Litter decomposition rates in Canadian forests

Global Change Biology (1999) 5, 75–82

Litter decomposition rates in Canadian forests

T . R . M O O R E , * J . A . T R O F Y M O W, † B . T AY L O R , ‡ C . P R E S C O T T , § C . C A M I R E , ¶L . D U S C H E N E , * * J . F Y L E S , † † L . K O Z A K , ‡ ‡ M . K R A N A B E T T E R , § § I . M O R R I S O N , §M . S I L T A N E N , ¶ ¶ S . S M I T H , * * * B . T I T U S , † † † S . V I S S E R , ‡ ‡ ‡ R . W E I N § § § andS . Z O L T A I ¶ ¶*Department of Geography and Centre for Climate and Global Change Research, McGill University, 805 Sherbrooke St. W.,Montreal, QC, H3A 2K6, Canada, †Pacific Forestry Centre, Canadian Forest Service, 506 West Burnside Rd., Victoria, BC,V8Z 1M5, Canada, ‡Taylor Mazier Associates, R.R. #3, St. Andrews, NS, B0H 1XO, Canada, §Faculty of Forestry,University of British Columbia, Vancouver, BC, Canada, ¶Faculte de Foresterie et Geomatique, Universite Laval, Ste. Foy, QC,Canada, **Great Lakes Forestry Research Centre, Canadian Forestry Service, Sault Ste. Marie, ON, Canada, ††Department ofNatural Resource Science, McGill University, Montreal, QC, Canada, ‡‡Pacific Forestry Centre, Canadian Forest Service,506 West Burnside Rd., Victoria, BC, V8Z 1M5, Canada, §§B.C. Ministry of Forests, Smithers, BC, Canada,¶¶Northern Forestry Centre, Canadian Forestry Service, Edmonton, AB, Canada, ***Agriculture Branch, Government of Yukon,Whitehorse, YK, Canada, †††Newfoundland Forestry Centre, St. John’s, NF, Canada, ‡‡‡Department of Biology, University ofCalgary, Calgary, AB, Canada, §§§Department of Biological Sciences, University of Alberta, Edmonton, AB, Canada

Abstract

The effect of litter quality and climate on the rate of decomposition of plant tissues wasexamined by the measurement of mass remaining after 3 years’ exposure of 11 littertypes placed at 18 forest sites across Canada. Amongst sites, mass remaining was stronglyrelated to mean annual temperature and precipitation and amongst litter types the ratioof Klason lignin to nitrogen in the initial tissue was the most important litter qualityvariable. When combined into a multiple regression, mean annual temperature, meanannual precipitation and Klason lignin:nitrogen ratio explained 73% of the variance inmass remaining for all sites and tissues. Using three doubled CO2 GCM climate changescenarios for four Canadian regions, these relationships were used to predict increasesin decomposition rate of 4–7% of contemporary rates (based on mass remaining after3 years), because of increased temperature and precipitation. This increase may bepartially offset by evidence that plants growing under elevated atmospheric CO2concentrations produce litter with high lignin:nitrogen ratios which slows the rate ofdecomposition, but this change will be small compared to the increased rate ofdecomposition derived from climatic changes.

Keywords: climate change, decomposition, forests, lignin, litter, nitrogen

Received 27 October 1997; revised version received 23 February 1998 and accepted 20 April 1998

Introduction

Decomposing plant litter and soil organic matter play animportant role in the terrestrial global carbon (C) cycle(Schimel 1995). Of the 1500 Pg (1015 g) of global soilorganic C and 55 Pg C of global litter, about 20 and 70%,respectively, are found in boreal and temperate forests(Schlesinger 1997). Global potential net primary produc-tion in boreal and temperate forests amounts to 13 Pgannually (Melillo et al. 1993). Litterfall in Canadian forestsis estimated to be 17 Tg (1012) y–1, from a biomass of12 Pg (Kurz et al. 1992). The decomposing litter eventually

Correspondence: J. A. Trofymow, fax: 11/250-363-0775e-mail [email protected]

© 1999 Blackwell Science Ltd. 75

forms soil organic matter and the rate of decompositionis dependent on environmental controls such as climate,litter quality and soil characteristics.

