Carbon reservoir and biomass in Canadian prairie shelterbelts

Transcription

1 Agroforestry Systems 44: , Kluwer Academic Publishers. Printed in the Netherlands. Carbon reservoir and biomass in Canadian prairie shelterbelts J. KORT* and R. TURNOCK Agriculture and AgriFood Canada, PFRA Shelterbelt Centre, P.O. Box 940, Indian Head, Saskatchewan, Canada, S0G 2K0 (*Author for correspondence: Key words: carbon offsets, carbon sequestration, carbon sinks, greenhouse gases, windbreaks Abstract. Greenhouse gases in the atmosphere, mainly carbon dioxide (CO 2 ), can be mitigated by the planting of trees and shrubs. Appropriate agroforestry practices in Saskatchewan include field and farmyard shelterbelts, wildlife plantations, poplar plantations and managed woodlots. A study was conducted to determine the amount of carbon held in prairie shelterbelts. The effect of the soil type and tree species on biomass and carbon content was measured in shelterbelts in the brown, dark brown and black soil zones of Saskatchewan. For some of the main shelterbelt species, the mean aboveground carbon content was 79 kg/tree (32 t/km) for green ash, 263 kg/tree (105 t/km) for poplar, 144 kg/tree (41 t/km) for white spruce and 26 t/km for caragana. In the brown and the dark brown soils, which are more arid than the black soil zone, trees had 60.6% and 65.5%, respectively, of the biomass and carbon content of trees and shrubs in the black soil zone. Improved, fast-growing poplar clones had the greatest biomass at maturity and fixed the greatest amount of carbon. Simple equations were developed to calculate the carbon contents of prairie shelterbelts, based on easily measured tree or shrub parameters. This paper will discuss the results of this particular study and the broader implications of this work. Introduction Carbon dioxide concentration in the earth s atmosphere has increased from 280 ppm in pre-industrial times to 370 ppm in 1995, due mainly to human activities (IPCC, 1995). As a result of the increased concentrations of greenhouse gases, the Intergovernmental Panel on Climate Change (IPCC) concluded that: The balance of evidence suggests a discernible human influence on global climate (IPCC, 1995). For this reason, it is important to evaluate human activities that have the potential to reduce or counteract these effects. Natural forests, forest plantations and agroforestry plantations are recognized as an important reservoir for carbon. The IPCC says specifically that A number of measures could conserve and sequester substantial amounts of carbon (approximately GtC in the forestry sector alone) over the next fifty years. In the forestry sector, measures include sustaining existing forest cover; slowing deforestation; natural forest regeneration; establishment of tree plantations; promoting agroforestry (IPCC, 1995). This study was conducted to determine the quantity of carbon stored in prairie agroforestry plantations, mainly in shelterbelts. Shelterbelts have been planted on the Canadian prairies since 1903, as windbreaks to protect soils, crops and farmyards (Howe, 1986). These shel-

2 176 terbelts consist of a variety of tree and shrub species including conifers and deciduous trees as well as drought hardy deciduous shrubs. The more drought hardy species are used in the southern and western prairies, which have higher moisture deficits, while less drought hardy, but often faster-growing species are used in the black chernozemic soils in the northern and eastern prairies. Brandle et al. (1992) estimated the amount of carbon that could be sequestered in the aboveground biomass of shelterbelts in the United States. Their estimates for 20-year old, single row shelterbelts were 9.14 t/km of carbon for conifers, 5.41 t/km for hardwoods and 0.68 t/km for shrubs. The authors recognized that trees and shrubs at this age had reached only 30 to 60 percent of their final height and that their biomass and carbon content at maturity would also be substantially greater. Materials and methods Established shelterbelts were identified in prairie chernozemic soils in Saskatchewan and Manitoba in each of the brown, dark brown and black soil zones (Figure 1). In these regions, five common shelterbelt species, caragana (Caragana arborescens Lam.), Manitoba maple (Acer negundo L.), Siberian elm (Ulmus pumila L.), hybrid poplar (Populus x deltoides) and green ash (Fraxinus pennsylvanica Marsh.) were sampled in the fall and winter of 1995 and In the case of Manitoba maple, Siberian elm, poplar and green ash, forty trees in each shelterbelt were randomly selected and measured for height, number of stems, crown width and diameter at breast height (DBH). Three of the measured trees of each species were selected in each of three shelterbelts in each region. These trees were then cut at ground level and the total tree fresh weight was determined by loading each tree onto a trailer that was weighed at a nearby grain elevator. This work was done while the deciduous trees were in the leafless, dormant state as it was considered that the carbon contained in the leaf litter was a short-term pool compared to the wood component. The sampled shelterbelts ranged from 17 to 90 years of age, with 72% of the shelterbelts between 30 and 60 years of age. Although this age variation undoubtedly added to the variability of the results, analysis of results by age could not be done within this project but will be addressed in a future project. Caragana was sampled by cutting all aboveground biomass in a 10 m length in each of three randomly chosen locations in selected shelterbelts. Twentyfour shelterbelts which appeared to be representative of the shelterbelts in the immediate area, were sampled including eight in each of the black, dark brown and brown soil zones. The shelterbelts varied from 40 to 65 years in age. The cut stems from each sub-sample were weighed at a nearby grain elevator. Shelterbelt height and width were also measured in each case, as well as the number of stems in each sub-sample and their average diameter. Sub-samples of stems and branches for these deciduous trees and caragana

