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INVITED ARTICLE

FOREST ECOSYSTEMS ACT AS SINK AS WELL AS
SOURCE OF CARBON FOR THE ATMOSPHERE

S. Singh and J.S. Singh
Department of Botany
Banaras Hindu University
Varanasi - 221 005.

ABSTRACT

Carbon dioxide concentrations in the atmosphere have risen by about 25% since 1850, due to burning of fossil fuels, deforestation, and industrial revolution. There is a lack of empirical information on the response of forest ecosystems to the increasing abundance of CO2, particularly in reference to carbon flux. This issue is briefly examined in this paper.

INTRODUCTION

Atmospheric concentration of carbon dioxide never exceeded 300 parts per million for at least 160,000 years before the Industrial Revolution. Since 1850, however, CO2 levels have risen by about 25% due to burning of fossil fuels and widespread deforestation, and may rise more than 600 ppm in less than 50 years (Jardine, 1994). CO2 is the inherent by-product of more than 80% of the primary energy used by the world’s population (Starr et al., 1992). Industrial activities, deforestation and burning of fossil fuels yielded about 220 Gt of C as CO2 between 1750 and 1990, virtually all of this production occurred after 1850, and more than two-third occurred after 1950 (Sundquist, 1993). Forecasts of CO2 emissions for the years 1990 to 2100 vary widely, ranging from about 700 to 2100 Gt of C as CO<, depending on assumptions about such factors as population, economic growth, energy supplies and technologies, and land use (Laggett et al., 1992).

Direct measurement of the increase of atmospheric levels, when compared to the rates of fossil fuel combustion and cement production, indicate that about 57% of the CO2produced from these sources accumulated in the atmosphere, the other 43% of the industrially derived CO2is in either the biosphere or ocean (Quay et al., 1992). The modern oceans are rapid net CO2sink, whereas the oceans were a gradual source during the degla-citation. In 1992, CO2released by anthropogenic activities added about 7 Gt C yr-1 to the atmosphere, of which about 2 Gt C yr-1 is thought to be sequestered in the oceans (Sarmiento, 1992). Tracer-calibrated models of the total uptake of anthropogenic CO2by the world’s oceans give estimates of about 2 Gt C yr-1 compared to estimates of 0.3-0.8 Gt yr-1 based on synoptic air-to-sea CO2influx, however, the discrepancy is largely due to the net flux of C to the oceans by rivers and rain which is ignored in the synoptic air-to-sea CO2flux calculation (Sarmientoand Sundquist, 1992). Both of the above estimates imply substantial uptake of anthropogenically produced CO2 by terrestrial vegetation and/or soil.

Increasing CO2 and carbon storage

Elevated levels of carbon dioxide (CO2 are known to have a direct effect on vegetation (Krupa and Kichert, 1993). There have been many demonstrations of enhances rates of photosynthesis in C3 plants with elevated atmospheric CO2levels because of two opposing effects of carbon dioxide, one on stomata and the other on the enzyme ribulose bisphosphate carboxylase/oxygenase (Mooney et al., 1991). Cole and Monger (1994) proposed that the increase in global carbon dioxide concentration from ca. 180 ppm during the last glacial maximum (ca. 18000 yr BP) to ca. 275 ppm at ca. 10,000 yr bp as recorded in various Ice Cores contributed directly to the shift from C4to C3vegetation on the piedmont-fan area of Chihuahuan desert in New Mexico. The results of this palaeosol study are consistent with studies of modern ecosystems that indicate higher CO2levels favour C3over C4plants, and explain the replacement of open C3 grasslands and savanna by C3shrubland (Cole and Monger, 1994). For a coastal marsh system, open-top chamber studies on stands with a mixture of C3 and C4 plants indicated an increase of 15% in above-ground biomass and 80% in below-ground biomass in C3 plants due to enhanced CO2 while C4 species showed no effect of CO2 on either below or above-ground production (Drake, 1992).

