Abstract: Growth and yield of Populus were examined in a 5-year test of five spacing- harvest regimes in western Washington. Two hybrid clones (D-01 and H-11) were planted at two woodgrass spacings (0.18- and 0.3-m) and three wider spacings (0.5-, 1.0-, and 2.0-m). All treatments were replicated three times in a randomized block design; all plots were fertilized, irrigated, and weeded uniformly. Mean annual harvest yields in the woodgrass treatments did not differ significantly between the two clones (D-01 and H-11) or two spacings (0.18- and 0.3-m), averaging 6.4 to 7.0 Mg ha-1 (tonnes ha-1) over the 5- year period. The highest yield in woodgrass treatments was produced in the second year (first year of coppice), and it declined thereafter. Cumulative growth in the wider spacings was significantly greater than in the two woodgrass spacings. Per hectare yields of clone H-11 in the wider spacings at age 5 were two to three times greater than cumulative yields obtained with annually cut woodgrass, averaging 15.7 to 18.8 Mg ha-1 yr-1. Fifth-year increments in all wider spacings (0.5-, 1.0-, and 2.0-m) of clone H-11 exceeded 30 Mg ha- 1. Yields of clone D-01 in the wider spacings ranged from 8.1 to 10.9 Mg ha-1 yr-1 and thus were also substantially greater than cumulative yields of woodgrass. We conclude that annually harvested woodgrass shows little promise as a viable system for growing Populus for biomass. On the other hand, yields in the wider spacings with a longer harvest cycle were substantially higher than previously expected, especially for clone H-11. Thus, possibilities for application of the wide-spaced regimes and longer cutting cycles appear promising.

Keywords: Poplar, cottonwood, coppice, spacing, yields, bioenergy, stand density, intensive culture

Farm production of woody biomass was proposed more than two decades ago in the southeastern United States (McAlpine and others 1966; Herrick and Brown 1967). The concept, called “silage sycamore” by the proponents, included establishing rapid-growing trees at dense spacings, applying intensive cultural practices, harvesting in cycles of 10 years or less, regenerating subsequent crops via sprouts or coppice arising from stumps, and using a high degree of mechanization. Trials were soon initiated elsewhere in the country (Heilman and others 1972; Dawson 1976; Geyer and others 1985), with emphasis on Populus species and hybrids. During the first energy crisis, this short-rotation approach was suggested as a means to produce wood for energy (Szego and Kemp 1987). Subsequently, considerable research was conducted on biomass plantations for fiber and energy, much of it funded by the Short Rotation Woody Crops Program (now the Biofuels Feedstock Development Program) of the U.S. Department of Energy. Other public agencies and utilities have also funded or established projects, particularly in the northeastern and north central states. In general, such work indicated that spacings should be wider and harvest cycles longer than those evaluated in many of the early trials. Stand densities of 2500 to 4000 trees per hectare (i.e., square spacings of 2.0 to 1.6 m) and rotations of 5 to 8 years are now perceived as optimum for bioenergy crops (Ranney and others 1987). Other ongoing research suggests that even wider spacings and longer rotations may be preferable in some situations and for some objectives.

In the late 1970's and early 1980's, however, a nurseryman in Oregon proposed a radical departure from the above trends in woody biomass farming (Dula 1984). The proposed system entailed establishing a Populus hybrid (clone D-01) at densities of 100 thousand to 600 thousand rootstocks per hectare with annual harvests of coppice. Biomass yields were purported to exceed 100 Mg ha-1 (tonnes ha-1) annually. Economic analyses performed by coupling such yield data with estimated costs suggested that the system — dubbed “wood- grass”—compared favorably with other short-rotation density regimes (Vyas and Shen 1982). Considerable interest developed in the energy conversion community; some political representatives and agency administrators also became intrigued with the concept. Some forest biologists, however, remained skeptical. Clearly, a scientific evaluation of the woodgrass concept was needed. Our study compares two Populus hybrids (D-01 and H- 11) at five square spacings—ranging from two woodgrass spacings (0.18 and 0.30 m) to one approaching a conventional pulpwood spacing (2.0 m). This report describes growth and yield of the plantings over a 5-year period.

Methods

Our experiment was established in cooperation with the Washington State Department of Natural Resources at the Tree Improvement Center, located 12 km east of Olympia, Washington. Climate is mild with an average growing season of 190 frost-free days and a mean July temperature of 16º C. Precipitation averages more than 1000 mm per year, falling mostly as rain from October through May; summers are periodically dry. The land was previously farmed for strawberry and hay crops, and topography is relatively level. The soil is Nisqually loamy fine sand (a sandy, mixed, mesic Pachic Xerumbrept); it is a deep, somewhat excessively drained, medium acid (pH 5.6) soil formed in glacial outwash. The land was prepared for planting by plowing and disking in winter 1985-86.