Although the influences of climate and litter qualityon litter decomposition rates are well established (e.g.Meentemeyer 1978, 1984; Melillo et al. 1982; Meentemeyer& Berg 1986; Taylor et al. 1989, 1991; Aber et al. 1990;Berg et al. 1993; Aerts 1997), the findings are limited bythe ecological range of sites examined and the smallnumber of tissue types used. The Canadian IntersiteDecomposition Experiment (CIDET) presents the firstdata from a wide range of tissues (tree leaves and needles,herbs and wood) decomposing at sites ranging from the

76 T . R . M O O R E et al.

Fig. 1 The location of the 18 sites and the distribution of ecoclimatic provinces of Canada (after Ecoregions Working Group 1989).

arctic through forests to the prairie border. This studyshows that decomposition rates of 11 litter types over3 years at 18 sites across Canada can be related to simpleclimatic parameters (mean annual temperature and pre-cipitation) and litter quality, primarily the ratio betweenthe Klason lignin and nitrogen concentrations (Couteauxet al. 1995). These relationships are then used to predictthe effect of climatic change on litter decompositionrates in Canadian forests, assuming changes in annualtemperature and precipitation from three doubled CO2

General Circulation Model (GCM) scenarios, and changesin litter quality, resulting from doubled atmospheric CO2

concentrations.

Materials and methods

Rates of litter decomposition were determined at 18upland forest sites, representing the major ecoclimaticprovinces and regions across Canada (Fig. 1, Table 1).

© 1999 Blackwell Science Ltd., Global Change Biology, 5, 75–82

Details of the study, the sites and methods are containedin Trofymow (1998). Ten different kinds of litter werecollected in litter traps and from senescent tissues froma range of forest types (Table 2). In addition, woodenblocks (10 3 5 3 2 cm) were obtained from a singlewestern hemlock log, cut to avoid branch knots andsapwood. The litter was allowed to air-dry, thoroughlymixed and subsampled to determine the air-dry to oven-dry conversion factor. Characterization of the litter typeswas performed using total elemental analysis (for C, N,S, P, Ca, Mg and K), wet proximate analysis (for non-polar extractable, water-soluble, acid-soluble carbo-hydrates, ash and Klason lignin fractions) and 13C CPMASNMR analysis of C fractions on a composite sample thathad been milled to pass through a 0.2 mm mesh. Themethods employed are described in Trofymow et al.(1995).

Litter bags were made from 20 3 20 cm polypropylenefabric with openings of 0.25 3 0.5 mm. Bags were filled

L I T T E R D E C O M P O S I T I O N I N C A N A D I A N F O R E S T 77

Table 1 Litter mass remaining, averagedacross all 11 litter types, at 18 uplandsites after 3 years: figure in parenthesesis the standard error of the mean for alllitter bags collected at each site.Numerals in bold refer to the site inFig. 2.

Mass

Site remaining (%) Latitude Longitude

Chapleau, ON 12 52.8 (4.7) 47°389N 83°149WGander, NF 18 58.2 (6.2) 48°559N 54°349WGillam, MN 11 80.1 (4.5) 56°199N 94°519WHidden Lake, BC 6 59.1 (4.5) 50°339N 118°509WInuvik, NWT 1 86.9 (3.0) 68°199N 133°329WKananaskis, AB 7 68.1 (3.9) 51°009N 115°009WMorgan Arboretum, QC 14 44.1 (4.0) 45°259N 73°579WMontmorency, QC 15 60.0 (5.7) 47°199N 71°089WNelson House, MN 10 79.8 (4.3) 55°559N 98°379WPetawawa, ON 13 52.6 (3.5) 45°559N 77°359WPort McNeill, BC 4 46.5 (5.7) 50°369N 127°209WPrince Albert, SK 9 75.7 (4.1) 53°139N 105°589WRocky Harbour, NF 17 52.0 (5.1) 49°329N 57°509WSchefferville, QC 16 70.4 (5.0) 54°529N 66°399WShawnigan, BC 5 43.4 (3.7) 48°389N 123°429WTermundee, SK 8 71.9 (4.7) 51°509N 104°559WTopley, BC 3 68.6 (5.0) 54°369N 126°189WWhitehorse, YK 2 82.3 (3.6) 60°519N 135°129W

Table 2 Litter mass remaining in 11 tissue types, averaged across all 18 sites, after 3 years; figure in parentheses is the standard errorof the mean for all sites and original N content and Klason lignin:N ratio. Letters in bold refer to the tissue types in Fig. 2.

