Sensitivity of temperate grassland species to elevated atmospheric C0 2 and the interaction with temperature and water stress Michael B. Jones and Marjan Jongen Botany Department, Trinity College, University ofDublin, Dublin 2, Ireland The annual cycle of growth of many temperate grasses is limited by low temperatures during the winter and spring and water stress during the summer. Climate change, induced by increase in the concentration of greenhouse gases in the atmosphere, can affect the growth and community structure of temperate grasslands in two ways. The first is directly through changes in atmospheric concentra- tion of C0 2 and the second is indirectly through changes in temperature and rainfall. At higher lati- tudes, where growth is largely temperature limited, it is probable that the direct effects of enhanced C0 2 will be less than at low latitudes. However, interactions with increasing temperature and water stress are complex. Temperate grasslands range from intensively managed monocultures of sown species to species- rich natural and semi-natural communities whose local distributions are controlled by variations in soil type and drainage. The different species can show marked differences in their responses to in- creasing CO, concentrations, rising temperatures and water stress. This will probably result in major alterations in the community structure of temperate grasslands in the future. In addition to impacts on primary productivity and community structure, a long-term effect of elevated CO, on grasslands is likely to be a significant increase in soil carbon storage. However, this may be counteracted by increases in temperature. Key words: soil carbon, scaling, stomatal conductance, temperate grasses Introduction The effects of the projected changes in green- house gas concentrations on the global climate have been estimated using general circulation models (GCMs) (Viner et al. 1995). The atmos- pheric concentration of carbon dioxide (CO,) is projected to double from pre-industrial concen- trations (about 280 pmol mol') by the middle to late 21st Century (Carter 1996). Together, ris- ing concentrations of C02 and other greenhouse gases are predicted to result in an increase in global mean surface air temperature of between I°C and 3°C by 2100 (IPCC 1996). Global av- erage temperature can provide some indication of the magnitude of likely climate change, but in terms of the regional implications a global av- erage conveys rather little information. Howev- er, it is at the regional level that the effects and © Agricultural and Food Science in Finland Manuscript received February 1996 271 Vol. 5 <19961: 271-283. AGRICULTURAL AND FOOD SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=4KsdJ52ovklOGJlT.orVR8vDKmAFOVDMhlv-_tg.w9E8iyTc9nzmCWjq4w76gk4OtYOftwGjMJ-5RA_p026Vh_eLzjC94UdCq8gK_wGEE7Lsj1ebhfnP5qnOQkVqJCpKmTZlarWct8yAMKTy1FkZmR3N_-Nc3THIIA1znpRmMt1X1Y2SmcW5v3Y752k05v2tjFTnyVvlyDNxpWgN5Vs_ytq3elQ-kNn2idvUPUdmPE4nZU305D1i6E_C8DGBGRJ5JEZbetf4d86H7GfF_v3fTJD6S4KEUVUJ9tLsdUqL0QYA8w_8pR7jh22I5RCgwxxDQxM2Q_vPcA Jones, M.B. & Jongen, M.: Effects ofelevated C0 2 on temperate grasses impacts of global climate change on vegetation will be felt. Here, we will review the impact of predicted climate change on temperate grasses, and in particular those of the cool temperate cli- matic zones of Europe. While there is a good degree of confidence in the global estimates offuture climate, there is unfortunately, at present, much less agreement on projected changes at the regional level (Par- ry and Duncan 1995). One important factor is that proximity to the oceans generally operates as a moderating influence and regions close to the oceans may warm more slowly then else- where at the same latitude (Carter 1996). How- ever, in rather general terms, the predictions of greenhouse gas induced climate change in the higher latitudes of Europe suggest that (i) win- ter temperature increases will be greater than the annual global mean increase, (ii) summer tem- perature will increase by a similar or smaller amount than the global annual mean, and (iii) winter precipitation will increase while summer precipitation changes are less certain (Parry and Duncan 1995, Carter 1996). Grasslands are a characteristic, important and widespread type of vegetation in Europe (Knapp 1979, Weissbach and Gordon 1992). They range from intensively managed, single species, sown swards to natural and semi-natural communities. Even the natural and semi-natural grasslands are maintained in this state by man’s activities and those ofhis livestock, and their composition and structure has been determined by a combination of climatic, edaphic and anthropogenic forces. The result of this is a series of different grass- land communities (Knapp 1979,Rodwell 1992). These grasslands are typically composed of a complex mixture of perennial grasses, nitrogen- fixing legumes and non-fixing