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The potential of willow and poplar plantations as carbon sinks in Sweden
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The potential of willow and poplar plantations as carbon sinks in Sweden Rose-Marie Rytter* The Forestry Research Institute of Sweden (Skogforsk), Ekebo 2250, 268 90 Svalo¨v, Sweden
article info
abstract
Article history:
A large share, estimated at 12e25%, of the annual anthropogenic greenhouse gas emissions
Received 21 October 2010
is attributed to global deforestation. Increasing the forested areas therefore has a positive
Received in revised form
impact on carbon (C) sequestration and mitigation of high atmospheric CO2 concentra-
6 September 2011
tions. Fast-growing species, such as willow and poplar, are of high interest as producers of
Accepted 13 October 2011
biomass for fuel, but also as C sinks. The present study estimated the rate of C seques-
Available online 3 December 2011
tration in biomass and soil in willow and poplar plantations. Calculations were based on above- and below-ground biomass production data from field experiments, including fine
Keywords:
root turnover, litter decomposition rates, and production levels from commercial planta-
Afforestation
tions. Accumulation of C in woody biomass, above and below ground, was estimated at
Carbon sequestration
76.6e80.1 Mg C ha1 and accumulation of C in the soil at 9.0e10.3 Mg C ha1 over the first
Short-rotation plantations
20e22 years. The average rates of C sequestration were 3.5e4.0 Mg C ha1 yr1 in woody
Salix
biomass, and 0.4e0.5 Mg C ha1 yr1 in the soil. If 400,000 ha of abandoned arable land in
Populus
Sweden were planted with willow and poplar, about 1.5 Tg C would be sequestered annually in woody biomass and 0.2 Tg C in soils. This would be nearly one tenth of the annual anthropogenic emissions of C in Sweden today. These calculations show the potential of fast-growing plantations on arable land to mitigate the effect of high CO2 concentrations over a short time span. Knowledge gaps were found during the calculation process and future research areas were suggested. ª 2011 Elsevier Ltd. All rights reserved.
1.
Introduction
Land use change through deforestation, cultivation of grass lands and an increase of the pasture land area, has reduced the C stored in soils and plants globally by approximately 180e200 Pg [1]. This is of the same magnitude as fossil fuel emissions to the atmosphere from pre-industrial times until 2000 which were estimated at 280 Pg C [2]. A portion of this released C has been taken up by land plants and in the oceans. But losses to the atmosphere have been nearly 200 Pg C which has contributed to the elevated CO2 levels today [3].
Atmospheric emissions of green house gases are now regarded as the major cause to the observed global warming during the last century [4]. Current atmospheric models predict that if all fossil fuels were burned the atmospheric concentration of CO2 would raise from 390 ppm today to 1200e2000 ppm [3]. Apart from a worldwide reduction of the use of fossil fuels, increasing the forested areas may contribute to mitigate the high atmospheric CO2 concentration over a short time span. Afforestation of arable land is regarded as one of the major potential C sinks in Europe [5]. Recent figures show that forested areas in Europe had a net increase of 7% in the period
* Corresponding author. Present address: Rytter Science, Backava¨gen 16, 268 68 Ro¨sta˚nga, Sweden. Tel.: þ46 418471300; fax: þ46 418471329. E-mail address: [email protected]. 0961-9534/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2011.10.012
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1990e2005 [6] but a further expansion of forested areas is an essential component of an overall strategy for mitigation of increasing atmospheric CO2 levels. Recent reports suggest that about 300,000 to 500,000 ha of arable land in Sweden, and abandoned (not yet afforested arable land), may be cultivated with energy crops and fastgrowing tree species in the future [7,8]. Short rotation plantations have been regarded as one of the main alternatives in the shift towards a more sustainable energy supply in order to substitute for fossil fuels in Sweden. In this context, fastgrowing species such as poplar and willow are of high interest, mainly as producers of biomass for fuel, but also for sequestration of C. Generally, forests have a higher C density than arable land, mainly due to the presence of perennial vegetation with high biomass. By using fast-growing tree species for energy purposes, C is stored in biomass and soils, and sustainable systems with recirculation of CO2 may be established in the long term. Conversion of arable land to forests also implies a shift from a shorter to a longer residence time of C by replacing annual crops with more long-lived perennial woody species. Soils constitute the largest pool of terrestrial C in the biosphere and estimates indicate that 1500 to 1600 Pg is stored in the uppermost 1 m soil layer, and more than 2000 Pg in the top 3 m soil [9e11]. The potential importance of vegetation changes on soil organic C (SOC) pools for C sequestration strategies has been stressed [10]. The storage capacity and rate of C sequestration in soils at afforestation depend on various factors such as the climate, soil type, tree species used for afforestation, current forestry practices, pre-afforestation management and land use history [12,13]. Processes in the soil are complex and whether a soil becomes a C sink depends to a high degree on the production of leaf and root litter and their decomposition rates. Studies of C sequestration involving both biomass and soil following afforestation are few, especially in Europe [14]. Those studies imply that the woody biomass accounts for the major part of the stored C and about 20e30% will be stored in the uppermost soil during the first decades after establishment of a new forest [14,15]. Further studies are needed, however, which deal with the relationship between C storage in soils, tree species and population age at afforestation of arable land [14]. In a metaanalysis of the effects of land-use changes on C content in soils, there was a general increase in the C stock for afforestation of arable land (þ18%), and significant differences between tree species were observed [16]. It is essential to include the whole plant when C budgets are made. Carbon is sequestered in woody biomass, but also accumulated as organic matter in the forest floor by litter from leaves and fine root turnover. Estimates of fine root production are crucial in this context. Fine roots (i.e. 2 mm in diameter) constitute a large and very dynamic component of the C cycles of the terrestrial ecosystems [17]. They emerge, die, and decompose in the soil continuously, thus having an annual turnover which may contribute to 20e70% of the total net primary production [e.g. [18e22]]. Excluding fine root turnover may lead to underestimation of organic matter and nutrients by 20e80% [23]. A global budget calculation, assuming a fine root turnover rate of once per year, estimated that fine roots represented 33% of the global annual net
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primary production [24]. It is by now widely accepted that fine roots have a longevity of 1 year or less [17]. Their role in the soil of ecosystems has been compared with the role of leaves in the aerial environment [25]. Different methods for quantifying fine root turnover have been used and, in brief, they include mass balances, such as the N budget and C budget techniques, sequential soil coring, in-growth cores, minirhizotrons and the use of C isotopes. It is of utmost importance to choose a method that permits frequent sampling with short time intervals to cover the rapid fine root turnover [e.g. [20]]. The potential for the capture of C in biomass and soil during cultivation of short rotation forest willow and poplar in Sweden is still unknown. Willow has been cultivated on arable land for bioenergy purposes during the last 30 years and, so far, willow plantations have been established on about 15 000 ha or 0.5% of total arable land in Sweden [26]. Today commercial willow plantations in Sweden have a double-row design with 1.5 and 0.75 m spacing between the rows and approximately 0.75 m between plants in the row yielding a planting density of about 12,000 cuttings ha1. A yield model, based on a number of commercial plantations in Sweden, predicts mean annual yields of 4.0e7.1 tonnes dry matter ha1 for 25% of the best growers [27]. Whole-shoot harvest is usually conducted every 3e4 years if growth conditions are favourable. The interest in Populus species, and hybrid aspen in particular, can be traced back to the 1940’s when the Swedish Match Company was involved in breeding species and crossings suitable for match wood production. However, the land area dedicated to commercial production has remained small until recently. Over the past 5 years the areas planted with hybrid aspen and poplars have increased from about 500 to more than 2000 ha. The stands are usually established with 1100 to 1600 plants ha1 and the rotation periods will most probably exceed 10 years. Mean annual increment at a stem density of 1100 plants ha1 is expected to be above 20 m3 stemwood ha1 yr1 during a 20e25 year rotation period on fertile soil in southern Sweden, corresponding to 8e9 tons of DM ha1 yr1 including twigs and branches [28]. The calculations of C sequestration in willow and poplar plantations in the present study were based on above- and below-ground biomass production data from field experiments, including C allocation to different plant parts and fine root turnover (Fig. 1). Litter decomposition rates and limit values for decomposition were obtained from the literature [29,30]. Mean annual stem-production levels were obtained from commercial and experimental plantations in Sweden. The conditions were fertile sites, i.e. former arable land, and proper management. Arable land will generally have a low C stock, compared to pasture, and it was therefore assumed that the SOC-pool increased quite immediately at afforestation [e.g. [13]]. A whole plant perspective was the base for these calculations, thus, the investigated depth was the rooting depth. Approximations for poplar were done in accordance with the experimental results from willow, and were also compared to other published studies with poplar. The calculations did not include the influence of management practices or harvests on the C fluxes.
