[Advances in Ecological Research] Litter Decomposition: A Guide to Carbon and Nutrient Turnover Volume 38 || Climatic and Geographic Patterns in Decomposition

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Climatic and Geographic Patternsin DecompositionI. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227ADVA# 2006NCElES IN ECOLOGICAL RESEARCH VOL. 38 0065-250sevier Ltd. All rights reserved DOI: 10.1016/S0065-25044/06(05)3$35.08007-II. The Microbial Response to Temperature and Moisture . . . . . . . . . . . 228III. The Influence of Climate on EarlyStage Decomposition of ScotsPine Needle Litter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229A. EarlyStage Decomposition at One Forest Stand over Time . . . 229B. Decomposition Studies in Transects with Scots Pineand Norway Spruce . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231IV. The EVect of Substrate Quality on MassLoss Rates in ScotsPine Transects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240A. Early Stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240B. Decomposition over a Transect with Scots PineMonoculturesThe Late Stage . . . . . . . . . . . . . . . . . . . . . . . . . 242C. Respiration from Humus from Scots Pine Standsin a PanEuropean Transect . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245V. The Influence of Climate on Decomposition of Norway SpruceLitter in a Transect. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250A. General Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250B. Climate Versus FirstYear Mass Loss. . . . . . . . . . . . . . . . . . . . . 251C. LigninMediated EVects on Litter Decomposition Ratesduring Late Stages of Decomposition. . . . . . . . . . . . . . . . . . . . . 252VI. A Series of Limiting Factors for Decomposing Litter . . . . . . . . . . . . 255A. Factors Influencing Lignin Degradation Rates . . . . . . . . . . . . . . 255VII. The Influence of Climate on Decomposition of Root Litter. . . . . . . . 257VIII. Litter Chemical Changes as Related to Climate . . . . . . . . . . . . . . . . . 259A. Development of Litter N Concentration with Climate inDecomposing Scots Pine Needle Litter (Transects I and II) . . . . 259B. Development of Litter Lignin Concentration with Climatein Decomposing Needle Litter . . . . . . . . . . . . . . . . . . . . . . . . . . 260I. INTRODUCTIONFor a long time, climate has been assumed to have a dominant eVect on litterdecomposition rates on a regional scale, whereas litter quality should do-minate on the local level, for example, within a stand. Thus, at a givenforest stand and climate, one should expect the massloss rates of litter tobe related primarily to its chemical and physical properties. Several studieshave shown such general relationships (Fogel and Cromack, 1977; Aber and0X228 BERG BJORN AND RYSZARD LASKOWSKIMelillo, 1982; McClaugherty et al., 1985; Upadhyay and Singh, 1985; Dyer,1986). Still, this view is oversimplified. There is a variation in weather, intemperature, and moisture in the litter environment as well as in the litterchemical composition, resulting in a large variation in decomposition ratesamong years, even within one forest stand. In addition, the substrate changesduringdecomposition (Chapter 4) andwith accumulatedmass loss, its chemicalcomposition becomes increasingly diVerent from the initial one, progressivelycreating a new substrate with new properties. When the decomposition processprogresses through time, the factors that regulate the rate of mass loss dochange. In turn, the heat and moisture delivery to the litter control the rate atwhich the decay phases can proceed. Thus, for a given litter type in one climaticregime (say, boreal climate), the early, nutrientcontrolled phasemay span overa long time, while in other regimes, this phase can pass quickly.Studies of decomposition dynamics have been performed using diVerentlitter types, at sites in diVerent climatic regimes and in diVerent forest types,and thus control by climate versus litter quality is often confounded. Fur-thermore, often only the decomposition of fresh, newly shed litter is studied,thus overemphasizing the early stage (ct. Berg et al., 1993). At broad,regional scales, climatic variables often appear to regulate decompositionrates, at least initially, whereas litter properties appear, in general, to berelatively