Mass remaining N KlasonLitter type (%) (mg g–1) lignin:N

Aspen leaves (Populus tremuloides) Dpt 56.1 (2.9) 6.7 21.4Beech leaves (Fagus grandifolia) Dba 70.9 (2.9) 7.1 39.4Bracken fern (Pteridium aquilinum) Fbf 64.5 (3.2) 8.8 37.4Black spruce needles (Picea mariana) Csb 56.2 (3.6) 7.3 38.7Douglas fir needles (Pseudotsuga menziesii) Cdc 67.6 (3.6) 7.0 43.3Fescue grass (Festuca halii) Gfh 41.8 (1.7) 7.1 15.7Jack pine needles (Pinus banksiana) Cpj 60.4 (3.5) 12.8 25.6Tamarack needles (Larix laricina) Cll 66.0 (3.5) 5.9 40.6White birch leaves (Betula papyrifera) Dbw 49.4 (3.6) 7.2 33.3Western hemlock wood blocks (Tsuga heterophylla) Whw 88.8 (3.4) 2.4 122.6Western red cedar needles (Thuja plicata) Ctp 75.3 (3.6) 6.4 55.5

with 10 g of air dry litter, except for those containingwestern hemlock wooden blocks, which weighed about50 g. Litter bags were placed at the field sites in autumn1992 at four plots at each of the 18 sites. Each litter typewas placed at all sites. The bags were placed on the forestfloor, in contact with the underlying litter; where therewere standing grasses of thick lichen layers, the bagswere placed so that they were in contact with the organiclayers or moss surface. Single bags from the four plotsat each site were collected each year, at the same time asthe original emplacement. After collection, the litter bagswere oven-dried at 55 °C, the litter remaining in the bagwas weighed and the proportion of the original littermass calculated. In this paper, we use the data collectedfor decomposition after 3 years’ exposure, representedby the percentage of the original mass that remained.


The data, as quadruplicate samples for each of 11 littertypes at each of 18 sites, are used to identify the effect ofclimate (using the mean value of the 11 litter types at eachsite) and tissue quality (using the mean value of the 18 sitesfor each litter type). Data for each individual bag (n 5 789,three bags were lost) is then used to identify the combinedeffect of climate and litter quality on decomposition rate.The 3-year data were used because they showed majordifferences amongst sites and litter types, and representthe intermediate stage of decomposition.

Climatic data were collected from the nearest long-term meteorological station, as the 30-year mean annualtemperature and precipitation. An estimate of annualactual evapotranspiration for the 18 sites was derivedfrom the map in the Hydrological Atlas of Canada(International Hydrologic Decade 1978).


Fig. 2 Relationship between percentage mass remaining after3 years and mean annual temperature among the 18 forest sites.The line represents the regression from Table 3 and sites areidentified by numerals (Table 1).

Results and discussion

Controls on litter decomposition

The average litter mass remaining at each site after3 years ranged from 43 to 87%, generally increasing fromsouthern to northern sites (Table 1). The relationshipbetween the climatic variables and the mass remainingwas established by regressions analysis, using SAS (SASInstitute 1989). Based on the mean mass remaining ateach site, mean annual temperature and mean annualprecipitation accounted for 72–87% of the variance (Fig. 2,Table 3). Mean annual actual evapotranspiration,although significantly related to mass remaining, pro-vided no additional explanation, probably because mostof these Canadian forest sites experience only limitedmoisture deficit during the summer. A similar relation-ship was obtained when individual litter bag data (n 5

789) were used, although the r2 (0.378) was lower becauseof the inclusion of variation among litter types andreplicates.

