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How Increasing CO2 and Climate Change Affect Forests Author(s): Robin Lambert Graham, Monica G. Turner, Virginia H. Dale Source: BioScience, Vol. 40, No. 8 (Sep., 1990), pp. 575-587 Published by: University of California Press on behalf of the American Institute of Biological Sciences Stable URL: http://www.jstor.org/stable/1311298 . Accessed: 05/08/2011 23:03 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. University of California Press and American Institute of Biological Sciences are collaborating with JSTOR to digitize, preserve and extend access to BioScience. http://www.jstor.org How Increasing CO2 and Climate Change Affect Forests At many spatial and temporalscales, therewill be forest responsesthat will be affectedby humanactivities Robin LambertGraham,Monica G. Turner,and VirginiaH. Dale T he strongrelationship among climate, atmosphere, soils, biota, and human activities providesa solid basis for anticipating changes in terrestrialbiomes in response to changesin the global environment.A major impact of the projected doubling of atmosphericCO2 and increasesin trace gas concentrations is the potential for global climate change (e.g., Tangley 1988). Currentpredictions are for an average rise in global temperatureof 1.5? to 4.5? C, with greater warming in winter than summer and increased warmingwith increasedlatitude.But the magnitude, rate, and spatiotemporal characteristicsof the climatic responseremainuncertain.Increased precipitationin high latitudesand decreasedsummerprecipitationand soil moisture in middle latitudes of the northern hemisphere are also predicted. Global forests are likely to experience dramatic changes as a consequence of elevated CO2 and climate changes. Predictingthese changes is Forest responses will determinethe fate of many species processes (Table 1). Of course, many forest responses cross scales, and their not only a social and economicneces- assignment may be somewhat arbisity, but it is also a scientificchallenge trary. For example, forest succession, becauseof the many spatial and tem- which takes decades or centuries and poral scales involved.Forestsdirectly affect climate at the global scale by alteringthe earth'salbedo, hydrological regimes, and atmosphericCO2. Forests also affect climate at a local scale by alteringtemperature,humidity, and solar radiation. Similarly,forest responsesoccur at many scales. For example, processes that involveexchangesof water,heat, and CO2 at the leaf surfacehave fast n sponsetimes (secondsto days).Forest growth and communitycomposition have intermediateresponsetimes (decades to centuries), whereas the geographicdistributionof forestsis a long-termresponse (centuriesto millennia; Figure 1). Furthermore,all Robin LambertGraham,Monica G. these responsescan be alteredby huTurner,andVirginiaH. Daleareresearch man intervention. scientistsin the Environmental Sciences How can the fate of forests be Division,Oak Ridge NationalLabora- predictedin the face of what appears tory, Oak Ridge, TN 37831-6038. Gra- to be an inevitablerise in atmospheric ham is a forest ecologist interested in CO2 and temperature?It requiresexlong-termforest productivityand the use plicit considerationof the many temof remotely sensed data and geographic and spatial scales at which forinformation systems in assessing forest poral resources.Turneris a terrestrialecologist est responsesoccur, incorporationof interestedin the theoreticaland applied the links betweenthese scales in both problemsof describinglandscapepattern models and experiments,and a clear and processes. Dale is a mathematical understandingof human impacts on ecologist interestedin modelingeffectsof forest responses. disturbanceson forest succession. Four levels of biotic organization September1990 (the biosphere,the biome, the ecosystem, and the tree) provide a useful framework for examining forest responses to CO2 and climate. Each level has a characteristicspatial and temporal scale and set of ecological is generallystudied at the ecosystem level of organization, is a function of species-specificresponses to environmentalvariables(measuredat the tree level); competitive interactions amongindividualsfor light, nutrients, and water (measuredat the ecosystem level); and seed source availability, which may depend on species migration rates (measured at the biome level). The objective of this article is to examine potential forest responses to elevated CO2 in conjunction with cli- mate change. For each level of biotic organization (Table 1), we discuss the potential effects of changed climate and elevated CO2 on key ecological processes, how human intervention can affect those processes, and the role of modeling in elucidating and predictingforest responses.We conclude with a discussion of future research needs. Biosphere Severalprocessesare of interestat the biospherescale. They are the various feedbacks among atmospheric CO2 concentrations,climate, and carbon storage in global forests (Figure 2). 575 ORNLDWG90M11337 w m 0-j C/) z w z z 0 UJ C,) w 0 I- (c) port the hypothesis that the observed increase in the annual variation of atmospheric carbon, especially in the northern latitudes, is due to an increased growth rate in temperate and boreal forests (Barcastow et al. 1985, D'Arrigo et al. 1987). The regulation of CO2 is but one of the ways global vegetation influences global climate. Vegetation modifies energy fluxes by altering the earth's albedo; vegetation generally has a lower albedo than soil and consequently absorbs more radiant energy, thus warming the earth. w x Hydrologic regime. Evapotranspiration from vegetation is a major component of the global hydrologic regime (Shuckla and Mintz 1982). As vapor pressure deficit increases exponentially with temperature, a warmer climate will increase evaporative deYEARS DECADES CENTURIES MILLENNIA mand if air moisture is not greatly DAYS increased. Although laboratory studTIME ies have shown that increased CO2 generally reduces stomatal conductances and thus water losses from Figure1. Spatialand temporalscales of biotic levels of organizationiin forests. transpiration, it does not necessarily follow that elevated atmospheric CO2 Carbon cycle. Carbon, a major con- ways whose photosynthiesis process is will reduce global evapotranspirastituent of plants, is taken up from the not saturated at current atmospheric tion. The impact of stomatal conducatmosphere as CO2 through photo- CO2 concentrations (Figure 3). Such on transpiration water losses tance necessarnot an increase when tisreleased and might (but plant synthesis sue respires or dies and either decom- ily) result in more carbon storage in depends on the degree to which the poses or is burned. Because forests forest biomass and thus could serve as foliage is coupled to the atmosphere contain 90% of the carbon in terres- a drag on atmospheric CO2 rise. Lim- (Jarvis and McNaughton 1986). In trial vegetation, the accumulation of ited tree ring evidence suggests that laboratory tests, there is good airflow forest biomass is a major regulator of forest growth in some locations has over the foliage. Consequently, the increased during the past century in- foliage is tightly coupled to the atmoatmospheric CO2. Increased concentrations of atmo- dependent of climate (LaMarche