197 (TxD) 70.8 10.9 9.7 147.7 106.6
57-276 (TxD) 91.7 10.9 10.4 164.9 104.6
309-74 (TxN) 87.5 10.4 11.1 185.7 103.9
195-529 (TxD) 83.3 10.7 10.0 146.4 98.1
311-93 (TxN) 100.0 10.4 10.3 151.7 95.9
56-273 (TxD) 91.7 8.9 9.5 102.0 65.8
DTAC-7 (TxD) 95.8 8.4 8.8 84.7 57.1
NM-6 (DxM) 70.8 7.6 7.4 65.6 50.2
55-260 (TxD) 75.0 6.6 6.5 50.4 39.3
Eridano (DxM) 70.8 6.9 6.6 41.8 36.0
50-184 (TxD) 62.5 4.8 5.0 20.4 17.2
Mean‡ 77.9 10.4 9.7 153.5 100.1
p > F 0.0077 <0.0001 <0.0001 <0.0001 <0.0001
CV (%) 52.2 24.3 19.3 52.0 50.7
LSD (0.05) 23.0 2.0 1.4 61.5 38.7
†DBH = Diameter at breast height (~1.37 m).
‡Mean is calculated from 20 clonal entries, with each entry consisting of four replications
   of 6 trees for each plot.

A multi-locational trial, coordinated by Washington State University and with clonal stock supplied by GreenWood Resources, was established in the western United States as part of the Western Sun Grant Initiative (Emerson et al., 2010). Of the 12 sites selected, eight trials were established in 2005 and harvested at the end of the five-year cycle in 2009 (Table 4), including Farmington. Sites ranged in latitude from Puyallup, WA (47.20° N), to Farmington (36.69° N). All sites but Farmington were in the Pacific Northwest and four sites were in the Intermountain West, with Pullman, WA, and Farmington representing the northern and southern limits of this region, respectively. Elevation ranged from sea level at Westport, OR, to 1,720 m at Farmington. End of cycle biomass was determined on a per tree basis via destructive sampling and used to calculate biomass (Emerson et al., 2010). Biomass production was the lowest at Newberg, OR (15 Mg ha-1), and the greatest at Farmington (137 Mg ha-1). On an incremental basis, biomass ranged from 3 Mg ha-1 yr-1 at Newberg to 27 Mg ha-1 yr-1 at Farmington (Table 4).

Table 4. Locational Characteristics and Mean Productivity of 64 Hybrid Poplar Clones Grown Under Drip Irrigation at Eight Sites in the Western United States Participating in the Western Sun Grant Initiative; Two Additional Clones Were Grown in Farmington, NM, 2005–2009.

Site Latitude
(N)
Longitude
(W)
Elevation
(m)
Biomass
(kg tree-1)
Biomass
(Mg ha-1)
Incremental
Biomass
(Mg ha-1 yr-1)
Puyallup, WA 47.20° 122.30° 15 39 116.5 23.3
Pullman, WA 46.73° 117.00° 775 26 77.7 11.5
Westport, OR 46.10° 123.37° 6 43 128.4 25.7
Boardman, OR 45.83° 119.55° 92 29 86.6 17.3
Newberg, OR 45.21° 122.97° 59 5 14.9 3.0
Caldwell, ID 43.66° 116.23° 722 39 116.5 23.3
Klamath Falls, OR 42.20° 121.82° 1,249 23 68.7 13.7
Farmington, NM 36.69° 108.31° 1,720 46 137.4 27.5

Incremental biomass yields for Farmington, based on measurements of all stems and multiple stems using Equation 6 after Jenkins et al. (2003), are illustrated in Figure 9. Of the 58 clones that survived the full five-year trial at Farmington, the incremental TAB ranged from a low of 5 Mg ha-1 yr-1 for entry 10266 to a high of 69 Mg ha-1 yr-1 for entry 910. The trial mean was 24 Mg ha-1 yr-1, and 24 clones met or exceeded this value. Oak Ridge National Laboratory has calculated a conversion value of 10 Mg ha-1 yr-1 as the level above which hybrid poplar production is more profitable than traditional crops (Walsh et al., 2003). Using this metric, there were 52 clones out of the original 64 that were planted for the trial that met or exceeded 10 Mg ha-1 yr-1. After two growing seasons, four top-producing P. × canadensis entries (433, 544, 910, and 911) were selected for the 2007 differential water application trial.

