Much of the experimentation has been designed to simulate UV-B levels expected on clear days with unobstructed sunlight, whereas many areas have persistent cloud cover and, correspondingly, lower UV-B flux rates. However, there is some suggestion that plant responsiveness to UV-B may be influenced by the ratio of UV-B:visible sunlight as much as by the absolute level of UV-B radiation (Deckmyn et al., 1994; Deckmyn and Impens, 1997). Certain clouds tend to transmit more radiation at shorter wavelengths than at longer wavelengths (Bordewijk et al., 1995); therefore, the ratio of UV-B:PAR would be greater than under clear-sky conditions. Yet, this has not been documented over extended time periods in different environments. The potential importance of plant responsiveness to greater UV-B:PAR ratios during cloudy periods deserves further attention and ecosystems that occur in cloudy environments should not necessarily be dismissed from consideration for the ozone reduction problem.
Overall, the consequences of increased solar UV-B in forests, grasslands and other nonagricultural ecosystems may involve several complex pathways (Fig. 3.1) rather than simply a reduction in overall ecosystem primary productivity. However, the effects of these more involved pathways are difficult to predict without conducting experiments with assemblages of plant species and long-term study of ecosystem responses. This has, thus far, received very little attention in experimental research.
Where ecosystem-level studies of terrestrial responses to increased solar UV-B have been initiated, high-latitude ecosystems have been emphasized since the relative ozone reduction is more pronounced at high latitudes. Yet, the absolute UV-B flux is greater at low latitudes where ozone reduction is not very pronounced.
Further discussion of implications
for specific types of ecosystems follows later in this chapter.