et al. sphere, and transpiration is sensitive spheric CO2 could increase photosyn- 1984), although this conclusion is to CO2-mediated changes in stomatal thesis rates of existing forests, be- open to debate (Kienast and Lux- conductance. However, at the recause forests are composed largely of moore 1988). In addition, global 3-D gional scale, foliage is poorly coupled plants with C3 photosynthetic path- atmospheric tracer model studies sup- to the atmosphere. As a result, regional evapotranspirational water Table 1. Four biotic levels of organization that participate in forest response to climate and CO2 losses are mostly a function of the change. radiation balance. Thus it has been Level of Temporal Spatial that regional or larger hypothesized Human activities scale scale Major processes organization scale water losses will probably be Years to Globe Energy, carbon, water Deforestation, fossil Biosphere insensitive to changes in atmospheric fuel burning fluxes millennia CO2 (Jarvis 1989). Plant breeding, land Biome Subcontinental Centuries to Evolution/extinction, In addition, the absolute amount of millennia management, migration, disturbance water returned to the atmosphere via conservation -I Ecosystem Tree 576 10 to 104 ha Years to centuries 10-2 to 103 m2 Minutes to decades Disturbance, nutrient cycling, production, water use, succession, competition Phenology, reproduction, physiological processes, death Pollution, exotic pests, fire suppression, flood control, forest management, soil management Fertilizing, watering, weeding, breeding evapotranspiration is a function of leaf area. If climate change or elevated CO2 alters leaf area, this change would have a profound effect on hydrologic regimes and river flows (Wigley and Jones 1985). Human impacts. Recent human activ- BioScienceVol. 40 No. 8 ORNL DWG 89M-1841 O I/ 7'\ CO2 andthe forest. betweenatmospheric Figure2. Thecomplexrelationships ities are affecting the feedbacks among CO2 concentrations, global climate, and global vegetation. The largest global net fluxes of carbon from 1800 to the present are from fossil fuel burning(150 to 190 x 1015 grams; Rotty 1987) and land use changes,in particularthe conversion of forestland (highcarbonstorage)to agriculture(low carbon storage; 135 to 228 x 1015 grams of carbon; Houghton et al. 1983). The use of fossil fuel for energy is heaviestin the temperatezone of the northern hemisphere, whereas current land-use changes are most dramatic near the equator, where large areas of tropical forests are being cleared or degraded. However, human activitieswithin any region that cause increases in atmosphericCO2 concentrationscan affect global climate patterns. Extensive planting of intensively managed forests or tree plantations has been discussed as a method for recapturing this "excess" atmosphericCO2 and thus slowing down the increasein CO2 and consequently climate change (Marland 1988). However, as Jarvis (1989) pointed out, it would take a new plantation with an area twice the size of Europe to absorbthe currentglobal fossil fuel emissions. Furthermore, after 80 September1990 years, the carbon uptake ability of such a plantationwould begin to decrease due to stand maturity,a new plantationwould have to be initiated, and the wood from the previousone would be stored in perpetuity. Switchingfrom fossil fuels to biomass-based fuels is a much more effective approach for using forests to slow down the buildup of excess atmosphericcarbon. The net release of carbon from a biomass fuel is close if not equal to zero. The US Congress Office of Technology Assessment (OTA 1980) estimatedthat biofuels could potentially generate 6-17 quads of energy annually in the United States. In 1988, the most recent data available, US fossil fuel energy use was approximately 71 quads. and the redistributionof forest types in responseto global changeare complex issues (c.f. Brubaker1986, Davis and Botkin 1985, Pennington1986). Some organismstrack climatechange closely, reacting to conditions each year, whereas others respond so slowly that only long-term climatic trendshave any observableeffect(Davis 1984). Tree species are expected to migrategraduallyto trackhospitable environmentsas climate changes. However, although the average rate of tree species migration in Europe and North America during the Ice Ages was approximately300 m/year (Woodward 1987), future migration rates are difficultto predict, because the projectedrate of climatechangeis an order of magnitude greater than previousclimate changes. Davis (in press) suggests that the spatial displacementof habitats can be visualizedif one assumesthe same general configuration of climate as today. With a latitudinallapse rate in the Great Lakes region of approximately 100 km/?C,isothermswould be displacednorthward300 km in the next 100 years. This rate is ten times greater than the documented range extensions for trees in a similartime interval.Furthermore,there are now new barriersto migrations (e.g., cities, agriculture,and roads) and new modes of migration(e.g., cars, trains, transplantsfor horticulture,forestry, and agriculture.) Range extension in the futuremay be less efficientthan in the past, because advancedisjunctcolonies have been extirpated by human disturbances, and propagule sources have ORNL-DWG 90M-1797 *I50- Biome I- / The majorecologicalprocessesof in25 terest at the biome level are species 0_ A migration,evolution, and extinction. Disturbances, such as introduced U /. blights, occur at both the biome and IC I I I the ecosystemlevel.To avoid duplica900 1100 700 300 500 100 tion, we will review these disturATMOSPHERICCO2 (ppm) bances in the discussion of ecosystems. response Figure3. Typicalphotosynthesis of a C3 plant(sugarbeet)to increasing CO2. Migration. The migrationof species atmospheric 577 tions (e.g., city parks) far south of their naturalrange. Common garden tests, widely used with both tropical and temperatedomesticatedtree species, could provide more specific informationon geneticvariabilityin the climate response (Kellisonand Weir 1987). Tree specieswith broad ranges,includingmost commercialspecies, are most likelyto surviveclimatechange. The manytree speciesthat are rareor restricted in occurrence, including 115 of the 679 native tree species in the United States (Little 1979), will likely be at greaterrisk of extinction. The evolutionaryresponseof trees to increasingatmosphericCO2 is unknown. The within-species genetic variabilityof CO2 sensitivityin CO2responsivetraitssuch as photosynthesis and stomatal control is unknown also. However,short-termdifferences in physiological responsiveness of seedlingsand leaves of differenttree species have been documented (e.g., Tolley and Strain1984). Tree speciesare likely to have high variabilityin CO2sensitivity,because naturaltree populationshave shown high variabilityin other characteristics, genetic variabilityin CO2 sensitivity has been demonstratedin herbaceous agricultural species, and earlyevolutionof hardwoodtree species took place in the Cretaceousera when atmospheric CO2 concentrations were three to ten times higher than today (Gammon et al. 1985). Positive changes in individual tree water-useefficiency(increasein biomass per unit of watertaken up), as a resultof stomataland photosynthesis responsesto elevatedCO2concentrations, could