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9 TERRESTRIAL ECOSYSTEMS Peter M. Vitousek INTRODUCTION o from the atmosphere Terrestrial ecosystems are metabolic systems whose activity produces and consumes many of the gases that drive global change. Plants use nitrogen from the soil to capture energy from the sun and carbon dioxide . Soil microorganisms ultimately utilize much of that energy, in the process releasing carbon dioxide and methane as end products and nitrogen-containing trace gases as by-products of their activity. The amounts involved can be very large; terrestrial plants take up more than 100 PG (billion metric tons) of carbon annually, and plants and microorganisms return approximately as much to the atmosphere in respir- ation. This exchange is 20 times greater than the amount of carbon re- leased by fossil fuel combustion. Similarly, fluxes of both methane and nitrous oxide from terrestrial ecosystems are well in excess of fossil fuel sources (Mooney et al. 19871. The systems ~ ., . . large absolute amount of material exchanged by terrestrial eco- does not mean that such systems control ongoing changes in the composition of the atmosphere and hydrosphere. Gases released by ter- restrial ecosystems may be more or less balanced by uptake in those sys- tems (as for carbon dioxide), or they may be balanced by natural proces- ses in the atmosphere (as for methane and nitrous oxide). However, terrestrial ecosystems are capable of driving change when their own dynamics are altered by human activity or by climate change. They are equally capable of responding to global changes in ways that feed back (positively or negatively) to those changes. I will develop three points in this brief presentation. First, although terrestrial ecosystems appear stable in the absence of human intervention, they are in fact dynamic in ways that interact strongly with the atmosphere and ocean. Second, human activity is now changing the earth system in wholly novel ways and at rates far in excess of any in the past several million years. These changes not only alter ~ - ~ ~ ~ '= ~ they also interfere with the capacity of those systems to respond to change. Third, our ability to understand the workings of the earth system may at last be developing as fast as our ability to alter the earth unintentionally. In that there is hope. terrestrial ecosystems Directly. nut 78
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79 TERRESTRIAL ECOSYSTEM DYNAMICS Terrestrial ecosystems vary more or less repeatably on temporal scales ranging from days to hundreds of millennia. Some of the short-term changes, including seasonal and interannual variation in the absorption of photosynthetically active radiation, can be imaged directly by satellite sensors. When summed globally, these seasonal changes cor- relate very strongly with seasonal changes in carbon dioxide concentra- tions (Fung et al., 1987~--an exciting demonstration that satellite- based earth observation now allows truly global-scale research. Repeated observations over periods of years and decades will allow us to determine the effects of occasional events such as E1 Nina or the 1988 drought and of directional changes in the distribution of vegetation. However, despite the value of such time-series measurements and the great interest in global change, they are very difficult to support--as is indicated by the ongoing drama over the continuation of LANDSAT. On the other extreme, terrestrial ecosystems vary on time scales of tens of thousands to hundreds of thousands of years as a consequence of glacial-interglacial cycles. "Ice ages" have been a cyclical feature of the earth for millions of years; the cause of these regular cycles is variations in the earth's orbit (Hays et al., 1976; Imbrie et al., 1984~' although not all of the mechanisms between orbital cause and climatic effect have been worked out in full. Compared to the present, full-glacial periods were (obviously) much cooler, especially at higher latitudes (CLIMAP Project, 1976~. The sea level was lower due to accumulation of more of the earth's water in ice, and circulation patterns of the oceans differed substantially. There were also correlated patterns of reduced carbon dioxide (and methane) concentrations in the atmosphere (Barnola et al., 1987), and the con- sequent reduction in the greenhouse effect contributed to the overall cooling of the earth. These full-glacial conditions altered terrestrial ecosystems spec- tacularly. The major vegetation zones were often shifted thousands of kilometers from their present positions, the fraction of the earth's surface covered by different types of vegetation was altered substan- tially, and many ecosystems were composed of combinations of species wholly different from those found anywhere today (Davis, 1981~. The co-occurrence of reduced vegetation cover and higher wind speeds caused greatly increased wind erosion at times during glacial cycles, leading to the deposition of large amounts of terrestrially derived nutrients into the sea. Changes in the composition of the atmosphere could also have had direct effects on terrestrial ecosystems. There are two great photo- synthetic pathways in land plants, termed the C3 and C4 pathways for the number of carbon atoms in the first organic product of carbon dioxide fixation (Bjorkman and Berry, 1973~. The C4 pathway, found primarily though not exclusively in tropical grasses, actively concentrates carbon dioxide within leaves. Its activity is therefore less sensitive to external carbon dioxide concentrations than is that of the C3 pathway (Strain and Bazzaz' 1983~. Low concentrations of carbon dioxide in the full-glacial atmosphere should therefore have favored C4-dominated
