SCOPE 56 - Global Change: Effects on Coniferous Forests and Grasslands

3.1 INTRODUCTION

 This chapter is divided into five sections. The first section, 3.2, authored by Gower, Johnson and Berg, describes a general approach to site-specific C budgets for temperate coniferous forests and it considers one site in Sweden in some detail. The second section, 3.3, authored by Breymeyer, reviews recent literature on coniferous forests' C budgets in Europe and considers how a climate gradient affects litter production and decay in these forests. The next section, 3.4, authored by Berg, reviews soil organic matter budgets for a set of forest sites in Sweden and also for a transect from northern Sweden to central Europe. The final section, authored by Gower and Johnson, reviews C balance in coniferous forests of temperate North America. 

3.2 CARBON BUDGETS-EXAMPLES OF A MODEL AND EMPIRICAL MEASURES 

Although aboveground biomass and net primary production (ANPP) data for temperate coniferous forests of North America and the world are numerous (Cannell 1982), there are very few complete C budgets for forest ecosystems (Table 3.1). This is largely due to a lack of information on soil C flux and allocation of C to fine root production and respiration. 

The major components of a forest C budget can be estimated as follows:

Table 3.1 Carbon content and fluxes for temperate conifer forests

^aWatershed average. 
^b>5 mm diameter. 
^clncluded in coarse roots.
^d<5 mm diameter. 
^eCalculated from Figure 1 from Ryan (1990). 
Sources: (1) Kinerson et al. (1977); (2) Gholz et al. (1986); Ewel  et al: (1987a, b); (3) 
Grier et al. (1981); Vogt et al. (1980, 1982, 1983, 1990); Harmon et al; (1986); 
Edmonds (1987); Ryan (1990); (4) Grier and Logan (1977). 

Figure 3.1 Relationship between C belowground allocation, fine root production and mean annual temperature. After Gower et al. (1994b)

where C_veg is the annual accumulation of C in the vegetation, A is net daytime canopy photosynthesis by the over- and under story vegetation, R_wood is respiration for woody tissue, R_leaf is night respiration only, since most chamber measurements of photosynthesis are net photosynthesis, R_root is root respiration and D_wood, D_leaf and D_root are detritus inputs of dead wood, leaf and root tissue, respectively. The annual change in soil C (C_soil) can be described as

  where R_soil is microbial or heterotrophic respiration. Total CO₂ flux from the soil surface can be described as

however, it is extremely difficult to measure R_soil and R_root, separately. The net change in C in a forest ecosystem (vegetation + soil) or net ecosystem productivity is equal to the sum of C_veg and C_soil and can be described by combining equations (1) and (2) to yield equation (4): 

Soil surface CO₂ flux ( R_s) for conifer forests ranges from 259 to 1314 g Cm^-2yr^-1 (Raich and Nadelhoffer 1989) and is positively correlated to mean annual temperature (Figure 3.1). It is difficult to partition total soil CO₂ flux into R_soil and R_root; however, soil surface CO₂ flux data for control ( R_soil + R_root)  and trenched (R_soil) plots in warm temperate (Pinus elliottii) and cold temperate (P.resinosa) pine plantations suggests that root comprises 62 and 35% of R_s, respectively (Ewel et al. 1987b; Gower, unpublished data). Nakane et al. (1983) estimated that R_root comprised 47-51% of R_s in a 80-year-old P.densitlora forest in Japan. 

Fine root production for temperate conifer forests ranges from 30 g Cm^-2yr^-1 for a Picea rubens forest in Ontario, Canada to 710 g Cm^-2yr ^-1 for an Abies amabilis forest in Washington, USA. Although fine root production is an important component of forest C budgets, it is still one of the least understood processes. Assuming C_soil in equation (2) is near zero in the short term (e.g. year-to-year) in forests, then the total allocation of C to roots (root turnover and root respiration) can be estimated as R_soil -D_leaf + D_wood (cf. Raich and Nadelhoffer 1989). Rates of soil C accumulation (C_soil) are, in fact, typically smaller than litterfall or soil respiration rates. For example, Schlesinger (1990) calculated an average long-term soil C accumulation rate of 2.4 ± 0.7 g Cm^-2yr ^-l, compared to average litterfall rates of 62-319 g Cm^-2yr ^-l and soil respiration rates of 259-1314 g Cm^-2yr ^-1 for conifer forest ecosystems (Raich and Nadelhoffer 1989). However, forest floor accumulation rates can be of the order of 95 g Cm^-2yr^-1 (e.g. Turner 1981), making the assumption of negligible soil and litter C accumulation invalid in aggrading forest ecosystems. Using the C balance approach, Gower et al. (1994b) reported that total C allocated to roots was positively correlated to mean annual temperature; this relationship supports the previously reported relationship between fine root NPP (determined by sequential coring method) and mean annual temperature for pine forests (Figure 3.1) (Gower et al. 1994b). The difference between the two regression lines should approximate the amount of C allocated to coarse root NPP and root respiration. 

