Agroforestry Systems 59: 21–26, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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Biomass production by two-year-old poplar clones on floodplain sites in the Lower Midwest, USA Stephen G. Pallardy*, Daniel E. Gibbins and Julie L. Rhoads Department of Forestry, 203 Anheuser-Busch Hall, University of Missouri, Columbia, MO 65211, USA; *Author for correspondence (tel.: (573)-882-3548; fax (573)-882-1977; e-mail: [email protected])

Key words: Biomass production, Carbon sequestration, Leaf area index, Populus, Short-rotation intensive culture

Abstract Four Populus clones were grown for two years at 1 ⫻ 1 m spacing for study of total biomass production and carbon sequestration capacity on floodplain sites previously in forage grasses under climatic conditions of the lower Midwest, U.S.A. Total biomass 共above-and below-ground兲 in the first year ranged from 3.9 Mg ha–1 in a Populus deltoides x P. nigra clone 共I45/51兲 to 1.9 Mg ha–1 for a local-source Populus deltoides clone 共2059兲. Second year total biomass production was substantially higher, ranging from 13.9 Mg ha–1 in I45/51 to 7.4 Mg ha–1 in P. deltoides clone 26C6R51. Second-year leaf area index 共LAI兲 values for I45/51 plants reached 4 during mid-season, indicating essentially complete canopy closure in this clone by the second year after planting. In contrast, maximum mid-season, second-year LAI was significantly lower in P. deltoides clones 共 ⬍ 2.4兲. There was some evidence for differential allocation to roots and shoots among Populus clones, with 26C6R51 showing relatively more allocation to root biomass than other clones. Second-year growth in Populus deltoides clone 2059 accelerated substantially, and this genotype exhibited two-year biomass accumulation nearly equal to that of I45/51 despite having less leaf area. This result suggested a higher photosynthetic capacity or assimilation efficiency in the former.

Introduction Populus 共poplar兲 species have been incorporated into many managed systems for production of timber and fiber. Poplar culture provides a variety of benefits both economic and environmental. Poplars, and especially their hybrids, have displayed the capacity for rapid biomass accretion 共Anderson et al. 1983; Ranney et al. 1987兲, and there is increasing interest in poplar wood for use in long-term storage products such as lumber and oriented strand board. Hence a role for poplar plantations in CO2 sequestration schemes is feasible, with perhaps even greater effectiveness in use of short-rotation poplar as a replacement for fossils fuels in energy production 共Vitousek 1991兲. Genetic variation in many traits, including growth rate, within species and among hybrids of Populus species is well documented 共e.g., Heilman and Stettler 1985; Rhodenbaugh and Pallardy 1993兲.

Despite the popularity of poplar culture and study of poplar biology in other parts of the U. S. 共e.g., Pacific Northwest and the Lakes States兲, there has been little systematic study of the suitability of poplars for use in short-rotation plantations in the lower Midwest, U. S. A. Further, there has been no comprehensive study of how the distinct climatic patterns in this region interact with physiological processes and morphology of poplar genotypes to determine production potential, despite the large genotype x environment effects observed in poplar trials 共e.g., Host and Isebrands, 1994兲. The objectives of this research were to test different Populus genotypes in short-rotation plantations for biomass production and carbon sequestration capacity on floodplain sites under climatic conditions of the lower Midwest. Accordingly, we monitored Populus dimensional growth, leaf area development and estimated biomass through two growing seasons in a closely-spaced plantation established

22 Table 1. Origin and source information for four Populus clones grown in a short rotation plantation in the Missouri River floodplain, USA Clone

Parentage

Source

Origin

Latitude

Longitude

I45/51 2059 1112 26C6R51

P. P. P. P.

Iowa State Tree Nursery MDC* Nursery MDC Nursery MDC Nursery

⫺ Osage County, MO New Madrid County, MO Pope County, IL

⫺ 38° 27' N 36° 35' N 37° 25' N

⫺ 91° 52' W 89° 37' W 88° 34' W

deltoides x P. nigra deltoides deltoides deltoides

*Missouri Department of Conservation

on the Missouri River floodplain in central Missouri, USA.

watered periodically during the first growing season to maintain soil moisture in the top 30 cm, but not during the second year when growing season rainfall was adequate 共52.3 cm, 102 percent of average兲.