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The overall aim of this study was to assess the potential for C sequestration in fast-growing plantations of willow and poplar in Sweden today and, in this context, identify any knowledge gaps during the process. Biomass pools were estimated, both above and below ground, as well as the accumulation of C in the forest floor and soil by leaf litter fall and fine root turnover. The results were scaled up to a theoretical, but possible land use area for such plantations in Sweden today, and the impact on C sequestration and the mitigation of atmospheric CO2 were discussed.
2.
Materials and methods
2.1. Annual net primary production and relative allocation of biomass
Fig. 1 e Stepwise calculations of C sequestration in biomass and soil. (1) The annual biomass allocation pattern to different plant parts, including leaf litter, was estimated in lysimeter-grown willows during three years, using a total of 32 lysimeters [20]. (2) Annual fine root (£2 mm in diameter) production, on a whole plant basis, was calculated from annual harvests of lysimeters, and observations of fine root dynamics in 8 minirhizotrons during three years [20,33,34]. (3) The total amount of litter stabilized as soil organic material (SOM). The continuing accumulation in the forest floor and mineral soil was calculated according to the ‘limit-value’ concept [29,30]. Finally, annual and total biomass production during 20e22 years, and C sequestration in biomass and SOC were estimated by using mean annual stem production values from commercial and experimental plantations in Sweden as input [27,28,31], and a C-concentration of 500 mg gL1 biomass [37].
Mean annual stem-production levels were obtained from commercial and experimental plantations in Sweden and used as input the calculations [27,28,31]. The relative allocation of biomass to the different plant parts was estimated from whole plant C budgets in a short rotation stand with lysimeter-grown willows (Salix viminalis L., Fig. 1, step 1). Annual net primary production and C allocation, above and below ground, were estimated for three years following plantation. The stand was situated at Ultuna, Uppsala, (59 490 N, 17 400 E, alt. 5 m) where lysimeters, with a volume of 200 dm3 each, were arranged in double rows and buried in the ground. The lysimeters used in these calculations were filled with clay substrate from the site and an effort was made to reconstruct the original soil profile in the lysimeters. The soil was compacted during the winter and in the following spring 25 cm long and un-rooted cuttings were planted at a regular spacing of 0.7 m. Cuttings were planted in the lysimeters and also in the rows between to avoid stand edge effects. Water and fertilizer were supplied daily during the growing seasons through a computer-controlled drip irrigation system. Annual net primary production and C allocation of all plant parts were estimated by whole-plant harvests at the end of each growing season. Dry weights were determined for shoots, coarse roots, fine roots, the cutting and remaining leaves after drying at 70e85 C to constant weight. Leaf litter was sampled regularly during the growing seasons and dry weight was determined. From these measurements the relative distributions of primary production were calculated for each year by also including the fine root production (see below, Table 1). For more details see Rytter [20].
2.2.
Fine root production
Fine roots (diameter 2 mm) were monitored at 2-week intervals by using minirhizotrons (Fig. 1, step 2). The minirhizotrons were installed vertically in the lysimeters, had a length of 70 cm, and consisted of transparent acrylic tubes with a grid drawn with waterproof ink on the outside [cf. [32]]. The effective soil area observed by one minirhizotron was 0.072 m2 and the number of fine root branches growing to the minirhizotron surface was recorded. Fine root production was estimated by two different methods both based on observed fine root dynamics in minirhizotrons and fine root biomass
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Table 1 e Relative distribution of annual net primary production in lysimeter grown willows (%), n [ 16. From Rytter (2001). Lysimeter-grown willows Relative annual net primary production (%) First rotation period Stems Leaves Cutting above ground Total above ground production Coarse roots Fine rootsa Cutting below ground Total below ground production Total annual production
Year 1
Year 2
Year 3
38.0 9.5 3.6 51.2 5.3 39.0 4.5 48.8 100
50.3 9.8 3.3 63.3 2.4 30.3 3.9 36.7 100
47.8 12.8 1.4 61.9 2.2 34.1 1.7 38.1 100
a Calculated annual fine root production.
estimates from harvested plants during three consecutive years (Table 1, both methods were used in the calculations, see [33] for more details). Briefly described, the first method separated observed fine root branch number in each 2 6 cm unit of the minirhizotrons into growth and decay by calculating changes between two consecutive dates [20,34]. Growth was defined as positive net changes in root number and decay as negative net changes in root number. The sums of growth (Grt, tþ1) between two destructive harvests of whole lysimeters, performed annually at the end of each growing season (at time t, tþ1), and the ratios (Rmt , Rmtþ1) of the measured standing crops of fine roots (g DM) at the harvest dates and observed standing root number at the same or adjoining dates, were then used for calculation of annual fine root production (Prt, tþ1): Prt;tþ1 ¼
Rmt þ Rmtþ1 X Grt;tþ1 2
The second method was based on fine root branching dynamics and fine root turnover rates estimated from minirhizotron data. A polynomial of the third degree described the phases of growth and decay in a simple but realistic way according to observed fine root branching orders, i.e. with a faster increase by time in branch numbers during the growth phase and a slower disappearance at the end of the decay phase. Integrals were used for calculating the mean fine root ages during the growth (RAGr) and the decay phase (RADe). Fine root duration times (Tt) were defined as: Tt ¼ RAGr þ RADe þ RASt where RASt was the mean fine root age during a short period of maximum root number. Fine root turnover rates (Tr) as times per year were calculated, for each year, as: Tr ¼
365 Tt
Annual turnover of fine root branches (Tb) was calculated as the product of turnover rates and a mean of standing root number from all measurements between the destructive samplings at harvests. Turnover of fine root biomass was thereafter calculated by multiplying the mean ratio of fine root mass and number of fine root branches (Rmt , Rmtþ1) by
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annual turnover of fine root branches (Tb). The annual fine root production (Prt, tþ1) was estimated by adding the annual (Ftþ1Ft) to estimated turnover: change in fine root biomass Rmt þ Rmtþ1 Prt;tþ1 ¼ Tb þ Ftþ1 Ft 2
2.3.
Decomposition of litter
The relative proportion of leaf litter was estimated from the lysimeter study (above) and fine root litter was obtained by subtracting fine root standing crop from fine root production. Fine root standing crop was calculated from relative proportions [20]. The total amount of litter stabilized as soil organic material (SOM) and further accumulated in the forest floor was calculated by using limit values (Fig. 1, step 3 [29],). When the decomposition rates approach zero the accumulated litter mass approaches a final limit value for decomposition that can be described by an asymptotic function. This limit value is different for different species and depends partly on nitrogen (N) concentration. Limit values for litter from willow and poplar were not available for Swedish conditions (latitudes 56 e65 N), thus the limit value for birch litter (remainder 0.23) was used in the present calculations [30]. While willow fine roots have N concentrations that are about 60% of the leaf N concentration [20], a low decomposition rate (approaching zero) will be reached later and at a higher mass loss, according to the ‘limit-value’ concept [29,30]. The limit value for fine root litter in the present calculations was set to 50% of the limit value for leaf litter.
2.4.
Carbon sequestration
The reported production levels for willow and poplar, the allocation of annual biomass production (Table 1), the predicted amounts of leaf and fine root litter and accumulation to SOM according to the ‘limit-value’ concept were used for calculations of C sequestration in both willow and poplar plantations. Thus, for poplar the below ground production was approximated according to the willow proportions (year 3, Table 1). Mean annual yields of 5.2, 6.3 and 6.3 tonnes dry matter per ha during the first, second and third harvest cycles from commercial willow plantations in Sweden were used as input in those calculations [27]. Biomass distribution in the first rotation period was calculated for four years using proportions from year 1, 2 and 3. Proportions from year 3 were also used in the fourth year (Table 1). The later rotation periods, with duration of three years, were calculated using the proportions from year 2 and 3 where proportions from year 3 were used the two latter years of the rotation. The calculations for willow were based on seven rotations, which is about the time the stump systems are used in commercial plantations in Sweden today. The calculations for poplar were based on a growth period of 20 years, a stem density of 1100 plants ha1 and a mean annual stem production of 20 m3 ha1 [cf [28,31].]. The annual production of branches was set to 15% of the stem volume [cf. [31]]. Conversion of woody biomass to dry matter (DM) was calculated by using a basic density of 0.322 Mg m3 [35]. Above ground litter fall was restricted to leaves which constitutes about 90% of total litter fall in young stands [36]. The C content was calculated by using a common
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C-concentration of 500 mg g1 biomass [37]. Flows of dissolved organic carbon (DOC) from the forest floor to the mineral soil were not estimated in this study.
3.
