Terrestrial ecosystems include agricultural lands, agroecosystems, and less intensively managed lands such as forests, grasslands, savannahs, deserts, tundra, etc. In any of these environments, ecosystem function includes many attributes that could potentially be affected by increased solar UV-B radiation including plant biomass production, seed production, plant consumption by herbivores including insects, disease incidence of plants and animals, population fluctuations of plants and animals, and changes in species composition and mineral nutrient cycling. Treatment of some aspects of ecosystem function, e.g., nutrient cycling, overlap with Chapter 5, and effects on amphibians in aquatic components of terrestrial systems overlap with Chapter 4.
Study of ecosystem-level effects of solar UV-B increase has only been undertaken in the past few years. However, much attention has been directed to effects of UV-B radiation on higher plants since the first reports of potential stratospheric ozone reduction over 25 years ago (e.g., Johnston 1971, Crutzen 1972). Approximately 600 papers have appeared, but the majority of these deal with herbaceous, agricultural plants under laboratory or glasshouse conditions. Fewer than 5% of the studies have been conducted under field conditions, and fewer still with plants from forests and other nonagricultural systems.
Numerous reviews of this literature dealing primarily with effects on terrestrial plants have appeared in the last decade (Bornman, 1989; Caldwell et al., 1989; Krupa and Kickert, 1989; Tevini and Teramura, 1989; Stapleton, 1992; Bornman and Teramura, 1993; Caldwell and Flint, 1993, 1994a,b; Strid et al., 1994; Tevini, 1993, 1994, 1996; Teramura and Sullivan, 1994; Manning and Tiedemann, 1995; Bornman and Sundby-Emanuelsson, 1995; Björn, 1996; Jordan, 1996; Panten et al., 1996; Sullivan, 1997; Rozema et al., 1997a,b). Rather than a review of the literature, this chapter provides an overview with interpretation of the results for both agriculture and of other ecosystems such as forests, grasslands, etc.
Fig. 3.1. Scheme of direct photochemical effects of elevated solar UV-B radiation on plants, microbes and animals and indirect ecosystem effects mediated through changes in plant competition, microbial population changes, secondary chemistry, plant litter decomposition and air quality. The arrows indicate some of the potential interactions and consequences of UV-B radiation. See also Fig. 5.2 for related ecosystem-level effects and processes.
In terrestrial ecosystems, organisms apart from higher plants have received comparatively little attention with respect to direct effects of solar UV-B radiation increases. Some direct effects on microbes and animal life have been demonstrated (e.g., Blaustein et al., 1994; Gehrke et al., 1995; see also Chapters 2 and 4). Microbes perform many important ecosystem functions including litter decomposition, cycling of mineral nutrients, pathogenic action, and symbiotic interaction with both plants and animals. Direct UV-B effects on microbes have been extensively studied (Jagger, 1981), but the ecological relevance is not well understood. Animals are often thought to be generally well shielded from solar UV-B radiation by pigments such as melanin, body coverings such as feathers, fur, etc. In some animals, the eyes may be at risk if there is prolonged UV-B exposure (see Chapter 2). However, microbes exposed to sunlight are usually not so well shielded.
Figure 3.2. The influence of UV-B radiation on several damage and regulatory processes in organisms. Much of the initial damage can be repaired, (e.g., DNA damage). Damage and regulatory changes in plants and other organisms alter metabolism and ultimately growth, reproduction and survival. The arrows indicate some of the potential interactions and consequences of UV-B radiation.
The major anticipated effects of increased solar UV-B on agricultural and nonagricultural ecosystems (such as forests, grasslands, savannahs, deserts, tundra, etc.) may result from direct UV-B radiation effects on plants, insects and microbes, or indirect effects of UV-B on these organisms that mediate other effects (Fig. 3.1). Non-biological UV-B effects such as direct photochemical reactions in plant litter during decay (Chapter 5) or effects on the ambient air quality (Chapter 6) can influence other processes in ecosystems. Although the principal processes may be the same in highly managed agroecosystems (e.g., agronomic crops) and in nonagricultural ecosystems, their importance may differ. For example, effects on litter decomposition or plant competitive balance may be less important in annually cultivated crop fields.
For individual organisms, there are
several potential pathways of UV-B action in damage and regulatory processes
that affect whole organism performance, such as growth and reproduction