Tissue quality is of equal importance in determiningdecomposition rates as climate. The 11 tissue types,averaged over all sites, had between 41 and 89% massremaining after 3 years, with the fastest decompositionbeing observed for the herbs and the slowest for thewood block and slowest foliar type for western red cedar(Fig. 2, Table 2). This variation can be explained bydifferences in initial Klason lignin and nitrogen contentin the original tissues (Table 3). The higher the ratiobetween lignin and nitrogen content, the slower the initialrate of decomposition (Fig. 3). The wood block acts asan outlier: exclusion of the wood block from the regressionreduces the r2 from 0.74 to 0.65 and increases the regres-sion slope from 0.39 to 0.71. This may be related tothe physical characteristics of the block—their size anddensity, compared to the other litter types. Incorporation


Fig. 3 Relationship between percentage mass remaining after3 years and Klason lignin:N ratio among the 11 litter types. Theline represents the regression from Table 3 and litter types areidentified by letters (Table 2).

of other chemical properties of the litter (Trofymow et al.1995), such as Ca, Mg, C and S content, explained , 3.6%additional variance each, and NMR characteristics, suchas carboxyl and alkyl C content, explained a further 5and 2%, respectively, to the variance using all samples.

The observed relationship between decomposition ratesand climate and litter quality variables were combinedinto a multiple regression equation, which explained 64%of the variance of all litter bags or 73% using the meansof replicates (Table 3). The ability of these simple climaticand litter quality variables to explain differences inmedium-term litter decomposition is very good. How-ever, about 35% of the variance remains unaccounted forand there may be a number of reasons for this, beyondthe variation among replicate litter bags which accountsfor 9% of the variance:1 During the period of measurement of decomposition(1992–95), the annual climate at each site may havediffered from the long-term mean used in this analysis.2 The litter bags were placed on the forest floor or soilsurface, where the microclimate will be different fromthat recorded at the nearest meteorological station, andthis difference will vary with site.3 Soil conditions, such as acidity, nutrient availabilityand macro- and microbiological activity, will affect therate at which tissues decompose and varies from siteto site.4 There may be interaction effects amongst the regressionvariables used. For example, there is a site-specific vari-ation in the slope of the response to Klason lignin:Nratio, but this is not easily related to simple climatevariables.

Effect of global change on litter decomposition rates

Climatic change will affect the storage of organic C inCanadian forests in a number of ways. Changes in


Table 3 Relationship between litter massremaining after 3 years and climate andlitter quality variables: T 5 mean annualtemperature ( °C); P 5 mean annualprecipitation (mm year–1); L 5 lignincontent (%); N 5 nitrogen content (%);L/N 5 Klason lignin:N ratio. Figures inparentheses indicate the standard errorof the regression parameters.

Variable/Regression n r2

ClimateMass 5 65.55(1.88) – 2.39(0.37)T 18 0.722Mass 5 76.32(3.03) – 1.63(0.33)T – 0.015(0.004)P 18 0.865Mass 5 76.31(1.35) – 1.63(0.15)T – 0.015(0.002)P 789 0.378

Litter qualityMass 5 53.47(9.85) 1 0.123(0.031)L – 3.34(0.96)N 11 0.750Mass 5 45.24(3.96) 1 0.392(0.078)L/N 11 0.738Mass 5 53.40(2.82) 1 0.123(0.009)L – 3.33(0.28)N 789 0.270Mass 5 45.29(1.18) 1 0.391(0.023)L/N 789 0.265

CombinedMass 5 59.52(1.24) – 1.63(0.11)T

– 0.015(0.001)P 1 0.390(0.016)L/N 789 0.642Mass 5 59.44(2.04) – 1.63(0.18)T

– 0.015(0.002)P 1 0.392(0.027)L/N 197 0.729

Table 4 Changes in mean annual temperature ( °C) and precipitation (%) for four forest regions of Canada, based on three doubledCO2 General Circulation Model scenarios: CCC – Canadian Climate Centre GCM II (Boer et al. 1992); GISS – Goddard Institute forSpace Studies transient GCM (Russell et al. 1995); GFDL – Geophysical Fluid Dynamics Laboratory transient GCM (Manabe et al. 1991).