dicots of differ- ent growth forms. The complex composition of many grasslands and the possible differential responses of com- ponent species to changing climate makes the prediction of community responses very diffi- cult. One possible simplification is that the spe- cies can be classified as different functional types (Grime 1974, Grimeet al. 1988), which co-exist to form the grassland vegetation, and it is the response of the functional type which is investi- gated (Körner 1993). Perhaps because of their complexity, the potential effects of climate change on grasslands have received much less attention than forests, although they clearly play an important part in the global carbon cycle and budget. For example it is estimated that grasslands contain more than 10% of the global soil carbon stocks and that changes in the carbon storage of grasslands have long- lived effects on the global carbon cycle (Parton et al. 1995). The growth rate of plants is determined by a range of environmental variables and when any of these is less then optimal it imposes a limit on growth rate and may be considered to impose a stress on the plant. The term stress, when used in biology, has general connotations rather than a precise definition but each environmental var- iable can cause stress, such as low and high tem- perature stress and drought stress (Jones and Jones 1989). Temperate grasses typically expe- rience low temperatures during winter and spring, and high temperatures and water stress during summer (Jones 1988, Parsons 1988). The impact of the changing climate on the occurrence of these extremes is of major importance in un- derstanding how grassland vegetation will re- spond in a future climate. In this paper our aim is to review the knowl- edge on the impact of elevated atmospheric CO, on temperate grassland species and the interac- tions with temperature and water stress. Virtual- ly all of these species have the C, pathway of photosynthesis. We also briefly review the prob- lems of scaling from the level at which most experiments are performed (single plants and small plots) to grassland ecosystems. We work from the premise that theresponses ofgrasslands to climate change variables will be the result of both the direct effects of increasing CO, and the indirect effects of rising temperature and chang- ing patterns of rainfall. Further, there will also be interactions with other components of global change including an increase in nitrogen depo- sition and rising tropospheric ozone. 272 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Direct effects of elevated CO 2 The ‘business-as-usuaT scenario of the IPCC projects that the atmospheric CO, concentration will rise from 355 (imol mof in 1990 to 520 pmol mol' 1 in 2050 and will exceed 700 pmol mof by 2100 (see Carter 1996). Carbon diox- ide is the substrate for photosynthesis for all ter- restrial higher plants, with C 3 plants growing in adequate light requiring 800-1000 pmol mof C02 for saturation of photosynthesis (Lawlor 1993). As a consequence, increasing atmospheric C02 concentrations will directly increase therate of photosynthesis and dry matter production of terrestrial C 3 plants. Among the wide range of C 3 crops and non- agricultural species that have been examined, al- most all show significant increases in photosyn- thesis and dry matter production in response to an increase in atmospheric CO, of between 500 and 1000 pmol mof. Reviews by Kimball (1983) and Cure and Acock (1986) of experi- ments done under a wide range of conditions have shown that a doubling of atmospheric C02 from 330 to 660 pmol mof increases the pro- ductivity of C 3 crops by an average of 33%. Poorter (1993) found the growth stimulation of 156 species of C 3 plants, produced by a doubling of the atmospheric C02 concentration, to be 41%. Assuming a linear response to rising CO,, these observations suggest that dry matter pro- duction increases by between 0.10 and 0.12% per 1 pmol mof increase in CO,. This means that the C02 enrichment since the start of the industrial revolution, from 280 to 355 pmol mof, would be expected to have increased dry mat- ter production of C 3 crops by 7.5-9.0%. This in- crease will be very difficult to detect empirical- ly, particularly because new varieties have been selected throughout this period for higher yields independent of C02 effects, but also because of massively increased inputs, such as fertilisers. Furthermore, the assumption of a linearresponse between approximately present day C02 concen- trations and twice present day values is almost certainly erroneous, although at present we do not know at what C02 concentration whole plants or whole ecosystems will be saturated (Körner 1995). Despite the consistent evidence from short term experiments for the direct stimulation in growth by increasing C02 , there has been some reluctance to accept that prolonged growth in elevated C02 