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Implications for Agriculture, Forests and other Ecosystems

Crops

One of the primary concerns of future increases in solar UV-B radiation is its potential effect on global agriculture. Despite the obvious potential consequences of the issue, we cannot make quantitative predictions of anticipated effects resulting from stratospheric ozone depletion. This is due to the limitation in controlled-environment studies as discussed earlier and the overall paucity of well-replicated experiments performed in the field. Even in comparisons of field studies, there are large differences in temperature, precipitation, soil types, etc. from year to year and in different locations. This adds to the difficulty in making generalizations about the effects. Also, a common finding is that different varieties of the same crop species often react differently to elevated UV-B radiation (Fig. 3.4).

    The general procedure in such field experiments is to supplement ambient sunlight with special fluorescent UV lamps filtered to supply either extra UV-B radiation (treatment) or with the UV-B removed (control). The methodology has continuously been improved, e.g., by introduction of automatic systems that change the lamp output to more realistically simulate the UV-B supplement with proper balance with the existing sunlight. Therefore, older experiments, and especially those performed in glasshouse or growth chamber conditions, are presently considered to be less reliable.

    The compilation of harvestable yield in field experiments in Figure 3.4 indicates how variably different varieties responded and also that many varieties did not respond in a significant manner (statistically speaking) and a very few were even stimulated in production. From the entire population of studies, there is a tendency toward more negative effects.
 
Fig. 3.4. Relative changes in yield (seed production) of four crops evaluated for UV-B radiation responsiveness in 49 field trials with UV-B supplementation from lamps. Each bar represents results obtained with one variety in one field experiment in which ozone depletion was simulated (usually ca. 20% depletion). Soybean data from Teramura and Murali (1986), Sinclair et al. (1990), Teramura et al. (1990), D’Surney et al. (1993), and Miller et al. (1994); rice data from Nouchi et al. (1997) and Olszyk et al. (1996); pea data from Mepsted et al. (1996); and mustard data from Conner and Zangori (1997). Most effects smaller than 10% were not statistically significant, but small sample sizes and other environmental factors may have obscured differences.

In addition to quantitative changes in crop yield, evidence exists for qualitative changes as well. For instance, in the study mentioned above, UV-B radiation also resulted in small changes on the order of 1 to 5% in the protein and oil content of the soybean seed (Teramura et al., 1990).

    Because of the broad range of response patterns in crop species, plant breeding and genetic engineering for UV tolerance is an important aspect to consider in order to avoid significant crop production losses. There may, however, be some qualitative changes in seed or foliage characteristics that accompany the development or use of more UV-B-tolerant varieties. This remains to be explored. Other agroecosystem consequences of elevated UV-B radiation are likely to be more important, such as changes in insect or pathogen susceptibility of crops.

Forests

Relatively little information exists on the effects of UV-B radiation on forest tree species. Tropical forests, though representing nearly one half of global productivity and much of the total tree species diversity, have received very little attention with respect to the ozone reduction problem. Although little, or no, ozone reduction has thus far occurred in the tropics, only a small decrease of ozone at these latitudes would result in a very sizeable absolute increase of UV-B radiation since solar UV-B radiation is already very intense in these regions (see Chapter 1). One study has shown that excluding existing solar UV-B radiation with filters can result in increased growth of some tropical tree species (Searles et al., 1995). Otherwise, the effects of UV-B radiation on tropical tree species have not received much attention.

    Fortunately, there is some information for mid-temperate latitude tree species. Because they are long-lived, trees present the opportunity to observe the longer-term cumulative effects of UV-B exposure over several years for the same individuals. These cannot be explored in annual crop species. In a field study using loblolly pine (Sullivan and Teramura, 1992), seedlings from several different geographic regions were grown for three consecutive years under UV-B lamps in a field experiment. Seedlings were exposed to either ambient solar UV-B or ambient levels supplemented with the UV-B from lamps, similar to studies with soybean yield (Teramura et al., 1990). After the first year of UV-B exposure, reductions were observed in biomass of seedlings derived from several geographic areas. By the end of the third year, these biomass reductions were several-fold larger in one variety. These overall growth reductions were generally associated with small decreases in both roots and shoots, but not necessarily accompanied by reductions in photosynthesis. This may be due to changes in needle growth or shifts in allocation of biomass as has been found for some crop species. These results suggested that the effects of UV-B radiation may accumulate in long-lived plants such as trees.

    The fact that decreases in conifer needle biomass and needle length and leaf area of broadleaf trees were not accompanied by sizeable reductions of photosynthesis (Sullivan and Teramura, 1992; Dillenburg et al., 1995; Sullivan et al., 1996) may be due to the very low penetration of UV-B radiation into older foliage. It appears that the decreased growth of leaves and conifer needles upon exposure to enhanced levels of UV-B radiation may be due, in part, to epidermal cell wall thickening. This might prevent cell wall extension and, thereby, growth of these cells (Liu and McClure, 1995; Sullivan et al., 1996). Thus, changes at the level of the epidermis, the first leaf cell layer to receive the incident radiation, can have other important consequences.

Other Ecosystems at Mid and High Latitudes

Although absolute UV-B irradiance is naturally very low in high-latitude ecosystems, such as tundra and subarctic areas, there is experimental evidence that the plants in such systems react to increases in UV-B associated with realistic levels of ozone depletion. Some plant species exhibit growth inhibitions and others do not, thus, eventually altered community composition may be expected (Johanson et al. 1995, Rousseaux et al., 1998, Searles et al., 1998). Longer-term observations of species composition are being pursued in high-latitude subarctic systems in Sweden, a high arctic site on Spitzbergen Island and in southernmost Argentina (Tierra del Fuego). In the latter system, attenuating the naturally occurring solar UV-B radiation increased insect herbivory, decreased plant tissue nitrogen concentrations, and increased populations of some microfauna (amoeba and rotifers) that inhabit peat bogs (Rousseaux et al., 1998; Searles et al., 1998). The subarctic studies in Sweden have been underway for several years and these show several effects including decreased litter decomposition (Gehrke et al. 1995), increased fruit formation and greatly increased insect herbivory (Gwynn-Jones et al., 1997; Buck and Callaghan, submitted). Similar ecosystem studies are underway in a mid-latitude site in the Netherlands where dune grasslands are important (Rozema, et al. 1997a).

    Although terrestrial ecosystems at high latitudes are not highly productive for grazing, timber production, etc., the influence of ozone reduction on these systems may be important for several reasons. Carbon sequestration is generally quite high in these ecosystems, including the extensive peat formations which are also being studied in the Swedish subarctic and southern Argentinean systems. Compared with other locations, these ecosystems are under the greatest ozone depletion, especially in the Southern Hemisphere, and they also experience the greatest warming as the global greenhouse effect intensifies. Thus, they are sensitive indicators of several features of climate change. These high latitude ecosystems are also very important for the survival of indigenous ethnic groups in the Northern Hemisphere.


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