The study was established as a factorial design with two Populus clones and five spacing treatments, replicated in three blocks. One clone, D-01, was a Populus hybrid (taxonomic identity unknown, but suspected to be either P. trichocarpa x P. nigra or P. trichocarpa x P. angustifolia) developed originally at University of Idaho and subsequently selected from a Canadian planting by Dula's Nursery of Canby, Oregon (Dula 1984). The other clone, H-11, was a P. trichocarpa x P. deltoides (11-11) hybrid developed and tested by University of Washington and Washington State University (Heilman and Stettler 1985). Square spacings (m by m) were 0.18, 0.3, 0.5, 1.0, and 2.0 m. Equivalent trees per hectare were ca. 310,000; 110,000; 40,000; 10,000; and 2,500. The first two spacings (0.18 and 0.3 m) were woodgrass treatments recommended by Dula [i.e., three and one plant(s) per square foot, respectively]. Size of treatment plots varied with spacing; all plots were sufficiently large to provide at least 100 interior measurement trees (400 trees for woodgrass harvests) and a border approximately one-half as wide as the projected height of trees at harvest. Thus, trees spaced at 2.0 m were grown in the largest plots (32 m by 32 m) and plants in the 0.18-m woodgrass treatments were grown in the smallest plots (~ 10 m by 10 m). Clone-spacing treatments were assigned randomly within each replicate block, with one minor stipulation. The annually harvested woodgrass plots were always assigned to outside positions in the block, so as to minimize future shading as well as root competition from trees in wider spacings.

Both clones were planted by hand as unrooted, hardwood cuttings in late April 1986. All cuttings were 30 cm long and had a minimum upper diameter of 1 cm; they were planted 20 cm deep with at least two healthy axillary buds remaining above ground.

Supplemental nutrients and water were provided uniformly in plots of all treatments. A pre-planting application of fertilizer (16-16-16) provided the equivalent of 100 kg per hectare each of nitrogen, phosphorus, and potassium. Additional nitrogen fertilizer (ammonium nitrate) was applied at 100 Kg N per hectare in May 1988. Plots were irrigated throughout each summer by a drip system; amounts applied were equivalent to 400-500 mm per growing season. All plots were kept free of weeds during the first year by tilling and hoeing; in the second and third year, developing weed patches were controlled by spot applications of herbicides (oxyfluorfen, pronamide, and glyphosate) and hoeing. Little such work, however, was needed after the second year. At the end of the first year, all positions occupied by dead trees were replanted with unrooted cuttings; also, any secondary shoots on plants in the wider spacings (0.5, 1.0, and 2.0 m) were removed, resulting in stands composed solely of single-stemmed trees.

Survival, height, and basal diameter were recorded at the end of each of 5 growing seasons on the central 100 trees in each plot. Number of living and dead sprouts per rootstock were also tallied after the second and subsequent growing seasons in woodgrass plots. Yield data for the woodgrass treatments were based on annual harvests after leaffall of 400 trees in the center of each plot. Moisture contents were determined on subsamples to convert fresh weight to oven-dry (105º C) weight. Yields for the wider-spaced plots were estimated from oven-dry biomass component equations applied to diameter and height measurements of the trees. The equations were developed via destructive sampling of trees representative of the spectrum of sizes in each spacing of each clone. Equations of the form Ln(Y)=f (diameter, spacing, height, and age) were fit independently for each clone. R2 ranged from 0.972 for branch weight of D-01 to 0.997 for stem weight of H-11. Stem weights and branch weights were estimated by separate equations and summed to provide above-ground, woody dry biomass. Above-ground woody biomass estimates for all trees on each plot were summed, and the resulting plot sums were expanded by appropriate multipliers to provide yield per hectare.

Plot means were calculated for each variable and displayed in tables or figures to illustrate trends in development of the plantings. All data have been analyzed by standard ANOVA techniques, and treatment means were compared by Bonferroni's test using P £ 0.05 as the level of significance.