Model

CCC GIS GFDL

Region °C % °C % °C %

N. Alberta/N. Saskatchewan 1 4.3 1 10 1 1.9 1 13 1 3.3 1 8N. Ontario 1 4.6 1 12 1 1.8 1 6 1 3.5 1 8S.E. Ontario/S.W. Quebec 1 4.4 1 4 1 1.5 1 6 1 3.4 1 6Atlantic Provinces 1 4.0 1 1 1 2.0 1 5 1 3.4 1 10

temperature and precipitation will affect decompositionrates, and elevated atmospheric CO2 concentrations maylead to a change in litter quality. In addition, the netprimary productivity of forests can be expected to change,as well as the distribution of different forest types.

Predicted changes in mean annual temperature andannual precipitation for four regions in Canada, derivedfrom three different doubled CO2 GCM scenarios arepresented in Table 4. In general, these four regionsare likely to experience an increase in mean annualtemperature of between 1.5 and 4.6 °C and an increasein annual precipitation of between 1 and 12%. Thesechanges will affect decomposition rates. For example, thepredicted changes in temperature and precipitation fromthe CCC and GISS models results in a 3.1–8.5% decreaseafter 3 years in the mass of jack pine needles, a commonlitterfall component across Canada, compared with thepredicted mass loss using the combined equation inTable 3. The GISS transient GCM predicts smaller changesin temperature and precipitation, resulting in smallerincreases in the rates of decomposition compared withthe CCC model, averaging 3.6 and 7.5%, respectively.


Several laboratory experiments have been conductedon the effect of elevated atmospheric CO2 concentrationson plants (cf. Ceulemans & Mousseau 1994), includinglitter quality, especially nitrogen and, to a lesser extent,Klason lignin contents (see review by O’Neill & Norby1996). These results are limited by the short duration oftreatment, the small stature of most of the trees and thelimitations of N availability in the soil (Melillo 1996), andthey show variable responses (Table 5). The studiessuggest, however, that there could be relatively largedecreases in the N content and increases in the Klasonlignin:N ratio of litter, caused primarily by a decrease inN content. Changes in the Klason lignin:N ratio may affectdecomposition rates, if our finding of the importance ofthis ratio in explaining differences in decomposition rateamongst litter types can be applied to change within alitter type.

Several studies have found that decomposition rates oftissues grown at elevated atmospheric CO2 concentrationsare slower than those grown under ambient concentra-tions: in Lolium roots (Gorissen et al. 1995; Van Ginkelet al. 1996), in marsh Scirpus (Ball & Drake 1997), and


Table 5 Changes in tissue N and lignin content (%) and lignin:N ratio of plants grown under ambient and elevated atmospheric CO2concentrations (generally 350 and 500–700 ppm CO2, respectively), expressed as the ratio between elevated and ambient higher CO2treatments. nd 5 not determined.

Source Vegetation N Lignin Lignin:N

Ball & Drake (1997) Marsh Scirpus and Spartina 1.04–1.10 0.98–1.01 0.98–1.08Brown (1991) Poplar leaves 0.58–0.75 nd ndCotrufo et al. (1994) Deciduous tree leaves 0.67–0.81 1.08–1.21 1.33–1.76

Spruce needles 0.97 1.26 1.30Cotrufo & Ineson (1995) Birch roots 0.86 nd ndIneson (1995) Spruce roots 0.69 nd ndCouteaux et al. (1991) Chestnut leaves 0.52 nd ndFranck et al. (1997) Grass shoots and roots 0.80–1.79 nd ndHirschel et al. (1997) Senescent tissues 0.83–1.03 0.90–1.10 0.97–1.22Johnson (1993) Spruce needles 0.74 nd ndKemp et al. (1994) Tallgrass 0.86–1.08 0.94–1.02 0.95–1.09Luxmoore et al. (1986) Pine needles 1.0 nd ndMelillo (1996) Deciduous tree leaves 0.62–0.96 1.05–1.08 1.18–1.71Norby et al. (1986) White oak leaves 0.77 0.64 0.83O’Neill et al. (1987) Poplar seedlings 0.67 nd ndRobinson et al. (1997) Grass shoots and roots 0.89–0.93 0.89–1.02 0.96–1.15Rouhier et al. (1994) Chestnut leaves 0.64 nd ndVan Ginkel et al. (1996) Grass roots 0.65–0.81 1.12–1.13 1.18–1.72