also extend some species' Evolution.Evolutionor extinctionof ranges into warmer, drier climates tree species may be affected by cli- than are acceptable under current mate changeor increasingconcentra- CO2 conditions. tions of atmospheric CO2. The genetic resilienceof most tree speciesto Modeling. Severaldifferentmodeling climatechangemust be inferredfrom approaches have predicted biomethe currentclimaticconditionswithin level responsesto changes in climate their natural ranges (Johnson and and to increased atmosphericCO2. Sharpe 1982). However, the bound- Biome changes at the scale of 0.5aries of a species' range may reflect degree by 0.5-degree grid cells were competitive ability rather than cli- simulatedby using the Holdridgelife matic constraints (Woodward 1987, zone classification and the current c.f. Michaels and Hayden 1987) or and predictedtemperatureconditions climaticrestraintsthat areonly signif- undera doublingof CO2 (Emanuelet icant during the reproductivephase. al. 1985a, 1985b). The simulations Many northern tree species grow predicted biome type changes quite well in noncompetitive situa- throughout34% of the world's land- often been reduced (Davis 1989a). The current spatial distributionand abundanceof a speciesis expectedto influence its ability to migrate successfullyto regionsof suitableclimate and soils (Petersand Darling 1985). The rate of habitat displacement and the rate of species dispersaland migrationwill influencethe composition and structureof forested landscapes, especially if time lags occur (e.g., Davis 1984, Pennington1986, Webb 1986). Time lags in species responses to historical climatic changeshave been well documented. For example, beech (Fagusgrandifolia), which has animal-dispersedseeds and tends to move as a front, showed a time lag of 500 to 1000 years in crossingfrom the easternto the western shore of Lake Michigan (Davis 1989a). In contrast, hemlock (Tsuga canadensis), whose wind-dispersed seeds can travel 100 km beyond the main species front, showed no time lags attributableto crossingthe Great Lakes. Davis (1989a) also proposes that time lags in the adjustmentsof species abundancesto climate will be small (approximately 10-20 yrs) in the heavily disturbed communities that cover most of the landscape. The most common species will be dispersed to new habitats by humans, but time lags will be a problem for unmanaged forests, natural areas, and preserves. The climate change during the next century may not be enough to kill dominant species directly. Thus forestedlandscapesmay appear superficiallyunchangedeven though a differentcommunityis becoming establishedin the understory. 578 mass. Boreal forest decreased by 37%, tundradecreasedby 32%, subtropical forest decreased by 22%, subtropical thorn woodland increased by 37%, and subtropical desertsincreasedby 26%. In addition to temperatureeffects, CO2 and soil moisture effects were includedin a model that predictedthe rangesof eight commercialnorthwest North Americantree species under a doubling of CO2 (Leverenzand Lev 1987). The model combined a projectedpositive effect of elevatedCO2 on the water-use efficiencyand photosynthesis of individual trees, with regional-scalecalculationsof site water-balance and temperatureconditions predicted under a doubling of atmosphericCO2. The rangesof lower-elevationspecies were projectedto expand to higher elevations at the expense of the currenthigh-elevation species.The requirementsfor chilling in several of the species, ratherthan their physiological tolerance to drought and heat, determinedtheir low-elevation and southern-range limits. As in the Emanuelmodel, migration, succession, and competition are not included. The usefulnessof both these models lies in their ability to provide insight into the rangeof possibleconsequences of climate change. As the authorsthemselveshave pointed out, the actual predictive value of each model is quite limited. The effectsof temperatureand precipitation change were predictedfor forests across easternNorth America by using FORENA, an individualbased forest stand model (Solomon 1986). The ability of the model to projectsuccessionalshifts in response to climatehas been corroboratedwith historicpollen records(Solomonand Webb 1985). The effects of climate change on forest growth of mixedaged, mature forests were simulated for 21 sites across boreal, deciduous, and tundra biomes. The FORENA model predictedslower growth rates of most deciduous tree species throughout much of their range; a universal but centuries-longdieback of the original forest; an invasion of the southernboreal forest by temperate deciduous trees; a shift in the generalpatternof forest biomes similar to the patternobtainedby Emanuel et al. (1985a, b); and, most interBioScienceVol. 40 No. 8 esting, time lags in shifts of community species and biome boundariesof up to 1000 years from the initial change in climate. Species migration is not considered in FORENA, and none of the models included the potential effects of naturaldisturbances or human activities. Human impact. Human activitiesare expectedto influencemany forest responses. Activities with biome-level impact include breeding programs, land management,and active conservation. Breedingprograms, a viable option for many commercialspecies, may permit certain species to maintain theircurrentranges(Kellisonand Weir 1987). Landuse and ownershipwill affect not only the migrationrate of species but also their ultimate range and abundance. For example, the predicted range of loblolly pine (Pinus taeda)undera warmerclimatewould extend well into Indiana and Ohio (Solomon1986). However,the actual rangeis not likely to extend that far, becausethe northernland would still be used for agriculture(Miller et al. 1987). Furthermore,the spatial displacement of forest types may alter their ownership and thus their commercial productivity (Wallace and Newman 1986). Human use of forests for fuel wood could also change with a warmerclimate,therebyaltering rates of forest degradation in some areas. The effectivenessof parks and wildernessareas in preservingrare species may change if the spatial distribution of a biome shifts. Humans may need to assist the migration of some species if those species are to survive (Peters and Darling 1985). Active conservationis most likely to be necessaryif the range of a species occurs at the edge of a biome, if the species has limited dispersal ability, or if the habitat of the species is disjointly distributed within the biome. Ecosystem Severalecological phenomena are of special interest within the ecosystem level of organization. These include disturbance,nutrientcycling, competition and succession, production, and water use. September1990 Disturbanceregimes.The interaction between climatic change and disturbance regimes,which can rapidlyalter forest structure,potentiallycould be as importantas the directeffectsof global warming in controlling shifts in species distributionsand local extinctions.Global warmingmay result in an increasedfrequencyof dry years and a consequentincreasein the risk of large fires (Sandenburghet al. 1987), perhaps increasingthe likelihood of fires of the magnitude observedin the northernhemispherein 1987 (e.g., China and North America) and 1988 (e.g., YellowstoneNational Park and elsewhere in North