The availability of nitrogen, the most common limiting nutrient in temperate conifer forests (Gessel et al. 1973; Johnson et al. 1981), influences the assimilation and allocation of C. Three major mechanisms by which N availability increases aboveground net production are: (1) increased net photosynthesis (Smolander and Oker-Blom 1990; but see Teskey and Whitehead 1994); (2) increased leaf area index (LAI) (Vose and Allen 1988; Gholz et al. 1991; Gower et al. 1992); and (3) reallocation of C from fine roots and mycorrhizae to aboveground tissues (Keyes and Grier 1981; Vogt et al. 1990; Gower et al. 1992). 

3.2.1 One site-the SWECON site, general background and site data 

We have selected a Scots pine forest of the former Swedish Coniferous Forest Project (SWECON) to create a dynamic budget for the surface organic soil layers, also known as the F and H layers. The background data and site history are very favourable which makes it possible to use it as a starting point for budgets in a regional perspective. 

Amounts of organic matter measured in the stand and some flows (Figure 3.2) give a static organic-matter budget for the forest at an age of about 120 years. The numbers given are based on a good set of measurements and the original compilation of data was published by Andersson et al. (1980). Litterfall data were measured for the period 1973-80 (cf. Flower-Ellis 1985; Berg et al. 1993). Root production was measured and estimates were made for root-litter formation as well as for that from the shrubs and mosses. Decomposition studies were long  term, using different litter components, such as needles (e.g. Berg et al. 1982) and root litter (Berg 1984) as well as other ones. Also the chemical composition of different litter components was investigated (Berg 1981). With this background information it appears reasonable to try to create a budget for organic matter (C). 

Figure 3.2  A budget for organic matter in a mature (120-year-old) Scots pine monoculture (SWECON site). Based on data from Andersson et al.(1980). Units are in kg of organic matter per ha. Att. -attached; Surf. -surface; min. -mineral; and veg. -vegetation  

The stand was a 140-year-old (in 1993) Scots pine monoculture located at Jädraãs in central Sweden (60°49'N; 16°30'E) at an altitude of 185 m a.s.1. on a flat area of deep sand sediments. The mean annual precipitation is 609 mm and the mean annual temperature is + 3.8°C. The forest has been described in earlier papers (e.g. Axelsson and Bråkenhielm 1980; Berg et al. 1982). 

A very loose A₀₀ horizon, interwoven with living mosses and lichens, covers a combined A₀₁-A₀₂ horizon of 5-10 cm. This latter organic layer was very well separated from the mineral soil. The parent mineral material as well as the whole soil is considered to be very poor in essential nutrients. In the middle part of the last century the forest at this site was burned completely, including the existing organic layers, leaving coal at the bottom of today's organic layer. 

3.3 EUROPEAN TRANSECT STUDIES ON ORGANIC MATTER PRODUCTION/DECOMPOSITION 

3.3.1 Trends of coniferous organic matter accumulation in the light of new European literature 

Cannell et al. (1992) reviewed the new information on C flux and storage in forests of Europe. Coniferous forests comprise about 50% of European forest. In comparison with broad-leaved forests they cover a smaller area (except in the extreme northern latitudes), but their C density is high. In central Europe they store large amounts of C in foliage (12 t ha^-1) and litter (45 t ha^-1). The total amount of C currently stored in the biomass and litter in these coniferous forests is 2.8 Gt. Since the last glacial maximum reforestation followed the ice sheet retreat at the rate of 0.01 million ha per year, equivalent to 7 x 10⁸ kg annually. From the Middle Ages until the beginning of this century, Europe was deforested due to conversion of forests to agricultural land. The average rate of deforestation is evaluated at 0.2-0.4 million ha yr^-1; thus, the forest was eliminated 20-40 times faster than the rate of forest increase during deglaciation. In recent decades, intensive afforestation is noted in Europe (land-use changes, CO₂ and N fertilization) and EU forests are now a net sink for C. 

Adams et al. (1990) indicate that the amount of organic C in vegetation and soils of the globe has more than doubled (from 0.96 to 2.3 Gt) during the present interglacial (last 10 000 years). The class of forest called by the authors 'dry open pine woodland' exactly doubled its C storage (from 7 to 14.1 Gt) during this time. 

Every year there is a massive exchange of C between the atmosphere and temperate vegetation. The amount of C in vegetation and soils is around two to three times as much as that in the atmosphere and is particularly important as a relatively rapidly responding pool.

 Kauppi et al. (1992) surveyed the growth patterns and stock of a third of European forest through the 1970s and calculated that forest resources increased as much as 25% (Figure 3.3). According to forest surveys in European countries, growing stock increased in Europe by 12% between 1971 and 1980. The increase was projected to continue at the same rate in the 1980s yielding a 25% larger growing stock in 1990 than in 1971. The fertilization effect of pollutants override the adverse effects, and forest biomass increased from 70 to 105 million tonnes of C accumulated annually during the 1970s and 1980s in European forests. These results suggest that the European forest is a C sink in the global C budget. 

Brubaker and Graumlich (1989) found a similar tendency of increased net primary production (NPP) for coniferous forests in the Pacific Northwestern USA during the past 100 years (Figure 3.4). The authors conclude that NPP increased by 60% over the period 1880-1979; the natural variability of productivity is great and controlled primarily by variations in summer temperature. 

