Special Issue

Nature Conservation 2009 — 1. 9. 2009 — Special Issue

Study on the global climate impacts at the ecosystem level

author: Michal V. Marek

The carbon cycle is of fundamental importance for the biosphere as it is inseparably connected with the climate, water and nut­rient cycles and the biomass production on the land and in the oceans. It is useful to recall that carbon is one of the most important crossroads of the non-living and living world.

Through photosynthesis, inorganic carbon becomes a part of organic hydrocarbon molecules, which form the backbone of all organic compounds. Therefore, proper understanding of the carbon cycle is of crucial significance for understanding the history of our planet, its human population and particularly for predicting and managing the common future of the environment and human beings.

Humans have been influencing the glo­bal carbon cycle for thousands of years. Man has been affecting it through agricultural activities, forestry, industrial and energy production and transportation. However, it is only in the last two centuries that anthropogenic emissions of atmospheric carbon have been measurable at the same scale as natural carbon fluxes.The fact that global climate changes are primarily a consequence of human activities is supported by nume­rous data in the literature, describing the increase in the concentrations of CO₂and the other greenhouse gases since the middle of the 18^thcentury. At that time, the CO₂concentration in the atmosphere equaled to 270–280 mmol mol^-1, while the present concentration of CO₂corresponds to 382 mmol mol^-1, i.e.an increase of 35 % compared to the pre-industrial period, i.e.the period prior to 1750. The industrial revolution began in England at that time and was accompanied by a period of rapid industrial growth, connected with massive exploitation of fossil fuels. The dynamics of the annual increase in CO₂concentrations in the atmosphere has also been substantial. While this increase equaled to 1.3 mmol mol^{-1 in 1970–1979, it corresponded to 1.9} mmol mol^{-1 in 2000–2006.}Humans emit approx. 8 gigatonnes of carbon (Gt C) into the atmosphere annually. Of this amount, 5 Gt C is reabsorbed by terrestrial ecosystems and the oceans; nonetheless the CO₂level in the air is increasing at a rate of approx. 1.5 ppm p.a. The trend accelerates the feedback of the global carbon cycle, which, together with anthropogenic increases in the amounts of other greenhouse gases, will have serious consequences for the Earth’s future. The Intergovernmental Panel on Climate Change (IPCC) came to the conclusion that climate change is most probably a result of the increased greenhouse effect of the atmosphere with the related consequences for ecosystems and biodiversity (Parry et al.2007, Solomon et al. 2007).

Current measurements indicate that the terrestrial biosphere absorbs more atmospheric carbon than it releases. In the 1980s, terrestrial absorption had equaled to 1.9 Gt C p.a., while it increased to 2.3 Gt C p.a. in the 1990s; however, the spatial and temporal distribution of this sink is almost unknown (Houghton et al.2001). The data can differ from one year to the next. As emissions from fossil fuels fluctuate annually by less than 4 % and the marine carbon cycle is quite stable, most of the inter-annual varia­bility in the CO₂flux is connected with the terrestrial cycle.

The terrestrial carbon cycle is determined to a substantial degree by annual fluctuations in the weather (Miller 2008). Generally, the effect of temperature could be important in the boreal forest zone and possibly also in the temperate zone; in the Mediterranean, the amount of precipitation is the limi­ting factor. For the Czech Republic, we can ge­neralize that lack of precipitation predominates in forest vegetation zones below zone 4, while forest production is limited by the temperature above forest vegetation zone 4. However, the final effects of the photosynthesis or respiration response to a change in the input of solar radiation, temperature and precipitation differ in carbon fluxes. In addition, various ecosystems react to external factors in a different way. Thus, in a certain area, even a short fluctuation in the weather during the vegetation season can unex­pectedly cause a substantial change in the over--all annual carbon sequestration.

It is assumed that the forests areas of the northern hemisphere temperate zone are among the most important carbon sinks worlwide. However, because of the substantial differences in the results obtained by various methods, this could be a highly controversial statement. For example, based on forest inventories, annual capture of carbon equal to 0.1 Gt has been estimated for the European Unions Member States, while the results based on local measurements of carbon fluxes revealed 0.4 Gt of captured carbon and the results from measurement of the carbon concentrations in the atmosphere concluded at 0.7 Gt (Lindner et al.2004, Ciais et al.2008, Dolman et al.2008, Kauppi et al.2008, Nabuurs et al.2008).

In spite of the above discrepancies in the research outputs, it has been demonstrated that the European forest carbon sinks have clearly increased in the past few decades. The drivers in the above trend are apparently connected with natural and artificial ecosystem fertilization by both the increasing nitrogen as well as carbon dioxide level in the air. Other positive factors include land--use changes, forest management changes connected with age structure changes, with overall forest restoration as a consequence of the substantial reduction in air pollution and possibly also the predominantly favourable responses of ecosystems just to the climate change in Europe (EEA 2008).