Results
Two different plant biomass pools were handled when C budgets for the plant and soil systems in willow and poplar plantations were calculated. One pool consisted of leaves and fine roots, which both have a fast turnover as most of them emerge and die on an annual basis. Thus, leaves and fine roots were producing litter almost immediately after they were formed (Table 2a,b) and this litter was directed to the decomposition pool in these calculations where they contributed to the accumulation of C in SOM from the first year and onwards (Table 3a,b). The other biomass pool included stems, branches and the coarse root system. Those plant parts possess a longer turnover time compared to leaves and fine roots and they were expected to have a positive annual increment during the whole rotation period (Table 4a,b). Some losses of biomass from branches and coarse roots do occur, but they are considered as a minor loss in younger stands and were not included in the estimates [e.g. [36]]. The annual mean fine root production was 4.2 and 5.1 Mg DM ha1 yr1 in plantations with willow and poplar, respectively (Table 2a, b). Since fine root turnover is rapid most of the produced fine root biomass was transferred to the fine root litter pool which reached a total of 90.9 and 101.9 Mg DM ha1, in willow and poplar plantations, respectively. A standing fine root crop was present in all years, but it was not supposed to have an annual increment. The remaining standing fine root crop from the last year, measuring 0.7e0.9 Mg DM ha1, was not transferred to the fine root litter pool during this time period, which covered a 20-year rotation for poplar and seven rotations for willow. It was instead assumed to be left after the final harvest and thus transferred to the litter pool in the following rotation period. However, since the size of the subtracted standing fine root crop from the last year was comparatively small in the time scale of 20e22 years, the contribution of fine roots to litter almost equalled fine root production.
The input from fine root turnover and shedding leaves to the litter pool was 123.6e140.4 Mg DM ha1 during the 20e22 year period (Table 3a, b). By using limit values for decomposition the total accumulation of C in the soil (SOC), i.e. the remainder from both leaf and fine root litter, was estimated at 9.0 and 10.3 Mg C ha1, in willow and poplar plantations, respectively. It is notable that the amount of fine root litter was 2.6e2.8 times larger than the leaf litter amount and the contribution of fine root litter to SOC was nearly 60% of the total C accumulation in SOC. The calculated average rate of C sequestration in SOC was 0.41 Mg C ha1 yr1 in the willow stand and the corresponding rate for poplar was 0.52 Mg C ha1 yr1. The woody biomass pool above ground, including stem, branches and cutting or stool, reached a production of about 139.8 Mg DM ha1 and 148.1 Mg DM ha1, in willow and poplar plantations, respectively (Table 4a, b). The corresponding C sequestration rates in above-ground woody biomass were 3.2 and 3.7 Mg C ha1 yr1. The woody biomass below ground consisted of the coarse root system and for willow also a cutting. The C sequestration rates in woody biomass below ground were 0.3 Mg C ha1 yr1 for both willow and poplar. By summing up above- and below-ground rates, total C sequestration rates in woody biomass reached 3.5 Mg C ha1 yr1 in a short rotation willow plantation and 4.0 Mg C ha1 yr1 in a poplar plantation.
4.
Discussion
4.1.
Carbon sequestration and methods
4.1.1.
The crop
Mean annual stem production was used as input in these calculations of C sequestration and for Salix it was based on estimates of the best growers from 2082 commercial plantations in Sweden [27]. Commercial plantations of Populus in Sweden have been rare until recently and, consequently, the estimated stem production was based on well managed experimental stands [31]. Thus, the estimated C sequestration in stems can be considered as realistic on fertile soils with
Table 2 e Production of fine roots and fine root litter in plantations with (a) willow and (b) poplar. Production of fine roots and fine root litter (Mg DM ha1) (a) Salix Rotation period Years Fine root production Fine root standing cropa Fine root litter (b) Populus Rotation period Years Fine root production Fine root standing cropa Fine root litter
First 4 14.8
Second 3 12.8
Third 3 12.8
14.8
12.8
12.8
4’the7’th 12 51.2 0.7 50.5
Annual mean (Mg DM ha1 yr-1)
Total (Mg DM ha1)
1’the7’th Annual 4.16 0.7 4.13
1’the7’th 22 91.6 0.7 90.9
First Annual 5.14 0.9 5.10
First 20 102.8 0.9 101.9
a Mg DM ha1, this pool did not have an annual increment and was transferred to the litter pool in all years, except in the last year.
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Table 3 e Annual and total production, accumulation in soil organic matter (SOM) and soil organic carbon (SOC) in (a) willow and (b) poplar plantations. Annual production and accumulation in SOM (Mg DM ha1 yr1) (a) Salix Rotation period Years Leaf litter production Remainder,a accumulates Fine root litter production Remainder,b accumulates
First 4 1.27 0.29 3.70 0.43
Second 3 1.53 0.35 4.27 0.49
Third 3 1.53 0.35 4.27 0.49
4’the7’th 12 1.53 0.35 4.20 0.48
Total litter production Total remainder, accumulates (b) Populus Rotation period Years Leaf litter production Remainder,a accumulates Fine root litter production Remainder,b accumulates
First Annual 1.92 0.44 5.14 0.59
Total litter production Total remainder, accumulates
Total SOM (Mg DM ha1)
Total SOC (Mg C ha1)
Annual SOC (Mg C ha1 yr1)
1’the7’th 22 32.7 7.5 90.9 10.5
1’the7’th 22 16.3 3.8 45.4 5.2
1’the7’th Annual 0.74 0.17 2.07 0.24
123.6 18.0
61.7 9.0
2.81 0.41
First 20 38.5 8.8 101.9 11.8
First 20 19.2 4.4 50.9 5.9
First Annual 0.96 0.22 2.55 0.30
140.4 20.6
70.2 10.3
3.51 0.52
a limit value 0.23 (Berg 2000). b limit value 0.115 (estimated).
proper management under Swedish conditions (Table 4a, b). The allometric relationships, obtained from the lysimeter study under non-limiting conditions with respect to water and nutrients, generated the proportions of the below ground production [20]. Accordingly, the calculated C sequestration below ground shows a possible outcome from such plantations. There may be differences in below ground allocation caused by, for example, different soil conditions, climate zones, clones, and management practices. More studies are therefore needed which deals with changes in allocation pattern and root-system development under varying site conditions and management. However, a study of different tree species in the boreal zone indicate that root system architecture appear to be more dependent on species than on site properties and stand development [38]. The assumption that the below ground allocation of willow was applicable for poplar was made. In willow root:shoot ratios, excluding leaves and cuttings, declined from 0.40 to 0.55 in the first year following plantation to 0.11e0.15 in the subsequent years [20]. In poplar, corresponding leafless root:shoot ratios were 0.5e1.7 and 0.14e0.17, respectively, in a North American study [39]. Clonal differences in growth were observed for poplar, but no different C allocation pattern was detected [40]. Furthermore, the estimated fine root production was within the same range in plantations with poplar and willow, and also in the lysimeter-stand used in the present study [20,34,41]. According to these estimates the approximations made for poplar would be within an acceptable range. The rate of sequestration of C in woody biomass below ground, i.e. the coarse root system and for willows also a cutting part, was 0.3 Mg C ha1 yr1 (Table 4a,b). Coarse root
turnover has received little attention in root research [42]. It has been suggested that coarse root turnover should be considered under conditions of strong competition or other external stress, but it was not included in the present calculations. By summing up the above- and below ground rates, total C sequestration rates in woody biomass reached 3.5 and 4.0 Mg C ha1 yr1, for willows and poplar, respectively, which are within the same range as those in young oak and spruce stands on former arable land [14].
4.1.2.