Klimo (1992) provided the data which can be used for the following calculation of organic matter budget in a Norway spruce forest (stand of Picea abies L.Karst established around 1900 in the Czech Drahonska Uplands): dry matter of aboveground parts of trees equals 273.43 t ha^-1; dry matter of underground parts of trees -40.50 t ha^-1; total 319.93 t ha^-1. Underground parts of spruce trees do not exceed 13% of the whole tree biomass. 

Figure 3.3  Relative change of growing stock based on the best available information from different regions in Europe. The value for 1970 has been adjusted to 1.0. After Kauppi et al. (1992)

Above-ground biomass (273.43 t ha^-1) produces litterfall, which, measured over 10 years (1977-88), fluctuates from 3.39 to 6.76 t ha^-1 yr^-1. The highest value of litterfall was registered in the year 1986 when Cephalia abieties L., the fir pest, occurred. In the other 'normal' years the sequence of organic matter portions in this trophic chain are as follows in units of t ha^-1 yr^-1: (1) ABVG biomass of trees, 273.43; (2) litterfall, 4.55; (3) herbivore feces fall (average year), 0.156; (4) herbivore feces fall (Cephalia year), 0.676. On the basis of physiological studies we can assume that food digestibility of herbivorous invertebrates oscillates around 30%; consequently, the biomass of their food is 43% higher from registered feces biomass and equals 0.222  (average year)-0.966(Cephalia year) t ha^-1 yr^-1. Thus, in spruce forest more than 0.2-1.0 t ha^-1 of plant matter is annually consumed by canopy herbivores; we say 'more than' because in this calculation the amount of organic matter utilized for costs of herbivorous life is not considered. The metabolism of small invertebrates is intensive and its energetic costs are high. Moreover, Funke and Roth-Holzapfel (1991) have found that the invertebrates of a spruce forest accumulate the elements K, Mg, Ca, Cu, Zn, Cr, Ni, Se, Sb, Hg and Cd from their food. Most elements are accumulated by primary consumers, some by saprophages and predators. The accumulation factor (the ratio of element concentrations in animals to those in food) can be as high as 4, 10, 15 or even 33. So important elements accumulated in invertebrates' bodies must influence the allocation and flows of organic matter in the forest system and its parts. 

Figure 3.4  Brubaker and Graumlich (1989) studies on 80 years time series of NPP, annual precipitation and summer temperature. NPP factor score obtained from the principal component analysis of the NPP measurements at four forest sites (Seattle, Washington)

In summary, the quoted literature suggests that coniferous forests accumulated organic C during recent decades and were a C sink in its global budget. The sink is a consequence of a combination of regrowing forests, CO₂ fertilization and N fertilization from poluted rain.

3.3.2 Response of coniferous litter production/decomposition to a climate gradient along a north-south transect

Breymeyer conducted studies (1991a, b, 1993) on production and decomposition of organic matter along a north-south transect of pine forests distributed from northern Sweden to central Poland. Temperature was considered to be the main climatic factor influencing both of these ecological processes. In this study, litterfall was treated as a production index.

The transect extends between 50 and 68° N, or approximately 18° latitude. Efforts were made to choose the stands in such a way that longitudinal deviations were avoided: in the central part of Europe, the shifting of the transect to the east or west would mean changing the effect of continentalism. The difference in temperature between the most northern and southern stand is approximately 8.4°C. All forests studied on this transect occur on sandy soils; near the ground, they shelter shrub berries and mosses. Admixtures of the other tree species are insignificant. The studies were carried out in the years 1986-88; some characteristics of all sampled stands are presented in Table 3.2.

The most northern stand, Tornetrask, is outside the acknowledged range of pine forests; it is in the basin of the large Tornetrask Lake, ca. 200 km north of the Arctic Circle, at a height of about 300 m a.s.l. Mean long-term temperatures for this area range from -1.0 to -0.5°C (at the Tornetrask climatic station -0.7°C), precipitation ranges from 300 to 1000 mm annually (at Tornetrask 470 mm). In 1986 the mean annual temperature and precipitation in Tornetrask averaged -1.10°C and 476 mm, respectively. The Tornetrask study site is situated several hundred metres from the edge of a lake; fog and groundfrosts often occur in this area. Soils were derived from poor, acid, glacial-fluvial sands originating from the weathered material of siliceous rocks (Sonesson 1979). This is the edge of the forest; the undulating terrain around the lake is covered by bogs and birch tundra (Betula pubescens Ehrh. f. Tortuosa Led.). Among the rather homogeneous low birch tundra occur stands of Pinus sylvestris L.forest, most frequently in the vicinity of the Tornetrask railway station and in the Abisko Valley. These are thought to be relict stands of pine, as years with sufficiently warm temperatures for this species occur very seldom. However, our studies indicate that in stands of pine forest around Tornetrask there are numerous young trees. Sonesson (1979) came to the same conclusion. Selected for the study was one of the largest stands of pine forest (about 10 000 m²). The age of the oldest pines is estimated at 80 years. The undergrowth observed were: Betula nana, Empetrum hermaphroditum, Vaccinium victicoides and V. myrtylli, Losselueria procumbens and Artous alpina. It seems that this forest is in good condition and ought to be functioning similarly to other pine forest ecosystems. The next stand is the pine forest in Jädraãs, in the central part of Sweden. The five stands from Poland studied in the transect are situated in Borecka Forest, in Mierzwice and Litewniki forests, in the Bialowieza Forest (Czerlonka stand) and in two pine forests in central Poland (Kampinos and Pinczow stands).

