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Effects on Ecosystems

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Effects on Ecosystems 10 Effects on Ecosystems J.M. MELILLO, T.V. CALLAGHAN, F.I. WOODWARD, E. SALATI, S.K. SINHA Contributors: /. Aber; V. Alexander; J. Anderson; A. Auclair; F. Bazzaz; A. Breymeyer; A. Clarke; C. Field; J.P. Grime; R. Gijford; J. Goudrian; R. Harris; I. Heaney; P. Holligan; P. Jarvis; L. Joyce; P. Levelle; S. Linder; A. Linkins; S. Long; A. Lugo, J. McCarthy, J. Morison; H. Nour; W. Oechel; M. Phillip; M. Ryan; D. Schimel; W. Schlesinger; G. Shaver; B. Strain; R. Waring; M. Williamson. CONTENTS Executive Summary 287 10.2.2.2.3 Decomposition 10.2.2.2.4 Models of ecosystem response to 10.0 Introduction 289 climate change 10.2.2.3 Large-scale migration of biota 10.1 Focus 289 10.2.2.3.1 Vegetation-climate relationships 10.2.2.3.2 Palaeo-ecological evidence 10.2 Effects of Increased Atmospheric CO2 and 10.2.2.4 Summary Climate Change on Terrestrial Ecosystems 289 10.2.1 Plant and Ecosystem Responses to 10.3 The Effects of Terrestrial Ecosystem Changes Elevated C02 289 on the Climate System 10.2.1.1 Plant responses 289 10.3.1 Carbon Cycling in Terrestrial Ecosystems 10.2.1.1.1 Carbon budget 289 10 3.1 1 Deforestation in the Tropics 10.2.1.1.2 Interactions between carbon dioxide and 10.3.1 2 Forest regrowth in the mid-latitudes of temperature 290 the Northern Hemisphere 10.2.1.1.3 Carbon dioxide and environmental 10 3.1.3 Eutrophication and toxification in the stress 290 mid-latitudes of the Northern Hemisphere 10.2.1.1.4 Phenology and senescence 291 10.3.2 Reforestation as a Means of Managing 10.2.1.2 Community and ecosystem responses to Atmospheric CCb elevated carbon dioxide 291 10 3 3 Methane and Nitrous Oxide Fluxes 10.2.1.2.1 Plant-plant interactions 291 10 3 3 1 Methane 10.2.1.2.2 Interactions between plants and animals 291 10 3 3.2 Nitrous oxide 10.2.1.2.3 Interaction between plants and microbes 291 10.3.4 Ecosystem Change and Regional 10.2.1.2.4 Decomposition 291 Hydrologic Cycles 10.2.1.2.5 Whole-ecosystem exposure to elevated 10 3.5 Summary carbon dioxide 292 10.2.1.3 Summary 292 10.4 Marine Ecosytems and Climate Change 10.2.2 Plant and Ecosystem Responses to Changes 10.4.1 Climate Change and Community Response in Temperature and Moisture 294 10.4.2 Interaction Between the Land and the Ocean 10.2.2.1 Plant responses to changes in temperature 10.4.3 Interactions Between the Ocean and the and moisture 294 Atmosphere 10.2.2.1.1 Carbon budget 294 10.4.4 The Carbon System and the Biological Pump 10.2.2.1.2 Phenology and senescence 294 10.4.5 Summary 10.2.2.2 Community and ecosystem responses 295 10.2.2.2.1 Plant community composition 295 References 10.2.2.2.2 Interactions between plants and animals 295 EXECUTIVE SUMMARY Ecosystem Metabolism and Climate Change change In past times, species migrations were largely unaffected Photosynthesis, plant and microbial respiration tend to increase by human land use Barriers to migration now exist (e g , human with increasing temperatures, but at higher temperatures settlements, highways, etc ) Therefore, inferences from previous respiration is often the more sensitive process As a consequence, migrations cannot be applied without caution to the present and global warming may result in a period ot net release of carbon future situations from the land to the atmosphere The magnitude of this release is uncertain Factors that will influence the amount of caibon Human Activities, Ecosystem Changes and the Climate released include local patterns of climate change and the System responses of the biota to simultaneous changes in soil moisture Human activities such as deforestation in the tropics and forest and atmospheric CO2 concentration harvest and regrowth in mid latitudes of the Northern Hemisphere Increased soil water availability will tend to stimulate plant are influencing the climate system by affecting greenhouse gas growth in dry ecosystems and inciease carbon storage in cold and fluxes Deforestation in the tropics is releasing 1 6 ± 1 Pg C wet ecosystems like lowland tundra A number of recent annually to the atmosphere modelling studies have predicted that water stress will be a The net exchange of carbon between the land and the primary cause of tree death in the southern temperate forests of atmosphere due to forest harvest and iegrowth in the mid latitudes the Northern Hemisphere as climate changes Forest death and of the Northern Hemisphere is uncertain These regrowing loiests replacement by grasslands would lesult in a net flux ot caibon may be accumulating 1-2 Pg C annually One analysis suggests from the terrestrial biosphere to the atmosphere that an equivalent amount ol carbon is released back to the Increased atmosphenc CO2 has the potential to inciease plant atmosphere thiough the burning and decay ol previously