insensitive indicators of regional patterns (Meentemeyer, 1984).When the analysis is confined, however, to one or a few sites with similarclimates, the influence of litter quality becomes apparent. With the increas-ing emphasis on understanding the impact of climate change, and the broadscale patterns of biological processes, the issue of geographic scale versusdecomposition patterns versus litter chemical composition becomes critical.This chapter focuses on litterdecomposition in standswithmonocultures andwe use results from five main transects with either only Scots pine or diVerentpine species and one with Norway spruce in which foliar litter decompositionwas studied. The results are possibly contrasting enough to illustrate thatdiVerent patterns should be expected among species under varying climates.To illustrate this, we have described the eVect of climate on diVerent decompo-sition stages, that is, early stage and late stage separately. In addition, we giveresults from a transect in which root litter decomposition was studied. We alsodescribe respiration from humus at seven sites from one of the transects.II. THE MICROBIAL RESPONSE TO TEMPERATUREAND MOISTUREThe communities of soil microorganisms encompass several thousands ofspecies in the soil of a given stand (Bakken, 1997) and have high adaptabilityto diVerent moisture and temperature regimes. This has, in part, beencommented on in Chapter 3. Still, both moisture and temperature can belimiting. At low moisture, say, below 10% waterholding capacity, waterCLIMATIC AND GEOGRAPHIC PATTERNS IN DECOMPOSITION 229supply becomes so limiting that an increase in temperature does not result inhigher microbial activity. Likewise, in an energylimited system, for example,due to low temperatures, higher moisture does not necessarily result inhigher activity. An example of this is the boreal forest.The microbial response to temperature should be regarded as the sum ofresponses from all microorganisms. Those bacteria and fungi that have theirtemperature optima at, say, 15 C are less active at 10 C and very little activeclose to 0 C. Still, at 0 C and below, there is a clear heterotrophic activitycarried out by psychrophilic microorganisms, which are of completely di-Verent species from those active at higher temperatures but normally with-out diVerences in function. In a system under a given climate, themicroorganisms thus are adapted to the prevailing climatic conditions.Further, the soil of a given forest stand under boreal or temperate climatemay have large variation in soil temperature over a year, say, from 0 C,representing unfrozen soil under a snow cover, to maybe 15 C at summer-time. The diVerent temperatures under diVerent periods support the devel-opment and maintenance of a microflora with numerous species that havetemperature optima over this whole range of temperatures.A microbial response to climate variability depends also on the availabili-ty of nutrient and carbon sources. The lack of an available carbon source oran essential nutrient as compared to the needs of the microbial communityresults in a lack of response to an increasing temperature and higher precipi-tation (Panikov, 1999). Thus, if decomposition is limited by what somewhatunspecifically is called substrate quality, a change in weather has relativelylittle eVect on the decomposition rate.III. THE INFLUENCE OF CLIMATE ONEARLYSTAGE DECOMPOSITION OF SCOTSPINE NEEDLE LITTERA. EarlyStage Decomposition at One Forest Standover TimeAt a given site, there is a clear variation in litter decomposition rates amongyears, which may be related to variation in annual weather. When local,annually collected Scots pine needle litter was incubated at its own site, thevariation among years for the firstyear mass loss as determined over 21measurements ranged from 21.1 to 33.8% (Fig. 1), the highest value being60% higher than the lowest one. However, there was no diVerence in annualmass loss between litter incubated in the spring and that incubated in lateautumn just after litter fall. Average annual mass losses for both groups wereclose to the overall average of 27.8% mass loss. This means that theFigure 1 Firstyear mass loss from Scots pine needle litter incubated annually in anutrientpoor Scots pine forest over a time range of 23 years, starting when the forestwas 120 years