Table 3.2 North-south transect studies of litter budget in coniferous forests. A. Litterfall; B. Decomposition; C. Organic matter lost annually from forest floor (Breymeyer 1991a, b)

^aAfter Jozefaciukowa (1975).
^bCalculated as ratio of differences between extreme litter masses and temperatures.

B

C

Litterfall was caught on 10-20 surfaces; they were smooth circular areas, cleared of vegetation and covered by sand, each 0.1 m². In such a way litterfall was gathered from the ground surface and additionally the production of the undergrowth was observed. Once a year, the plant litter was collected and transported to the laboratory, sorted, dried at a temperature of 90°C, and weighed.

The decomposition of organic matter was evaluated by the litter bag method. Litter produced at a site was decomposed there. Three types of litter were used in the decay studies; pine needles, mixed litter (needles, leaves and so on in the proportions in which they fell to the ground), and pine cones. The litter bags were made of nylon netting with a mesh size of 1 mm; 10 g of material was put into each litter bag.

The comparison of litterfall and decomposition rates for the P. sylvestris forest along the whole transect, from Tornetrask to Pinczow, is presented in Table 3.2. Litterfall was lowest in Tornetrask (55-61 g m^-2). This litter also decayed the slowest, decomposing at an average rate of 11-14% per year. In Jädraãs, in central Sweden, the litterfall was twice as large, and the decay rate was also higher, although it did not change as greatly as litterfall. Mixed litter (composition identical to that which falls on the bottom of the forest) has a decomposition rate ranging from 11% per year in the north to 28% per year in the south. Pine needles decay most rapidly-from 14% per year in the north to 40% per year in the south. The biggest differences were observed in the decay rates of pine cones. Pine cones increase their decomposition rates as much as five-fold after shifting 18° south from Tornetrask. On average, the decomposition of mixed litter doubles, of needles and small twigs almost triples, and of cones, quadruples.

Five summary points can be drawn from this transect study. First, the decay rate of cones is very low, requiring about 30 years for complete decomposition. Second, the decomposition of cones reacts most to temperature changes along the transect stretching from 50°N to 68°N. Third, the decomposition of litter includes at least two phases: fast initial decay of carbohydrates and slow decay of lignin and cellulose, while warming will make a difference in the ratio of decay in both phases, it will more strongly stimulate the slow lignin-cellulose phase. Fourth, litterfall for the northern stands was comprised of a greater percentage of lignified, slowly decaying woody tissue and fifth, as one moves south, the composition of litter changes: it contains more remains of soft, green parts of plants (Breymeyer 1991a, b).

Table 3.3 Relationship between annual total litterfall and needle litterfall as dependent on stand age in Scots pine monoculture stands at the SWECON site, Jädraãs. Basic data from Axelsson and Bråkenhielm (1980), Berg et al. (1993) and Flower-Ellis (1985). Significance levels to the coefficient (b); n.s. = not significant

 3.4 A BUDGET APPROACH FOR SOIL ORGANIC MATTER IN SCOTS PINE STANDS-FROM ONE SITE TO A REGION

In this attempt to set up a budget for organic matter we first use information from a site supplying very good data: this is the Jädraãs SWECON site described above as an example of the empirical C budget. Using this approach we attempt to generalize our observations on soil organic matter accumulation to a region.

Two younger stands (18 and 55 years old) on the same nutrient-poor ground were investigated for litterfall. Further data on these are given by Axelsson and Bråkenhielm (1980).

3.4.1 Litter production

3.4.1.1 Trends in total litterfall

Litterfall was observed for 7-10 years in three Scots pine stands, initially 18,55 and 120 years of age (Table 3.3). Some trends were evident within stands, and there were clear differences between the different-aged forests. A detailed analysis of annual litterfall as well as of single litter components such as needles, cones, branches, and fine litter has been given by Berg et al. (1993) and by Flower-Ellis (1985) (cf. Figure 3.2). Total litterfall in all three stands increased with stand age (cf. Berg et al. 1993). The stands thus represented the age periods of 18-25 years, 55-65 years and 120-130 years, giving a relatively good distribution among ages.

The methods used by Flower-Ellis (1985) to determine total litterfall was very much adapted to litter type and the reader is referred to Flower-Ellis (1985) and Flower-Ellis and Olsson (1978) for a full description.

3.4.1.2 Linear relation for litterfall vs stand age

Total litterfall was positively correlated to stand age, using data for the Jädraãs site for the two younger stands only (18 and 55 years old). For example, for the 18-year-old stand, total litterfall was positively correlated to age. The 55-year-old stand also showed a significant, positive relationship for the 10 years during which it was followed (Table 3.3). These data suggest that the amount of litter was still increasing. However, this trend seems not to be an effect of age alone, but includes a temporal trend which is common to all three stands (Berg et al. 1993). The old stand did not follow a similar pattern. At an age of 120 years, it may no longer have been expanding its needle mass but there was no indication that this was declining during the period of observation.