Table 6 Predicted change (using combined regression in Table 3) in 3-year mass remaining (%) of jack pine needles using predictedchanges in mean annual temperature and precipitation (from Table 4) and an increase in Klason lignin:N ratio (L:N) of 30%, for fourregions in Canada.

Climate change alone Climate change 1 L:N

Region Current CCC GISS CCC GISS

N. Alberta/N. Saskatchewan 68.2 60.7 64.4 63.7 67.4N. Ontario 61.1 52.6 57.7 55.6 60.7S.E. Ontario/N.W. Quebec 51.0 43.6 47.9 46.6 50.9Atlantic provinces 47.9 41.3 43.9 44.3 46.9

Betula leaves and roots (Cotrufo et al. 1994; Cotrufo &Ineson 1995), but there was no significant effect in eithertallgrass prairie (Kemp et al. 1994) or spruce roots (Cotrufo& Ineson 1995). Robinson et al. (1997) found that decom-position rates of Festuca shoots and leaves grown atelevated CO2 were slower than ambient CO2 sampleswhen placed in a High Arctic site, but the difference wasreversed or insignificant at a Low Arctic site. Franck et al.(1997) found that the C:N ratio in the litter of four annualgrasses increased when grown under elevated CO2, butthat there was little effect on decomposition rates andthere was strong variability among species. Hirschel et al.(1997) found little difference in lignin:N ratio of senescedleaf litter grown under elevated CO2 concentrations,and there was little difference in decomposition rates,compared with materials grown under ambient CO2

concentrations. In experiments in which CO2 concentra-tion has been elevated under field conditions, there is


little evidence of major changes in the N content of litter(P. Canadell, pers. comm.).

Assuming a 30% increase in Klason lignin:N ratio, themaximum likely from the laboratory experiments, themass remaining of jack pine needles after 3 years’ decom-position reveals an average reduction of 4.5% for the fourregions with the CC scenario, equivalent to a decomposi-tion rate increase of 7.9%, compared with present values(Table 6). With the GISS changes in climate and Klasonlignin:N ratio, the average reduction in mass remainingis only 0.6, equivalent to an increase of 1.1% in the rateof decomposition. However, increased rates of soil Nmineralization because of warmer temperatures mayreduce the predicted increase in Klason lignin:N ratios.Thus, even in the most extreme conditions, changes inlitter quality are unlikely to compensate for increaseddecomposition rates associated with warmer and wetterclimatic conditions.


The change in net primary production (NPP) associatedwith both warmer and wetter climates and elevatedatmospheric CO2 concentrations is probably of greatersignificance to the C budget of forests. Melillo et al.(1993) used a process-based terrestrial ecosystem modelto predict global changes in NPP for major vegetationtypes. They showed that changes in climate and CO2

may lead to increases in NPP of 30–40% in boreal forestsand woodlands, whereas increases were predicted to bebetween 5 and 15% in temperate forests. As increasedNPP leads to increased rates of litter fall, this will increasethe store of C both in vegetation and on the forest floor,especially if the net increases in decomposition rateare minimal.

Acknowledgements

The study was supported by funding from the Climate ChangeProgram of the Canadian Forest Service. Technical support wasprovided by T. Brown, B. Ferris, R. Leach, E. Andersen, S. Dixon,J. Kinnis, A. Coneys and A. van Niererk. M. Weber, C. Moreal,D. Anderson, R. Trowbridge and D. White helped locate sitesor collected initial litter samples. The advice and encouragementof M. Harmon of the US LTER intersite decomposition study isalso gratefully acknowledged.

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Documents

Litter decomposition rates in Canadian forests