of age. The stand was that of the former Swedish Coniferous ForestProject (SWECON), located at Jadraas, Sweden. The first incubation was made in1973 and the latest in 2000. In those cases, the same year appears twice: oneincubation was made in May and one in October. Data from B. Berg (unpublished)and B. Andersson (unpublished). With kind permission of Springer Science andBusiness Media.230 BERG BJORN AND RYSZARD LASKOWSKIdecomposition process is generally not sensitive to the point in time forlitter fall.In the same stand, there are diVerences in decomposition rates amongperiods of the year as determined by patterns and intensity in temperatureand rainfall. A model for daily soil moisture and temperature was found topredict the early stage decomposition rates quite well over periods of months(Jansson and Berg, 1985), with R2 values ranging between 0.85 and 0.99,indicating that the variation in climate may dominate the variation in massloss at that stand. The predictive power of the two factors, namely, the soilmoisture and soil temperature combined, was clearly superior to separatesinglefactor models (Table 1). The soil climate was modeled over a period of6 years, representing a substantial variation with respect to soil moisture andtemperature and indicating that periods with high and low decompositionrates did not follow any simple pattern. Two summers were characterized aswarm with extended drought periods, whereas the other summers weremoist. The variations in soil temperatures were much more pronouncedbetween diVerent winters than between summers. Three of the winters hadsoil temperatures well below zero degrees, which also caused high waterTable 1 CoeYcients of determination (R2) obtained from correlations betweenobserved decomposition rates and diVerent soil climate estimates as independentvariableIndependent variable1st incubation yrn 92nd incubation yrn 8Both yearsn 17Actual evapotranspiration(AET)0.41 0.74 0.55Soil temperature 0.37 0.77 0.52Soil water tension 0.78 0.97 0.81Soil water content 0.68 0.96 0.77Soil temp and water tension 0.90 0.98 0.89Soil temp and water content 0.85 0.99 0.87*From Jansson and Berg (1985). Unified Scots pine needle litter was used and incubatedannually.CLIMATIC AND GEOGRAPHIC PATTERNS IN DECOMPOSITION 231tension in the soil. During the other winters, the soil was both moister andwarmer, mainly because of thicker snow packs, which prevented the uppersoil layer from freezing. Under these conditions, the soil water was alwaysunfrozen, which means that decomposition took place under the snow cover.In fact, for one of the oneyear periods, the main part of the decompositiontook place during the winter when the ground had a snow cover.As indicated in Table 10, Chapter 2, there was a certain variation in initiallitter chemical composition at this site, for example, in N and P values; still,the model based on just temperature and moisture could explain the decom-position quite well, supporting the theory that an annual variation in weath-er can be responsible for the annual variations in decomposition rate withina stand. It deserves to be emphasized that the response to temperature andmoisture was observed mainly for the early stage.B. Decomposition Studies in Transects with Scots Pineand Norway SpruceAmong studies on decomposition in diVerent climatic transects, in NorthernEurope, there are at least five using needle litter and one using root litter. Wehave indicated them in Fig. 2 and numbered them I thru VI (Textbox 1). Thedecomposition data from the transects Nos. IIV and from one for rootlitter were related to both climate and substrate quality, using actual evapo-transpiration (AET) as a climatic index (Meentemeyer, 1978). The mainclimate indices used in this book are listed in Table 2 with often usedabbreviations.Figure 2 Map of western Europe with transects indicated and numbered from Ithrough VI. Transect No. I in Scots pine forests along Sweden had local needle litterincubated at 20 stands. Transect No. II, in Scots pine forests, had unified needle litterincubated at 13 stands as did transect III with an extension to southernmost Europe,encompassing 39 pine stands. Transect IV was a latitudinal one along 52 and 53 N,ranging from Berlin in the west (12 250E) to the RussianWhite Russian border inthe east (32 370E). Transect V had about