The soil
The rate of forest floor development depends partly on above ground litter fall, but the organic input from fine root turnover cannot be neglected when C sequestration to soils is estimated. Fine root production exceeded leaf production by nearly three times and was the largest biomass pool except for stem biomass (Table 2 a,b and 4a,b). The fine root share of net primary production (NPP) was estimated at 30e39% which was of the same magnitude as estimates reported for a Populus forest (Table 1 [21]). Consequently, the contribution of fine root litter to SOM was about 60% of the total litter production. Few studies have dealt with the contribution of tree root litter to SOM and the time span for most available studies are short [e.g. [43,44]], and there is no indication that decomposition of leaf litter is correlated with fine root litter within species [45]. Thus, there is an obvious need for further studies concerning the role of fine roots in forests and tree plantations and their contribution to SOM. Since no long-term decomposition studies of leaf litter from willow and poplar under Swedish conditions (latitudes 56 e65 N) were available, the limit value from birch litter was used in the present calculations for estimating the remainder
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Table 4 e Annual and total (20e22 years) biomass production and C sequestration in (a) willow and (b) poplar plantations. Annual biomass production (Mg DM ha1 yr1)
Total biomass (Mg DM ha1)
Total C (Mg C ha1)
Annual C (Mg C ha1 yr1)
(a) Salix Rotation period Years Stem Cutting above ground Leaves Total above ground
First 4 5.20 0.24 1.27 6.71
Second 3 6.30 0.26 1.53 8.09
Third 3 6.30 0.26 1.53 8.09
4’the7’th 12 6.30 0.26 1.53 8.09
1’the7’th 22 134.2 5.6 32.7 172.5
1’the7’th 22 67.1 2.8 16.3 86.3
1’the7’th Annual 3.05 0.13 0.74 3.92
Coarse roots Cutting below ground Fine roots Total below ground
0.29 0.30 3.70 4.29
0.30 0.32 4.27 4.88
0.30 0.32 4.27 4.88
0.30 0.32 4.27 4.88
6.5 6.9 91.6 105.0
3.3 3.4 45.8 52.5
0.15 0.16 2.08 2.39
Woody, above ground Woody, below ground Total woody biomass
5.44 0.59 6.03
6.56 0.62 7.18
6.56 0.62 7.18
6.56 0.62 7.18
139.8 13.4 153.2
69.9 6.7 76.6
3.18 0.31 3.49
(b) Populus Rotation period Years Stem Branches Leaves Total above ground
First Annual 6.30 1.11 1.92 9.33
First 20 125.9 22.2 38.5 186.6
First 20 63.0 11.1 19.2 93.3
First Annual 3.15 0.56 0.96 4.66
Coarse roots Fine roots Total below ground
0.60 5.14 5.74
11.9 102.8 114.7
6.0 51.4 57.4
0.30 2.57 2.87
Woody, above ground Woody, below ground Total woody biomass
7.41 0.60 8.01
148.1 11.9 160.0
74.1 6.0 80.1
3.71 0.30 4.01
(0.23) that accumulates in the forest floor [30]. A limit value from litter of Populus grandidentata Michx., estimated at Blackhawk Island (43 N 89 E), gave a remainder of 0.20 [46]. Hence, the use of limit values for birch litter may function as an approximation, but litter decomposition studies applied to short rotation forest species, such as willow and poplar, and for various climate zones, are urgently needed for improving the prediction of C sequestration in such plantations. A combination of the minirhizotron technique and the measured fine root biomass from harvested plants were used to estimate fine root production. The minirhizotron technique has proven to be a useful tool in numerous root studies. However, there are still some questions concerning the accuracy of the technique. It has been shown that fine root densities at the minirhizotrons differ from those in the bulk soil [47]. This could be due to, for example, voids, compaction, soil shrinkage, light leaks or differences in soil temperature or moisture [e.g. [23,47e49]]. The installation procedure may be critical and several studies suggest a minimum of one-year wait for the adjustment of root systems [48,50e53] and different installation-angles has been tested [49,50,54,55]. Discrepancies in estimates of fine root longevity from minirhizotron observations and C isotope studies have been found, and the question of how to handle the heterogeneity of the fine root system has arisen [17]. Whether the minirhizotron
interface affects the decay rate of roots has also been discussed [56,57]. In the present study observations from minirhizotrons were included in the estimates of fine root production, but the limit-value concept was used for estimating the remainder of organic material from fine roots and the long-term build-up of SOC from fine root litter. The calculated average rate of sequestration into SOC was 0.41 Mg C ha1 yr1 in a willow plantation and the corresponding rate for poplar was 0.52 Mg C ha1 yr1, the above ground input rates, from leaf litter, were 0.17 and 0.22 Mg C ha1 yr1, respectively (Table 3a,b). These above ground values may be compared with a reported calculated mean value for C accumulation of 0.18 Mg C ha1 yr1 to forests soils, from above ground litter input on different latitudes and for different tree species, under Swedish conditions [58]. The specific values for spruce, pine and birch were 0.20, 0.15 and 0.15, respectively. In a Danish study, afforestation of arable land with spruce was found to give a total input of 0.35 Mg C ha1 yr1 to the forest floor over 32 years [14]. In a review the mean accumulation of soil C was estimated at 0.34 Mg C ha1 yr1, for various species and various time periods, when arable land was afforested [12]. A poplar short rotation forest increased the organic C storage in soil by about 23% in the second rotation cycle compared to the previous annual crop [59]. Thus, the present estimated rates of C
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sequestration in the soil were of the same magnitude as those from several other studies, suggesting there is a significant potential to increase the SOC-pool in short-rotation plantations with willow and poplar on arable land. However, using soils as C sinks remains a short time solution since most of the gain is made initially at the conversion of land use from, for example, annual crops to perennial tree plantations. In a longer perspective the reduction of the use of fossil fuels and development of alternative energy sources is required to stabilize the atmospheric CO2 levels. The fate of DOC was not included in the present calculations. A long-term study of the effect of afforestation with Norway spruce on DOC dynamics suggested that more than 90% of DOC leached from the forest floor was retained within 0e60 cm of the mineral soil [60]. However, more studies of DOC are needed which also include data from the time before afforestation, under various climatic conditions and for different species and management. The current knowledge of the potential of arable land to mitigate greenhouse effects is limited and the literature is not consistent. Afforestation of arable land is expected to result in significant sequestration of C due to accumulation of both woody biomass and organic matter in the soil [61], but it is still debated whether soil C stocks generally will increase following afforestation [15,62,63]. Vesterdal et al. [64] found that C sequestration occurred mainly in the biomass of trees within a short time span of about 30 years. They also noted that soil C stores were clearly higher in older plantations. Current research on poplar plantations, established on former agricultural land, found losses or no changes in SOC during the first 5e12 years [40,65]. However, data concerning spatial and temporal variability in the SOC pool under short rotation woody crops are limited due to lack of well-designed longterm studies [66]. Whether a soil becomes a source or a sink for C is strongly influenced by former land use, and also by soil disturbing activities such as harvest [e.g. [40,67]].
4.2.
Future potential
If 400,000 ha of abandoned arable land in Sweden were planted with willow and poplar, about 1.5 Tg C would be sequestered annually in woody biomass, including both above and below ground production, and 0.2 Tg in soils. This would be nearly one tenth of annual anthropogenic emissions of C in Sweden today. In the Swedish forests, which cover 22.7 Mha of the country, the annual sequestration of C to soils has been estimated to 4-5 Tg [58]. The present estimates show the potential of fast-growing tree plantations on arable land to mitigate the effect of CO2 over a short time span. In a longer perspective such plantations may provide fuel in a renewable system under environmental sustainable conditions. A study comparing rotation periods under Swedish conditions, found that short rotations (15 years) of Populus tremula L. would optimize the reductions in CO2 emissions to the atmosphere [68]. When regarding the costeffectiveness of plantations aimed at capturing and storing C, it was suggested that short-rotation bioenergy plantations may be more economically efficient for stringent C targets (around 400 ppm) compared to long-rotation forests [69]. Results concerning the positive effects of bioenergy crops on C
93
sequestration, planted on potentially available land area, are also reported from other parts of the world [66,70,71].
4.3.
Future research in Sweden
To meet the growing demand for biofuels in Sweden, and to increase the knowledge concerning fast-growing tree species planted on arable land, an experiment was established in 2009. The purpose is to test cultivation practices, involving shorter rotation periods and fewer management measures than currently recommended for pulp and timber production. Six tree species, including willow and poplar, suitable for biofuel production were planted with a spacing adapted to the specific species in order to optimize productivity. The experiment was established on five different latitudes, from north to south, in Sweden. Initial samplings and analyses of the soil, including all sites, have been performed. The main objectives of the soil research activities are to [72]: follow the sequestration of C in biomass and soil for different tree species during afforestation of arable land, follow the process over a north-south gradient in Sweden which will show the climatic impact on C sequestration rates, evaluate the species-specific effects on soil chemical parameters and use the results to formulate recommendations for future actions such as liming, nitrogen fertilization or return of ashes.
5.
Conclusions
Willow and poplar plantations on abandoned arable land will most probably increase the rate of C sequestration and thereby mitigate the negative effects of increasing atmospheric CO2 concentrations. The sequestration rates will be affected by former land use, climate and management of the plantations. Those effects on C sequestration rates need further investigations and more long-term studies are desirable. Changes in SOC are difficult to estimate and in most cases there are no soil data available from the time before afforestation. Further research is needed on this subject. The magnitude of the contribution of root litter to soil C sequestration is relatively unknown. More studies concerning root production, including fine root turnover rates, and estimates of decomposition rates are needed. The fate of DOC also needs further investigations that include data from the time before afforestation, under different climatic conditions, and for different species and management.
Acknowledgements Preparation of this paper was funded by The Swedish Energy Agency and The Swedish Research Council Formas. I thank Lars Rytter and four anonymous referees for helpful discussions and valuable comments on the manuscript.