Greater sequestering of carbon by terrestrial ecosystems with high levels of CO2 in the future has been predicted (Strain and Cure, 1985). Short-term exposure experiments have shown increased leaf photosynthesis in several evergreen species grown under CO2 enriched conditions seedling growth of evergreen and deciduous tree species is enhanced with elevated levels of atmospheric CO2 (Mooney et al., 1991). Most studies thus indicate a large effect of CO2 on production but that the effect is mainly one of increasing the amount of carbon transferred below-ground, rather than to above ground production (Mooney and Koch, 1994).

However, increased photosynthesis in the short-term may not necessarily lead to increased carbon storage in the ecosystem biomass (Dutton et al., 1988). In a three-year study of yellow poplar, Norby et al. (1992) found no change in carbon storage (dry mass) in trees, above or below ground even though there was a large increase in photosynthesis under increased level of CO2 (500 and 650 ppm). Korner and Arnone (1992) selected a humid tropical ecosystem for observing the effect of elevated CO2 on vegetation, because this biome represents about 40% of the global biomass and because plant responses to CO2 are predicted to be more pronounced under high temperatures. The above authors constructed four models of ecosystem by enclosing identically structured population of 15 tropical plant species (trees, shrubs, climbing vines, ground creepers, and herbaceous monocots) in 17 m3 and ground area of 6.7 m2. The 2-6 individuals of each species in each ecosystem were located in exactly the same position in the four houses. All ecosystems were allowed to stabilize under low CO2levels (340 ćl of CO2per liter of air) for 30 days. Daytime CO2was maintained at 340 µl of CO2per liter in the two of houses while the other two received 610 µl of CO2per liter during the experiment. Experiment was terminated after 3 months after the LAI stabilized at a valve typical for humid tropical forest. Total ecosystem biomass at both CO2levels more than doubled within 3 months, but there was no evidence of greater biomass storage under increased CO2 level. Massive starch accumulation in the tops of canopy as a result of overflow of carbohydrates due to reduced activity of carbon sinks, increased production of rapidly turning-over fine-roots, and doubling of soil respiration were the main effects of elevated CO2. Increased CO2 concentration stimulated rhizosphere activity leading to C loss from the soil. Korner and Arnone's data thus indicate that elevated CO2 may not necessarily lead to greater carbon sequesteringby forest ecosystem.

Species composition and turnover

Increasing atmospheric CO2 is expected to change forest productivity and species composition by altering the distribution of key climate parameters such as temperature and soil moisture (Graumlich, 1990). Philips and Gentry (1994) assessed turnover rates of 40 tropical forest sites. The trend of accelerated turn over observed has implications for global change due to links between the global carbon cycle and tropical forests. With continued increase in forest turnover rates, climbing plants and gap-dependent tree species, best positioned to benefit from increased disturbances and atmospheric CO2 may become more abundant in primary forests (Philips and gentry, 1994). Lianas and fast-growing trees have less dense wood than shade-tolerant species, and a net carbon source, rather than a sink. In the Indian Central Himalaya, replacement of slow-growing, all-aged oak forests by fast-growing, even-aged pine forests has been recorded (Saxena et al., 1984). These replacements are tied with pertur bation of nutrient cycling (Singh et al., 1984).

Biodiversity and carbon storage

Faster turnover of forests may also lead to a decline in large-scale biodiversity levels because of gradual disappearance of slow-growing, shade-tolerant species and of other organisms whose life cycles are tied with them. Higher diversity communities are expected to consume more CO2 than lower-diversity communities; the loss of plant biodiversity means a reduction in the ability of ecosystem to fix CO***** (Naeem et al., 1994). Loss of diversity may thus reduce the ability of terrestrial ecosystems to absorb anthropogenically produced CO2. However, microcosm experiment of Korner and Arnone (1992) using a fairly diverse system indicated that these systems may already be overloaded with CO2, and their sink capacity may not expand with increasing CO2 concentration.