stimulates yield under normal ag- ricultural conditions (Jenkinson et al. 1994). This is largely because it has been assumed that pho- tosynthesis is often limited by other environmen- tal variables such as temperature, water and nu- trient availability during much of the year, so that plants do not respond to increased atmos- pheric C0 2 (Melillo et al. 1990). Also, there is much evidence that the initial C02 stimulation of photosynthesis is not maintained and that down-regulation of photosynthesis occurs after prolonged exposure to high C02 concentration (Bowes 1993, Bowes et al. 1996). There are, however, very few observations from field ex- periments to confirm these views. Recently, results for well-fertilized and irri- gated Lolium perenne swards, grown at elevat- ed (700 pmol mof) and ambient atmospheric CO, in open-top chambers in Dublin, Ireland for a three year period, showed marked seasonal variations in the aboveground stimulation in yield (Figure 1) (Jones et al. 1996a). During the growing season the grass was managed by har- vesting at frequent intervals to simulate a fre- quently cut sward. The largest increases in yield were recorded in the early and late growing sea- son, with an overall stimulation in yield from the elevated C02 treatments of about 20% in 1992, 28% in 1993 and 42% in 1994. Further- more, after three years of exposure to elevated C02 , there is no evidence of down-regulation in the form of a decline in the effect of CO, on aboveground harvestable dry matter production (Jones et al. 1996b). In these experiments only aboveground growth was measured. However, in experiments conducted at the FACE (Free-Air C02 Enrich- ment) facility in Zurich, Switzerland well-ferti- lized Lolium perenne and Trifolium repens showed differential responses of above- and be- 273 Vol. 5 (1996): 271-283. AGRICULTURAL AND FOOD SCIENCE IN FINLAND Jones, M.B. & Jongen, M.: Effects ofelevated CO2 on temperate grasses lowground biomass to elevated C02 (Jongen et al. 1995), with a much larger proportion of the extra dry matter produced at elevated C02 being allocated to the roots (Figure 2). There are, unfortunately, few other examples of long-term exposure of temperate plant com- munities to elevated C02, but the picture emerg- ing from these studies is of rather complex re- sponses which depend on interactions with oth- er climatic conditions. For example, an estua- rine marsh community in Chesapeake Bay, Mar- yland, USA showed sustained carbon gain (Drake and Leadley 1991) but an arctic tundra community exhibitedmarked down-regulation of photosynthesis after only three weeks at elevat- ed CO, (Oechel et al. 1994). Furthermore, Owensby et al. (1993) showed that in a tallgrass prairie ecosystem, C 4 tallgrass species had in- creased production under elevated C02 , but C 3 grass species had not. They concluded that, in water-stressed environments dominated by C 4 species, increased water-use-efficiency resulting from elevated CO, was responsible for increased productivity. Fig. 1.The seasonal change in yield stimulation over a three year period for Lolium perenne swards grown in Dublin, Ireland in open-top chambers at elevated (700 umol mol ') C02 compared with an ambient CO, treatment (from Jones etal. 1996b). Fig. 2. The response of above- ground (□) and belowground (B)biomass production ofLolium perenne and Trifolium repens swards grown in a FACE (Free Air C0 2 Enrichment) facility near Zurich at elevated C0 2 (600 pmol mol 1) compared with an ambient C02 treatment (data from Jongen et al. 1995). 274 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 5 (1996): 271-283. Interactions of elevated C0 2 and temperature In higher latitudes, where grass growth is large- ly temperature limited throughout much of the growing season, it is probable that the direct ef- fects of enhanced C0 2 will be less than at low latitudes. Here, temperatures are no longer lim- iting and growth is consequently more likely lim- ited by available C02 (Lemon 1983). There are, however, many experimental observations that run contrary to this argument and show that op- timal conditions are not a prerequisite for en- hanced growth at elevated C02 . In fact, the en- hancement can be maintained when other fac- tors are co-limiting. For example, some temper- ate crops growing under low irradiance have been found to have a larger response to C02 than those growing at higher irradiance (Sionit et al. 1982, Gifford 1992, Ziska et al. 1990). This is thought to be due to the suppression at increas- ing CO, concentrations of photorespiration, which is proportionately more significant at low irradiance, and could be important for crops grown in overcast conditions during the spring at high latitudes (Long 1991).Also, there is some evidence that elevated C02 may lower the mini- mum temperature at which some plants grow and complete their life-cycle (Potvin 1985). This re- sponse could be very