Biomass forest ecosystems

Under low-dose conditions, forest ecosystems act as sinks for atmospheric pollutants and in some instances as sources. As indicated in Chapter 7, the atmosphere, lithosphere, and oceans are involved in cycling carbon, nitrogen, oxygen, sulfur, and other elements through each subsystem with different time scales. Under low-dose conditions, forest and other biomass systems have been utilizing chemical compounds present in the atmosphere and releasing others to the atmosphere for thousands of years. Industrialization has increased the concentrations of NO2, SO2, and CO2 in the "clean background" atmosphere, and certain types of interactions with forest systems can be defined. 

Table IV. Biomass and nutrient losses associated with wood harvest (fuel wood or timber export) and fire in selected forest ecosystems.

Much of the surface soil erosion and hence nutrient loss occurs when deforestation and biomass burning removes and/or consumes the organic materials that protect the soil surface. Significant losses may occur by dry ravel or overland water erosion associated with precipitation events. Under a shifting cultivation system in a tropical deciduous forest ecosystem in Mexico, Maass et al. 61) reported first year losses of N, P, K, and Ca were 187, 27, 31, and 378 kg ha" respectively. In contrast, losses in adjacent undisturbed forests were less than 0.1 kg ha for all nutrients except Ca (losses were 0.1-0.5 kg ha for Ca). 

In soils of non-agricultural ecosystems, above ground biomass (foliar uptake) and metal cycling is considered important (see Figure 8), due to large impact on the metal distribution in the humus layer and mineral soil profile. Especially in soils of Forest ecosystems, it may affect the accumulation in the humus layer, which is considered a very relevant compartment regarding the calculation of a critical load. In these soils, however, a steady-state element cycle is assumed, which implies that mineralization, Minj, equals litterfall, Mjf. 

In forest ecosystems these symbols stand for CL(M) is critical load of a heavy metal (g ha-1 a-1) Mu is metal net uptake in wood biomass under critical loads conditions (g ha-1 a 1) Mle(crit) is critical leaching flux of metal with drainage water (from the uppermost 10 cm soil layer) (g ha-1 a-1). 

The plant species biomass of Boreal and Sub-Boreal Forest ecosystems accumulates a significant part of living matter of the whole planet. This value is about 700 x 106 tons of dry weight. The biomass per unit area of different Forest ecosystems varies from 100 to 300 ton/ha and even 400 ton/ha in the Eastern European Oak Forest ecosystems. The annual net primary productivity, NPP, varies from 4.5 to 9.0 ton/ha (Table 1). 

Figure 3. The general nitrogen model for illustrating the bio geochemical cycling in Forest ecosystems. Explanations for the fluxes 1, ammonia volatilization 2, forest fertilization 3, N2-fixation 4, denitrification 5, nitrate respiration 6, nitrification 7, immobilization 8, mineralization 9, assimilatory and dissimilatory nitrate reduction to ammonium 10, leaching 11, plant uptake 12, deposition N input 13, residue composition, exudation 14, soil erosion 15, ammonium fixation and release by clay minerals 16, biomass combustion 17, forest harvesting 18, litterfall (Bashkin, 2002).

The dominant species are the spruce (Picea excelsa), the birch (Betula verrucosa, B. pubescens), the aspen (Populus tremula), and the alder (Aims incana). The moss and low bush layer is represented by the blueberry-bush (Vaccinium myrtiilus), hypnic mosses, separate species of cowberry (Vaccinium uliginosum) and flowering plants. The biomass of these Spruce Forest ecosystems reaches 10 ton/ha at the age of 100-150 years (Table 4). 

We can see from Table 4 that most of the Spruce Forest ecosystem biomass is accumulated in trees, with trunk mass predominating. The values of annual Net Primary Production (NPP) and litterfall production are more connected with needles. In living matter, the mass of moss and bush species makes up to 2-3% of the tree biomass, whereas in dead matter (litterfall), it is up to 10%. 

Table 4. Biomass and total ash mass distribution in Spruce Forest ecosystems of the Karelia region, Russia (after Dobrovolsky, 1994).

We can see that the tree vegetation absorbs annually from soil tens of grams per hectare of Zn and Ba, units of grams of Ni, V, and Co. The absorption of trace metals by low bush species is smaller by an order of magnitude. Simultaneously, a similar amount of metals is released from the living biomass of the Spruce Forest ecosystems. 

The Broad-Leaved Forest ecosystems are widespread in regions of Sub-Boreal climate zone with a well balanced precipitationrevapotranspiration ratio. The southern periphery of the vast belt of Eurasian boreal and Sub-Boreal Forest ecosystems is represented by Oak Forest ecosystems. These ecosystems exhibit both the largest biomass and annual NPP rates in comparison with other forest ecosystems of this zone. However, the dead mass surface organic matter is 2-3 time less than that of coniferous forests. 