94
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Available online at www.sciencedirect.com
http://www.elsevier.com/locate/biombioe
The potential of willow and poplar plantations as carbon sinks in Sweden Rose-Marie Rytter* The Forestry Research Institute of Sweden (Skogforsk), Ekebo 2250, 268 90 Svalo¨v, Sweden
article info
abstract
Article history:
A large share, estimated at 12e25%, of the annual anthropogenic greenhouse gas emissions
Received 21 October 2010
is attributed to global deforestation. Increasing the forested areas therefore has a positive
Received in revised form
impact on carbon (C) sequestration and mitigation of high atmospheric CO2 concentra-
6 September 2011
tions. Fast-growing species, such as willow and poplar, are of high interest as producers of
Accepted 13 October 2011
biomass for fuel, but also as C sinks. The present study estimated the rate of C seques-
Available online 3 December 2011
tration in biomass and soil in willow and poplar plantations. Calculations were based on above- and below-ground biomass production data from field experiments, including fine
Keywords:
root turnover, litter decomposition rates, and production levels from commercial planta-
Afforestation
tions. Accumulation of C in woody biomass, above and below ground, was estimated at
Carbon sequestration
76.6e80.1 Mg C ha1 and accumulation of C in the soil at 9.0e10.3 Mg C ha1 over the first
Short-rotation plantations
20e22 years. The average rates of C sequestration were 3.5e4.0 Mg C ha1 yr1 in woody
Salix
biomass, and 0.4e0.5 Mg C ha1 yr1 in the soil. If 400,000 ha of abandoned arable land in
Populus
Sweden were planted with willow and poplar, about 1.5 Tg C would be sequestered annually in woody biomass and 0.2 Tg C in soils. This would be nearly one tenth of the annual anthropogenic emissions of C in Sweden today. These calculations show the potential of fast-growing plantations on arable land to mitigate the effect of high CO2 concentrations over a short time span. Knowledge gaps were found during the calculation process and future research areas were suggested. ª 2011 Elsevier Ltd. All rights reserved.
1.
Introduction
Land use change through deforestation, cultivation of grass lands and an increase of the pasture land area, has reduced the C stored in soils and plants globally by approximately 180e200 Pg [1]. This is of the same magnitude as fossil fuel emissions to the atmosphere from pre-industrial times until 2000 which were estimated at 280 Pg C [2]. A portion of this released C has been taken up by land plants and in the oceans. But losses to the atmosphere have been nearly 200 Pg C which has contributed to the elevated CO2 levels today [3].
Atmospheric emissions of green house gases are now regarded as the major cause to the observed global warming during the last century [4]. Current atmospheric models predict that if all fossil fuels were burned the atmospheric concentration of CO2 would raise from 390 ppm today to 1200e2000 ppm [3]. Apart from a worldwide reduction of the use of fossil fuels, increasing the forested areas may contribute to mitigate the high atmospheric CO2 concentration over a short time span. Afforestation of arable land is regarded as one of the major potential C sinks in Europe [5]. Recent figures show that forested areas in Europe had a net increase of 7% in the period
* Corresponding author. Present address: Rytter Science, Backava¨gen 16, 268 68 Ro¨sta˚nga, Sweden. Tel.: þ46 418471300; fax: þ46 418471329. E-mail address: [email protected]. 0961-9534/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2011.10.012
b i o m a s s a n d b i o e n e r g y 3 6 ( 2 0 1 2 ) 8 6 e9 5
1990e2005 [6] but a further expansion of forested areas is an essential component of an overall strategy for mitigation of increasing atmospheric CO2 levels. Recent reports suggest that about 300,000 to 500,000 ha of arable land in Sweden, and abandoned (not yet afforested arable land), may be cultivated with energy crops and fastgrowing tree species in the future [7,8]. Short rotation plantations have been regarded as one of the main alternatives in the shift towards a more sustainable energy supply in order to substitute for fossil fuels in Sweden. In this context, fastgrowing species such as poplar and willow are of high interest, mainly as producers of biomass for fuel, but also for sequestration of C. Generally, forests have a higher C density than arable land, mainly due to the presence of perennial vegetation with high biomass. By using fast-growing tree species for energy purposes, C is stored in biomass and soils, and sustainable systems with recirculation of CO2 may be established in the long term. Conversion of arable land to forests also implies a shift from a shorter to a longer residence time of C by replacing annual crops with more long-lived perennial woody species. Soils constitute the largest pool of terrestrial C in the biosphere and estimates indicate that 1500 to 1600 Pg is stored in the uppermost 1 m soil layer, and more than 2000 Pg in the top 3 m soil [9e11]. The potential importance of vegetation changes on soil organic C (SOC) pools for C sequestration strategies has been stressed [10]. The storage capacity and rate of C sequestration in soils at afforestation depend on various factors such as the climate, soil type, tree species used for afforestation, current forestry practices, pre-afforestation management and land use history [12,13]. Processes in the soil are complex and whether a soil becomes a C sink depends to a high degree on the production of leaf and root litter and their decomposition rates. Studies of C sequestration involving both biomass and soil following afforestation are few, especially in Europe [14]. Those studies imply that the woody biomass accounts for the major part of the stored C and about 20e30% will be stored in the uppermost soil during the first decades after establishment of a new forest [14,15]. Further studies are needed, however, which deal with the relationship between C storage in soils, tree species and population age at afforestation of arable land [14]. In a metaanalysis of the effects of land-use changes on C content in soils, there was a general increase in the C stock for afforestation of arable land (þ18%), and significant differences between tree species were observed [16]. It is essential to include the whole plant when C budgets are made. Carbon is sequestered in woody biomass, but also accumulated as organic matter in the forest floor by litter from leaves and fine root turnover. Estimates of fine root production are crucial in this context. Fine roots (i.e. 2 mm in diameter) constitute a large and very dynamic component of the C cycles of the terrestrial ecosystems [17]. They emerge, die, and decompose in the soil continuously, thus having an annual turnover which may contribute to 20e70% of the total net primary production [e.g. [18e22]]. Excluding fine root turnover may lead to underestimation of organic matter and nutrients by 20e80% [23]. A global budget calculation, assuming a fine root turnover rate of once per year, estimated that fine roots represented 33% of the global annual net
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primary production [24]. It is by now widely accepted that fine roots have a longevity of 1 year or less [17]. Their role in the soil of ecosystems has been compared with the role of leaves in the aerial environment [25]. Different methods for quantifying fine root turnover have been used and, in brief, they include mass balances, such as the N budget and C budget techniques, sequential soil coring, in-growth cores, minirhizotrons and the use of C isotopes. It is of utmost importance to choose a method that permits frequent sampling with short time intervals to cover the rapid fine root turnover [e.g. [20]]. The potential for the capture of C in biomass and soil during cultivation of short rotation forest willow and poplar in Sweden is still unknown. Willow has been cultivated on arable land for bioenergy purposes during the last 30 years and, so far, willow plantations have been established on about 15 000 ha or 0.5% of total arable land in Sweden [26]. Today commercial willow plantations in Sweden have a double-row design with 1.5 and 0.75 m spacing between the rows and approximately 0.75 m between plants in the row yielding a planting density of about 12,000 cuttings ha1. A yield model, based on a number of commercial plantations in Sweden, predicts mean annual yields of 4.0e7.1 tonnes dry matter ha1 for 25% of the best growers [27]. Whole-shoot harvest is usually conducted every 3e4 years if growth conditions are favourable. The interest in Populus species, and hybrid aspen in particular, can be traced back to the 1940’s when the Swedish Match Company was involved in breeding species and crossings suitable for match wood production. However, the land area dedicated to commercial production has remained small until recently. Over the past 5 years the areas planted with hybrid aspen and poplars have increased from about 500 to more than 2000 ha. The stands are usually established with 1100 to 1600 plants ha1 and the rotation periods will most probably exceed 10 years. Mean annual increment at a stem density of 1100 plants ha1 is expected to be above 20 m3 stemwood ha1 yr1 during a 20e25 year rotation period on fertile soil in southern Sweden, corresponding to 8e9 tons of DM ha1 yr1 including twigs and branches [28]. The calculations of C sequestration in willow and poplar plantations in the present study were based on above- and below-ground biomass production data from field experiments, including C allocation to different plant parts and fine root turnover (Fig. 1). Litter decomposition rates and limit values for decomposition were obtained from the literature [29,30]. Mean annual stem-production levels were obtained from commercial and experimental plantations in Sweden. The conditions were fertile sites, i.e. former arable land, and proper management. Arable land will generally have a low C stock, compared to pasture, and it was therefore assumed that the SOC-pool increased quite immediately at afforestation [e.g. [13]]. A whole plant perspective was the base for these calculations, thus, the investigated depth was the rooting depth. Approximations for poplar were done in accordance with the experimental results from willow, and were also compared to other published studies with poplar. The calculations did not include the influence of management practices or harvests on the C fluxes.