America; Turner and Romme in press).Fireregimesmay be more sensitive to climate change than some forest processes, because fire is responsive to fuel moisture, which depends in turn on precipitation and relativehumidity.The directeffectsof climate change on forest production and decomposition might be overshadowed by more drastic effects of climate on fire regimes (Clark in press). Past climaticchangesof small magnitude have caused significant changes in fire regimes (e.g., Clark 1988, 1990). In northwesternMinnesota, for example, fire was most frequent (approximately8.6-yearreturn interval) during the warm, dry fifteenth and sixteenth centuries and less frequent (approximately 13.2year return interval) during the cooler, moister periods from 12401440 A.D. and during the period (1640-1840 A.D.) called the Little Ice However,the projectionof changesin fire frequency,intensity, and severity with a changingclimateremainschallenging because of the complex relationship among weather, fuel availability, and ignition (Turner and Romme in press). Biotic disturbancessuch as forest pests and pathogensmay also change. Because of their short reproductive cycles,such organismsmay be able to adapt or evolve much more quickly than forests. Furthermore,the ranges of forest pests and pathogens are often limited by climatic factors, and speciesdistributionsmay changewith climate. For example, spruce budworm populationsmay increasewith a warmer climate because they are favored by warm, dry, early spring conditions(Haynes1982). Insectherbivory may also increase because of CO2-inducedchanges in plant tissue quality.In controlledtests, leaf-eating insects that fed on agriculturalplant tissuegrown underelevatedCO2generally had higher consumptionrates per individualbecause of the poorer plant tissue quality (high C:N ratio) (e.g., Butleret al. 1986). Trees experiencingstress due to climate change may also have lower resistance to pests such as bark beetles (e.g., reducedresinexudation).Possibleshifts in the range and relative competitiveness of weedy plants may be important to managed forests and nature reserves (Peters and Darling 1985). Because long-lived trees may survive short-termclimatic fluctuations, forest speciesthat are best adaptedto the currentclimate may only be able to become establishedin open habitats after disturbance events. Thus forest composition may respond to climatic changes primarilyafter disturbance (Dunwiddie 1986). For example,althoughthe ultimatecauseof postglacial vegetation change in the Pacific Northwest was climate change, the proximatecause of some vegetationchangeswas an alteredfire regime (Cwynar 1987). It is likely that a small changeto a drierclimate triggereda relativelylarge change in the disturbance regime (Cwynar 1987). The implicationsfor the present are extremelyimportant,because Age (Clark 1990). Almost all major forest fires in Mt. Rainier National Park, Washington, since 1300 A.D. corresponded with major droughts that were reconstructedfor regions east of the Cascade Range (Hemstrom and Franklin1982). Fire regimes are sensitive to the averageclimate over decades to centuries,as well as the interannualvariabilityof water balance(Clark1990). In northwestern Minnesota, fires were clustered during times of extendedlow effectiveprecipitationand soil moisturestoragein the 1880s and 1910s (Clark 1989). Similarly, the durationof dry periods is one of the best predictorsof the area burnedin an altereddisturbance regimemaybe contemporary Canadian forests one of the earliestsignalsof climate (Flannigan and Harrington 1988). change. 579 Competition and succession. Experimental and paleoecological studies that indicate that competitive abilities of species will shift as a result of respecies-specific physiological sponses to climate change and elevated CO2. In a microcosm study of bottomland and upland tree communities, seedlings of different tree species were grown together for 90 days under elevated CO2 and two levels of light. Elevated CO2 had no effect on the overall growth of seedlings in either simulated community, although there were differential species responses to CO2 within the communities (Williams et al. 1986). In another growth chamber study with seedlings, a fast-growing pioneer tree species showed a 79% increase in biomass under an enriched CO2 atmosphere, whereas a slower-growing climax species showed only a 30% increase (Oberbauer et al. 1985). Such studies suggest that successional paths may change and new community types may evolve under conditions of elevated atmospheric CO2, just as they have in areas subjected to chronic ozone air pollution (Taylor 1984). However, the specific results of such studies with seedlings cannot be interpreted as reliable predictions of future stand responses, because other factors, such as temperature and evaporative demand, that will change concurrently with elevated CO2 were not considered in the designs of the experiments, nor were the studies designed to consider effects on seedling establishment or ecosystem feedbacks. Paleoecological studies also strongly support the hypothesis that individual species response to climate change and elevated CO2 and seed source availability will work to create completely new community types (Davis 1989b). For example, such studies have identified community types such as a spruce-oak woodland in North America that existed in the past but cannot be found now. The rate at which communities will change is difficult to predict, because it depends on not only competitive differences but also species longevity, changes in disturbance patterns, and seed source availability. Productivity. Site-specific ecosystem productivity will change as a conse- 580 ORNL-DWG90M-1796 50 (A 4 E 40 o 0 E IUJ 30 z 0 20 - 0 0 o 0 900 300 600 CO2 (pbar) INTERCELLULAR 1200 Figure 4. The relationshipbetween photosynthesis and intercellularCO2 partial pressurein two C3 speciesgrown at atmosphericCO2 partial pressureof 300 pLbars (filledsymbols)or 900-1000 ,xbars(open symbols) (300 txbarsis the normalpartial pressureof CO2 in the atmospherein Reno, Nevada, where the experimentstook place). The arrows indicate the intercellularpartial pressure of CO2 when the experimentalatmosphericCO2 was set equal to the partialpressureof CO2 at which the plants were grown. Note that the adaptionto elevatedCO2 resultedin a greatly reducedrateof photosynthesisin the cabbageplant (Brassicaoleracea)but an increased rateof photosynthesisin the potato plant (Solanummelongena).