important in temperature- limited growing conditions. Future elevated atmospheric C02 concentra- tions will probably be associated with warmer temperatures. Unfortunately, limited experimen- tal evidence is available on the interactive ef- fects of enriched C02 and temperature but the observed and predicted responses are generally complex and varied. Using a mechanistic model ofcarbon exchange, Long (1991) has shown that elevated CO, concentrations could alter both the magnitude of the response of leaf and canopy carbon gain to rising temperature, and some- times, the direction of response. Newton et al. (1994) suggested that, because of the modifying effects of temperature, the influence of C02 on plant growth in temperate regions will be differ- ent during the changing seasons. Experiments carried out in solardomes, which allow studies on the effects of both changes in C02 and tem- perature on vegetation (Jones et al. 1993), show that exposure to an increase in temperature of 3°C above present ambient has a larger effect on the annual aboveground biomass production of Lolium perenne than does a doubling of present day C02 concentrations (Figure 3) (Ash- enden pers. comm.). However, the combined ef- fects of elevated C02 and elevated temperature are less than additive, indicating a decline in re- sponse to elevated C02 as temperature increas- es. Newton et al. (1994), using turves taken from a ryegrass/white clover based pasture, found that ryegrass growth rates declined as temperature increased from 10/4°C (day/night) to 22/16°C and furthermore, this decline was greater at ele- vated C0 2 . In contrast, white clover growth rate increased with temperature and was stimulated by elevated CO . Fig. 3. The yield response ofLolium perenne swards grown in Solardomes at the Institute of Terrestrial Ecology, Ban- gor, Wales to elevated CO, (+CO,) and/or elevated temper- ature (+T) as compared with an ambient CO, and ambient temperature treatment. The elevated C02 treatment was 700 pmol mol' 1, elevated temperature was ambient +3 °C (data from Ashenden pers. comm.). 275 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Jones, M.B. & Jongen, M.: Effects ofelevated C02 on temperate grasses The interactions between elevated C02 and elevated temperature will be further complicat- ed by the fact that the effects of temperature on grass growth are the results of both an increase in the length of the growing season and higher growth rates as temperatures rise. In higher lat- itudes there is evidence that the largest effect of rising temperatures is mediated through the long- er growing season rather than increased growth rate at higher temperatures (Bergthörsson et al, 1988). Furthermore, because of the seasonal re- sponse to elevated C02 (mentioned above) we might expect a changing interaction between C02 and temperature during the season as indicated by Newton et al. (1994). The response is more complex in mixed-species grass swards where individual species show different responses to temperature (Newton et al. 1994). When the sea- sonal change in C02 effectiveness is put togeth- er with seasonal changes in temperature/C02 in- teractions and changes in species composition, an even more complex pattern of response to C02 becomes likely. nteractions of elevated C0 2 and water stress The seasonal pattern of growth of temperate grasses generally shows a decline during the summer months, which is partially developmen- tally related to the onset of flowering (Parsons 1988),but can in many cases be attributed to de- veloping water stress (Jones 1988). The stress is due to an excess of evapotranspiration over rain- fall and is a consequence of the increase in the soil water deficit. The decline in summer rain- fall projected in some climatic scenarios would be expected to increase this effect under climate change. However, a direct effect of increasing C02 concentrations is to decrease stomatal con- ductance in many species. A reduction in sto- matal conductance will be expected to reduce transpiration and increase water-use efficiency at the leaf scale, and in effect conserve water for continued plant growth. There have been many experimental demonstrations of reduced stomatal conductance at elevated C0 2 (Morison 1985, Ea- mus 1991) and for a doubling ofpresent day C02 concentrations the reduction in conductance fre- quently varies from 20-50% (Eamus 1991,Field et al. 1995). With the temperate grasses the re- duction in conductance varies from 43% for Triticum aestivum (Tuba et al. 1994) to 50% for Avena barbata (Jackson et al. 1994) and 51% for L. perenne (Jones et al. 1996a). It is likely, however, that a number of proc- esses interact at the ecosystem scale to reduce the magnitude of the response of ecosystem eva- potranspiration to increasing CO, as compared with transpiration at the leaf scale (Eamus 1991). This difference arises from the effect of stomat- al