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• W2114033416 abstract "By the mid 1980s, concerns about ozone depletion by chlorofluorocarbons (CFCs) and ensuing increases in ultraviolet-B radiation (UV-B) reaching the earth's surface prompted urgent research to examine the effects of UV-B on plant processes. Although international agreements have now slowed the production of CFCs, whether stratospheric ozone and UV-B levels will revert to the preozone-depletion levels of the 1970s by the end even of this century remains uncertain because of noncompliance and the positive feedback that greenhouse gases may have on ozone depletion (Madronich et al., 1998; Shindell et al., 1998). However, our ability to predict the impacts of changing UV-B levels on organisms and ecosystem processes is limited because few field studies have examined effects across multiple trophic levels. In this issue, Searles et al. (2001b) (see pp. 213–221) examine how solar UV-B influences plants and microbes at the southern extremity of South America, the Tierra del Fuego archipelago – an area that is experiencing enhanced solar UV-B levels during spring and summer due to the Antarctic ozone hole. Counterintuitively – since UV-B is deemed detrimental to most organisms – they found that some microbes, in particular testate amoebae, occur in higher numbers in a Sphagnum peatland under near-ambient UV-B than under reduced UV-B levels. Less surprisingly, these findings reemphasize that what we know about UV-B effects on plants cannot easily be extrapolated to predict how UV-B will affect other trophic levels in a system. ‘Ecology has taught us that seemingly small changes in one process might ultimately have large effects at other levels of organization’ What have we learned since the 1980s from these research efforts? Early work showed that UV-B exposure indoors, while providing useful information on mechanisms and short-term responses, often exaggerates the effects of UV-B on plants because of low visible and UV-A irradiance. This concern was largely responsible for an emphasis on outdoor studies conducted under a background of sunlight. The two approaches employed in these outdoor UV-B studies involve either reducing some of the ambient UV-B reaching plants through the use of UV-B absorbing filters (exclusion studies), or supplementing ambient UV-B levels with UV-B lamps (lampbank studies). Regarding lampbank studies, when the levels of UV-B supplements are given at a constant dose over the day, unrealistically high levels of UV-B can be supplemented against low visible and UV-A irradiance, and UV-B effects may be exaggerated with these so-called squarewave exposures. Modulated lampbank systems, which continuously measure solar UV-B and adjust lamp output to compensate for low background irradiance, have largely overcome this problem. A survey of recent studies employing filter exclusions or modulated supplements found that, in the case of exclusion studies, among those that examined biomass, one-half found that production was compromised by ambient UV-B exposure (Day, 2001). Additionally, one-half of the exclusion studies detected reductions in the area of individual leaves and increases in the bulk concentration of soluble leaf UV-B absorbing or screening compounds (primarily phenolics). These reductions in individual leaf area usually carry over to the whole plant, and occur in the absence of reductions in CO2 assimilation rates per unit leaf (Allen et al., 1998; but see Keiller & Holmes, 2001). The mechanisms responsible for UV-B induced leaf stunting and leaf area reductions remain unclear. In the case of modulated lampbank studies, while all of the nine studies surveyed detected at least one significant plant response, the most consistent response was an increase in concentrations of soluble leaf UV-B absorbing compounds; while the majority of studies that examined this parameter found an increase, in some cases it was only detected in the epidermis or during high levels of background solar UV-B. In no case did supplemental UV-B reduce total biomass production In summary, exposure to ambient UV-B (assessed in these exclusion studies) does elicit responses in many plants, particularly reductions in individual and whole-plant leaf area and increases in UV-B absorbing compounds, and in some cases, it is responsible for impressive reductions (10–35%) in biomass production (Krizek et al., 1997, 1998; Mazza et al., 1999a; Day et al., 2001; Xiong & Day, 2001). As Paul (2001) recently alluded to, these findings speak of current UV-B levels as a significant factor in controlling some plant processes – however, how overall plant performance is influenced by the large natural variability in outdoor UV-B levels, both spatially and temporally, remains unknown. Regarding responses to supplemental UV-B, results from modulated lampbank studies suggest that some plants are indeed responsive to supplemental UV-B; the majority of these studies reported an increase in UV-B absorbing compound concentrations. Compared with exclusion studies, plant responses to UV-B supplements tended to be more subtle and total biomass production was not impaired. The greater responsiveness of plants in exclusion studies was not necessarily the result of greater absolute differences in biologically effective UV-B levels between treatments (Day, 2001). Instead, plants may be more responsive to increases in UV-B when added to relatively low, subambient levels of UV-B (exclusion studies), than when added to relatively high, ambient levels of UV-B (supplementation studies); targets and mechanisms may become saturated as UV-B exceeds ambient levels, and at above-ambient supplements some responses may be insignificant or difficult to detect. The majority of recent exclusion and modulated supplement studies surveyed found that concentrations of soluble UV-B absorbing compounds increased with UV-B level. Corroborating this, in a recent meta-analysis of 103 outdoor lampbank studies (including both modulated and squarewave supplements) Searles et al. (2001a) found that an increase in bulk UV-B absorbing compounds was the most consistent response to UV-B supplements. The implications of these UV-B induced increases in concentrations are unclear. While these compounds attenuate or screen UV-B, the protection that increases in bulk concentrations afford plants remains difficult to assess given their spatially heterogeneous compartmentalization and the optically complex nature of leaves, along with our poor understanding of the identity and