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The overall aim of this study was to assess the potential for C sequestration in fast-growing plantations of willow and poplar in Sweden today and, in this context, identify any knowledge gaps during the process. Biomass pools were estimated, both above and below ground, as well as the accumulation of C in the forest floor and soil by leaf litter fall and fine root turnover. The results were scaled up to a theoretical, but possible land use area for such plantations in Sweden today, and the impact on C sequestration and the mitigation of atmospheric CO2 were discussed.
2.
Materials and methods
2.1. Annual net primary production and relative allocation of biomass
Fig. 1 e Stepwise calculations of C sequestration in biomass and soil. (1) The annual biomass allocation pattern to different plant parts, including leaf litter, was estimated in lysimeter-grown willows during three years, using a total of 32 lysimeters [20]. (2) Annual fine root (£2 mm in diameter) production, on a whole plant basis, was calculated from annual harvests of lysimeters, and observations of fine root dynamics in 8 minirhizotrons during three years [20,33,34]. (3) The total amount of litter stabilized as soil organic material (SOM). The continuing accumulation in the forest floor and mineral soil was calculated according to the ‘limit-value’ concept [29,30]. Finally, annual and total biomass production during 20e22 years, and C sequestration in biomass and SOC were estimated by using mean annual stem production values from commercial and experimental plantations in Sweden as input [27,28,31], and a C-concentration of 500 mg gL1 biomass [37].
Mean annual stem-production levels were obtained from commercial and experimental plantations in Sweden and used as input the calculations [27,28,31]. The relative allocation of biomass to the different plant parts was estimated from whole plant C budgets in a short rotation stand with lysimeter-grown willows (Salix viminalis L., Fig. 1, step 1). Annual net primary production and C allocation, above and below ground, were estimated for three years following plantation. The stand was situated at Ultuna, Uppsala, (59 490 N, 17 400 E, alt. 5 m) where lysimeters, with a volume of 200 dm3 each, were arranged in double rows and buried in the ground. The lysimeters used in these calculations were filled with clay substrate from the site and an effort was made to reconstruct the original soil profile in the lysimeters. The soil was compacted during the winter and in the following spring 25 cm long and un-rooted cuttings were planted at a regular spacing of 0.7 m. Cuttings were planted in the lysimeters and also in the rows between to avoid stand edge effects. Water and fertilizer were supplied daily during the growing seasons through a computer-controlled drip irrigation system. Annual net primary production and C allocation of all plant parts were estimated by whole-plant harvests at the end of each growing season. Dry weights were determined for shoots, coarse roots, fine roots, the cutting and remaining leaves after drying at 70e85 C to constant weight. Leaf litter was sampled regularly during the growing seasons and dry weight was determined. From these measurements the relative distributions of primary production were calculated for each year by also including the fine root production (see below, Table 1). For more details see Rytter [20].
2.2.
Fine root production
Fine roots (diameter 2 mm) were monitored at 2-week intervals by using minirhizotrons (Fig. 1, step 2). The minirhizotrons were installed vertically in the lysimeters, had a length of 70 cm, and consisted of transparent acrylic tubes with a grid drawn with waterproof ink on the outside [cf. [32]]. The effective soil area observed by one minirhizotron was 0.072 m2 and the number of fine root branches growing to the minirhizotron surface was recorded. Fine root production was estimated by two different methods both based on observed fine root dynamics in minirhizotrons and fine root biomass
b i o m a s s a n d b i o e n e r g y 3 6 ( 2 0 1 2 ) 8 6 e9 5
Table 1 e Relative distribution of annual net primary production in lysimeter grown willows (%), n [ 16. From Rytter (2001). Lysimeter-grown willows Relative annual net primary production (%) First rotation period Stems Leaves Cutting above ground Total above ground production Coarse roots Fine rootsa Cutting below ground Total below ground production Total annual production
Year 1
Year 2
Year 3
38.0 9.5 3.6 51.2 5.3 39.0 4.5 48.8 100
50.3 9.8 3.3 63.3 2.4 30.3 3.9 36.7 100
47.8 12.8 1.4 61.9 2.2 34.1 1.7 38.1 100
a Calculated annual fine root production.
estimates from harvested plants during three consecutive years (Table 1, both methods were used in the calculations, see [33] for more details). Briefly described, the first method separated observed fine root branch number in each 2 6 cm unit of the minirhizotrons into growth and decay by calculating changes between two consecutive dates [20,34]. Growth was defined as positive net changes in root number and decay as negative net changes in root number. The sums of growth (Grt, tþ1) between two destructive harvests of whole lysimeters, performed annually at the end of each growing season (at time t, tþ1), and the ratios (Rmt , Rmtþ1) of the measured standing crops of fine roots (g DM) at the harvest dates and observed standing root number at the same or adjoining dates, were then used for calculation of annual fine root production (Prt, tþ1): Prt;tþ1 ¼
Rmt þ Rmtþ1 X Grt;tþ1 2
The second method was based on fine root branching dynamics and fine root turnover rates estimated from minirhizotron data. A polynomial of the third degree described the phases of growth and decay in a simple but realistic way according to observed fine root branching orders, i.e. with a faster increase by time in branch numbers during the growth phase and a slower disappearance at the end of the decay phase. Integrals were used for calculating the mean fine root ages during the growth (RAGr) and the decay phase (RADe). Fine root duration times (Tt) were defined as: Tt ¼ RAGr þ RADe þ RASt where RASt was the mean fine root age during a short period of maximum root number. Fine root turnover rates (Tr) as times per year were calculated, for each year, as: Tr ¼
365 Tt
Annual turnover of fine root branches (Tb) was calculated as the product of turnover rates and a mean of standing root number from all measurements between the destructive samplings at harvests. Turnover of fine root biomass was thereafter calculated by multiplying the mean ratio of fine root mass and number of fine root branches (Rmt , Rmtþ1) by
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annual turnover of fine root branches (Tb). The annual fine root production (Prt, tþ1) was estimated by adding the annual (Ftþ1Ft) to estimated turnover: change in fine root biomass Rmt þ Rmtþ1 Prt;tþ1 ¼ Tb þ Ftþ1 Ft 2
2.3.
Decomposition of litter
The relative proportion of leaf litter was estimated from the lysimeter study (above) and fine root litter was obtained by subtracting fine root standing crop from fine root production. Fine root standing crop was calculated from relative proportions [20]. The total amount of litter stabilized as soil organic material (SOM) and further accumulated in the forest floor was calculated by using limit values (Fig. 1, step 3 [29],). When the decomposition rates approach zero the accumulated litter mass approaches a final limit value for decomposition that can be described by an asymptotic function. This limit value is different for different species and depends partly on nitrogen (N) concentration. Limit values for litter from willow and poplar were not available for Swedish conditions (latitudes 56 e65 N), thus the limit value for birch litter (remainder 0.23) was used in the present calculations [30]. While willow fine roots have N concentrations that are about 60% of the leaf N concentration [20], a low decomposition rate (approaching zero) will be reached later and at a higher mass loss, according to the ‘limit-value’ concept [29,30]. The limit value for fine root litter in the present calculations was set to 50% of the limit value for leaf litter.
2.4.
Carbon sequestration
The reported production levels for willow and poplar, the allocation of annual biomass production (Table 1), the predicted amounts of leaf and fine root litter and accumulation to SOM according to the ‘limit-value’ concept were used for calculations of C sequestration in both willow and poplar plantations. Thus, for poplar the below ground production was approximated according to the willow proportions (year 3, Table 1). Mean annual yields of 5.2, 6.3 and 6.3 tonnes dry matter per ha during the first, second and third harvest cycles from commercial willow plantations in Sweden were used as input in those calculations [27]. Biomass distribution in the first rotation period was calculated for four years using proportions from year 1, 2 and 3. Proportions from year 3 were also used in the fourth year (Table 1). The later rotation periods, with duration of three years, were calculated using the proportions from year 2 and 3 where proportions from year 3 were used the two latter years of the rotation. The calculations for willow were based on seven rotations, which is about the time the stump systems are used in commercial plantations in Sweden today. The calculations for poplar were based on a growth period of 20 years, a stem density of 1100 plants ha1 and a mean annual stem production of 20 m3 ha1 [cf [28,31].]. The annual production of branches was set to 15% of the stem volume [cf. [31]]. Conversion of woody biomass to dry matter (DM) was calculated by using a basic density of 0.322 Mg m3 [35]. Above ground litter fall was restricted to leaves which constitutes about 90% of total litter fall in young stands [36]. The C content was calculated by using a common
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C-concentration of 500 mg g1 biomass [37]. Flows of dissolved organic carbon (DOC) from the forest floor to the mineral soil were not estimated in this study.
3.