Results and Discussion

Establishment year (general)

Survival at the end of the first growing season averaged 96% for D-01 and 98% for H-11. Average heights for D-01 and H-11 were 1.4 and 1.8 m, respectively. In the two woodgrass spacings, mean heights of the two clones were very similar, averaging 1.3 m. As spacing widened from 0.18 to 1.0 m, mean height of both clones increased to 2.3 m for H-11 and 1.7 for D-01. Trees of both clones, however, were shorter at 2.0-m than at 1.0- m spacing. Mean heights for the clones at spacings of 0.5, 1.0, and 2.0 m differed by 50 cm or more. Effects of spacing on basal diameter were of greater magnitude than effects on height. Both clones had similar diameters at the 0.18- and 0.3-m spacings (6 mm and 8 mm, respectively); mean diameter of both clones was greater at wider spacings and response to increased spacing was greater for H-11. Diameters of the latter clone were nearly four times greater at the 2.0-m spacing (i.e., 22 mm) than at 0.18-m spacing. Patterns of leaf, bud, and branch production also differed markedly between clones and among spacings during the first growing season. Average distances between leaves (hence, axillary buds) were greater in H-11 (about 4 cm) than in D-01 (about 3 cm). Moreover, H- 11 exhibited sylleptic growth; that is, branches developed from axillary buds during the same growing season in which the buds formed. The proportion of buds producing sylleptic branches ranged from none in the densest woodgrass spacing to 31% in the widest (2.0-m) spacing. Growth in D-01 was predominantly proleptic with axillary buds remaining dormant until the next growing season; no buds produced sylleptic branches in the two woodgrass spacings and only 3.3% and 2.0% produced sylleptic branches in the 1.0-m and 2.0-m spacings.

The substantially reduced first-year growth in the woodgrass spacings as compared with growth at wider spacings indicated that competition among plants was sufficient to depress individual tree growth. Contrasted with trees in the 1.0-m spacing, trees in the densest woodgrass spacing averaged 41% shorter in height and 71% smaller in basal diameter. Moreover, leaf area per tree in the densest spacing was less than one-fifth of that in the 1.0-m spacing. Because of the intense competition in the woodgrass plots, and, in accord with Dula's (1984) procedures, we harvested trees in all woodgrass treatments at the end of the growing season to establish coppice.

The growth patterns, morphological traits, and competitive stresses observed in the establishment year were harbingers of major differences in performance among clones and spacings in subsequent years.

Woodgrass

Yields from the first (non-coppice) harvest of the woodgrass spacings and those of four subsequent (true coppice) harvests are shown in Table 1. First-year yields of the two clones were nearly identical, averaging 3.5 Mg ha-1. Dry-matter production averaged 4.0 Mg ha-1 in the 0.18-m spacing, and 3.0 Mg ha-1 in the 0.3-m spacing, but differences were not statistically significant (p=0.09).

In April following winter harvest, vigorous coppice developed on the stumps, and growth was excellent throughout the season. Yields from the second cutting were more than double those of the first cutting, and ranged from 7.7 to 9.6 Mg ha-1 (Table 1). Although yields did not vary significantly at p < 0.05, production tended to be greater for clone D- 01 than for H-11 (p=0.16, 9.1 vs. 8.0 Mg ha-1) and in the 0.18-m than in the 0.3-m spacing (p=0.31, 9.0 vs. 8.2 Mg ha-1).

Coppice development on stumps after the second harvest (or first coppice harvest) was also vigorous, but it became increasingly less so with each successive harvest. Third- and fourth-year yields continued to be somewhat greater (p=0.35 and p=0.17, respectively) in the 0.18-m than in the 0.3-m woodgrass spacing (Table 1). Fourth-year yields of H-11 were significantly greater (p=0.02) than those of D-01 (7.6 vs. 5.7 Mg ha-1). Production was declining, however, and by the fifth harvest, average yield was about 25% lower than that obtained in the second harvest. The reductions in yield (5th- versus 2nd-year) were greater for clone D-01 (-28%) and in the densest (0.18-m) spacing (-34%). By the fifth year, annual production in the 0.3-m spacing was somewhat, but not significantly, greater (p=0.08) than that at the 0.18-m spacing.

Increased biomass production in the second harvest and patterns of production in subsequent harvests were associated with sprouting characteristics—number of sprouts per rootstock, size of dominant sprouts on the rootstocks, and number of rootstocks surviving (Table 2, Fig. 1). The tendency for clone D-01 to produce higher yield per hectare in the second harvest was related mostly to dramatic differences between the clones in the total number of sprouts initiated (p < 0.01) and the number surviving at harvest (p < 0.01) (Table 2). Averaged across spacings, D-01 produced 7.4 sprouts per rootstock whereas H-11 had only 4.6.