3.4.1.3 Litter decomposition-maximum decomposition-limit values

For most litter types it has been observed that the decomposition rate decreases as decomposition proceeds and the more easily metabolized components are decomposed and the recalcitrant compounds remain (Berg et al. 1982, 1987). For some litter types it has even been observed that the decomposition proceeds at such a low rate that is not detectable.

Howard and Howard (1974) presented estimated final values for mass loss of some leaf litter types that were decomposing in the absence of soil fauna. Based on their approach, they found that asymptotic functions described their data well. The interpretation of their finding was that in the absence of soil animals, the decomposition could stop or at least come close to a halt and reach a limit value well below 100% mass loss. Using data from a nutrient-poor forest, almost free from soil animals, Berg and Ekbohm (1991) tested long-term mass-loss values for such limits. They found that for seven, chemically very different, litter types, limit values could be estimated ranging from about 50% remaining, down to none for one specific litter type. A later study (Berg and Ekbohm 1993) gave results that confirmed the tendency for some litter types to reach certain, specific limit values. They found when comparing 12 different sets of decomposing needle litters (Scots pine and lodgepole pine) that there was a considerable homogeneity in limit values between species. For Scots pine needles an average amount of 11 % remained.

3.4.2 Method of estimation

In their study, Howard and Howard (1974) compared several statistical models for mass-loss data and preferred an asymptotic nonlinear model with three parameters. Later, Berg and Ekbohm (1991) modified their function somewhat and obtained

where m.l. is the accumulated mass loss (in %) and t is time in days. The parameter m represents the asymptotic level which the accumulated mass loss will ultimately reach, and the parameter k is the decomposition rate at the beginning of the decay.

3.4.2.1 A possible interpretation of the phenomenon of limit values

Howard and Howard (1974) used an experimental environment, with no soil animals present. This may very well be a good basic condition for the phenomenon as the mechanical treatment of the litter is minimized or nonexistent. The main published results are from systems which were either free from soil animals or had a soil fauna with a very low activity toward litter, at least in the early decomposition stages. It is thus possible that the asymptotes reflect a stage at which microbial decomposition comes to a halt and some other agent is needed for the process to continue. However, even if the phenomenon as such may be connected to the absence of soil animals, the observation that there is a considerable range of limit values suggests that a low level of soil animals is a necessary condition though it is not necessarily the only one. Berg et al. (1995) used existing data and examined the relationships between chemical composition and limit values for decomposition of litters. They observed a highly significant, negative relationship to initial N concentration (r = -0.706; n = 28; p < 0.001). In this analysis data from both field studies were used and they seemed to fit the same line. The higher the initial N level, the more organic material should be recalcitrant and thus be stored.

There may be a causal relationship. As decomposition of the litter proceeds, the recalcitrant component lignin is enriched. Before its decomposition has started and afterwards, lignin is subject to chemical transformations such as the chemical incorporation of N in a first step of humification ( cf. Nömmik and Vahtras 1982). Such chemical incorporation may result in structures that are not necessarily broken by soil microorganisms.

The microflora able to decompose lignin completely is limited and those fungi and bacteria able to do this are called 'white rot'. The composition of the microbial community is very dependent on the chemical quality of the litter input into a system. This includes not only the availability of the energy source but also of the concentrations of different nutrients. The interaction of nutrients may be complex and although, for example, a good supply of N may stimulate the initial decomposition rate of litter, the same supply of N may have a reversed effect in somewhat later stages. Properties like these will affect the selection of decomposing organisms including 'white rot'. One example was given above and another one is the observation by Keyser et al.(1978) that a white-rot fungus did not synthesize its lignin-degrading enzyme system in the presence of low-molecular, N-rich compounds such as ammonium and amino acids. Fungi with such properties would naturally be hampered in their development when growing on a highly decomposed, lignin-rich litter which also is rich in N. Because a good number of white-rot fungi have this property (P. Ander, pers. comm.) it follows that the effect of high N in the litter could hamper lignin degradation and consequently, the decomposition of litter in several soil systems.

When testing the relationship between initial concentrations of manganese and the limit values for all available decomposition data, we observed a highly significant, positive relation (r = 0.757; n = 17; p < 0.001). This nutrient has been suggested by some scientists to promote lignin decomposition by stabilizing the enzymes of the lignin degrading system and by increasing the production of the enzyme (Perez and Jeffries 1992; cf. Berg et al. 1995). Although these causal relationships are not clear, we may still see that there was a clear connection, and we cannot exclude this as a possible factor that may determine limit values for decomposition.

3.4.2.2 Soil organic matter data for the stand

In a measurement on the amounts of soil organic matter in the 120-year-old stand, Staaf and Berg (1977) estimated-with the use of large diameter samplers-the amount of 'humus' in the combined A₀₁-A₀₂ layer to be 1.54 kg ha^-1. This estimate did not include visible litter remains.