(AdaptedfromSageet al. 1989 with permission.) the boreal forest duringthe past century is evidence of this type of relationship (Josza and Powell 1987). Long-termdecreases in productivity will occur in ecosystems where the current most limiting environmental factor becomes even more limiting. Forestecosystemsin the southeastern United States appearto be quite vulnerableto a warming,becausethereis crease. In the short term, ecosystem currentlya delicate balance between productivity will be a function of the potential evapotranspirationand acresponses of the trees currently pre- tual precipitation (Neilson et al. sent and of the abundance and 1989). The degree to which elevated CO2 growth rates of invading individuals. Thus the short-term response is not will directly alter forest responsesis necessarily indicative of the long-term the wild card in all climate change scenarios. Species adapt to elevated condition. Long-term positive increases in CO2 in differentways. Some species productivity are likely where current display high assimilation rates at quenceof changesin multiplefactors, such as species composition, altered climateconditionsfor growth, and/or CO2 fertilization effects (Leverenz and Lev 1987). Furthermore,the direction of the change may shift over time. For example, if ecosystem change is initiated by catastrophic disturbance, productivity will undoubtedlyfirst decreaseand then in- growth is strongly limited by length of the growing season. The correlation between warmer climate and an increase in stem wood productivity in twice ambient CO2, regardless of the CO2 concentrations under which the individualshave beengrown (e.g., the potato plant in Figure 4). In other BioScience Vol. 40 No. 8 species, the positive assimilation response to elevated CO2 is lost if the individuals have been grown under elevatedCO2 (e.g., the cabbageplant in Figure4). For an increasein productivityin responseto elevatedCO2, not only must the species be responsive to elevatedCO2 even if adapted, but there must also be a carbon sink for the extra photosynthate,that is, the ecosystem must be environmentally capableof increasingits biomass (Drakeet al. in press). The complexities of the CO2 response are apparentin the published reports of two long-term ecosystemlevel studieson the impactof elevated CO2.Arctictussocktundra,an exceptionally environmentallylimited ecosystem, showed no increase in productivityduringthree years of in situ exposure to double ambient CO2 (Oechel and Reichers 1986, Tissue and Oechel 1987). Furthermore,laboratory studies with tundra species have shown no positive growth responses to elevated CO2, even with the addition of nutrients (Billingset al. 1984, Oberbauer et al. 1986). Thus, in the arctic tundraecosystem, both species characteristicsand environmentallimitationsappearto keep ecosystemproductivityfromresponding positivelyto elevatedCO2. In contrast,a salt marshdominated by a C3 species showed increased productivityduringeach of the three contiguousyearsthat it was subjected in situ to elevated CO2 (Curtiset al. 1989a,b, Drake et al. 1989, in press). Both above-ground and belowground productivitywere higher underelevatedCO2. Quite interestingly, the increase in productivitywas not only a consequenceof increased assimilationrates but also of decreased respiration rates (Drake et al. in press).These two studiessuggestthat potential forest ecosystem responses to elevated CO2 are also likely to be complex. Even if all other things were constant (which, of course, will not be the case under a changed climate), it is quite doubtful that forest productivity responses to elevated CO2 would be uniform. Young or aggrading forests would be more likely than mature forests to display increased productivity in response to elevated CO2, because younger stands should provide more of September1990 a carbon sink for extraphotosyn- mental evidence that tree litter thate. lignin:N ratios, which are also negatively correlatedwith decomposition Wateruse. It is difficultto predicthow rates and generallya better indicator water use will changein specificeco- of decay rates than C:N ratios, resystems.Althoughwater-useefficiency main the same or decreaseundereleof individual seedlings subjected to vated CO2 (Norby et al. 1986). The elevated CO2 generallyincreasesdue interaction of all these positive and to stomatalandphotosynthesiseffects, negativeeffects and the difficultiesin total ecosystemwateruse maystaythe extrapolating results across ecosyssame or increase,becausewater use is tems limits currentability to predict a functionof manyfactorsin addition the overall impact of climate change and elevated CO2 on decomposition to efficiency. Leaf area is positively correlated processes. The effectof elevatedCO2on many with stand water use; thus a CO2induced increase in stand leaf area aspects of internalnutrientcycling is would increase water use, all other largely unknown for forest ecosysfactorsbeingequal (Jarvis1989). Wa- tems. No studies have followed the ter use is also strongly related to effect of elevated CO2 on woody evaporative demand, which will in- plant nutrition for longer than one crease under a warmer climate. In growingseason. Limiteddata suggest addition, water use is a function of that the return of nitrogen through precipitationand timing of precipita- litter fall may remainconstantor detion. If climate change alters either, creasedue to an observedincreasein then forestwateruse will change,and nutrient-useefficiency (Norby et al. this changecould have profoundcon- 1986). Species shifts associated with sequenceson streamflow (Wigleyand climate change may also change ecoJones 1985). Finally,if the forest can- system nutrient cycling patterns and opy is dense and deep, and thus rates at specificsites (Pastorand Post poorly coupled with the atmosphere, 1988). water use will be largelya functionof net radiationabsorbedby the canopy Modeling. Experimental studies of rather than of canopy conductance the impactof climatechangeand elevated CO2 on forest ecosystems are (Jarvisand McNaughton 1986). limited by inabilityto easily manipuNutrient cycles. Climate change and late climate and atmosphere at the elevated CO2 will affect forest nutri- spatial scale of a forest ecosystem. ent cycles by alteringlitter decompo- Thereare no experimentalstudiesexsition rates, plant nutrient uptake, posing intact forests ecosystems to and/or internal cycling. Decomposi- elevatedCO2with or withoutwarmer tion rates will change in responseto climate conditions. Thus predicting alterations in the physical environ- forest ecosystemresponseto changes ment, litter quantity and quality, or in CO2 concentrations and climate abundanceand typesof decomposers. requires extrapolating results from A warmerclimate would tend to in- finer to broader scales, probably crease the rate of decompositionby through modeling (Johnson and enhancing fungal and bacterial Sharpe1982, Solomon 1986). The effects of climatic change on growth (Meentemeyer1978), but a drier climate would tend to retard processessuch as net primaryproducdecomposition.Forest arthropods,a tion and decompositionmay be elucikey componentof the decomposition dated by ecosystem simulationmodprocess, could have their ranges al- els. Individual-basedstandsimulation tered and their activitylevels affected models, which incorporate climatic by changingplanttissueor litterqual- factors as driving variables for tree ity (Kimball1985). growth, have been widely used to If the higher C:N ratio found in predict the response of forest stands plant tissues grown under elevated to climate change (e.g., Davis and CO2 results in higher C:N ratios in Botkin 1985, Pastor and Post 1988). litter, then decomposition may be Model results have supportedpaleoslowed because decomposition rates ecologicalevidencethat speciesabunare negatively correlated with litter dances are not always in equilibrium C:N ratios. However, there is experi- with climate (Woodward1987), and 581 competitiveinteractionscan strongly influence community response (Solomon 1986). Simulations also suggest that there may be a transition period of up to 200 years during