closure on leaf temperature and the drying of the boundary layer in response to decreasing sto- matal conductance which increases the driving gradient for transpiration (Jarvis and Mc- Naughton 1986, McNaughton and Jarvis 1991, Field et al. 1995). Furthermore, the effects of increasing C0 2 on canopy evapotranspiration are likely to be smallest in aerodynamically smooth canopies such as grasses. Field et al. (1995) sug- gest that the decrease in evapotranspiration in this type of canopy may be only 25% as large as the decrease in leaf conductance. Also, the in- crease in leaf area associated with greater above- ground biomass at elevated C02 will diminish the effect of reduced single leaf conductance. However, a small number of ecosystem meas- urements of evapotranspiration of grasses at el- evated C02 have shown that although canopy evapotranspiration may not, as predicted, decline significantly at elevated C02 , there is neverthe- less an increase in the water-use efficiency (ex- pressed as the ratio of canopy CO, uptake/water vapour loss) which is a consequence of higher C02 assimilation rates (Nijs et al. 1989, Diemer 1994). Despite an increase in instantaneous wa- ter-use efficiency, if canopy evapotranspiration does not decline at elevated C02 , water stress may develop at the same rate compared with ambient conditions. Also, in circumstances 276 AGRICULTURAL AND FOOD SCIENCE IN FINLAND where leaf area increases at elevated C0 2, we might expect water stress to develop more rap- idly. Overall, we therefore expect that the effect of elevated C02 on grassland ecosystem eva- potranspiration, and therefore on developing water stress, will be rather small. Species specific responses to elevated C02 As already made clear, temperate grasslands are typically composed of a complex mixture of per- ennial grasses, nitrogen-fixing legumes and non- fixing dicots of different growth forms. Most studies so far have concentrated on the response of single plant and/or single species at elevated CO,. However, there is a major concern that it will not be possible to extrapolate to communi- ty responses from the results of these experi- ments (Pitelka 1994).This is because the differ- ent species are likely to show marked differenc- es in theirresponses to increasing C02 (Kimball 1983, Cure and Acock 1986, Poorter 1993) and climatic stresses, and this will probably result in major alterations in community structure in the future. As an example. Figure 4 shows the response of four species representative of the Lolio-Cy- nosuretum grassland association (Rodwell 1992) to elevated CO,. These results show large dif- ferences between species in the sensitivity of aboveground biomass production to elevated C02 . In addition the sensitivity alters during the growing season. The most marked difference is between Cynosurus cristatus, which shows no significant growth response at elevated C02 and Lolium perenne, showing an almost 150% in- crease in aboveground biomass at elevated C02 during September. Further, differential respons- es at elevated C02 were also recorded for the C:N ratio of aboveground biomass and the spe- cific leaf area (Figure 4). Baxter et al. (1994) have also found large differences in sensitivity to elevated C02 be- tween component species for montane Agrostis- Fescue grassland in Snowdonia, N. Wales. They have demonstrated that, whereas wholeplant dry weight of Agrostis capillaris and Poa alpina in- creased at elevated C02 , there was a decrease in growth rate of Festuca vivipera under similar conditions. F. vivipera also partitioned more as- similates to roots at elevated C02 and the leaves showed marked discoloration and senesced faster (Baxter et al. 1994). Little information is available on therespons- es of mixed species communities when exposed to elevated C02, where competition between the different species will interact with the differen- tial responses to elevated C02 . Preliminary re- sults from experiments in which this type of in- teraction has been investigated have shown that it is unlikely to be possible to predict the re- sponse ofa mixed community from the observed responses of species grown in monoculture (Jon- gen 1996). Recently, Stewart and Polvin (1996) have investigated the effect of C02 enrichment on plant-plant interactions in an artificial grass- land community dominated by Trifolium repens and Poa pratensis. Their results show that ele- vated C0 2 increased the strength and number of plant-plant interactions and that Trifolium, a ni- trogen-fixing legume, exploited the C02-en- riched atmosphere more effectively than Poa, a non-fixing C, grass. In addition to differential responses to ele- vated C0 2 , component species of grasslands show species-specific responses to ambient tem- perature and stress factors, including water stress (Jones 1988). The different temperature respons- es are the result of each species having its own