As it was mentioned, terrestrial biota (the living component of terrestrial ecosystems) stores about 2.3 Gt C in the current annual balance. The amount almost balances the carbon loss caused by tropical forest defo­restation. Approximately 2.0 Gt C enters the atmosphere annually through deforestation. Current knowledge indicates that managed forests annually capture up to 6 tonnes of carbon/ha. The primary Siberian forests and the Amazonian rain forests currently capture approximately the same amount of carbon annually as the managed forests, although it was assumed that the carbon balance of the climax ecosystems would be close to zero (Luyssaert et al.2008). However, an increa­sing atmospheric CO₂level set-up conditions for equilibrium in these ecosystems.

Terrestrial and especially forest ecosystems are potential “storage sites” for atmospheric carbon. Compared to oceans, terrestrial ecosystems constitute small carbon sinks or storage sitesbut the annual carbon fluxes between the surface of terrestrial ecosystems and the air are comparable with the fluxes between oceans and the atmosphere. In addition, at least 20 % of the carbon mo­lecules in atmospheric CO₂are exchanged between the atmosphere and terrestrial biota annually.

Why do forests play such an important role in the global terrestrial carbon cycle? This is primarily because of the proportion of the forest area to the overall terrestrial ecosystem area; according to current estimates, the global forest area covers approx. 4.1 ´ 10⁹hectares and the amount of carbon deposited in forest vegetation and soils is approx. 1,146 PgC, where approx. 37 % is fixed in tropical forests. The tree longevity ensures that a substantial part of the carbon is stored in forests for a certain time. In addition to sinks in trees, the forest soil sinks are also extremely important (Naaburs et al.l.c.). Thus, forest ecosystems, forest tree growths, can really be considered to be an enormous system of pumps transporting atmospheric oxygen into biomass, soil and, on the other hand, releasing it through respiration and release from the soil into the air.

The importance of the equilibrium between pumping carbon into the forest ecosystem and its release from forests into the air should be mentioned there. The global terrestrial carbon cycle consists of CO₂fluxes between the ecosystem and the atmosphere. The fluxes directly reflect the instantaneous balance between CO₂diffusion into the leaves during photosynthesis and CO₂diffusion from tissues and the soil in its production as a consequence of autotrophic and heterotrophic respiration. Therefore, we most frequently suggest the ecosystem carbon exchange (NEE – net ecosystem exchange). Over a longer period of time, this is the ba­lance between processes of increase in the amount of carbon in the ecosystem (photosynthesis, increase in biomass, carbon accumulation in the soil) and processes of carbon release (autotrophic respiration, microbial litter decomposition, soil carbon oxidation, damage to and destruction of forests): thus, the term net ecosystem production (NEP) is applied.

As it was mentioned, NEE is the instantaneous balance between the assimilation and dissimilation processes. At any moment, a certain amount of atmospheric carbon is absorbed by the plant and simultaneously a certain amount of carbon is released through autotrophic and heterotrophic processes. The rate of carbon uptake in assimilation processes can be expressed as the rate of gross ecosystem assimilation (P_E). The rate of dissimilation processes and also the respiration loss of carbon in the ecosystem are described as the ecosystem respiration (R_E).

For forest growths, NEE can be expressed by the relationship:

NEE = P_E+ R_E

In evaluation of the CO₂flux between the biosphere and atmosphere, the carbon flux into the ecosystem is usually expressed as a negative value. Consequently, the P_Evalues are negative in the above relationship. Losses of CO₂from vegetation through respiration have the opposite flux direction and are expressed as positive values. From the viewpoint of absolute values of the individual fluxes, NEE is thus equal to the difference between P_Eand R_E. The NEE values are negative; if assimilation is greater than dissimilation, carbon flows into the vegetation.

The carbon flux into forest ecosystems has been studied at the Experimentální ekologické pracoviště (EPBK) Bílý Kříž (Bílý Kříž/White Cross Experimental Ecological Workplace), located at the Bílý Kříž site in the Moravskoslezské Beskydy/Moravian-Silesian Beskids Mts. at 973 m a.s.l. It was established in 1987 and in 2002 was selected by the European Commission, Directorate-General for Research Brussels as the only European research infrastructure for ecological research. The EPBK is operated by the Laboratory of Plants Ecological Physiology at the Institute of Systems Biology and Ecology of the Academy of Sciences of the Czech Republic. A substantial part of the research is performed due to financial assistance for projects in the 5^thFramework Programme of the European Community (EC) for Research, Technological Development and Demonstration Activities CARBOEUROPE and ESFRI infrastructure ICOS. Domestic support was provided by the Ministry of the Environment of the Czech Republic (CzechCarbo project – Various authors 2007).