growth in a variety ol ways stimulation of photosynthesis harvested wood depression of respiration relief of water and low light stresses The issue of carbon storage in the mid latitudes of the Noithein relief of nutrient stress by several mechanisms (greater nutrient Hemisphere is lurther complicated by the eutrophication ol the use efficiency, increased nutrient uptake through root-microbial region with nitrogen Nitrogen in agncultuial fertilizers and in associations, increased symbiotic nitrogen fixation), and delay ol acid lain may be promoting carbon storage at the rate ot 0 5 10 senescence that prolongs the giowing season Some ot the Pg C annually, but there is considerable uncertainly in this mechanisms that promote incieased giowth could be paiticulaily estimate important in and/semi and and infertile areas However there is great uncertainty about whethei 01 not these mechanisms operate Reforestation as a Means of Managing Atmosphenc CO2 for prolonged periods in natural ecosystems For example there Reducing the atmospheric CO2 concentration through an are no field data from whole ecosystem studies of forests that afforestation program would require the planting ol a vast aiea of demonstrate a ' CO2 feitilization etlect It elevated CCb does forest Approximately 370 x 10& ha of tempeiate loiest would stimulate the growth ot woody vegetation, this could lead to long have to be planted in order to accumulate I Pg C annually I his term net carbon storage in teirestial ecosystems assumes a forest with an annual carbon accumulation rate ol 2 7 t per hectare The carbon accumulation would continue for almost a Ecosystem Structure and Climate Change century After that time the forest would be mature and would not Because species respond ditfeicntly to climatic change, some will sequester more carbon increase in abundance while others will decrease Ecosystems will therefore change in structure Ovei time some species may be Methane and Nitrous Oxide Fluxes displaced to higher latitudes or altitudes Rare species with small Microbial activity is the dominant source to the atmosphere of ranges may be prone to local 01 even global extinction methane and nitrous oxide Warmei and wcttei soil conditions Warming rates aie piedicted to be uipid (0 VC per decade) and may lead to increased fluxes ot these gases to the atmosphere there is gieat uncertainty about how species will iespond to these Changes in land use and fertilizer and atmosphenc inputs ol rapid changes Ecosystems ot laige stature such as loiests may nitrogen, also have the potential to affect methane and nitious not be able to migrate fast enough to keep pace with climate oxide fluxes 288 Effects on Ecosystems 10 Deforestation and Regional Hydrology to solar radiation required for photosynthesis. Since nutrients are The conversion of large areas of tropical forest to grassland will an important controller of net primary production in marine likely change the hydrological regime of the region. Rainfall will environments, production will be changed to the degree that upper be reduced and surface water flow will be affected. ocean physical processes change in response to climate change. Different nutrient and mixing regimes are characterized by Marine Ecosystems different plankton communities, which have wide ranging Climate change will probably affect ocean circulation and mixing efficiencies of processing carbon, with important implications for patterns. Circulation and mixing control nutrient availability to long term ocean storage of organic carbon. the oceans' microscopic plants (phytoplankton) and their access i 10 Effects on Ecosystems 289 10.0 Introduction 10.2 Effects of Increased Atmospheric CO2 and On the basis of current evidence from climate modelling Climate Change on Terrestrial Ecosystems studies it appears that the change in globally averaged Increases in atmospheric CCb, warming and changes in surface temperature due to doubling CCb probably lies in precipitation patterns all have the potential to affect the range 1 5 to 4 5°C (Section 5) Temperature changes ol terrestrial ecosystems in a variety of ways Here we ieview this magnitude in the Earth s history have been associated some of the major effects and identify some of the ways with shifts in the geographic distribution of terrestrial biota that these three factors interact to influence ecosystems For example, the boreal forests of Canada extend well north of the current timber line during the Medieval Warm 10.2.1 Plant and Ecosystem Responses to Elevated CO2 Epoch (800 to 1200 AD) a time when temperatures in that Current climate models estimate that even if man made region were about \°C wannei than todays At the same emissions of CO2 could be kept at present rates time, farmers in Scandinavia grew cereal crops as far north atmospheric CO?
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