Landuse changes and carbon storage

The strongest sources of anthropogenic pollutants are located at mid latitudes of the Northern Hemisphere. In the tropical region also considerable change in land use is occurring, of which wet forest deforestation and the conversion of natural savanna ecosystem to agricultural soils are the most significant changes (Keller et al., 1992). Although the harvesting of wood and conversion of forests to pasture and crops will always produce CO2, some may be released immediately by burning, while some may be released slowly by decay of wood and soil organic matter. Some CO2 may be reassimilated through afforestation and regrowth after logging or abandonment of agricultural land. Thus an accurate budget requires accounting for not only the shifting geographic patterns of human land use, but also the temporal fate of vegetation, soils, debris, and wood products following disturbances (Houghton et al., 1983). In Tables 1 and 2, results from two case studies in India, one from the Central Himalaya and the other from the Vindhyan hills, are given to illustrate the net release of CO2 from the forest ecosystems due to deforestation and land use change. The deforestation and land use changes are evidently turning the forested lands from being a carbon sink to carbon source, although intact forests in both of the above regions agreed carbon over the annual cycle (Rawat and Singh, 1988; Singh and Singh, 1991).

Indirect effect of increasing CO2 abundance is illustrated through the influence of climate change on the boreal forests. Boreal forest has been considered a single largest net terrestrial sink for carbon, absorbing on average, 0.7 billion tonnes of carbon annually during 1980-90 (see Jardine, 1994). However, logging and clearance for agriculture, fire and insect outbreaks and increasing frequency and intensity of lightning and storms due to global warming, are rapidly depleting the boreal forests. Global warming will also result in displacement of the boreal forest to the south reducing its total size. Assuming a doubling of pre-industrial CO2 levels, studies predict that existing boreal forest cover could decline by 50-90%. Thus the boreal forest is about to become the world’s largest carbon source (Jardine, 1994).

CONCLUSIONS

In addition to the oceans, forest ecosystems are important sink of CO2. However, rampant deforestation and landuse changes have turning these systems, at many places, from a sink to a net source of carbon for the atmosphere. There is evidence to indicate that although increasing atmospheric abundance of CO2 may promote photosynthesis, the capacity to sequester carbon in the biomass of extant vegetation for long-term storage may not increase and most of the excess CO2 assimilated may escape through below-ground processes. There is a need to undertake long-term studies and whole-ecosystem experiments to determine the sink-source efficiency of forested ecosystems in view of the predicted global climatic change.

Table 1: Carbon dynamics for total forest (50x103 km2) of Indian Central Himalaya for 1972-73
(based on Singh et al., 1985).

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                                Range                  Area-weighted average
-----------------------------------------------------------------------------
Net Production (x 1012 g C/yr)
        Tree                            6.5-14.9                10.7
        Shrub+herb                      0.7-16.5                 1.1
        Total                           7.2-16.5                11.8

Net accumulation (x 1012 g C/yr)
        Vegetation                      3.4-7.7                  5.6

Carbon release due to forest clearing (x 1012 g C/yr)
        Aboveground                     4.8-11.1                 7.9
        Belowground                     0.4-0.9                  0.7
        Agriculture expansion           1.0-2.2                  1.6
	Total				6.2-14.2		10.2

Net release                             2.8-6.5                  4.6
(total release - net accumulation)
-----------------------------------------------------------------------------

Table 2: Vegetation C, Soil C and total C injected into the atmosphere due to vegetation conversion in 65x104 ha area of dry deciduous forest of vindhyan hills during 1982-1989 (based on Raghubanshi et al., 1991).

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From			To		Net release	Net release	Total release 
					of vegetation C	of soil C
                                        (x1012 g C/yr)  (x 1012 g C/yr) (x 1012 g C/yr)
-------------------------------------------------------------------------------------
Mixed forest with	Mixed forest	0.685		0.032		0.717
crown cover 50%		with crown
			cover 50%-30%

Mixed forest with	Tree savanna	0.223		0.0004		0.223
crown cover 50%-30%

Grass savanna          Degraded savanna 0.338           0.125           0.163

-------------------------------------------------------------------------------------
Total					0.946		0.158		1.104
-------------------------------------------------------------------------------------

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