Results
Two different plant biomass pools were handled when C budgets for the plant and soil systems in willow and poplar plantations were calculated. One pool consisted of leaves and fine roots, which both have a fast turnover as most of them emerge and die on an annual basis. Thus, leaves and fine roots were producing litter almost immediately after they were formed (Table 2a,b) and this litter was directed to the decomposition pool in these calculations where they contributed to the accumulation of C in SOM from the first year and onwards (Table 3a,b). The other biomass pool included stems, branches and the coarse root system. Those plant parts possess a longer turnover time compared to leaves and fine roots and they were expected to have a positive annual increment during the whole rotation period (Table 4a,b). Some losses of biomass from branches and coarse roots do occur, but they are considered as a minor loss in younger stands and were not included in the estimates [e.g. [36]]. The annual mean fine root production was 4.2 and 5.1 Mg DM ha1 yr1 in plantations with willow and poplar, respectively (Table 2a, b). Since fine root turnover is rapid most of the produced fine root biomass was transferred to the fine root litter pool which reached a total of 90.9 and 101.9 Mg DM ha1, in willow and poplar plantations, respectively. A standing fine root crop was present in all years, but it was not supposed to have an annual increment. The remaining standing fine root crop from the last year, measuring 0.7e0.9 Mg DM ha1, was not transferred to the fine root litter pool during this time period, which covered a 20-year rotation for poplar and seven rotations for willow. It was instead assumed to be left after the final harvest and thus transferred to the litter pool in the following rotation period. However, since the size of the subtracted standing fine root crop from the last year was comparatively small in the time scale of 20e22 years, the contribution of fine roots to litter almost equalled fine root production.
The input from fine root turnover and shedding leaves to the litter pool was 123.6e140.4 Mg DM ha1 during the 20e22 year period (Table 3a, b). By using limit values for decomposition the total accumulation of C in the soil (SOC), i.e. the remainder from both leaf and fine root litter, was estimated at 9.0 and 10.3 Mg C ha1, in willow and poplar plantations, respectively. It is notable that the amount of fine root litter was 2.6e2.8 times larger than the leaf litter amount and the contribution of fine root litter to SOC was nearly 60% of the total C accumulation in SOC. The calculated average rate of C sequestration in SOC was 0.41 Mg C ha1 yr1 in the willow stand and the corresponding rate for poplar was 0.52 Mg C ha1 yr1. The woody biomass pool above ground, including stem, branches and cutting or stool, reached a production of about 139.8 Mg DM ha1 and 148.1 Mg DM ha1, in willow and poplar plantations, respectively (Table 4a, b). The corresponding C sequestration rates in above-ground woody biomass were 3.2 and 3.7 Mg C ha1 yr1. The woody biomass below ground consisted of the coarse root system and for willow also a cutting. The C sequestration rates in woody biomass below ground were 0.3 Mg C ha1 yr1 for both willow and poplar. By summing up above- and below-ground rates, total C sequestration rates in woody biomass reached 3.5 Mg C ha1 yr1 in a short rotation willow plantation and 4.0 Mg C ha1 yr1 in a poplar plantation.
4.
Discussion
4.1.
Carbon sequestration and methods
4.1.1.
The crop
Mean annual stem production was used as input in these calculations of C sequestration and for Salix it was based on estimates of the best growers from 2082 commercial plantations in Sweden [27]. Commercial plantations of Populus in Sweden have been rare until recently and, consequently, the estimated stem production was based on well managed experimental stands [31]. Thus, the estimated C sequestration in stems can be considered as realistic on fertile soils with
Table 2 e Production of fine roots and fine root litter in plantations with (a) willow and (b) poplar. Production of fine roots and fine root litter (Mg DM ha1) (a) Salix Rotation period Years Fine root production Fine root standing cropa Fine root litter (b) Populus Rotation period Years Fine root production Fine root standing cropa Fine root litter
First 4 14.8
Second 3 12.8
Third 3 12.8
14.8
12.8
12.8
4’the7’th 12 51.2 0.7 50.5
Annual mean (Mg DM ha1 yr-1)
Total (Mg DM ha1)
1’the7’th Annual 4.16 0.7 4.13
1’the7’th 22 91.6 0.7 90.9
First Annual 5.14 0.9 5.10
First 20 102.8 0.9 101.9
a Mg DM ha1, this pool did not have an annual increment and was transferred to the litter pool in all years, except in the last year.
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Table 3 e Annual and total production, accumulation in soil organic matter (SOM) and soil organic carbon (SOC) in (a) willow and (b) poplar plantations. Annual production and accumulation in SOM (Mg DM ha1 yr1) (a) Salix Rotation period Years Leaf litter production Remainder,a accumulates Fine root litter production Remainder,b accumulates
First 4 1.27 0.29 3.70 0.43
Second 3 1.53 0.35 4.27 0.49
Third 3 1.53 0.35 4.27 0.49
4’the7’th 12 1.53 0.35 4.20 0.48
Total litter production Total remainder, accumulates (b) Populus Rotation period Years Leaf litter production Remainder,a accumulates Fine root litter production Remainder,b accumulates
First Annual 1.92 0.44 5.14 0.59
Total litter production Total remainder, accumulates
Total SOM (Mg DM ha1)
Total SOC (Mg C ha1)
Annual SOC (Mg C ha1 yr1)
1’the7’th 22 32.7 7.5 90.9 10.5
1’the7’th 22 16.3 3.8 45.4 5.2
1’the7’th Annual 0.74 0.17 2.07 0.24
123.6 18.0
61.7 9.0
2.81 0.41
First 20 38.5 8.8 101.9 11.8
First 20 19.2 4.4 50.9 5.9
First Annual 0.96 0.22 2.55 0.30
140.4 20.6
70.2 10.3
3.51 0.52
a limit value 0.23 (Berg 2000). b limit value 0.115 (estimated).
proper management under Swedish conditions (Table 4a, b). The allometric relationships, obtained from the lysimeter study under non-limiting conditions with respect to water and nutrients, generated the proportions of the below ground production [20]. Accordingly, the calculated C sequestration below ground shows a possible outcome from such plantations. There may be differences in below ground allocation caused by, for example, different soil conditions, climate zones, clones, and management practices. More studies are therefore needed which deals with changes in allocation pattern and root-system development under varying site conditions and management. However, a study of different tree species in the boreal zone indicate that root system architecture appear to be more dependent on species than on site properties and stand development [38]. The assumption that the below ground allocation of willow was applicable for poplar was made. In willow root:shoot ratios, excluding leaves and cuttings, declined from 0.40 to 0.55 in the first year following plantation to 0.11e0.15 in the subsequent years [20]. In poplar, corresponding leafless root:shoot ratios were 0.5e1.7 and 0.14e0.17, respectively, in a North American study [39]. Clonal differences in growth were observed for poplar, but no different C allocation pattern was detected [40]. Furthermore, the estimated fine root production was within the same range in plantations with poplar and willow, and also in the lysimeter-stand used in the present study [20,34,41]. According to these estimates the approximations made for poplar would be within an acceptable range. The rate of sequestration of C in woody biomass below ground, i.e. the coarse root system and for willows also a cutting part, was 0.3 Mg C ha1 yr1 (Table 4a,b). Coarse root
turnover has received little attention in root research [42]. It has been suggested that coarse root turnover should be considered under conditions of strong competition or other external stress, but it was not included in the present calculations. By summing up the above- and below ground rates, total C sequestration rates in woody biomass reached 3.5 and 4.0 Mg C ha1 yr1, for willows and poplar, respectively, which are within the same range as those in young oak and spruce stands on former arable land [14].
4.1.2.