The amount of nitrogen in the biomass of the Oak Forest ecosystem reaches 900-1,200 kg/ha and the sum of ash elements is about 2,000-3,000 kg/ha, e.g., greatly in excess of nitrogen. The corresponding values for accumulation of nitrogen in annual growth are 80-100 and for ash elements, 200-250 kg/ha. An essential point is that the green leaves store 70-80% of the mass of uptaken elements, and the fallen leaves,... 

Amongst the ash elements the most abundant in biomass is calcium, which is accumulated in leaves, in trunk wood, and in twigs. Potassium is dominant in annual NPP. The masses of trace metals in biogeochemical cycling of this Oak Forest ecosystem are roughly in correspondence to their respective average values for the... 

The biomass of arid ecosystems is significantly less than that of Forest ecosystems and changes from 10 to 25 ton/ha, by dry weight, in Steppe, from 4.0 to 4.5 ton/ha in Desert and from 2 to 3 ton/ha in Extra-Desert Ecosystems of the Central Asia. The overall biomass of Arid Steppe and Desert ecosystems is an order of magnitude less than that of Forest ecosystems (Rodin et al., 1975). 

The ash content of Arid Steppe and Desert ecosystem vegetation is about 2 times higher than that of forest species. Accordingly, the biogeochemical fluxes of elements are similar to those in the forest ecosystems, in spite the smaller biomass (see above). The compartments of biogeochemical turnover in Steppe and Desert ecosystems are shown in Table 1. 

The characteristic biogeochemical feature inherent in all Steppe and Desert ecosystems is the most intensive cycling of different chemical species in comparison with forest ecosystems. For a Steppe ecosystem the biogeochemical cycle is 2-3 years and this means that the complete renewal of all ecosystems biomass takes place over this period. Remember that in Forest ecosystems the biogeochemical cycling is about 3->25 years and even about 50 years in Forest Swamp ecosystems. The turnover is the highest in Ephemeral Desert and gradually decreases to the north. 

Table 6. Plant biomass parameters in Tropical Rain Forest ecosystems, ton/ha.

The average sum of total ash elements in the biomass of Tropical Rain Forest ecosystems is about 8,000 kg/ha. The annual ash element turnover and heavy metal exposure rates are shown in Table 8. 

The total ash content accounts for 11-23% of the dry weight of plant biomass. We can remember for comparison, that these values for the terrestrial forest ecosystems on similar limestone soils are 5-6% only. These differences can be attributed to the adaptation of Mangrove ecosystems to saline marine waters and relevant exposure to the chemical species. 

Structural changes to the land cover are not exclusively due to human activity. In some regions of the globe, hurricanes introduce considerable changes in the carbon balance of forest ecosystems. For example, in the U.S.A. two hurricanes happen on average every three years, which accelerates the transition of the living biomass of... 

Mobilization of water from the soil is closely related to root depth and root density in each layer of soil. Fine roots of active B. brizantha pastures, established in deeply weathered clayey soils in eastern Amazonia, reach depths of 8 m or more (Nepstad et al. 1994). In abandoned pastures (50% B. humidicola and P. maximum cover and 50% invading shrubs and small trees), fine roots ( < 1 mm in diameter) were found at depths of 12 m (Nepstad et al. 1994). Fine-root biomass in the superficial soil layers of an active pasture in Paragominas, eastern Amazonia, was 3 times higher than that found in an adjacent primary forest area. Fine root biomass in the active pasture decreased by a factor of 100 between the surface and 6 m depth. In an abandoned pasture area, the distribution pattern of fine-root biomass was similar to that observed in the deeper soil layers of the forest ecosystem. This pattern is associated with the fine roots of the existing dicotyledonous invading species. 

Wadsworth, F. H., 1983. Production of usable wood from tropical forests. In Tropical Rain Forest Ecosystems. Structure and Function. Ecosystems of the World 14A, ed. F. B. Golley (Elsevier, New York), pp. 279-288. Wang, D., F. H. Bormann, A. E. Lugo, and R. D. Bowden,. 1991. Comparison of nutrient-use efficiency and biomass production in five tropical tree taxa." Forest Ecology and Management 46 1-21. 

Generally, in humus the ratio of C N P S is close to 140 10 1.3 1.3 (Stevenson, 1986). As a result of its high nutrient content, humus plays a role of biogeochemical barrier in soil profile and dominates the storage of biogeochemical species in most ecosystems. For instance, in Temperate Forest ecosystems, the aboveground biomass... 

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