which time stand biomass drops (often later to recover) as the forest developedunder the original climate is replacedby species bettersuited to the modifiedclimate(Solomon1986). Empiricalmodels of the relationship between climate and ecosystem processes such as decomposition (e.g., Meentemeyer1978) may also be useful, particularly at broad spatial scales. The combined impact of climate change and elevated CO2 on forest stand structureand growth has been simulated using individual-based models by incorporatinghypothetical effectsof elevated CO2 on individual tree growth. For example, Solomon and West (1987) modifiedFORENA by increasingthe maximumpotential growth by 20% for deciduousspecies and 11% for coniferous species and by decreasing deciduous-treewater use by as much as 18%. These modifications were designed to account for possible productivityand wateruse efficiencyeffectsassociatedwith a doublingof CO2. Three forest types were simulated: boreal forest, coniferous-deciduous transitionforest, and southerndeciduous forest. Temperature was assumed to increase by 6? to 9? C in the winter and 4? to 5? C in the summer, and precipitationwas assumedto decrease by 25% in all but the boreal site. The incorporationof CO2effects modified the projected forest responses to climate by shorteningthe transitionperiod,decreasingthe associated decline in biomass, and increasing the projected equilibrium stand biomass by 50% in the waterand heat-stressedsoutherndeciduous forest and by 16% in the coniferousdeciduoustransitionforest. Because understandingof ecosystem responsesto CO2 is so limited, the simulation results must be regardedas highly speculativeand useful largely for their heuristic value. Nonetheless, if reliable information becomes available,these models will help us examinethe potentialecosystem implications of climate change underconditionsof elevatedCO2. The impact of elevated CO2 on 582 short-termcarbon assimilationin Picea sitchensis plantations was modeled by using MAESTRO,a physiologically based canopy model (Jarvis 1989). Althoughthe stand structure, soil respiration,and leaf area parameters of the model were not modified for the elevated CO2 simulation,the photosynthesisand stomatalparameters were alteredon the basis of physiologicaldatacollectedfromseedlings adapted to elevated CO2 concentrations. Such seedlings display somewhat lower photosynthesisratesthan unadaptedseedlingsat a givenlevel of CO2.Nonetheless,the modelstill predictedan increasein carbonassimilation at the stand level underelevated CO2. The modeling exercise demonstratedthe importanceof considering CO2 adaption effects in modeling even at the largerscale of the canopy. Models at differentspatial or temporal scales can be linked in various ways. Forexample,ecosystemmodels can be joinedwith standmodels,with the ecosystem model defining available resources and the stand model allocatingresourcesand growing individualtrees.Pastorand Post (1988) exploredthe impactof climatechange on mature forests of eastern North Americaon a varietyof soils with an individual-based stand simulation modelin which the nitrogencycle and water balance were explicitly modeled. In the simulations,productivity and biomass increasedon soils that retained adequate water but decreasedon soils with inadequatewater. The model results illustrate the importanceof spatialheterogeneityof soils in predictingforest responsesto climatic change at the ecosystem scale. Human impact. Humans influence forest ecosystems primarilythrough forest management activities, although local air and water pollution, poor soil management (erosion or compaction), introduction of new pests, and human modifications of natural disturbanceregimesthrough activitiessuch as firesuppressionand flood control can also be important. Silviculturaltools such as planting stock, density of planting, mulch, fertilization, and thinning regimes may be used in managed stands to counteractnegativeeffectsof climate changeor to augmentpositive effects (Sandenburghet al. 1987). Forest response to elevated CO2 and climate change may be easier to manage in mixed-speciesstands under uneven-agedmanagementthan in monospecificstands undereven-aged management.Individualtreeswill respond to climatechangeon a gradual basis and at uneven rates. Unevenaged managementallows harvesting in accordance with the differential timing of those responses. In stands undereven-agedmanagement,species composition and genotype can be switched only at planting time. In short-rotation,intensive-cultureplantations, where genotypes and/or cultural regimes can be easily changed every 10 to 20 years, the risk may be minimal.However, even-agedstands having longer rotation times will require decisive management with muchmore risk. Alreadythereis concern within the United States timber industry about the fate of loblolly pine stands planted to the south and west of their natural range if the climate in that region becomes even drierand hotter (Grahamet al. 1986). Regionalland-usechangeswill also influence forest ecosystem responses (e.g., Turner 1987). Hierarchy and landscapeecology theorysuggestthat an ecosystem's response to disturbance may be a function of the system's setting in the larger landscape (e.g., Turneret al. 1989b). Direction of changes. Although ecosystem phenomena are likely to change in response to elevated CO2 and climate change, the directionof the changes will depend on highly specificcircumstances.The prediction of the impact of these changes on forest resourceswill depend on integrating experimentalresults from a finer scale and linking them through concepts of ecosystem function (Woodward1987). The most significant long-termeffect of elevatedCO2 and climate change on forest ecosystems is likely to be changesin disturbanceregimes(Turnerand Rommein press) and in successionalpatternsin the unmanaged,mixed-speciesstands that dominateglobal forest resources (Solomon et al. 1984). The role of elevated CO2 in these changes is highly uncertain because of our inability to predict whole, maturetree responses to elevated CO2 and the BioScienceVol. 40 No. 8 many feedbacks between CO2 and ecosystem function and structure. Both currentmodelingresultsand paleoecological evidence suggest that during times of climate change, the responseof naturalforests in the absence of disturbanceis delayed because of the longevity of the individual trees. Thus we may not perceive climateeffectson communitycomposition until long after the climatehas changed. Managed mixed-species stands and plantations will also be sensitiveto elevatedCO2 and climate change, but with foresight, human interventioncan potentially mitigate many of the economically and socially detrimentalresponses. quency of such events increases. responses to these factors (Kramer Information on the effect of ele- and Sionit 1987). The lack of longvated CO2 on treesis limitedprimar- term studies precludesmuch data on ily to leaf-levelstudies, a 3-week ex- the impact of elevated CO2 on tree posure of the canopy of 12-year-old life-cycleevents or phenology. trees (Wong and Dunin 1987), and short-term (two growing seasons) Modeling. Even at the much finer studieson seedlingsor young saplings spatialand temporalscale of the tree, of less than 30 tree species (Eamus modeling is essentialfor understandandJarvis1989). The