Materials and Methods Sampling for growth Plantation design and establishment Hardwood cuttings 共20 cm long兲 of three eastern cottonwood 共Populus deltoides Bartr.兲 and one P. deltoides Bartr. x Populus nigra L. hybrid clone 共Table 1兲 were planted in May 1999 in a former tall fescue 共Festuca arundinacea Schreb.兲 pasture on the Missouri River floodplain at the University of Missouri’s Horticulture and Agroforestry Research Center at New Franklin, MO, USA. 共Lat. 39° 01' N, Long. 92° 46' W兲. The hybrid clone 共I45/51兲 is recommended for and widely planted in the upper Midwest; the three P. deltoides clones were collected by the Missouri Department of Conservation and distributed through the state tree nursery. Soil at the site is a Nodaway silt loam 共fine-silty, mesic, Mollic Udifluvent兲 共Grogger et al. 1978兲. This soil is fertile, moderately well-drained and permeable with 0-5 percent slope, low surface runoff and occasionally flooded. Depth to water table is 90 to 150 cm. Before planting the site was treated with a non-selective herbicide to eliminate grass competition to establishing cuttings. A randomized complete block design similar to that of Scarascia-Mugnozza et al. 共1997兲 was employed, with six replicates 共blocks兲 each consisting of four single-clone plots 共7 rows ⫻ 10 columns兲. Spacing was 1 ⫻ 1 m 共10,000 trees/ha兲. Planted cuttings showed good shoot emergence in May, but in June there was substantial mortality in some clones. This required replanting with potted cuttings and transplanting from some blocks so that a number of complete blocks would be available for each clone. Ultimately the number of complete blocks 共of an original six兲 available per clone were: six for I45/51, five for 1112, three for 2059 and two for 26C6R51. Plants were

To relate dimensional growth to biomass accumulation, some plants were harvested throughout the plantation 共30 in the first year and 10 in the second year兲. Cutting, stem and branches were harvested first; soil was then excavated so that roots could be separated and collected from a 1 ⫻ 1 ⫻ 1 m volume around the plant base. Although it is obvious that some roots of each plant extended beyond the limits of the excavated volume, it was assumed that there were as many roots entering the volume from adjacent trees as were growing out of it. Hence, the excavation provided a reasonable estimate of the total root mass associated with each tree 共ScarasciaMugnozza et al. 1997兲. Biomass data for roots, stems, branches, and cuttings were obtained after drying samples to constant weight at 75 °C. Stepwise regression procedures were then employed, with all possibly meaningful independent variables 共clone, size, age 共in days from planting兲 at excavation兲 and their interactions, to predict root, shoot and total dry weight as dependent variables. The clonal variable was coded using three dummy variables for the four clones. Height and stem base diameter were measured for 79 “permanent plot” trees throughout the plantation 共17-24 trees per clone兲 at the end of the first and second growing seasons. The experiment was designed so that these trees were ⬙buffered⬙ by the presence of all eight adjacent trees 共i.e., there were no adjacent excavated trees兲. These data and biomass regression equations were then employed to estimate biomass components for each plant. During the second growing season, Leaf Area Index 共LAI兲 and canopy gap estimates were obtained monthly for each plot using

23 Table 2. Results of stepwise regression procedure for biomass predictive equations for four Populus clones grown in the Missouri River floodplain, U.S.A.. Columns are arranged in order of variable selection with cumulative R2 values shown down each column. d2h ⫽ stem base diameter 共cm兲 squared multiplied by height 共m兲 Selected variable

Root

Shoot

Total

d2h Age d2h*clone

0.849 0.922 0.9481

0.993 0.994 –

0.986 0.992 0.9922

One clone dummy variable selected in the final equation; 2Two clone dummy variables selected in the final equation 1

a LI-COR LAI-2000 Canopy Analyzer 共LI-COR, Inc., Lincoln, NE, USA兲. Statistical analysis Height, stem base diameter, biomass parameters, root-shoot ratio and LAI were analyzed via Analysis of Variance 共ANOV兲 using an incomplete block design. Root-shoot ratio data were arcsine transformed before analysis. Least squares means were computed and means comparisons 共using the PDIFF option of the LSMEANS statement兲 were conducted if overall ANOV results provided a significant F-test 共p ⱕ 0.05兲.

Results Regression models Final models developed using stepwise regression had R2 values ranging from 0.95 for root- to over 0.99 for shoot and total weight 共Table 2兲. The proxy volume variable d2h 共d⫽stem base diameter; h⫽tree height兲 accounted for between 85 and 99% of variation in the data for all clones, with small but statistically significant 共p ⱕ 0.05兲 improvements in fit by adding age and d2h ⫻ clone factors. Plots of predicted vs. actual biomass 共Figure 1兲 indicated good fit and high predictive ability across clones. Populus growth rates and patterns During the first year after planting average height and diameter across all clones were 1.71 m and 2.25 cm, respectively 共Table 3兲. In the second year, mean plant height almost tripled and diameter nearly doubled. First year biomass accumulation averaged 2.7 Mg