location of UV-B targets (Day, 2001), although some progress in this arena has been made (Barnes et al., 2000; Mazza et al., 2000). The vast number of these phenolic compounds, which probably differ in their roles, further complicates our understanding of the significance of this generalized response. Many additional roles have been proposed for some of these compounds including as antioxidants (Bornman et al., 1997), constraints to cell and leaf expansion (Liu & McClure, 1995), deterrents against herbivores and pathogens, and allelopathic compounds (Klein & Blum, 1990). These diverse functions suggest that UV-B induced changes in plant phenolic chemistry could have a wide range of effects on the exposed plant, adjacent plants and other trophic levels. Compared with effects of UV-B on plants, effects on terrestrial herbivores, consumers and decomposers have received less attention, especially in an ecological context, and a framework for predicting the influence of UV-B on these other trophic levels, as well as what feedbacks these might in turn have on plant performance, has yet to emerge. However, it is likely that the effects of alterations in the UV-B environment of a system will take several years to manifest themselves, in part because of the time required for feedbacks between trophic levels to develop. The handful of longer-term UV-B studies reported have found that plant responses to higher UV-B levels tend to increase or be cumulative over successive years (Sullivan & Teramura, 1992; Johanson et al., 1995; Björn et al., 1998; Phoenix et al., 2000; Day et al., 2001). Along with the need to examine UV-B effects over several years at several trophic levels is the need to incorporate other climatic factors into experimental designs, since UV-B sometimes has strong interactions with other climate change factors such as CO2 (Beerling et al., 2001) or warming/precipitation (Kiesecker et al., 2001). Work to date has found both negative and positive UV-B effects on organisms at other trophic levels, and these appear to depend on a complex array of biotic and abiotic factors. For example, exposure of leaf litter to enhanced UV-B can either accelerate, slow or have no noticeable effect on litter microbial respiration, litter mass loss rates or chemistry (Gehrke et al., 1995; Newsham et al., 1997, 1999, 2001; Rozema et al., 1997). Contrasting effects of UV-B have also been found when the indirect effect of different plant UV-B exposure has been assessed on subsequent litter decomposition. Some studies have also focused on the influence of UV-B on insect herbivory. Exposure of field plants to UV-B often reduces insect herbivory, abundance and performance (Ballaréet al., 1996; Rousseaux et al., 1998; Mazza et al., 1999b), although the causes for this are largely unknown. This may involve both indirect effects of plant UV-B exposure on leaf physiochemical properties, as well as direct effects of UV-B on insects. Mazza et al. (1999b) found that thrips perceive and avoid UV-B, and this effect was detectable in response to small changes in the level of outdoor UV-B supplements. Hence, failure to detect reductions in plant growth under UV-B supplements might in some cases be partly attributable to the insect deterrent afforded by supplemental UV-B. Relatively few studies have examined the influence of UV-B on multiple trophic levels. In this issue, Searles et al. (2001b) (pp. 213–221) examine how 3 yr of solar UV-B exposure influenced plants and microbes in Tierra del Fuego. They show that testate amoebae in a Sphagnum peatland occur in higher numbers under near-ambient than reduced UV-B levels. As already mentioned, greater numbers of amoebae under higher UV-B levels is counterintuitive, based on our current understanding of UV-B as generally detrimental to organisms, but highlights our lack of understanding of UV-B effects across multiple trophic levels. Such findings are not uncommon; researchers examining multiple trophic levels in aquatic systems have sometimes found complex, counterintuitive results: algae can increase in abundance in response to solar UV-B because they are less sensitive than their consumers to UV-B (Bothwell et al., 1994), and bacteria can increase in abundance due to photochemical changes in dissolved organic matter which render it more easily consumed (Herndl et al., 1997). Searles et al. (2001b) also found that changes in amoebae abundance was affected by UV-B treatments not only in upper layers of Sphagnum, but also in lower layers where UV-B levels are negligible, suggesting that not only direct, but indirect effects of UV-B were responsible. Such evidence of indirect effects seems to reiterate the idea that UV-B effects need to be studied at multiple trophic levels if we are to understand the mechanisms responsible for alterations in the performance and abundance of organisms. While one can find ample examples in the literature that ambient and enhanced levels of UV-B do not have detectable effects on various plant and litter parameters, this should not be taken as a verdict that UV-B has no effect on other, as yet unstudied parameters in a system, or on any parameters in other systems. Furthermore, many plant species are responsive to ambient UV-B, as well as supplemental UV-B, although total biomass production does not appear to be compromised by the latter. Much of the UV-B research to date has focused on plant performance, largely because of concerns about ozone depletion effects on agricultural productivity. More research is now needed that focuses on UV-B effects over multiple trophic levels, and over longer time frames, if we are to understand how UV-B levels influence plant performance, as well as how ecosystem processes feedback to primary producers. While many of the UV-B effects detected to date appear subtle, and some researchers may have concluded that changing UV-B levels are of little consequence to plant performance, ecology has taught us that seemingly small changes in one process might ultimately have large effects at other levels of organization, and have the potential to develop feedbacks that are initially difficult to envision. The work by Searles et al. (2001b) shows us that UV-B can have significant effects on other trophic levels, and that our existing UV-B paradigm is not robust enough to predict such effects." @default.
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• W2114033416 title "Multiple trophic levels in UV-B assessments - completing the ecosystem" @default.
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