The soil
The rate of forest floor development depends partly on above ground litter fall, but the organic input from fine root turnover cannot be neglected when C sequestration to soils is estimated. Fine root production exceeded leaf production by nearly three times and was the largest biomass pool except for stem biomass (Table 2 a,b and 4a,b). The fine root share of net primary production (NPP) was estimated at 30e39% which was of the same magnitude as estimates reported for a Populus forest (Table 1 [21]). Consequently, the contribution of fine root litter to SOM was about 60% of the total litter production. Few studies have dealt with the contribution of tree root litter to SOM and the time span for most available studies are short [e.g. [43,44]], and there is no indication that decomposition of leaf litter is correlated with fine root litter within species [45]. Thus, there is an obvious need for further studies concerning the role of fine roots in forests and tree plantations and their contribution to SOM. Since no long-term decomposition studies of leaf litter from willow and poplar under Swedish conditions (latitudes 56 e65 N) were available, the limit value from birch litter was used in the present calculations for estimating the remainder
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Table 4 e Annual and total (20e22 years) biomass production and C sequestration in (a) willow and (b) poplar plantations. Annual biomass production (Mg DM ha1 yr1)
Total biomass (Mg DM ha1)
Total C (Mg C ha1)
Annual C (Mg C ha1 yr1)
(a) Salix Rotation period Years Stem Cutting above ground Leaves Total above ground
First 4 5.20 0.24 1.27 6.71
Second 3 6.30 0.26 1.53 8.09
Third 3 6.30 0.26 1.53 8.09
4’the7’th 12 6.30 0.26 1.53 8.09
1’the7’th 22 134.2 5.6 32.7 172.5
1’the7’th 22 67.1 2.8 16.3 86.3
1’the7’th Annual 3.05 0.13 0.74 3.92
Coarse roots Cutting below ground Fine roots Total below ground
0.29 0.30 3.70 4.29
0.30 0.32 4.27 4.88
0.30 0.32 4.27 4.88
0.30 0.32 4.27 4.88
6.5 6.9 91.6 105.0
3.3 3.4 45.8 52.5
0.15 0.16 2.08 2.39
Woody, above ground Woody, below ground Total woody biomass
5.44 0.59 6.03
6.56 0.62 7.18
6.56 0.62 7.18
6.56 0.62 7.18
139.8 13.4 153.2
69.9 6.7 76.6
3.18 0.31 3.49
(b) Populus Rotation period Years Stem Branches Leaves Total above ground
First Annual 6.30 1.11 1.92 9.33
First 20 125.9 22.2 38.5 186.6
First 20 63.0 11.1 19.2 93.3
First Annual 3.15 0.56 0.96 4.66
Coarse roots Fine roots Total below ground
0.60 5.14 5.74
11.9 102.8 114.7
6.0 51.4 57.4
0.30 2.57 2.87
Woody, above ground Woody, below ground Total woody biomass
7.41 0.60 8.01
148.1 11.9 160.0
74.1 6.0 80.1
3.71 0.30 4.01
(0.23) that accumulates in the forest floor [30]. A limit value from litter of Populus grandidentata Michx., estimated at Blackhawk Island (43 N 89 E), gave a remainder of 0.20 [46]. Hence, the use of limit values for birch litter may function as an approximation, but litter decomposition studies applied to short rotation forest species, such as willow and poplar, and for various climate zones, are urgently needed for improving the prediction of C sequestration in such plantations. A combination of the minirhizotron technique and the measured fine root biomass from harvested plants were used to estimate fine root production. The minirhizotron technique has proven to be a useful tool in numerous root studies. However, there are still some questions concerning the accuracy of the technique. It has been shown that fine root densities at the minirhizotrons differ from those in the bulk soil [47]. This could be due to, for example, voids, compaction, soil shrinkage, light leaks or differences in soil temperature or moisture [e.g. [23,47e49]]. The installation procedure may be critical and several studies suggest a minimum of one-year wait for the adjustment of root systems [48,50e53] and different installation-angles has been tested [49,50,54,55]. Discrepancies in estimates of fine root longevity from minirhizotron observations and C isotope studies have been found, and the question of how to handle the heterogeneity of the fine root system has arisen [17]. Whether the minirhizotron
interface affects the decay rate of roots has also been discussed [56,57]. In the present study observations from minirhizotrons were included in the estimates of fine root production, but the limit-value concept was used for estimating the remainder of organic material from fine roots and the long-term build-up of SOC from fine root litter. The calculated average rate of sequestration into SOC was 0.41 Mg C ha1 yr1 in a willow plantation and the corresponding rate for poplar was 0.52 Mg C ha1 yr1, the above ground input rates, from leaf litter, were 0.17 and 0.22 Mg C ha1 yr1, respectively (Table 3a,b). These above ground values may be compared with a reported calculated mean value for C accumulation of 0.18 Mg C ha1 yr1 to forests soils, from above ground litter input on different latitudes and for different tree species, under Swedish conditions [58]. The specific values for spruce, pine and birch were 0.20, 0.15 and 0.15, respectively. In a Danish study, afforestation of arable land with spruce was found to give a total input of 0.35 Mg C ha1 yr1 to the forest floor over 32 years [14]. In a review the mean accumulation of soil C was estimated at 0.34 Mg C ha1 yr1, for various species and various time periods, when arable land was afforested [12]. A poplar short rotation forest increased the organic C storage in soil by about 23% in the second rotation cycle compared to the previous annual crop [59]. Thus, the present estimated rates of C
b i o m a s s a n d b i o e n e r g y 3 6 ( 2 0 1 2 ) 8 6 e9 5
sequestration in the soil were of the same magnitude as those from several other studies, suggesting there is a significant potential to increase the SOC-pool in short-rotation plantations with willow and poplar on arable land. However, using soils as C sinks remains a short time solution since most of the gain is made initially at the conversion of land use from, for example, annual crops to perennial tree plantations. In a longer perspective the reduction of the use of fossil fuels and development of alternative energy sources is required to stabilize the atmospheric CO2 levels. The fate of DOC was not included in the present calculations. A long-term study of the effect of afforestation with Norway spruce on DOC dynamics suggested that more than 90% of DOC leached from the forest floor was retained within 0e60 cm of the mineral soil [60]. However, more studies of DOC are needed which also include data from the time before afforestation, under various climatic conditions and for different species and management. The current knowledge of the potential of arable land to mitigate greenhouse effects is limited and the literature is not consistent. Afforestation of arable land is expected to result in significant sequestration of C due to accumulation of both woody biomass and organic matter in the soil [61], but it is still debated whether soil C stocks generally will increase following afforestation [15,62,63]. Vesterdal et al. [64] found that C sequestration occurred mainly in the biomass of trees within a short time span of about 30 years. They also noted that soil C stores were clearly higher in older plantations. Current research on poplar plantations, established on former agricultural land, found losses or no changes in SOC during the first 5e12 years [40,65]. However, data concerning spatial and temporal variability in the SOC pool under short rotation woody crops are limited due to lack of well-designed longterm studies [66]. Whether a soil becomes a source or a sink for C is strongly influenced by former land use, and also by soil disturbing activities such as harvest [e.g. [40,67]].
4.2.
Future potential
If 400,000 ha of abandoned arable land in Sweden were planted with willow and poplar, about 1.5 Tg C would be sequestered annually in woody biomass, including both above and below ground production, and 0.2 Tg in soils. This would be nearly one tenth of annual anthropogenic emissions of C in Sweden today. In the Swedish forests, which cover 22.7 Mha of the country, the annual sequestration of C to soils has been estimated to 4-5 Tg [58]. The present estimates show the potential of fast-growing tree plantations on arable land to mitigate the effect of CO2 over a short time span. In a longer perspective such plantations may provide fuel in a renewable system under environmental sustainable conditions. A study comparing rotation periods under Swedish conditions, found that short rotations (15 years) of Populus tremula L. would optimize the reductions in CO2 emissions to the atmosphere [68]. When regarding the costeffectiveness of plantations aimed at capturing and storing C, it was suggested that short-rotation bioenergy plantations may be more economically efficient for stringent C targets (around 400 ppm) compared to long-rotation forests [69]. Results concerning the positive effects of bioenergy crops on C
93
sequestration, planted on potentially available land area, are also reported from other parts of the world [66,70,71].
4.3.
Future research in Sweden
To meet the growing demand for biofuels in Sweden, and to increase the knowledge concerning fast-growing tree species planted on arable land, an experiment was established in 2009. The purpose is to test cultivation practices, involving shorter rotation periods and fewer management measures than currently recommended for pulp and timber production. Six tree species, including willow and poplar, suitable for biofuel production were planted with a spacing adapted to the specific species in order to optimize productivity. The experiment was established on five different latitudes, from north to south, in Sweden. Initial samplings and analyses of the soil, including all sites, have been performed. The main objectives of the soil research activities are to [72]: follow the sequestration of C in biomass and soil for different tree species during afforestation of arable land, follow the process over a north-south gradient in Sweden which will show the climatic impact on C sequestration rates, evaluate the species-specific effects on soil chemical parameters and use the results to formulate recommendations for future actions such as liming, nitrogen fertilization or return of ashes.
5.
Conclusions
Willow and poplar plantations on abandoned arable land will most probably increase the rate of C sequestration and thereby mitigate the negative effects of increasing atmospheric CO2 concentrations. The sequestration rates will be affected by former land use, climate and management of the plantations. Those effects on C sequestration rates need further investigations and more long-term studies are desirable. Changes in SOC are difficult to estimate and in most cases there are no soil data available from the time before afforestation. Further research is needed on this subject. The magnitude of the contribution of root litter to soil C sequestration is relatively unknown. More studies concerning root production, including fine root turnover rates, and estimates of decomposition rates are needed. The fate of DOC also needs further investigations that include data from the time before afforestation, under different climatic conditions, and for different species and management.
Acknowledgements Preparation of this paper was funded by The Swedish Energy Agency and The Swedish Research Council Formas. I thank Lars Rytter and four anonymous referees for helpful discussions and valuable comments on the manuscript.
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