Figure 1. Actual vs. predicted total biomass of Populus clones grown in the Missouri River floodplain, U.S.A.. Data obtained from destructive harvest during the growing seasons of 1999 共n⫽30兲 and 2000 共n⫽10兲. Symbols represent individual plants of different clones 共䊱-I45/51; "-26C6R51; *-2059 䉬-1112兲. Line illustrates a 1:1 ratio.

ha–1 across all clones. By the end of the second year biomass accumulation was substantially greater, averaging over 13 Mg ha–1 and reflecting mean growth across all clones of 10.5 Mg ha–1 yr–1, almost four times the first-year rate. Significant clonal variation in growth rates also was observed in both years. In Year 1 the hybrid clone generally exhibited superior dimensional and biomass growth, although variability in the data did not always render these differences statistically significant 共Table 3兲. In the first year of growth P. deltoides clones 1112 and 26C6R51 exhibited greater growth than 2059, but allocation of biomass differed among clones. Whereas clone 1112 showed proportionally more aboveground growth, which was reflected in height, diameter and shoot weights 共Table 3兲, clone 26C6R51 allocated relatively more biomass below ground. Total biomass yields of clone 2059 were lowest of all in Year 1. By the end of the second growing season, clonal biomass rankings had shifted, particularly with respect to P. deltoides clones. The hybrid clone I45/51 maintained its top ranking in diameter and biomass accumulation 共Table 3兲. However, clone 2059, which had the poorest biomass yields in Year 1, exhibited growth rates that approached those of the hybrid clone I45/51 and was ranked second in total biomass accumulation over two years 共slightly behind I45/51兲. Continued greater proportional allocation to roots in 26C6R51 resulted in a higher second-year root-shoot

0.26b 0.30ab 0.28b 0.36a 0.31 3.87a 1.91ab 2.61b 2.49b 2.72 13.9a 12.5abc 8.45bc 5.71c 10.14 3.34a 3.48a 2.17b 2.74ab 2.93

2.84a 1.45a 2.21a 0.86b 1.85

17.7a 16.1ab 10.6b 8.4b 13.20

shoot ratio Yr 2 Yr 1 Yr 2 Yr 1 Yr 2

Figure 2. Leaf Area Index of four Populus clones grown in plantation culture in the Missouri River floodplain, U.S.A. during the second growing season. For a given sample date, data points not associated with the same letter are significantly different 共p ⱕ 0.05兲

ratio in this clone compared to all others. By the end of Year 2, this clone and 1112 had only about 60 percent of the total biomass present as did clones I45/51 and 2059.

1.66a 0.60b 0.59b 1.63a 1.12 4.67a 4.15ab 3.67b 3.52b 4.00 4.67a 5.19a 4.33ab 3.58b 4.44 Populus deltoides x P. nigra-I45/51 P. deltoides-2059 P. deltoides-1112 P. deltoides-26C6R51 Mean

1.83a 1.69a 2.22a 1.08b 1.71

Yr 1 Yr 2 Yr 1

2.64a 1.98b 2.55a 1.83b 2.25

Yr 1

Second-year LAI patterns

Yr 2

Yr 2 Root: Total weight 共Mg ha–1兲 Shoot weight 共Mg ha–1兲 Root weight 共Mg ha–1兲 Diam 共cm兲 Height 共m兲 Clone

Table 3. Height, diameter and biomass accumulation in four Populus clones through two growing seasons in the Missouri River floodplain, USA. Within a column, means not followed by the same letter are significantly different 共p ⱕ 0.05兲

24

During the second growing season the P. deltoides x P. nigra clone maintained substantially higher LAI through August than did P. deltoides clones 共26C6R51, 2059, 1112兲 共Figure 2兲. By the September and October sample dates 共Yeardays 256 and 292, respectively兲, autumn leaf abscission had begun and clones had more similar LAI, although some significant differences were still present. Estimates of gap fraction in the I45/51 plots 共data not shown兲 indicated that canopy closure had been achieved in this clone. The P. deltoides clones had sparser branching characteristics that were reflected in lower LAI values 共Figure 2兲 and higher canopy gap estimates 共data not shown兲.