3 177 Figure 1. Shelterbelt carbon and biomass sampling locations in Saskatchewan, Canada. were taken by cutting three discs from each tree and weighing them fresh, then weighing them dry to determine the tree s fresh weight to dry weight ratio. The discs for each tree included a basal disc, a mid-stem disc and a branch disc. To determine the disc fresh weight, the discs were put into a plastic bag immediately after cutting and weighed as soon as possible in the laboratory. The discs were dried in a forced-air drying cabinet at C and weighed intermittently until no further weight loss was observed. Sub-samples of green ash, Manitoba maple, Siberian elm, caragana and poplar were sent to a wood analysis laboratory to determine carbon content in the wood. For caragana, three subsamples were sent for analysis to determine whether there was any apparent difference in carbon content by soil zone. The results (50.2%, 50.1% and 49.9% for the black, dark brown and brown soil zones, respectively) showed little variability so that a single carbon content was determined for each of the other deciduous species based on a pooled wood sub-sample. The sub-sample for each species consisted of five

4 178 logs of cm diameter randomly selected from all the cut material. Carbon content at the laboratory was determined by combustion followed by gas chromatography and measurement of carbon dioxide using a thermal conductivity detector. Coniferous species sampled included Scots pine (Pinus sylvestris L.), white spruce (Picea glauca (Moench.) Voss.) and Colorado spruce (Picea pungens Engelm.). All of the conifer sampling was done at Indian Head, Saskatchewan in the black soil zone. Tree height, crown spread and DBH were measured. Forty trees were measured in each of four shelterbelts for each conifer species. Three of the forty trees measured in each shelterbelt were cut in the fall and weighed immediately, a total of twelve trees for each species. The weights included the weights of the needles. Disc sub-samples of the stems and the branches of the felled trees were measured fresh and dry to determine moisture content. Shelterbelt age varied from 33 to 74 years of age. Other deciduous shrubs that were sampled included choke cherry (Prunus virginiana var. melanocarpa (Sarg.)), villosa lilac (Syringa villosa L.), buffaloberry (Shepherdia argentea Nutt.) and sea buckthorn (Hippophae rhamnoides L.). For these species, all sampling was done at Indian Head, Saskatchewan. Three shelterbelts were sampled for each of choke cherry, villosa lilac and buffaloberry while two sea buckthorn shelterbelts were sampled. Shelterbelt ages ranged between 17 and 52 years. Height and width were measured in each shelterbelt. Three sub-samples, each 5 m in length, were cut in each shelterbelt and weighed. Less material was sampled for these species than for caragana due to a shortage of shelterbelts. Stem and branch pieces were randomly selected and weighed fresh and after drying to determine moisture content. Numbers of stems and their average DBH were also determined. For the trees and shrubs that had been cut and weighed, regression analysis was used to determine what easily measured tree or shrub parameters were most important in determining aboveground biomass. The results of the regression analysis were used to develop equations by which measured parameters could be used to calculate tree or shrub aboveground biomass. Using the equations so developed, the biomass was calculated for the forty randomly selected trees in each shelterbelt. The mean calculated biomass of these trees was taken as the mean tree biomass within that shelterbelt. Results For the deciduous and coniferous tree species, linear regression analysis showed that stem cross-sectional area at breast height was the growth parameter that best related to aboveground biomass. This parameter for multi-stemmed trees was calculated by summing the cross-sectional area at breast height for all of the stems. Multiple regressions which included height as a parameter did not improve the regression coefficients (R 2 values).