optimum temperature regime for growth, which leads to a different seasonal pattern of growth for the component species. For example, in mixed grass-clover swards, grasses may be dom- inant early in the season with clover forming a significant component during mid-season. The effects of increasing C02 concentration and rising temperature on the composition of these swards are therefore complex on a tempo- ral scale and require more detailed studies. Future studies must address this issue by carry- 277 Vol. 5 (1996): 271-283. AGRICULTURAL AND FOOD SCIENCE IN FINLAND Jones, M.B. & Jongen, M.: Effects ofelevated C02 on temperate grasses Fig. 4. The response of biomass production, carbon to nitrogen (C:N) ratio and specific leaf area of four grass species from the Lolio-Cynosuretum association exposed to elevated (700 umol mol 1) CO, com- pared with an ambient CO, treatment. Plants were grown as monocultures in pots in open-top chambers (from Jongen 1996). 278 AGRICULTURAL AND FOOD SCIENCE IN FINLAND ing out experiments on complete grassland eco- systems (Wolfenden and Diggle 1995). Because of the additional interactions in ecosystems be- tween plant and soil, these experiments will need to be carried out either in situ or using mono- liths representative of the natural communities transferred to controlled environments. One ap- proach which may help to simplify these studies is to determine whether the growth response to elevated C02 and its interaction with tempera- ture and water stress can be predicted on the basis of ecological functional types. Different func- tional types can be identified, based on the es- tablished growth phase of the plants; for exam- ple competitors, ruderals and stress tolerators (Grime 1974, Grime et al. 1988). Hunt et al. (1991) predicted a high C0 2 responsiveness within the competitive functional type, with the ruderal strategy showing a smaller response. In the case of grasslands, alternative functional types may be more relevant, for example nitro- gen fixers and non-fixers. It has been hypothe- sised that nitrogen fixers may benefit more from elevated C02 than non-fixers, and there is some evidence to support this (Stewart and Potvin 1996). Scaling up Processes in vegetation occur over a wide range of spatial and temporal scales, but because of the current interest in global change there is a need to scale upwards to predict the responses to change (Wessman 1992, Jarvis 1995). There may be general agreement that increasing glo- bal carbon dioxide concentrations will have di- rect physiological effects on plants, but the du- ration of these effects and their impact at the lev- el of the population and ecosystem is still rela- tively unknown. This is largely because much of the experimental work on plant responses to climate change has involved investigation of sin- gle plant responses over time periods of days to weeks. Many of the results reported above have been obtained using either single plants or small plots which have been grown for periods of, most frequently, a few weeks and, very rarely, as much as 2-3 years. In order to understand how large geographi- cal areas, such as northern Europe, will respond to climate change, there is clearly a need to scale up from single plants and plots with dimensions in the order of 1 to 100 m 2, to patches (100- 10000 m 2), to landscapes (1-100 km 2 ) and ulti- mately to regions (10000 km 2 ) as well as from relatively short-term to long-term time scales (years to centuries). The scaling process involves taking information at one scale and using it to derive processes at another scale (Wessman 1992, Jarvis 1995). A major problem with this approach is the non-linearity between processes and variables; for example transpiration at any spatial scale is non-linearly related to stomatal conductance (see above). Also, there are hetero- geneities in properties that determine the rate of processes. Heterogeneity may be random or ex- ist as patches; for example a grass sward may consist of a strictly random arrangement of grass and clover plants or it may consist of patches where the species composition and dominance of individual species varies over a given area. Ultimately however, the key to scaling is deter- mining what to ignore. The object is not to ana- lyse all of the smaller scale aspects of a process under observation, but to focus instead only on those that have direct importance to the scale under consideration (Wessman 1992). Predicting the response ofEuropean temper- ate grasslands to climate change on a spatial scale of landscape and region presents particularly difficult problems because of the marked patch- iness of different grassland ecosystems (Knapp 1979). On a temporal scale, the difficulties are possibly greater, because most experimental work on which the predictions are based has been done over very short time periods. However, mechanistic models of vegetation growth and