Discussion Although detailed data on cutting mortality were not presented, the widely varying number of surviving blocks 共33 to 100 percent兲 indicated that there were obvious differences among clones in capacity to establish on the site, especially within P. deltoides. These differences were likely attributable to variation

25 in rooting capacity which is known to vary widely with genotype within Populus 共Kozlowski and Pallardy 1997兲. The hybrid clone has been in production and distribution as hardwood cuttings for many years, and so it would be expected to have high success in establishment 共otherwise it would have been discarded兲. In our study the cuttings were planted rather late in the season 共early May兲 due to lack of access to the site and the young plants may have been under substantial water stress as high vapor pressure deficits placed great evaporative demands on the rapidly emerging shoots. Earlier planting and perhaps preplanting treatments 共e.g., rooting hormone application兲 might be considered to improve survival and establishment. Once established, Populus cuttings grew rapidly through the second year after planting. Height, diameter and biomass accumulation obtained here were equivalent to those of some clones planted in the Pacific Northwest of the United States, although the best-producing clones in the PNW grew taller and produced considerably more biomass than ours 共Harrington and DeBell 1984; DeBell et al. 1996; Scarascia-Mugnozza et al. 1997兲. For example, two-year total woody biomass yields for Populus trichocarpa Torr. and Gray, P. deltoides and P. trichocarpa x P. deltoides hybrids were in the range of 14 to 35 Mg ha–1 for slightly larger cuttings 共25 cm兲 planted on a fertile alluvial soil at the same 1 ⫻ 1 m spacing and irrigated as needed 共Scarascia-Mugnozza et al. 1997兲. Populus growth and biomass yields here also were comparable to those reported in other temperate regions for similar types of plantation culture 共e.g., U. K., Cannell et al. 1988; Belgium, Deraedt and Ceulemans 1998;兲. Hence the early results presented here suggest culture of Populus in the lower Midwest appears feasible and competitive with other regions, at least with respect to productivity. There are relatively few studies available that present below-ground biomass production for Populus clones for comparison. Because root-shoot ratio tends to decline with age 共Pallardy and Kozlowski 1979; Scarascia-Mugnozza et al. 1997兲 and is sensitive to a host of site conditions, close comparisons are difficult to make. Hence it is not unexpected that the root-shoot ratios reported here 共0.26 to 0.36兲 are somewhat different and higher than those reported for similar Populus plantings in the PNW 共0.17-0.25, Scarascia-Mugnozza et al. 1997兲. It is noteworthy that clonal variation in root-shoot ratio within P. deltoides was clearly evident, with clone 26C6R51 placing

relatively more biomass below ground than other clones. This pattern might be exploited on sites where a higher root-shoot ratio might be desirable from a water relations perspective 共e.g., unirrigated, droughtprone sites兲. This strategy, however, would likely reduce above-ground biomass yields because this clone, even if well-supplied with water and nutrients, placed less of its carbon in productive leaf area. The LAI data indicated that the best clone 共I45/51兲 had developed a full canopy by the middle of the second growing season 共LAI~4兲. These results are similar to those for Populus plantations planted at 1 m spacing in the PNW, where LAI ranged from slightly more than 2 to 5 共DeBell et al. 1996兲. The increase in solar radiation interception associated with incremental increase in LAI is nearly nil when LAI exceeds 4 共Cannell et al. 1988兲; therefore, clone I45/51 exhibited the capacity to fully capture the available radiation at the site in Year 2. This capacity obviously contributed to its high productivity. Interestingly, P. deltoides clone 2059, while exhibiting substantially lower LAI than I45/51, accumulated nearly as much biomass as this clone, suggesting that it might possess greater photosynthetic or assimilative capacity, as has been shown in other Populus 共e.g., Rhodenbaugh and Pallardy 1993; Deraedt and Ceulemans 1998兲. A detailed short-term 共one month兲 study by Green et al. 共1991兲 also examined biomass productivity in three-year-old, closed-canopy stands of Populus deltoides and Populus x euramericana hybrid clones as a function of intercepted radiation and light use efficiency 共LUE兲. These authors observed that aboveground biomass productivity was most closely related to LUE 共and presumably photosynthesis兲 and less so to intercepted radiation. Similar to the Populus deltoides clone 2059 in the present study, P. deltoides clones of Green et al. had higher presumed photosynthesis 共inferred from growth compared with LAI and LUE, respectively兲 than did P. x euramericana clones. The rapid growth of P. x euramericana clone I45/51 in the present study likely accrues from its more rapid acquisition of canopy closure, a factor not addressed by Green et al.’s 共1991兲 study of older, closed-canopy stands.

Acknowledgments This work was funded through the University of Missouri Center for Agroforestry under cooperative

26 agreements AG-02100251 with the U. S. D. A. Agricultural Research Service Dale Bumpers Small Farms Research Center, Booneville, AR. and CR 82670401-0 with the U. S. Environmental Protection Agency. The results presented are the sole responsibility of the co-authors and the University of Missouri and may not represent the policies or positions of the Agricultural Research Service or the Environmental Protection Agency.

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