5 179 Relationships between aboveground biomass and stem cross-sectional areas at breast height were determined for all tree species (Figure 2 a g). For shrub species, the shelterbelt height, width and length, when multiplied together to give a shelterbelt volume (i.e. the space occupied by the growing shelterbelt) proved to be the best predictor of aboveground biomass. Stem cross-sectional area at breast height was difficult to determine for shrub species because of the large number of stems and the variability in stem diameter and it was not as closely related to aboveground biomass as was shelterbelt volume. Relationships between biomass and shelterbelt volume and the associated regression R 2 values for shrubby shelterbelts are shown in Figures 3 (a d). Sea buckthorn is not shown in this figure as only two sea buckthorn shelterbelts were sampled. The resulting biomass equations for all twelve species, shown in Table 2, were used to calculate the mean aboveground biomass values for the sampled shelterbelts. In the tree shelterbelts, the calculations were based on the stem cross-sectional areas of the forty randomly selected trees in each shelterbelt. The mean aboveground biomass of deciduous trees ranged from kg/tree for green ash to kg/tree for hybrid poplar (Table 1). Biomass values for the coniferous species ranged from kg/tree for Scots pine to kg/tree for white spruce. Shrub species ranged from a mean of 21.3 t/km of aboveground biomass for sea buckthorn to a mean of 51.6 t/km for caragana. Since the shelterbelts varied considerably in age, results were likely much more variable than they would have been if they had all been of the same age. There was also no attempt to correlate shelterbelt biomass to other factors that could significantly affect the standing biomass such as shelterbelt design, soil texture, landscape topography, or maintenance intensity. The species which were sampled in all three soil zones generally had a lower biomass in the drier brown soil zone, an intermediate biomass in the dark brown soils and a higher biomass in the black soils (Figures 4 and 5). One exception to this was hybrid poplar grown in the brown soil zone where the only poplar that could be found occurred in depressional areas where ground water was available to the tree roots. In this case, since water was not limiting to growth, the trees responded positively to the growing conditions in this region. Under the conditions of this study, it was found that, in brown and dark brown soils, the aboveground biomass were 62% and 72%, respectively, of that in the black soil zone (Figure 5). The aboveground biomass of the coniferous tree species and the other shrub species listed previously are shown in Table 1. These species were sampled in one location only in the black soil zone at Indian Head, Saskatchewan. It is expected that the biomass for these species in shelterbelts in the brown and dark brown soil zones would be less than those in the black soil zone similar to the ratios found for the other species (Figure 5). The carbon content of those species for which samples were analyzed varied from 48.0% to 50.1% (Table 3). It was therefore assumed that the carbon percentage in the wood of the species which were not analyzed (conifers and

6 180 Figure 2. Linear regressions for biomass of shelterbelt tree species in Saskatchewan, Canada, as a function of stem cross-sectional area. Regression lines were forced through the origin.

7 181 Figure 3. Linear regressions for biomass of shelterbelt shrub species in Saskatchewan, Canada, as a function of shelterbelt volume. Regression lines were forced through the origin. Table 1. Shelterbelt parameters and biomass contents of shelterbelts in Saskatchewan, Canada. Species Mean Mean Mean Diameter Mean shelterbelt shelterbelt shelterbelt at breast aboveground age (yrs) height (m) width (m) height (cm) biomass (kg/tree) Green ash Manitoba maple Hybrid poplar Siberian elm White spruce Scots pine Colorado spruce Choke cherry N/A a Villosa lilac N/A a Buffaloberry N/A a Caragana N/A a Sea buckthorn N/A a a Aboveground biomass values expressed in terms of kg/10 metres of shelterbelt. shrubs other than caragana) was 50% (Freedman and Keith, 1995). These carbon contents were then used to calculate the aboveground carbon content of shelterbelts of the species studied (Table 3). It has been reported that the