3.4.2.3 Budget

From this site we have good access to data for total litter input to the mature forest. Using this information and knowledge of an approximate linear relationship (Table 3.3) between litter input and stand age we estimated the annual litter input for the period 0-120 years. We assumed that the canopy development stopped after about 100 years although there was no real canopy closure and that the litter input thus became constant and similar to the estimated value of 0.280 kg m^-2yr^-1. This figure was based on long-term measurements and includes litter formation from shrubs, mosses and roots (Andersson et al. 1980).

With regard to the importance of the chemical composition of the litter, it is essential to emphasize that the litter produced from the pines was chemically similar including all parts of the trees (with the exception of the rough bark) and the main part of the shrub litter had a chemical composition that did not deviate much (Berg 1981). In addition, litter formation from the pines dominated the litter input in the stand, and we may thus assume that the decomposition process in the late stages was similar among the litter types and on the average leaving recalcitrant or stabilized remains of about 11% of the input. This figure was estimated for this stand as based on a separate determination. We further assumed that in this stand, 10 years are required before the litter has decomposed far enough to lose its structure enough to become called soil organic matter. Based on this assumption we estimated a total accumulation of 1.82 kg m^-2 which compares favorably with the measured value of 1.54 kg m^-2 (Staaf and Berg 1977). When the former stand at this site burned in the middle of the nineteenth century, almost all of the surface organic matter (L + F + H layers) was burned, leaving just coal remains, a fact that allows us to suggest this comparison. The difference between the estimated and the determined amount was 15%.

In this regard, we may consider whether we thus have a method to estimate the accumulation of soil organic matter in forests in other soil types. This kind of approach is, of course, sensitive to the accuracy of litter input data (both root litter and litterfall data) as well as to the limit value for litter decomposition.

3.4.3 Trends in a region from the Arctic Circle to the arch of the Alps and the Carpathians

3.4.3.1 Litterfall -a transect of Scots pine sites across Scandinavia

In an attempt to construct a regional model we compared the annual fall of needle litter as well as that of total litter, to stand age, basal area, site index and latitude (Tables 3.4 and 3.5).

The litterfall data used, originating from 33 Scots pine sites, was based on three main studies. Litterfall was measured from 3 to 10 years at each site. The geographical range for these sites varied from north of the Arctic Circle (66° 32' N) to south Sweden (58° 07' N) which covers about half of the north/south distribution of Scots pine in Europe. For all of these sites information was presented about needle litterfall (Albrektsson 1988; Johansson 1986; Flower-Ellis 1985), and about half of them also had total litterfall measured (Johansson 1986; Flower-Ellis 1985). A compilation of these data was made by Berg et al. (1993).

Mean annual needle litterfall ranged from 530 to 2312 kg ha^-1. The lowest amounts were found for nutrient-poor sites (sediment soil) in the north while litterfall was commonly greater for the more nutrient-rich sites with till deposits or clay soils. Among sites of similar fertility, needle litterfall was lower for sites located in the north than for sites at a more southern latitude. This is evident when needle litterfall at two extreme sites, at latitudes 66° 32' N and 58° 04' N, are compared. These sites had nearly identical site qualities and basal areas (17.5 and 18.3 m² ha^-1, respectively) but the needle litterfall at the northern site (608 kg ha^-1 yr^-1) was only about one-third of the amount obtained at the site located farther to the south (1571 kg ha^-1 yr^-1).

The mean annual total aboveground litterfall (larger twigs and branches excluded as limited by the method-70 cm litter trap) varied from 735 to 4198 kg ha^-1. The proportion of needle material in the total litterfall, which comprises needles and fine litter, varied between 33 and 75% in the stands studied. However, in most of the stands the needle litter fraction accounted for about 50-60% of the total annual litterfall. According to Bonnevie-Svendsen and Gjems (1957) and Mälkönen (1974), needle litterfall is approximately 75% of the total litterfall in Scots pine stands. Their values are higher than ours, despite the fact that larger twigs and branches are not included in our figures. Although the methods used in the two transect studies were not ideal for e.g. cones and branches we have used data for estimates in a budget (below) to indicate trends.

Table 3.4 Simple and multiple linear relations for average annual amounts of needle litterfall (kg ha^-1) as dependent on latitude, stand age, site index and basal area in Scots pine stands in Scandinavia. All available average values for Scots pine were used (data from Albrektsson 1988; Flower-Ellis 1985; Johansson 1986; cf. Berg et al. 1993). Levels of significance refer to the coefficient (b); n.s. stands for not significant

Table 3.5 Relationships between total litterfall (kg ha^-1) as dependent on latitude, stand age, site index and basal area for Scots pine stands in Sweden. All data from Flower-Ellis (1985); Johansson (1986); cf. Berg et al. (1993). Levels of significance refer to the coefficient (b); n.s. stands for not significant

3.4.3.2 Linear relations of litterfall in Scandinavia to stand age, latitude, basal area, and site index

Stand age: When we compared all available data for needle litter to stand age we obtained an r² value of 0.27 (n = 33; ns) for needle litter (Table 3.4). For total litter the r² value was 0.19 (n = 17: ns) (Table 3.5).