Clonal differences in total sprout production may be related in part to differences in patterns of bud and branch production during the establishment and subsequent years. Numbers of axillary buds per centimeter were about one-third greater in D-01 than in H- 11; also, a larger percentage of the H-11 buds formed sylleptic branches. The combined effect of these two growth characteristics resulted in greater numbers of vigorous buds (axillary and suppressed) on the rootstocks of D-01 than on H-11; such buds play a significant role in sprout development. The number of sprouts initiated increased in the third and fourth year, more so on D-01 than on H-11. In number of sprouts living at harvest (and thus included in yield), even greater differences existed between the clones (p < 0.01). Such differences were especially evident in the 0.3-m spacing where clone D-01 averaged 7 to 12 living sprouts per rootstock each year and clone H-11 had only 1 or 2.

Rootstock survival also accounted for some of the trends in sprout development and thus in yield. Survival declined overall, but the mortality differed greatly by clone (p < 0.01) and spacing (p < 0.01) (Fig. 1a). By year 5, only 3% of clone D-01 rootstocks planted at 0.3-m spacing had died; rootstock mortality increased to 23%, however, at 0.18-m spacing. Such losses were much greater for clone H-11; more than two-thirds of the rootstocks planted at 0.18-m and about one-half of those planted at 0.3-m were dead after 5 years.

These mortality losses were accompanied by enhanced growth of sprouts on surviving rootstocks of clone H-11 and similar or slightly declining growth on surviving D-01 rootstocks. Heights of dominant sprouts were rather similar for the two clones in the first harvest, and both increased at the second harvest (Fig. 1b). Thereafter, height growth of clone H-11 tended to increase with successive harvests, and growth of clone D-01 remained constant or declined. The major aberration in the latter general trend is an improvement of growth of clone D-01 spaced at 0.3-m in the fifth year, when it equaled that attained in the second season. Diameter growth of dominant sprouts followed the same general pattern as height growth, including the exception (Fig. 1c). Overall, the clonal differences in sprout growth tended to balance differences in root stock survival, leading to similar production.

Figure 1. Cumulative five-year a) survival of rootstocks, and b) height and c) diameter of tallest sprout of two Populus clones (D-01 and H-11) in woodgrass.

Total 5-year production in the woodgrass treatments ranged from 32.0 to 35.0 Mg ha-1 (Table 1). All treatments attained their highest current annual yield in the second harvest (first coppice harvest). Mean annual increment, however, peaked in both spacings of clone D-01 in the third year, and in both spacings of clone H-11 in the fourth year. At age 5, mean annual increment ranged from 6.4 to 7.0 Mg ha-1. Although annual production averaged about 0.5 Mg ha-1 more in the denser (0.18-m) spacing, yields of the two spacings were not significantly different (p=0.16). Cumulative 5-year yields of the two clones were essentially equal, averaging 33.4 Mg ha-1.

Wider Spacings

Tree growth in the wider spacings (0.5-, 1.0-, and 2.0-m) also accelerated during the second year, and even more so in the third year in some of the wider spacings (Fig. 2). Height and diameter growth slowed in all clonal and spacing treatments in the fourth year. At age 5, trees of clone H-11 were substantially larger in height (32%, p < 0.01) and diameter (14%, p < 0.01), and greater in woody biomass (91%, p < 0.01) than those of clone D-01 (Fig. 2). Tree size increased with increasing spacing, with differences becoming greater with time (Fig. 2). Clonal and spacing differences were also observed in branch retention; clone D-01 retained its branches much longer than clone H-11, and trees of both clones retained their branches longer in the wider spacings.

Effects of spacing and clone × spacing interactions were evident beginning in the second growing season (Fig. 2). In the first two seasons, diameter growth of both clones increased with spacing, and the improvement in growth was substantially greater for clone H-11. By the third year, however, height and diameter increment of the two clones were rather similar. During the fifth year, height and diameter increment of clone D-01 was equal to or better than clone H-11 in the widest two spacings. This change in relative clonal performance appears related primarily to differences in previous growth rate and resulting changes in intensity of competition in the plots. Clone H-11 has superior growth potential, but the realization of that potential is dependent upon adequate growing space and other growth factors. During years 4 and 5, competition intensified greatly; because of past growth trends, trees were larger and competition was greater in plots of clone H-11. Survival remained at 100% in all spacings of clone D-01, but 11% and 2% of the trees in 0.5- and 1.0-m spacings, respectively, of clone H-11 died and many more were suppressed. Even the 2.0-m spacing provided less than adequate growing space for maximum individual tree growth at this size and age, as indicated by our observations of superior growth of trees in the border (non-measured) rows of the plots.