carbon fluxes, which can be run to simulate changes over decades and centuries, provide a valuable tool to predict long term changes. To illustrate, the Hurley-Pasture model of 279 Vol. 5 (1996): 271-283. AGRICULTURAL AND FOOD SCIENCE IN FINLAND Jones, M.B. & Jongen, M.: Effects ofelevated C02 on temperate grasses Thomley and Verberne (1989) has been used by Thomley et al. (1991) to estimate the terrestrial carbon storage resulting from an increase in C02 concentration and temperature in temperate grasslands. The results suggest that the C02 fer- tilization that has occurred since the industrial revolution, and which will continue in the fu- ture, has resulted in increased carbon sequestra- tion in soils and may contribute appreciably to the so-called missing carbon sink (Figure 5). However, a rise in temperature tends to act in the reverse direction, and it may be that increas- ing temperature will counteract the CO, effect. Similar model predictions have been made by Parton et al. (1995) using the CENTURY mod- el. The outcome of these modelling exercises are further examples of the important interactions between rising C02 and increasing temperature as climate change occurs. Conclusions Temperate grassland productivity shows a sig- nificant, but variable, positive response to in- creasing ambient C02 concentrations. There ap- pears to be a larger response of belowground growth compared to aboveground and if this is sustained it suggests that grassland soils will become an increasing sink for carbon in the fu- ture, unless higher temperatures counteract the effect. There are important interactions between rising CO, and increased temperatures which in some cases are not simply additive. If the tem- perature increase reaches 3°C globally, as pro- jected in some scenarios, it is likely that the tem- perature effects will exceed the CO, effects in temperate grasslands (Parton et al. 1995). How- ever, elevated CO, has important differential ef- fects on the component species ofgrasslands and as a consequence we are likely to see significant changes in sward composition. Similar conclu- sions have been reached by Wolfenden and Dig- gle (1995) working with upland grassland vege- tation in Britain. In order to predict responses of grasslands to climate change over Europe there is a need to scale up from the experimental work which has been done at the single plant and small plot scale. There are considerable difficulties associated with this scaling and preliminary analysis shows that some effects seen at the plot scale, such as a reduction in evapotranspiration at elevated C0 2 , will not be seen or will be reduced in magnitude at the landscape and regional scale. Modelling is the most likely solution to many of these dif- ficulties, as experiments are impossible on the large spatial and temporal scales required to an- swer many of the questions about the conse- quences of climate change for grasslands. Acknowledgements. Trevor Ashenden and John Thomley are thanked for kindly providing some of the results presented here. Fig. 5. Carbon sequestration in grassland soils estimated using the Hurley Pasture Model. The sequestered carbon is plotted against time for two scenarios. In the first, a step change in the atmospheric C02 concentration from 280 to 350 pmol mol '; in the second, it is assumed that the atmos- pheric CO, concentration increases linearly from 280 to 350 pmol mol 1 over 130 years. The dashed line indicates the asymptote for the step change (from Thomley et al. 1991). 280 AGRICULTURAL AND FOOD SCIENCE IN FINLAND References Baxter, R., Ashenden,T.W., Sparks,T.H. & Farrar, J.H. 1994. 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Ilmaston muutos, jonka aiheuttaa ilmakehän kasvihuonekaasujen pitoisuuden kasvu, voi vaikuttaa suoraan ja epäsuorasti nurmien kasvuun ja niiden laji- koostumukseen. Ilmakehän hiilidioksidipitoisuuden muutos vaikuttaa suoraan kasvien kasvuun. Toisaal- ta ilmaston lämpeneminen ja sademäärän muutokset vaikuttavat epäsuorasti. On todennäköistä, että hiili- dioksidipitoisuuden nousu vaikuttaa enemmän matalil- la leveysasteilla kuin korkeammilla leveysasteilla, missä lämpötila usein rajoittaa kasvua. Lämpötilan ja veden saannin muutosten yhteisvaikutukset ovat kui- lenkin monimutkaisia. Lauhkean kasvuvyöhykkeen nurmet vaihtelevat voimaperäisesti viljellyistä monokulttuureista moni- lajisiin kasvustoihin, joiden paikalliseen esiintymi- seen vaikuttavat maalaji ja maan kuivatusolosuhteet. Kohonnut hiilidioksidipitoisuus, lämpötila ja kuivuus voivat vaikuttaa hyvin eri tavoin eri nurmikasveihin, mikä todennäköisesti tulee muuttamaan suuresti nur- mien lajikoostumusta tulevaisuudessa. Kohonnut hiilidioksidipitoisuus saattaa pitkällä aikavälillä merkittävästi lisätä maahan varastoituneen hiilen määrää. Lämpötilan kohoaminen voi kuitenkin kumota tätä ilmiötä. 283 Vol. 5 (1996): 271-283. AGRICULTURAL AND FOOD SCIENCE IN FINLAND