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80 550~ 450 g a_ Cod O 350 ban 1501 . ;. 160 120 80 40 0 1000 years before present c 0 FIGURE 9.1 Past and projected variations in the concentration of carbon dioxide (solid line). The 160,000-year record is derived from the Vostok ice core (Barnola et al., 1987~; the modern values are measured (to 350 ppm) or projected. Carbon dioxide in the atmosphere has already increased nearly as much (in 200 years) as the entire range of the 160,000-year record. The dashed line is the human population--past and projected. ecosystems such as tropical savannas over C3-dominated ecosystems such as tropical forests. These two represent sharply defined alternative states in many tropical regions today; they differ strikingly in their carbon storage, albedo, effect on the local climate, and fire regime. A sub- stantial expansion in savanna caused by reduced CO2 in the atmosphere could therefore feed back to climate and the composition of the atmo- sphere. MODERN GLOBAL CHANGE AND TERRESTRIAL ECOSYSTEMS How does human-caused change compare with past changes such as the glacial-interglacial cycle? At least in terms of the composition of the atmosphere, the ongoing change is both much larger and much faster (Figure 9.1~. The concentration of carbon dioxide varied from 200 to 285 ppm during the glacial-interglacial cycle; it is now approximately 350 ppm, and it is increasing rapidly. This increase will accentuate the greenhouse effect, the more so because greenhouse gases other than CO2
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81 are also increasing rapidly as a consequence of human activity. The climatic effects are predicted to be similar in distribution but opposite in direction to those during full-glacial periods; temperatures will likely increase substantially at high latitudes and relatively little at low latitudes. In addition, all else being equal, the increase in carbon dioxide concentration should favor C3-dominated forest vegetation over C4- dominated savanna. The net result would be an increase in carbon storage on land and therefore a buffering of the rate of increase in the atmosphere. However, all else is decidedly not equal. Humans are clearing and burning tropical forests at unprecedented rates. Much of the land so cleared is converted to cattle pastures through planting of C4 grasses, either immediately upon clearing or after 1 or 2 years of cropping. This activity can itself increase the amount of carbon dioxide in the atmosphere-ocean system. It also changes local, and possibly regional, climate because pastures (like savannas) have higher albedo, higher surface temperatures, and lower near-surface humidities than do the forests from which they are derived. Tropical deforestation has many other effects. It increases transport of dissolved and particulate nutrients to water systems. Biomass burning adds nitric oxide to the remote atmosphere, where it catalyzes the production of tropospheric ozone--and during the dry season ozone concentrations in the Amazon and Zaire basins are approaching the intolerable regional levels of eastern North American and northern Europe (Browell et al., 1988~. (They are still far from those in southern California.) Nitrous oxide produced in tropical pastures may be a significant source of global increase in that greenhouse gas, and cattle themselves are a globally significant source of methane. (Tropical and subtropical rice paddies--themselves derived by land conversion--are the most important source of methane worldwide (Cicerone and Oremland, 1988~. Further, deforestation is causing the extinction of numerous species--a global change that is significant in its own right, and one that forecloses forever any possibility of the reconstitution of tropical forests as they exist today. I have concentrated on changes in the tropics, but changes to and within terrestrial ecosystems in other areas may be equally significant globally (Schimel et al., 1989~. Of particular concern are the following: 1. The effects of increased carbon dioxide concentrations on the functioning of terrestrial ecosystems everywhere. Increased concen- trations are known to affect plant growth, water use efficiency, nu- trient use, decomposition, and herbivority under controlled conditions; their long-term interactive effects on the ecosystem level are worth ~ . exploring. 2. The possibility that global warming will catalyze the release of vast amounts of organic carbon stored in high-latitude soils. To the extent that this takes place in wetlands, an increase in methane in the atmosphere will result; to the extent that it occurs in upland sites, carbon dioxide concentrations in the atmosphere-ocean system will increase .