Fig. 9: Bar graph of final incremental aboveground biomass of 58 poplar clones grown under drip irrigation at the NMSU Agricultural Science Center at Farmington, NM, 2005�2009.

Figure 9. Final incremental aboveground biomass of 58 poplar clones grown under drip irrigation at the NMSU Agricultural Science Center at Farmington, NM, 2005–2009. The remaining eight clones did not survive the full five-year trial. White bars indicate four DxN clones selected for the 2007 differential water application trial.

Annual growth characteristics of the four entries selected for the 2007 differential water application trial grown under four drip irrigation levels are presented in Table 5. Note that entry 433 in Figure 9 is, in reality, clone OP-367. Averaged over entries, DBH tended to increase with water application, although the 120% and 130% of ET applications were not significantly different. By the end of the 2011 season, clone OP-367 and entry 544 both had an average DBH of 17 cm. As with DBH, height averaged over the four entries increased with water application, although the 120% and 130% of ET treatments were not significantly different. OP-367 had the greatest height of 13.8 m at the end of the fifth season, followed by 544 (13.0 m), 910 (12.3 m), and 911 (12.2 m). WVol also increased with water applications, but again, 120% and 130% of ET applications were not significantly different after five years, with values of 99 m3 ha-1 and 103 m3 ha-1, respectively. WVol for clone OP-367 was the greatest after the 2011 season at 91 m3 ha-1. Entry 544 had the next greatest WVol at 83 m3 ha-1, followed by 910 and 911, each with a WVol of 69 m3 ha-1.

Table 5. Diameter at Breast Height (cm), Height (m), and Wood Volume (m3 ha-1) for Four Hybrid Poplar Clones Grown Under Four Drip Irrigation Regimes at the NMSU Agricultural Science Center at Farmington, NM, 2007–2011.