the same extension as transect I, butencompassed 14 stands with Norway spruce. A transect (No. VI) with incubated rootlitter had extension from the Arctic Circle in Scandinavia to Berlin in NorthernGermany.232 BERG BJORN AND RYSZARD LASKOWSKI1. Transects with Local Litter in Scots Pine MonoculturesInvestigating the data of transect I (Fig. 2) ranging over Scandinavia,Johansson et al. (1995) determined the eVect of climate and litterqualityvariables on massloss rates. Using longterm climatic mean values andrelating firstyear mass loss to climate variables (Table 2), they found thatof single climate factors, average annual temperature (AVGT) gave the bestTextbox 1 Description and extent of the climatic transects referred to inthe textThe northern end of thre NS transects was at the Arctic Circle in Scandinaviaor northernmost Finland and the extent varied (Fig. 2). A transect (No. I) with Scots pine stands in Scandinavia, located mainlyon till, in which local Scots pine needle litter was incubated once or twice.Twentyeight stands at 22 sites were located between 66 080N, close to theArctic Circle and 55 390N, close to the latitude of the city of Copenhagen (seealso Tables 2, 3, 8, and Figs. 2 and 7).A transect (No. II) with Scots pine stands on sediment soil, in which unifiedScots pine needle litter was incubated annually for a period of approximately 6to 19 years. The transect had 13 sites between northernmost Finland (69 450N)and central Holland (52 020N) and had highly standardized sites with nutrientpoor Scots pine stands on sandy sediments and thus on flat ground. In additionto unified litter at each of these sites, however, a special set of experimental litterwas incubated (cf. Table 4; Figs. 5, 6).A pine forest transect (No. III), located on mainly sediment soils in whichunified Scots pine needle litter was incubated. Transect No. II was included andsites with stands of stone pine, Austrian pine, maritime pine, Corsican pine, andMonterey pine. The transect with, in all, 39 sites ranged across Europe (fromnorthernmost Finland at 69 450N to southernmost Spain at 38 070N andsouthernmost Italy 39 240N (Tables 4 to 7; Figs. 3, 4, 5).A latitudinal (around 5253 N) Scots pine transect (No. IV) with increas-ing degrees of continentality, ranging from Berlin in the west (12 250E) to theRussian/White Russian border in the east (32 370E).A transect (No. V) with Norway spruce stands located on till soil in whichlocal litter was incubated once. Fourteen sites were used, located between66 220N close to the Arctic Circle in Scandinavia and 56 260N in southernmostSweden (Tables 11, 12; Fig. 10).A northeast to southwest transect (No. VI) with root litter encompassingpine sites (Scots pine and lodgepole pine) (n 25) and sites with Norway spruce(n 12), ranging from the Arctic Circle in Scandinavia to Berlin (at 52 280N).Table 13.CLIMATIC AND GEOGRAPHIC PATTERNS IN DECOMPOSITION 233fit with an R2 value of 0.536 (Table 3), and annual actual evapotranspiration(AET) gave almost as good a fit, with an R2 value of 0.523. Potentialevapotranspiration (PET) and average temperature in July (JULT) werealso significant whereas annual precipitation did not give any significantrelationship. AET has previously been distinguished as a superior climateindex at broad, continental scales (Meentemeyer, 1978, 1984; Berg et al.,Table 2 Climatic and substrate quality variables toward which litter mass loss wasregressed in the studies of decomposition in the climate transects nos. IIV and atransect with root litter (no. VI)aDescription of variable AbbreviationAverage temperature for July (C) JULTAverage annual temperature (C) AVGTTotal annual precipitation (mm) PRECIPPotential annual evapotranspiration (mm) PETActual annual evapotranspiration (mm) AETInitial concentration of water soluables (mg g1) WSOLInitial concentration of mitrogen (mg g1) NaThe climate variables, based on longterm averages were calculated according to Meentemeyer(1978) and Thornthwaite and Mather (1957). See also Berg et al. (1993). For convenience, theabbreviations are used in this chapter.Table 3 Linear relationships between firstyear litter mass loss and climate factorsin a climatic transect (No. I) from the Arctic Circle in Scandinavia (northeast) to thelatitude of Copenhagen in the southwestaClimatefactor Slope (SE) Intercept (SE) r R2 p