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82 3. The possibility that atmospheric transport and deposition of nitrogen-containing compounds resulting from human activities (fossil fuel combustion, fertilizer use) will alter the metabolism of extensive areas of temperate forests and grasslands downwind of industrial and agricultural areas. This process ultimately may alter the amounts and kinds of compounds exchanged between terrestrial ecosystems and the atmosphere or hydrosphere. CONCLUS IONS These are exciting times--the ability to understand many aspects of the earth system is within our reach for the first time, and public, . educational, and scientific interest in global change is overwhelming. However, the global changes that have taken place to date are small relative to what can be expected in the next 50 years. Unless surprising progress is made, carbon dioxide concentrations soon will be more than 50 percent greater than the preindustrial values; most tropical forests worldwide will be a memory. Scientific conclusions, partial though they inevitably will be, must be transformed into global action with unpre- cedented speed if our increased ability to understand the earth is to hold out any hope against the exponential increase in human-caused global change. REFERENCES Barnola, J.M., D. Raynaud, Y.S. Korotkevich, and C. Lorius. 1987. Vostok ice core provides 160,000-year record of atmospheric CO2. Nature 329: 408-414. Bjorkman, O., and J. Berry. 1973. High-efficiency photosynthesis. Scientific American 229 (Oct): 80-93. Browell, E.V., G.L. Gregory, R.C. Harriss, and V.W.J.H. Kirchoff. 1988. Tropospheric ozone and aerosol distributions across the Amazon Basin. Journal of Geophysical Research 93: 1431-1451. Cicerone, R.J., and R.S. Oremland. 1988. Biogeochemical aspects of atmospheric methane. Global Biogeochemical Cycles 2: 299-327. CLIMAP Project. 1976. The surface of the ice-age earth. Science 191: 1131-1137. Davis, M.B. 1981. Quaternary history and the stability of forest communities. Pp. 132-153 in West, D.C., H.H. Shugart, and D.B. Botkin (eds.), Forest Succession. Springer-Verlag, New York. Fung, I.Y., C.J. Tucker, and K.C. Prentice. 1987. Application of Advanced Very High Resolution Radiometer vegetation index to study atmosphere-biosphere exchange of CO2. Journal of Geophysical Research 92: 2999-3015. Hays, J.D., J. Imbrie, and N.J. Shackleton. 1976. Variations in the earth's orbit: pacemaker of the ice ages. Science 194: 1121-1132. Imbrie, J., J.~. Hays, D.G. Martinson, A. McIntyre, A.C. Mix, J.J. Morley, N.G. Pisias, W.L. Prell, and N.J. Shackleton. 1984. The orbital theory of Pleistocene climate: support from a revised
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83 chronology of the marine 180 record. Pp. 269-305 in Berger, A.L. (ed.), Milankovitch and Climate. D. Reidel Publishers, Dordrecht The Netherlands. Mooney, H. A., P. M. Vitousek, and P.A. Matson. 1987. Exchange of materials between terrestrial ecosystems and the atmosphere. Science 238: 926-932. Schimel, D.S., M.O. Andreae, D. Fowler, I. Galbally, R.C. Harriss, H. Rodhe, B. Svenss on, and G. Zavarzin. 1989. Key areas for research in global trace gases exchange. In Andreae, M.O., and D.S. Schimel (eds.), Exchange of Trace Gases Between Terrestrial Ecosystems and the Atmosphere. John Wiley and Sons, Chichester, in press. Strain, B.R., and F.A. Bazzaz (chairmen). 1983. Terrestrial plant communities. Pp. 177-222 in Lemon, E.R. (ed.), CO2 and Plants: The Response of Plants to Rising Levels of Atmospheric Carbon Dioxide. Westview Press, Boulder, Colorado. the Atmoschere.
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