Irrigation (% ET)
or Clone (Taxon)
2007 2008 2009 2010 2011
  ------------------- DBH† (cm) -------------------
70 1.1 5.6 9.1 12.4 14.6
80 1.1 5.6 9.9 13.2 15.4
120 1.1 6.1 11.7 15.7 18.3
130 1.1 5.8 11.7 15.7 18.0
OP-367 (DxN) 1.1 5.8 11.4 15.2 17.2
544 (DxN) 0.9 5.6 10.7 14.5 17.2
910 (DxN) 1.1 5.8 10 13.7 15.9
911 (DxN) 1.2 5.8 10 13.7 16.0
Mean‡ 1.1 5.8 10.7 14.2 16.6
CV (%) 24.2 14.8 12.1 10.8 15.6
LSD (0.05) (Irr.) 0.05 0.18 0.25 0.58 0.58
LSD (0.05) (Clone) 0.05 0.18 0.25 0.30 0.52
p > F (Irr.) 0.4079 0.1089 <0.0001 <0.0001 <0.0001
p > F (Clone) <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
p > F (Irr. X) <0.0001 <0.0001 <0.0001 <0.0001 0.029
  ------------------- Height (m) -------------------
70 2.2 5.2 7.6 10.3 11.1
80 2.1 5.1 7.8 11.1 11.9
120 2.0 5.3 8.8 12.3 13.9
130 2.1 5.2 8.6 12.4 14.4
OP-367 (DxN) 2.2 5.3 8.6 12.6 13.8
544 (DxN) 2.0 4.7 7.9 11.5 13.0
910 (DxN) 2.0 5.3 8.1 11.0 12.2
911 (DxN) 2.0 5.5 8.2 11.1 12.3
Mean‡ 2.1 5.2 8.2 11.5 12.8
CV (%) 12.7 8.8 8.8 8.1 10.1
LSD (0.05) (Irr.) 0.05 0.09 0.15 0.57 0.66
LSD (0.05) (Clone) 0.05 0.09 0.15 0.19 0.26
p > F (Irr.) 0.3773 0.5097 <0.0001 <0.0001 <0.0001
p > F (Clone) <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
p > F (Irr. X) <0.0001 0.0013 <0.0001 <0.0001 <0.0001
  ----------- Wood Volume (m3 ha-1) -----------
70 0.1 3.4 14.0 34.7 51.5
80 0.1 3.4 16.1 42.5 60.3
120 0.1 4.2 25.4 65.1 98.7
130 0.1 3.9 24.4 64.9 102.9
OP-367 (DxN) 0.1 4.0 23.3 63.7 91.4
544 (DxN) 0.0 3.0 18.6 51.6 82.7
910 (DxN) 0.1 3.9 18.8 46.0 69.4
911 (DxN) 0.1 4.1 19.0 45.9 69.4
Mean‡ 0.1 3.7 19.9 51.8 78.3
CV (%) 78.5 29.9 25.5 23.2 30.0
LSD (0.05) (Irr.) 0.01 0.22 1.04 5.39 7.45
LSD (0.05) (Clone) 0.01 0.22 1.04 2.42 4.76
p > F (Irr.) 0.2434 0.0718 <0.0001 <0.0001 <0.0001
p > F (Clone) <0.0001 <0.0001 <0.0001 <0.0001 <0.0001
p > F (Irr. X) 0.2996 <0.0001 <0.0001 <0.0001 0.7953
†DBH = Diameter at breast height (~1.37 m).
‡Mean is calculated from four clonal entries and four irrigation levels, with four replications
   of 12 sample trees for each plot.

Discussion

Four drip-irrigated trials were carried out at ASC-Farmington from 2002 through 2011 to determine adaptation potential of hybrid poplar to the semi-arid Four Corners region of northwest New Mexico (Figures 10–12). A wide range of poplar germplasm was included over the 10 years of the trials (Table 1). There were eight different hybrid types using five poplar species. Over the four trials carried out during 10 years, there were 100 hybrid entries, sometimes repeated in various trials (e.g., OP-367, which was a DxN cross used as a check in all trials) and sometimes entered in a single trial (e.g., 10272, a DxB cross entered in the 2005 trial). Given the range of germplasm, an indication of adaptation potential can be surmised.

Fig. 10: Photograph of five-year-old hybrid poplar during the winter of 2007�2008.

Figure 10. Five-year-old hybrid poplar during the winter of 2007–2008.

Fig. 11: Photograph of the east end of the 2003-planted hybrid poplar trial after five growing seasons.

Figure 11. East end of the 2003-planted hybrid poplar trial after five growing seasons.

Fig. 12: Aerial view photograph of 2005-planted hybrid poplar biomass trial.

Figure 12. Aerial view of 2005-planted hybrid poplar biomass trial.

From these early results, it is apparent that OP-367 is well adapted to the region. It tended to have the greatest productivity throughout the four trials in both low- and high-population tests. After 10 years of growth in the 2002 trial (3.05 × 3.05 m spacing), OP 367 had a DBH of 28 cm, a height of 20 m, and a WVol of 473 m3 ha-1 (Table 2). By the end of the eighth year, this clone had reached a desired productivity recommended by a local sawmill, which could shorten forecasted rotations from 10 to eight years. In the high-density (1.52 × 1.52 m spacing) biomass trial of 2003 (Table 3), OP-367 attained a WVol of 319 m3 ha-1 and a TAB of 208 Mg ha-1 (42 Mg ha-1 yr-1).