The eddy covariance method is the most widely used tool for measuring primary flu­xes of energy and substances (CO₂) between vegetation and the atmosphere; the method is based on measuring the manifestations of atmospheric turbulence. Its principle is that a vertical flux of any scalar (a variable that only has magnitude) in the air, e.g.the CO₂or H₂O level, is the sum of the average vertical flow and its fluctuations. Basically, it is a pa­rallel measurement of the rate and direction of the individual atmospheric eddies and the related instantaneous concentrations of CO₂and water vapour. The covariance method, allowing long-term measurement of turbulent fluxes of energy and substances between the vegetation and the ground-level layer of the atmosphere, provides continuous and instantaneous information on ecosystems and their reactions to a disturbance of the environment and also evaluates the factors that cause variation in annual fluxes (Fig. 1). This is the most recent approach that is applicable to whole forest stands, allows to evaluate the data from a greater number of sources and very precisely monitoring, day-by-day, the exchange of the kinetic energy, apparent latent heat, water vapour and carbon dioxide between the vegetation and the ground-level layer of the atmosphere. The method can be used to determine the primary production of the forest vegetation, the effectiveness of CO₂incorporation into biomass and the utilization of water in the biomass production and, together with knowledge of the input of photosynthetically active radiation into the vegetation, also the effectiveness of solar radiation utilization in the biomass production (Marková et al.2008, Urban et al.2007, 2008). Therefore, such a research presents direct interconnection of physical research studies under the conditions of real forest vegetation with production forest ecology.

The rate of pumping atmospheric carbon is dependent on the amount of captured solar light radiation providing energy for photosynthetic assimilation. The relationship is presented by the NEE light curve. It is interesting by how much the rate of NEE on days with direct solar radiation differs from days when it is cloudy and diffuse solar radiation predominates. Diffuse radiation penetrates deeper into the canopy and thus a greater part of the assimilation apparatus is reached and actively fixes atmospheric carbon (Pavelka et al.2008, Fig. 2).

The dynamics of CO₂input/output between the forest vegetation and the air displays a clearly seasonal character, the period of input being related to seasonal trends in assimilation activity and in the forest growth respiration dynamics. The final balance results from the ratio of the periods when the forest is a carbon sink and, to the contrary, emits carbon (Fig. 3).

Conclusions

The Central European forests are of substantial importance for the carbon cycle. Research on the carbon flux into forest ecosystems supports not only improving our knowledge of their production ecology and the plant ecophysiology, but also the ways in which the CO₂level in the air can be reduced in a reasonable manner.

The author is Director of the Institute of Systems Biology and Ecology, Academy of Sciences of the Czech Republic Brno and Professor at the Mendel University of Agriculture and Forestry in Brno.

A sink is when ...

Carbon capture (sequestration) is simply the efforts to slow down greenhouse gas production through keeping carbon dioxide out of the air as a gas or its carbon component. Any process, activity or mechanism that removes, from the air, a greenhouse gas, aerosol or substances from which greenhouse gases or aerosols are formed (precursors) is called a sink.Carbon sinks are natural or man-made systems that absorb carbon dioxide from the atmosphere and store them: they take-in and store more carbon than they release. Therefore also ecosystems that, in contrast to sources of greenhouse ga­ses, capture and remove carbon from the air, have come to be called sinks. Carbon enters the atmosphere primarily through plant respiration, soil organic matter decomposition or forest fires: by far the greatest amount of CO₂is released into the air by volcanoes and respiration of marine biota.

What can be accomplished by one hectare of mountain Norway Spruce stand in the Czech Republic?

• An area of 22 hectares of needles is distributed over 1 hectare of the growth, capturing more than 90 % of incident solar radiation.
• Approx. 2 % of the incident radiation is stored in the biomass. Thus, during the vegetation season, a single hectare captures energy corresponding to 8 tonnes of brown coal.
• One hectare of tree stand produces 10 tonnes of oxygen each year, which is sufficient for the annual respiration of 38 people.
• One hectare of forest growth includes: 44 tonnes of dry wood in trunks (current annual growth increment 4 tonnes),
• 20 tonnes of dry wood in branches (current annual growth increment 2 tonnes),
• 22 tonnes of needle dry matter (current annual growth increment 2 tonnes).
• The vegetation on one hectare absorbs the same amount of carbon dioxide (CO₂) as that produced by a passenger car travelling 90,000 km (15 tonnes of CO₂).
• During a clear day, one hectare of vegetation pumps out and evaporates up to 40,000 litres of water.
• During a clear day, the cooling effect of one Norway Spruce corresponds to the output of 10 refrigerators.