Latitude: We obtained a negative relationship between latitude and litterfall (both total litterfall with r² = 0.39; n = 17; p < 0.01) (Table 3.4) and needle litterfall (r² = 0.48; n = 33; p < 0.001) (Table 3.5).This may be compared with the results of Albrektson (1988), who also found a negative relationship for needles (r² = 0.46; n = 16).

Basal area: A comparison of litterfall to basal area gave a coefficient of determination (r²) of 0.52 for total litterfall (n = 17; p < 0.001) (Table 3.5). When all available data for needle litterfall were combined (Table 3.4), basal area gave a highly significant, but poorer relationship (r²=0.26; n=33; p<0.01).

Site index: Site index values were available for all the Scandinavian sites. All needle litter data combined gave an r² value of 0.42 (n = 33; p < 0.001;Table 3.4). Total litter gave an r² value of 0.50; n = 17; p < 0.01 (Table 3.5).

Multiple linear relations for litterfall of both needles and total litter, using all Scandinavian data: Relationships on latitude and site index were highly significant (n= 33) and gave r² values of 0.48 and 0.42 respectively (p<0.001). In multiple linear relationships, the combined factors gave significant or highly significant relationships with r² values from 0.49 for site index plus basal area, to 0.71 for latitude plus basal area. A three-variable model that included latitude, site index and basal area, gave an r² value of 0.73, thus explaining about three-quarters of the variation in needle litterfall (Table 3.4). For total litterfall an r² value of 0.77 was reached when latitude, site index and basal area were included (Table 3.5).

Decomposition of litter and limit values: Also for the Scots pine needle litter decomposing in the transect of Johansson (1986) limit values were estimated for the whole climatic range of sites (Johansson, pers. comm.). Although some trends in limit values apparently were dependent on climate and the nutrient status of the sites (Berg, Johansson, unpubl.) they have so far not been observed to be of a magnitude to be considered in the present budget estimate. We have therefore used the value of 11% litter remaining, estimated for the Jädraa006s site for the whole transect range.

Accumulation of soil organic matter on a regional level-network studies: With the use of the presented data and by using the equations of Tables 3.4 and 3.5 we estimated the accumulation of organic matter layers for a larger region. We have preferred to illustrate this with the use of a change in latitude from, say, northern Scandinavia to central Germany or south Poland.

Let us take first needle litter and then total litter. Needle litterfall would be expected to range from a low value of about 810 kg ha^-1 for forests at 66° N to 3316 kg ha^-1 at 49° N (cf. Figure 3.5). These data suggest that soil organic matter may accumulate about four times faster in the south just due to the needle component. For total litter we would have a figure of 3.5 times as fast, assuming that the limit values would be similar in both cases and that the stands would have similar nutrient status.

Figure 3.5 A linear regression giving needle litterfall as compared to latitude. The dashed line gives the extrapolation of data to central Europe. Circles = total litter; squares = needles

If we extend the calculations and use the fact that the stand in the south would grow faster and reach a maximum canopy in 40 years as compared to 110 years in the north we can extend the estimates. We use the simple model, suggesting a linear increase in litterfall and a remaining difficult-to-decompose component of 11%. We may thus estimate that in the northern stand the needle component in litterfall would have added about 4900 kg ha^-1 to the soil organic matter layer in 110 years. In the south the stand would have added 7500 kg ha^-1 in 40 years, in both cases until the canopies reached a maximum.

3.4.3.3 Concluding remarks and recommendations

The extension of Scandinavian data to the northern half of continental Europe lacks validation data. Our results indicate a considerable difference in the growth rate of the soil organic matter layer as a consequence of a difference in latitude, and the magnitude may seem alarmingly large with respect to the possibility of a corresponding storage of nutrients.

The example given above is valid for Scots pine stands. We may expect more pronounced differences between northern and southern latitude forests for other coniferous species, e.g. spruce and also for deciduous trees. We would therefore make the following recommendations:

• It appears self-evident that a good basic knowledge about lignin-degrading organisms and their ecology is vital for further progress in the research field of soil organic matter dynamics.
• As budgets of dead organic matter make up the basic starting point for budgets of nutrients in forest stands some basic approaches are necessary: (1) well-coordinated litter input measurements in selected stands in transects; and (2) a generalization of the concept of limit values as well as an understanding of the causal relationships for their magnitude.

3.5 NORTH AMERICAN CONIFEROUS FORESTS AND STUDIES ON CARBON BALANCE

Temperate conifer forests are common in the western half of North America, across extreme moisture and temperature gradients from west to east resulting from the Cascade, Sierra Nevada and Rocky Mountains. The greater abundance of evergreen conifers over broad-leaved deciduous forests is particularly striking in the Pacific Northwest where, except for the early successional species such as red alder (Alnus rubra) and occasional patches of aspen (Populus tremuloides), conifers dominate the landscape. The unique environmental conditions of the Pacific Northwest, namely dry, warm days and cool nights in the summer and mild, moist winter days are largely responsible for the dominance of evergreens in the Pacific Northwest (Waring and Franklin 1979). Old-growth Douglas fir in the western hemlock zone are unique in that biomass accumulation is as great or greater than anywhere else in the world and second-growth Douglas fir forests are among the most productive in the world (Waring and Franklin 1979; Long 1981). In the eastern USA, conifer forests are common after disturbance or on nutrient-poor soils. The mild climate, long growing season and infertile soils common to the southeastern USA favor evergreen conifer species (Hicks and Chabot 1985). Both western and eastern conifer forests are commonly nitrogen deficient, and productivity can be greatly enhanced by N fertilization (Gessel et al. 1973; Johnson et al. 1981; Vose and Allen 1988; Gower et al. 1994a).