Woody biomass accumulation per tree did not decline as did height and diameter growth (Fig. 2c). Mean tree woody biomass continued to increase substantially with spacing, and at age 5 was about 12 times greater in the 2.0-m spacing than in the 0.5-m spacing. As a result, biomass accumulation per hectare was much more similar among spacings (Fig. 3). Biomass accumulations in the 0.5-m and 1.0-m spacings are essentially equal, with those in the 2.0-m spacing being about 25% lower for clone D-01 and 16% lower for H-11. Relative amounts of stems and branches differed greatly by clone and spacing (p < 0.01); the proportion of woody biomass comprised of branches ranged from 5% in the 0.5-m spacing of clone H-11 to 28% in the 2.0-m spacing of clone D-01. Branch weights in the two densest spacings also differed significantly (p < 0.01) by clone, averaging 8.0 and 5.7 Mg ha-1 for clone D-01 and clone H-11, respectively. At 2.0-m spacing, however, branch biomass of the two clones was nearly identical (11.3 and 11.4 Mg ha-1).

Figure 2. Cumulative five-year a) height growth, b) diameter growth, and c) mean tree yield growth of two Populus clones (D-01 and H-11) in wider-spaced treatments.

Woodgrass vs. Wider Spacings

Cumulative 5-year woody biomass production is shown in Figure 3 for all treatments. Yields for woodgrass spacings (0.18- and 0.3-m) include live woody biomass from five harvests; values for the 0.5-, 1.0-, and 2.0-m spacings represent estimates of live woody biomass standing after each growing season. Production increased in the woodgrass treatments with the second (coppice) harvest; yields in subsequent harvests tended to be equal to or lower than those of the second harvest. Cumulative yields for woodgrass over the 5-year period were 32 to 35 Mg ha-1. Woody biomass production was significantly greater (p < 0.01) in the wider spacings than in the woodgrass spacings, regardless of clone. Cumulative yield of the least productive treatment in the wider spacings (clone D- 01 at 2.0-m) was 41 Mg ha-1. Yields of that clone in the 0.5- and 1.0-m spacing averaged 55 Mg ha-1, or 50 to 60% greater than woodgrass yields. Cumulative yields of clone H-11 at the three wider spacings were two to three times greater than those of woodgrass treatments, ranging from 78 to 94 Mg ha-1.

Figure 3. Cumulative five-year biomass production of two Populus clones (D-01 and H-11).

In terms of mean annual increment, production of woodgrass over the 5-year period was similar for both spacings and both clones—6.4 to 7.0 Mg ha-1. Mean annual production of clone D-01 in the wider spacings ranged from 8.1 to 10.9 Mg ha-1, and mean annual production of clone H-11 was from 15.7 to 18.8 Mg ha-1. For years 3 to 5, annual woody biomass accumulation in the wider spacings of clone H-11 averaged nearly 25 Mg ha-1 per year; fifth-year increment in all three spacings (0.5-, 1.0- and 2.0-m) exceeded 30 Mg ha-1. Current annual increment of clone H-11 did not decline; thus, mean annual production in the wider spacings of this clone would likely increase for at least another 2 to 3 years. Moreover, cumulative production in the widest (2.0-m) spacings of both clones presumably would approach that in the 0.5- and 1.0-m spacings.

Implications and Conclusions

General plantation performance

Growth and development of trees and stands during the 5-year period of this study were excellent. Height and diameter were equal to or greater than growth for comparable spacings at other locations in the Pacific Northwest. This successful performance presumably resulted from a favorable irrigation and fertilizer regime as well as excellent weed control imposed uniformly on all treatments.

Comparison of woodgrass yields

Because interest in the woodgrass concept was stimulated by the promise of much higher yields than had been attained with more conventional culture, it is appropriate to compare our experiment yields with purported yields. To make such a comparison, one must first place the yields suggested by Dula (1984) or Vyas and Shen (1982) on a common basis with those determined in our study. The report by Dula (1984) suggests that at least 112 wet Mg ha-1 (50 wet tons per acre) per year of total above-ground yield (including leaves) can be expected. His sample contained 37% leaves; thus, stems and branches weighed 71 Mg. Moisture content of the stems and branches was 71%; therefore, dry woody biomass weighed about 20 Mg. Dula's yield was measured in a nursery environment in which long, narrow beds occupied only two-thirds of the total land dedicated to woodgrass production. To be comparable to “solid” plantings, which occupy the total land area, the woodgrass yield should therefore be reduced by one-third. Thus, the woodgrass yield indicated by Dula is equivalent to about 14 Mg of dry woody biomass per hectare per year. Our best coppice yields (i.e., the second harvest) were about 9 to 10 Mg for D-01 and about 8 Mg for H-11, and thereafter declined. It is possible, however, that other clones and other locations may lead to somewhat higher annual yields.