resultsfromthe ing and predictingresponse to CO2 few longer-term studies are con- and climate.Tree size, longevity,and founded by experimental artifacts the large numberof species preclude (Jarvis 1989). Under well-watered taking an exclusively experimental and fertilizedconditions,nearlyall C3 approach to evaluating individual species, includingtrees, show signifi- tree responses.Models of physiologicantly greater growth with elevated cal processes such as photosynthesis CO2 (Kramerand Sionit 1987). and respiration(e.g., Farquharet al. The usefulnessof this information 1980) can be used to test hypotheses is limited,however,becausetreesgen- about the interactionsof temperature, erallygrow underless than ideal con- CO2, radiation, and moisture on a Tree ditionsand for manyyears.Thus data leaf. Thesemodelsmay help elucidate Processesof interest when consider- on the long-termresponseof trees to the mechanisms of whole-tree reing individual trees are phenology, CO2 under conditions of nutrient sponses to climate change and allow life-cycle events (e.g., reproduction stress,water stress,and light depriva- prediction of tree responses under and death), and physiological pro- tion are needed. These data will be conditions beyondthose of the expercesses(e.g., photosynthesis,transpira- difficultto obtain and will likely be iments. tion, respiration, carbon allocation, limited to a few species. nutrientuptake, and nutrient allocaAlthoughinitiallysome of the eco- Humanimpact.Humanscan influence tion). The key challengeis to identify logical literature invoked Liebeg's individual trees by modifying either the whole-tree effects of climate fac- Law of the Minimumto suggestthat the immediateenvironmentof the tree tors and/or atmospheric CO2 with CO2 will have no beneficialeffect if (e.g.,watering,fertilization,weed conand without other stresses (e.g., otherenvironmentalfactorsare limit- trol, or pollution abatement)or the drought, flooding, cold, shade, poor ing, short-termseedling studies have geneticmakeupof the tree (e.g.,breedsoil, and pollution). repeatedlyfound that elevated CO2 ing or geneticengineering).Of the four enhances growth even under condi- levels consideredin this the Climate. Current understanding of tions of nutrient, soil moisture, or individualtree seems mostarticle, amenable the effect of climate variables on light deprivation (e.g., Tolley and to experimental study. Currently, trees is based largely on qualitative Strain1984). In many cases, the relalong-termdata and data on life-cycle observations of mature trees and tive response of stressed plants to eventsand phenologicalresponsesare short-term(minutesto days) experi- elevated CO2 exceeds that of unneeded. especially ments on leaves or seedlings. Exper- stressed plants. Whether this enimental studies on whole, mature hancementcan be maintainedover a trees have been limited because of longer period is, however, unknown. Implicationsfor future research tree size and longevity. Mature-tree Severallong-termfield studies curresponses to climate are generally rently underway should help resolve and detecting forest reinferredby relatingobservedgrowth this most important issue. Elevated Predicting sponses to the projected changes in responses to past weather patterns CO2can also inducein seedlingssub- CO2 and the global climate present (e.g., Holdaway 1988). Field data on tle morphologicalchangessuch as in- significantchallengesthat require cregrowth responses to climate are of- creasedbranching,increasedratio of ative solutions. Major issues include ten qualitative, and growth data are fine to coarseroots, and greaternodchanges in the areal distributionof generally lacking from the tropics ule mass on nitrogen-fixingspecies forests and forest types, changes in (Solomon et al. 1984). In particular, (e.g., Norby 1987). Tissue quality productivityor other functions of inthereis little informationon how the also changeswith elevatedCO2; C:N terest, and the influenceof landscape frequencyof extreme climatic events ratios increase,and lignin contentde- heterogeneityon these responses.The affectstree-levelprocesses. This lack creases(e.g., Williamset al. 1986). challenges arise primarily from the is especiallyimportant,becauseafter The combined effect of elevated broad-scalenature of the questions, the predicted climate change trees CO2and otherenvironmentalfactors, the projectedrate of change, and the may be exposed more frequently to such as temperature,humidity,or air manyenvironmentalinteractions that extremeevents. Treesmay survivean pollution, have only been studied at must be considered. The and types infrequent extreme event such as a the individualleaf level in trees.Agri- combinationsof experimentsthat can severedroughtbut experiencedelete- culturalstudies suggest that elevated be conducted become increasingly rious effects-even death-if the fre- CO2 may partially mitigate negative limited as one goes to successively September1990 583 Table 2. Timingof forestresponsesto global climatechangeand elevatedCO2. Feature Likelyresponse Earlychanges(yearsto decades) Disturbanceregimes Seedlingestablishment Ecotonemovement Hydrologicregimes Intermediateto long responsetime (decadesto centuries) Productivityrates Changesin pools of carbonand otheressentialelements Species composition in interior biomes they are not, why. More information on below-ground responses to elevated CO2 is also needed, as is information on tropical forest responses. Changesin the frequency,intensity,and extent of It may be prudent to focus experidisturbancessuch as fire,pest or pathogen mental work on tree species that are outbreaks,and stormeventsmay lead to rapid economically important or characterchangesin forestedlandscapes. istic of particular forest ecosystem Changesin the relativeabundanceof seedlingsof differentspecies(i.e., failedor decreased types. Research on water use and recruitment)suggestthe initiationof a longCO2 response is particularly relevant termchangein speciescomposition. for species that reside in regions such Biome-levelchangesin the locationof boundaries as the southeastern United States that betweenforestedand nonforestedsystemsmay are likely to be water limited under be observedearly,becausespeciesat the edges of theirrangesare likelyto be less tolerantof global warming. Experimental work climaticchanges. should also focus on variables that Changesin precipitation,temperature,and stand provide linkages between spatial wateruse will alterrunoffpatterns,although scales and biotic levels. For example, potentialregional-levelchangesin water use et al. (1986) focused on paNorby are not well understood. efficiency rameters linking tree-level physiological phenomena with ecosystem-level decomposition processes. Because of Forestproductivityover largespatialareaswill the increased likelihood of drought in changeslowly, becausechangesin species some of the major temperate forest compositionmay reducethe rateof changein areas and the societal importance of productivity.Increasedwateruse efficiencyin water availability, the interactions betreesmay help mitigatepotentialproductivity losses due to drought. tween CO2 and water use and forests As productivity,decomposition,and nutrient at larger spatial scales needs particucyclingchangewith climate,storageof lar exploration. in