3.5.1 Carbon balance and stand development

Deforestation and natural disturbances are important influences on the global C cycle because ecosystem energetics are altered by disturbance. Despite our incomplete understanding of how forest C budgets change during stand development, it is useful to infer patterns based on the limited data. The annual net C change or net ecosystem productivity (NEP) of a forest is defined by equation (4). Aboveground net primary production (ANPP) for conifer forests follows a sigmoid curve during early stand development and often exhibits a decrease in many overmature conifer forests (see Sprugel 1985; and Gower et al. 1994a, for reviews). The exact reason for the decline is not precisely known, but decreased nutrient availability (van Cleve and Viereck 1981; Miller 1981; Gholz et al. 1985) or reduced stomatal conductance due to inefficient transport of water to the canopy in older trees (Ryan and Waring 1992) may in part explain the decrease in ANPP. Few data for below ground net primary production (BNPP) are available, but it appears maximum BNPP coincides with canopy closure (Vogt et al. 1986). A third factor that may help explain decreased NPP in overmature conifer forests is increased autotrophic respiration costs and constant or decreased net canopy photosynthesis. This hypothesis was first proposed by Kira and Shidei (1967), but few empirical data are available to rigorously examine this hypothesis, and the data that are available are often the result of extrapolating a few chamber measurements to the stand. Autotrophic respiration, as a percentage of gross primary productivity, either does not differ between overmature and aggrading forests (Sprugel 1984) or is slightly greater for overmature forests (Ryan and Waring 1992; Table 3.1, this study).

Immediately after a forest is harvested or destroyed, GPP (Gross Primary Production) is very low because LAI is small or near zero; the duration of the period of low LAI and GPP is related to the severity of the disturbance. The GPP increases rapidly during the early stages of stand development and reaches a long- term steady state near canopy closure (Sprugel1985). During the period when GPP is very low, heterotrophic respiration reaches a maximum because of improved substrate and increased soil temperature and moisture conditions. Consequently, NEP is negative for some period immediately after disturbance and reaches a maximum near or before canopy closure when the foliage:woody biomass ratio is highest (Gower et al. 1994b) and resources are not strongly limiting. Although NEP is hypothesized to approach zero in overmature forests (Sprugel and Bormann 1981), few data are available to test this hypothesis. Grier and Logan (1977) reported that NEP for five old-growth conifer forest communities averaged 331 g Cm^-2 yr^-1 (Table 3.1), but given the uncertainty in many of their measurements it is difficult to ascertain if these estimates of NEP differed from zero.

Despite our tenuous understanding of how the C budget changes during stand development, it is clear that any attempt to balance the global C budget must consider the effects of natural and anthropogenic disturbances on forest C budgets. Johnson (1992) reviewed the literature and found that losses of soil C after harvesting and reforestation were small in the vast majority of studies. However, Harmon et al. (1990) calculated that converting Douglas fir in the Pacific Northwest from old-growth to second growth forests released 1.5-1.8 x 10¹² g C as CO₂ to the atmosphere. Forests in the southeastern USA may have been a regional C source last century due to large-scale deforestation (Delcourt and Harris 1980). There is increasing evidence that northern latitude, temperate and boreal forests are currently an important C sink (Tans et al. 1990).

3.5.2 Oregon transect study

In the Pacific Northwest USA, very steep climatic gradients exist because of the orographic effect from the maritime influence of the Pacific Ocean and the high mountain ranges. Annual precipitation can exceed 3000 mm yr^-1 for coastal Picea sitchensis forests to less than 200 mm yr^-1 for Juniperus woodlands on the east slopes of the Cascades. Along this large climatic gradient, Grier and Running (1977) found that leaf area index (LAI) was positively correlated to site water balance, which they defined as precipitation minus open pan evaporation. Gholz (1982) also reported a similar relationship along the same Oregon transect and that ANPP was positively correlated to LAI. More recently, Runyon et al. (1993) reported that ANPP was positively correlated (r² = 0.82) to annual intercepted photosynthetic active radiation (IPAR) and this relationship was even stronger (r² = 0.98) when they accounted for the percentage of annual IPAR that was not used to assimilate CO₂ because of environmental constraints on photosynthesis. The positive relationship between ANPP and IPAR reported by Runyon et al. (1993) and others (e.g. Linder et al. 1985; Cannell et al. 1987; Daugherty et al. 1992) may be a useful tool to scale stand-level measurements of net primary production to larger scales, especially since IPAR has been shown to be positively related to the remotely sensed vegetation index (normalized difference vegetation index- NDVI) (Bartlett et al.1990; Goward and Huemmrich 1992; Waring et al. 1993).

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