The five-year multi-locational trial with 64 entries (66 at Farmington) was carried out at eight sites in the western United States from 2005 to 2009 (Table 4). A range of environments exist at these sites, representing irrigated and non-irrigated production opportunities from latitudes of 36.69° N to 47.20° N. Mean productivity for the eight sites, determined by destructive sampling, was 93.3 Mg ha-1 (18.7 Mg ha-1 yr-1). Farmington, the most southern and driest site, had the greatest mean site productivity of 137 Mg ha-1 (27 Mg ha-1 yr-1) (Emerson et al., 2010).

Along with four other clones, OP-367 had the greatest productivity of the 66 entries planted at Farmington in the 2005 trial (Figure 9). Incremental TAB for OP-367 in this high-density test (1.83 × 1.83 m), based on the sum of DBH measurements from stems and multiple stems, was 62 Mg ha-1 yr-1 after five seasons, somewhat greater than the incremental TAB of 42 Mg ha-1 yr-1 produced by OP-367 in the 2003 trial. Although both of these trials were conducted for five years, the 2003 trial was planted at a higher tree density than the 2005 trial, i.e., 4,328 versus 2,986 trees ha-1, which may have resulted in greater competition and less productivity in the 2003 trial. Mean incremental TAB for the 2005 trial at ASC-Farmington was estimated to be 24 Mg ha-1 yr-1, which was similar to the 27.5 Mg ha-1 yr-1 for incremental TAB determined by destructive sampling (Emerson et al., 2010). Of the 24 clones in the 2005 Farmington trial with an estimated incremental TAB equal to or greater than the mean of 24 Mg ha-1 yr-1, 19 were P. × canadensis crosses (DxN taxon). Walsh et al. (2003) at Oak Ridge National Laboratory determined that production greater than 10 Mg ha-1 yr-1 in a hybrid poplar system would be more profitable than land use systems under traditional crops. There were 54 entries out of 66 planted entries (82%) in the 2005 trial that met or exceeded 10 Mg ha-1 yr-1, indicating economic potential for these entries.

Four top-producing P. × canadensis entries (OP-367, 544, 910, and 911) from the 2005 trial were selected for testing in a differential water application trial planted in 2007 (Figure 13). Clones that were under-irrigated at rates of 70 and 80% of ET produced less DBH, height, and WVol than clones receiving 120 and 130% of ET (Table 5). By the end of the fifth growing season (2011), the clone OP-376 grew the tallest (13.8 m) and had the greatest WVol (91.4 Mg ha-1), which amounted to an incremental productivity of 18.3 Mg ha-1 yr-1. Given the range of poplar taxa tested at ASC-Farmington, the DxN taxon (P. × canadensis crosses) generally produced the greatest biomass in both low- and high-population density trials. The clone OP-367, a cross originally made in upstate New York during the 1920s, appears especially well adapted to the region. Through breeding and selection, a number of other clones also appear well suited to the region, especially the P. × canadensis crosses. Other poplar crosses should also be investigated for adaptation to conditions in northern New Mexico and the surrounding Four Corners region, as well as other locations with similar climatic and edaphic conditions. Species for selection in further crosses should include Rio Grande cottonwood (P. deltoides ssp. wislizeni) and Fremont cottonwood (P. fremontii), two species endemic to the region.

Fig. 13: Photograph of growth differences of hybrid poplar.

Figure 13. Growth differences of hybrid poplar are evident near the end of five growing seasons in the 2007-planted differential water application trial. Percentages represent ET replacement applications.


Fig. 14: Mick O'Neill, Professor of Agronomy at the NMSU Agricultural Science Center in Farmington.


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Acknowledgments

The authors wish to recognize the valuable contributions to this research provided by the support staff of the NMSU Agricultural Science Center at Farmington. The research was partially funded through the NMSU Agricultural Experiment Station, the Western Sun Grant Initiative, and the José Fernández Memorial Chair in Crop Production.


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