Perennial crops mostly include fruit trees, grapevines, and grasslands.
Assuming that climate variability does not increase, higher temperatures in late winter and early spring will hasten development stages. Concerning fruit trees, no experimental or simulation results are known, but it can be expected that budbreak will be promoted, as will later stages. On the other hand, higher temperatures could be detrimental to flowering quality for cold-requiring species and therefore could reduce fruit production.
With grapevines, simulation studies have shown that fruit biomass would decrease consistently in northern Italy under weather conditions foreseen by equilibrium climate scenarios (but would increase with some transient scenario outputs), even if the fertilizing effect of CO2 is accounted for. The promoting effect of CO2 has been experimentally demonstrated under current conditions by Bindi et al. (1996), who show that berry weight and total acidity increase, whereas total sugars decrease.
Higher temperatures probably will be beneficial to grasslands (see, e.g., Tchamitchian, 1994 for perennial ryegrass), at least early in the season, through increased early biomass production (earlier senescence also is expected). Higher temperatures during the summer may decrease the growth capabilities of grass. Higher CO2 tends to abate the negative effects of temperature, as demonstrated experimentally by Soussana et al. (1994). The interaction between nitrogen level and carbon distribution in crops nevertheless is a key factor in forecasting the responses of grasses (Casella et al., 1996). Some recent years in Europe have illustrated the immediate impact of dry periods on grassland production because of low rooting depth, shallow soils, etc., leading to a reduction in yield. Similar events may become more common in the future. Through simulation experiments, Rounsevell et al. (1996) show that grassland production in England and Wales is resilient to small increases in temperature and precipitation-and even stimulated at higher elevations. Experiments in northern and western Europe have shown positive responses in total biomass production across a range of grass species to increased temperature (+3°C) and elevated CO2 concentrations (Jones et al., 1996; Jones et al., 1997). The interactive response may be less than additive, however, indicating a decline in response to elevated CO2 as temperature increases. A significant part of the temperature response is manifest through an extended growing period. The response of below-ground growth to elevated CO2 appears to be greater than that of above-ground growth, and there are differential responses between species-suggesting that sward conditions may change significantly in the future (Jones et al., 1996; Jones et al., 1997).
For woody species, the influence of changing climates will really depend primarily on how long increasing CO2 concentrations enhance growth and production (i.e., whether there is any acclimation, whether farmers are capable of adapting their strategy to such situations, etc.) and secondarily on the influence of CO2, temperature, and water shortage on fruit quality-which is the major issue in fruit production. No real information is available on irrigation needs for fruit trees under changing climate conditions. Although grasslands show very different responses according to species, they apparently display no acclimation to increasing CO2.
Weeds, pests, and diseases
Weeds are expected to benefit from higher CO2 concentrations. The expected result of the crop-weed competition will depend on their respective reactions to climate and atmospheric fertilization. C3 plant growth probably would be proportionally more enhanced than C4 growth as a result of increasing CO2 concentrations. This difference can lead to various consequences during the early crop vegetation period (fall or spring), depending on which species is weed and which is crop. Chemical control of weeds obviously remains possible, but this approach must be considered in the context of increasing incentives for environment-friendly agriculture in Europe.
Increasing precipitation and temperature in the northern half of Europe probably will be linked to increasing air humidity and possibly leaf wetness duration. All factors are favorable to early (fall, spring) disease outbursts for annual and perennial crops; the same holds for early pest attacks (Harrington et al., 1994). Sophisticated agricultural systems probably can cope with increasing weeds, pests, and diseases in a sustainable manner (better-targeted and more efficient chemicals, risk forecast models, adapted machinery, etc.). The global result will depend on how widespread such agriculture will be in Europe around 2050. Whether controls will be possible in autumn and spring (i.e., whether working days will be available) also is questionable and must be studied in more detail. (Some model results are available for Finland.) The risk of crop damage from pests and diseases increases in all regions under a warming of climate. Northward shifts in the distribution of certain pests could be of similar magnitudes and rates as those estimated for cereal crops. Additional generations of multivariate species also can be expected. The damage potential of diseases such as potato late blight could increase at a similar rate as the potential increase in yields of the crop host. This analysis implies an increased requirement for pest and disease control, with associated consequences for the environment (Carter et al., 1996).
None of the models that have been used to predict future agricultural yields have taken into account the possibility of future reductions in productivity associated with increased concentrations of ozone. Depending on the supply of precursors, ozone concentrations are likely to increase in warmer temperatures. For example, Legge et al. (1996) have shown that current yield losses of spring wheat in Hessen, Germany, may be as high as 15% in some years. Greater losses can be expected in parts of Europe where ozone concentrations are higher.
Increasing precipitation probably will induce greater risks of soil erosion, depending on the intensity of rain episodes (such information is not currently available from available climate scenarios). This possibility needs to be examined, as does the expected evolution of soil organic matter. If soil organic matter content decreases with increasing temperature as a result of a higher mineralization rate, soils will be more susceptible to slaking; consequently, early development of crops will be hampered. Some indication exists that the negative effect of temperature will be more or less compensated under European conditions by a positive impact of carbon fertilization (Balesdent et al., 1994) and that the temperature-dependent mineralization rate is larger for clay and sand soils than for loam (Houot et al., 1995). Increased rainfall amounts also could increase fertilizer leaching in already wet areas; for example, Peiris et al. (1996) have simulated decreasing wheat yields under Scottish conditions that are explained by such a lack of fertilizer. Changes in vegetation cover probably will play a role as well.
Concerning working days, Rounsevell and Brignall (1994) have suggested that opportunities for autumn soil tillage in Great Britain will be improved by global warming (as a result of increasing water demand), as long as the future precipitation increase is no more than 15%. This analysis must be verified for spring conditions and other areas.
Other reports in this collection