8 182 Table 2. Equations for determination of aboveground biomass for tree and shrub shelterbelt species in Saskatchewan, Canada. Species Green ash Manitoba maple Hybrid poplar Siberian elm White spruce Scots pine Colorado spruce Choke cherry Villosa lilac Buffaloberry Caragana d Sea buckthorn Biomass equation ABG a = CSA b ABG = CSA ABG = CSA ABG = CSA ABG = CSA ABG = CSA ABG = CSA ABG = SBV c ABG = SBV ABG = SBV ABG = SBV ABG = SBV a ABG = Aboveground biomass in kg/tree for tree species and in kg/5m shelterbelt length for shrub species. b CSA = Cross-sectional area (cumulative for multi-stemmed trees) of the tree s stem at breast height. c SBV = Shelterbelt volume calculated from the shelterbelt width, height and length. d Caragana sub-samples were 10 m in length but were expressed in this table on a 5 m basis to be consistent with the other shrub species. root biomass for deciduous trees was 40% of the aboveground biomass (Freedman and Keith, 1995), 30% for coniferous trees (Keyes and Grier, 1981; Van Lear and Kapeluck, 1994) and 50% for the shrubs (Young et al., 1987). Using these values it would be possible to calculate the total shelterbelt biomass by multiplying the aboveground carbon value by the appropriate factor to account for root biomass. Although the shelterbelts sampled varied in age from 17 to 90 years of age, they were considered to be well-established because of their size and condition at sampling. Shelterbelt age was likely an important uncontrolled variable in this study which affected shelterbelt biomass. Biomass was also likely affected by soil texture, the degree of maintenance and care that shelterbelts received, disease, insects, defoliation and other factors. Further work is required to determine how shelterbelt biomass depends on shelterbelt age and these other factors. Conclusions Established shelterbelts represent a significant carbon reservoir on the agricultural prairie landscape. In this study, the range of aboveground carbon stored in shelterbelts was from 11 t/km for sea buckthorn, a small shrub, to 105 t/km for hybrid poplar. Conifer shelterbelts in this study held 24 to 41 t/km of carbon. Green ash and caragana are the main field shelterbelt species

9 183 Figure 4. Mean biomass for tree and shrub species in three prairie soil zones in Saskatchewan, Canada. Figure 5. Relative biomass in shelterbelts in three prairie soil zones in Saskatchewan, Canada, based on data for caragana, green ash, Manitoba maple, Siberian elm and hybrid poplar.

10 184 Table 3. Aboveground carbon contents a of shelterbelts in Saskatchewan, Canada. Species Tree Carbon Brown soil Dark brown soil Black soil Mean spacing content (m) (%) kg/tree t/km kg/tree t/km kg/tree t/km kg/tree t/km Deciduous trees Green ash Manitoba maple Hybrid poplar Siberian elm Coniferous trees Colorado spruce b White spruce b Scots pine b Shrubs Caragana Choke cherry b Villosa lilac b Buffaloberry b Sea buckthorn b a Belowground carbon is not included but can be calculated assuming it to be, for deciduous, coniferous and shrub shelterbelts to be 40%, 30% and 50% of the aboveground carbon content, respectively. b Carbon percentages were not measured but were estimated at 50%.

11 185 used on the prairies and were found to hold 32 and 26 t/km, respectively. The amount of carbon held in shelterbelts was found to depend on soil zone and can be expected to depend, also, on soil texture, shelterbelt age, location in the local topography, and shelterbelt condition. The proportion of total tree biomass in roots has not been extensively studied because of the labour and resources required to excavate and measure roots so that quantification of this important component of shelterbelt biomass, reported at 30 50% of aboveground biomass, is based on limited data presented in the literature. For deciduous and coniferous trees, stem cross-sectional area at breast height was found to be well related to aboveground biomass. This result is expected to be valid for healthy, middle-aged to mature trees but may no longer be valid for deteriorating, older trees since biomass losses may equal or exceed biomass accumulation due to the increase in dead stems and branches and the development of rot in the trees heartwood. For shrub species, biomass was best related to shelterbelt volume, that is, the space occupied by the shelterbelt. In general, this can be explained by the tendency of shrubs to expand laterally until their growth is limited by contact with the crowns of neighbouring shrubs, especially in the case of shrubs which have a suckering growth habit. As the total leaf area available for photosynthesis would then be the same, independent of the number of shrubs in the shelterbelt, the biomass accumulation would depend more on the growing conditions. Moisture availability is an important determinant of tree and shrub growth and this study showed that biomass and carbon accumulation is greater in black soils in northern and eastern Saskatchewan than in the more arid brown soils in southwestern Saskatchewan and the dark brown soils of central Saskatchewan. Similarly, it may be expected that other factors that affect moisture availability to the trees, such as soil texture, soil salinity and location of the trees in the local topography are likely to affect biomass and carbon accumulation in trees and shrubs. Past literature (Brandle et al., 1992) has indicated that shelterbelts on the prairies control wind erosion, protect farmyards, trap snow and provide wildlife habitat. Their value as a carbon reservoir, therefore, is an incremental benefit which needs to be recognized, especially as the nations of the world attempt to stabilize and reduce net carbon emissions in order to slow global warming. Sedjo (1989) indicated that 465 million hectares of new forests would be needed worldwide to offset annual increases of 2.9 billion tonnes in carbon emissions while Dudek and LeBlanc (1990) suggested that treeplanting in the United States under the Conservation Reserve Program (CRP) could contribute significantly to sequestering carbon emissions from new electricity-generating stations. Brandle et al. (1992) indicated that there was a need for 115,000 ha of shelterbelts in the United States. In 1990, the total emissions from the Canadian Prairie Provinces of Alberta, Saskatchewan and Manitoba was 45.6 million tonnes of carbon (Environment Canada, 1995). By 1995, national emission levels had increased by 9.2% over