essentialelements forestswill also change. Treemigrationwill be a long-termresponse becauseof time lags in the responsesof mature trees.Matureindividualsmay persistfor decades,even thoughthe compositionof the understorychanges. larger spatial and temporal scales. Future research should include monitoring, experimentation, and modeling. Detection. Attempts to detect changes in forests should focus on sensitive processes and areas, recognizing the different time scales over which responses will be observed (Table 2). One of the earliest indications of forest response to global change may be alterations in the frequency and intensity of disturbances. Therefore, particular emphasis should be placed on increasing ability to predict the occurrence and effects of disturbances and on monitoring disturbance-prone forests. Seedling establishment may be particularly sensitive to climate change. Thus tracking tree recruitment in natural stands may provide an earlier indication of forest response than monitoring changes in the overstory composition, which may exhibit substantial time lags. Efforts to predict changes and monitor the distribution and boundaries of major ecosystem types should con584 tinue. In the absenceof disturbance, the firstperceptiblechangesin global vegetation patterns may be at the transitionsbetween major life zones (e.g., Nielson et al. 1989). The boreal forest transitionzone and its alpine counterpartmay be particularlysensitive. It is also criticalthat the effects of changeson the variancein temperatureandprecipitationbe considered. Mean values may not be meaningful, because extreme events may affect forestedareas even though the mean remainsunchanged. Experimental research. Experimental research needs include quantitative data on the water use, productivity, nutrient uptake, and phenological responses of mature trees to long-term exposure to elevated CO2, the sensitivities of tropical species to climate, and the interactions between exposure to elevated CO2 and other environmental stresses. We need to know if the adaption and acclimation responses observed in seedlings are characteristic of mature trees and, if Scaling. Research is especially needed on the scaling problems that permeate the entire discussion of forest responses to climate change and elevated CO2. Methods to extrapolate information from site-specific studies to the regional scale must be developed if the broad-scale questions are to be answered. Changes in the importance of the variables controlling ecological processes at different scales should be used to our advantage in developing hypotheses and designing experiments (Turner et al. 1989a). Ecological responses differ with the temporal and spatial scale at which they are studied, and the controlling variables at each scale are likely to be different. As spatial extent increases, for example, the level of discernible detail, the number of important variables, and the potential for experimental manipulation all decrease (Meentemeyer and Box 1987). Important variables at the broad scale tend to be abiotic rather than biotic, and the variance of parameters in the landscape may increase with scales (Meentemeyer and Box 1987). For example, the rates of litter decomposition at a particular site can be predicted knowing the chemical/ BioScienceVol. 40 No. 8 physical properties of litter species, but at broad spatial scales, climatic variablesare sufficientto predictrates (Meentemeyer1978, 1984). Thus we may be able to substantiallysimplify experimentsif we can identifya priori the key controllingvariablesat a particular scale. The development of good correlative relationships between abiotic and biotic variables,for example, could significantly reduce the numberof parametersthat must be measuredthrough time to predict forest responses. Large-scalestudies. The dynamicsof heterogeneouslandscapemust be understood to predict regional forest responses.Global change is likely to have substantialeffects on the structure and function of forested landscapes,yet the implicationsof spatial heterogeneity over large areas are only beginning to be understood (Turner 1989). The traditional process-orientedview of ecosystemsmust be complemented with other approaches as we seek to understand forest responses at larger and larger spatial scales. For example, manipulation of the factors that constraina species distributionor an ecological process may be more insightfulthan obtaininga more detailedview of all factors that influence the phenomenon of interest. Cross-site research is essential to understanding forest responses to global change. Observationsor manipulative experiments can be conducted in forests across a globally significantgradient(e.g., temperature or precipitation gradients in North America)to test hypothesesabout the mechanisms controlling ecological processesat broad scales. For example, studies conducted along a climatic gradientmight use a varietyof carefully chosen sites as surrogates for climatic change. One might test hypothesesabout the abilityof particulartree speciesto withstanddifferent climates, the time for recovery from disturbances,or changes in the variance of biological or physicalparameters.Identicalperturbationscould be imposed in differentforests across a major temperature or precipitation gradientin the United States and responses compared. Experimentscan also be conducted along altitudinal gradients across which climate September1990 changesrapidlyin responseto elevation. Comparativestudiesof the same phenomenaat differentscales would aid in the developmentof extrapolation rules. alteredby the displacementof forests at the biome scale, and local economies will certainlybe affected.Forest responsesto climate change and elevated CO2 will determinethe fate of many of the species dependent on Modeling. Currentmodelingcapabil- them, includinghumans. ities should continue to be used to explore hypotheses about ecological Acknowledgments responses, identify sensitive parameters that should be monitored, and We appreciatethe helpfuland insightprovide a means to extrapolatelocal ful comments of three anonymous responses to regional scales. Model- reviewers, our colleagues at Oak ing is particularlyimportantbecause Ridge National Laboratory,and the of the time lags in forest responses. BioScience editors. Research was Because of the potential economic sponsoredby the EcologicalResearch consequencesof climate change, it is Division, Office of Health and Envialso importantto predictforesteffects ronmentalResearch,US Department that are relevantto managementac- of Energy,under contractDE-AC05tions. From the perspective of the 840R21400 with Martin Marietta forest industry,informationmust be Energy Systems, Inc. This article is geographically and temporally spe- PublicationNo. 3476, Environmental cific. Sciences Division, Oak Ridge NaA comprehensiveresearchprogram tional Laboratory. should avail itself of currentcapabilities in remote sensing. The extrapo- Referencescited lation of empiricalresults and simulation studies to large spatial scales Barcastow,R. B., C. D. Keeling, and T. P. Wharf.1985. Seasonalamplitudeincreasein requires methods to measure and atmosphericCO2 concentrationsat Mauna track changes in broad patterns of Loa, Hawaii, 1959-1982. J. 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