We therefore believe that the minimal woodgrass yield suggested by Dula is not unreasonable, provided that it is expressed on a basis comparable to that conventionally used to report short rotation yields—above-ground, leafless, dry matter. But, neither is it particularly high. When compared on a common basis, the woodgrass yield of 112 wet Mg of total above-ground biomass per hectare annually is similar to rates of production measured in many short-rotation intensive culture trials. Mean annual production in the 1.0-m spacing of clone H-11 was about one-third greater than Dula's estimated annual woodgrass yield.

Comparison of woodgrass with wider spacings

Per-hectare production during the first season was closely related to spacing, with the dense woodgrass treatments greatly outproducing the wider spacings. Leaf canopy closure occurred in all spacings during the second year, and growth per tree and per hectare accelerated—especially in the wider spacings. By the end of the second year, cumulative production of both clones in the 0.5-m spacing and of H-11 in the 1.0-m spacing equaled or exceeded that of woodgrass. The growth advantages of wider spacings became even greater in subsequent years. The wider spacings were therefore much superior to woodgrass for growing woody biomass with both Populus clones. Because the growth attributes of these two clones are so different, superiority of the wider spacings is likely to be characteristic of Populus in general. Although yield of woodgrass plantings of Salix is more productive than those of Populus (White and others, in press), preliminary results from trials in New York State and Sweden indicate the higher productivity of wider spacings and longer rotations in this genus also (Personal communication with L. P. Abrahamson, 1993).

Potential of Populus woodgrass in bioenergy production

If yield and cost of production are the primary criteria for selection of a short-rotation density regime, spacings other than woodgrass are overwhelmingly superior. Yields of clone H-11 in the wider spacings are two to three times greater than those of woodgrass. Perhaps even more important are establishment costs which are substantially higher for woodgrass. Differences in cutting costs alone are tremendous; at 10¢ per cutting, such costs would be $31,000 and $11,000 per hectare for the two woodgrass spacings. This compares to $1000 per hectare for the 1.0-m spacing and $250 per hectare for the 2.0-m spacing. Even if cuttings were only 1¢ each, total cutting costs per hectare for the woodgrass spacings would be $3100 and $1100 versus only $100 per hectare for the 1.0- m spacing and $25 per hectare for the 2.0-m spacing—differences still amounting to at least $1000 per hectare. In order to overcome such differences in establishment costs, considerable savings would therefore be needed in other management, maintenance, harvest, or interest costs. Savings of sufficient magnitude are unlikely to be achieved.

Despite the disadvantages of woodgrass in terms of yield and production costs, the system could be desirable if characteristics of the produced biomass were superior in value to those of biomass grown by the wider-spaced, short-rotation systems. Because of its younger age and smaller size, woodgrass will have higher contents of bark, extractives, nutrients, and moisture and a lower content of cellulose than an equal biomass produced in a wider spacing on a somewhat longer rotation (Blankenhorn and others 1985a,b). Many of these differences would have negative value in various systems of energy conversion (Butler and others 1987), but they might be beneficial for some uses. Even so, the characteristics would have to be superior by many, many fold and the advantages derived therefrom reflected in raw material prices paid by the processing or conversion industry.

Conclusions

Our experiment, coupled with other current knowledge, indicates that woodgrass has little promise as a viable system for growing Populus biomass for energy. Other wider-spaced, short-rotation density regimes, especially those involving clone H-11 (and other P. trichocarpa × P. deltoides hybrids) appear superior in nearly all respects. They are producing higher yields than expected, and possibilities for commercial application of these systems seem much brighter.

Although the experiment was limited to Populus as are the above specific conclusions, the “woodgrass experience” has some implications with regard to other genera, clones, and biomass production systems. That is, a range of alternatives should be examined before any biomass production system is selected. The costs of—and the time required for—such evaluations are minimal when compared with unnecessary costs that may be incurred or productivity that may be foregone if decisions or commitments are made in the absence of such assessments.