12 levels (Environment Canada, 1997) which, if applied to the Prairie Provinces would equal an increase of 4.2 million tonnes of carbon. A shelterbelt planting program of six million trees and shrubs per year, based on the results of this study, is estimated to potentially sequester 0.4 million tonnes of carbon per year. From this perspective, it is apparent that shelterbelts have the potential to contribute a modest but significant amount to the goal of limiting net carbon emissions, while, at the same time, fulfilling their other environmental roles. Agroforestry plantations, such as shelterbelts, are clearly a quantifiable carbon reservoir. However, the duration of the carbon offset that a shelterbelt represents depends on its lifespan and its fate. If the shelterbelt was simply destroyed or removed at the end of its lifespan, the carbon removed from the atmosphere would be returned to the atmosphere. If wood from the shelterbelt was used in long-term products such as lumber, the duration of the offset could be lengthened. If the biomass in the shelterbelts was used as a fuel to substitute for fossil fuels in heating or electricity-generating systems, the offset could be considered to be a permanent one. If the shelterbelt was renovated after biomass harvesting, the fixation of carbon in the shelterbelt could be considered to be an on-going process and the fossil fuel replaced by each harvest could be considered an additional offset. Clearly the fate and management of a shelterbelt or any other agroforestry planting will determine its value as a carbon offset. References Brandle JR, Wardle TD and Bratton GF (1992) Opportunities to increase tree planting in shelterbelts and the potential impacts on carbon storage and conservation. In: Sampson RN and Hair D (eds) Forests and Global Change, Vol. 1: Opportunities for Increasing Forest Cover, Ch 9, pp American Forests, Washington DC Dudek DJ and LeBlanc A (1990) Offsetting new CO 2 emissions: a rational first greenhouse policy step. Contemporary Policy Issues VIII (July): Environment Canada (1995) Canada s National Action Program on Climate Change. Government of Canada. Ottawa, Ontario Environment Canada (1997) Canada s Second National Report on Climate Change. Government of Canada, Ottawa, Ontario Freedman B and Keith T (1995) Planting trees for Carbon Credits. Publ by Dalhousie University, Halifax, Nova Scotia, 42 pp Howe JAG (1986) One hundred years of prairie forestry. Prairie Forum 11(2): IPCC (1995) IPCC Second Assessment: Climate Change Publ by World Meteorological Organization (WMO) and United Nations Environment Programme (UNEP) Keyes MR and Grier CC (1981) Above- and below-ground net production in 40-year-old Douglas fir stands on low and high productivity sites. Can J For Res 11: Sedjo RA (1989) Forests to offset the greenhouse effect. J of Forestry (July): Van Lear DH and Kapeluck PR (1994) Above- and below-stump biomass and nutrient content of a mature loblolly pine plantation. Can J For Res 25: Young A, Cheatle RJ and Muraya P (1987) The potential of agroforestry for soil conservation. Part III. Soil changes under agroforestry (SCUAF): A predictive model. ICRAF Working Paper No. 44. International Council for Research in Agroforestry, Nairobi, Kenya, 90 pp