24. Ecosystem Climate Manipulations
dient (Dunne 1996). A major drawback of this approach is the physical effort needed to deploy the
tarpaulins on rapid response, or shading if they are
left in place. Additional rainfall is, of course, readily simulated by artificial watering. In the Throughfall Displacement Experiment (Hanson and Edwards 1996), soil moisture in an upland oak forest
in eastern Tennessee, U.S.A., is manipulated by intercepting throughfall with subcanopy troughs suspended above the forest floor of the dry plots and
transferring it by gravity flow across an ambient
plot for subsequent distribution onto the wet treatment plots.
Field Methods:
Enhancing UV-B Radiation
Separate from the issue of global warming, stratospheric ozone depletion from emissions of chlorofluorocarbons and other anthropogenic compounds
also poses a threat to ecosystems. The natural
stratospheric ozone layer attenuates solar UV-B radiation (290 to 320 nm), and therefore depleting the
ozone layer increases penetration ofUV-B radiation
to the earth's surface. UV-B radiation affects plant
chemistry and physiology (Caldwell et al. 1989), as
well as plant morphology and competition (Barnes
et al. 1995). Terrestrial plants are especially vulnerable due to their obligatory requirement for sunlight for photosynthesis (Greenberg et al. 1996).
There have been hundreds of studies of the impacts of elevated solar UV-B radiation on plants
(for a summary, see Caldwell and Flint 1994). Very
few have dealt with multiple species, much less
plant communities or whole ecosystems. Most have
been short-term studies in greenhouses or growth
chambers, but these often overestimate the UV-B
radiation effects (Mark et al. 1996). Results of several studies suggest that different species of plants
show large differences in sensitivity to UV-B radiation (Rau and Hofmann 1996), making it difficult to extrapolate results from chamber studies of
single-species response to UV-B radiation alteration to whole-ecosystem response. As is the case
with ecosystem manipulations of climate, there is a
relative scarcity of whole-ecosystem, long-term
field manipulations of UV-B radiation (Barnes et
al. 1995; Caldwell and Flint 1994; Flint and Caldwell 1996).
363
Field studies of UV-B radiation have taken one
of three approaches: excluding UV-B radiation by
filtering sunlight, transplanting to high altitudes
(where UV-B radiation intensity is greater due to
reduced screening from natural ozone), or enhancing ambient UV-B radiation artificially. Excluding
UV-B radiation in the field is done with a shade or
greenhouse. Exclusion experiments are simpler and
cheaper than enhancement manipUlations, and if
carefully conducted, can suggest whether ambient
UV-B radiation is influential (Caldwell and Flint
1994). They cannot be used to realistically simulate
ozone depletion, however, since extrapolating results from UV-B radiation exclusion experiments
to enhanced levels of UV-B radiation is not necessarily justified. The effects of elevated UV-B radiation can also be examined by studying plant responses at high elevation or in low latitudes, since
the amount of UV-B radiation reaching the earth's
surface increases with altitude above sea level and
with lower latitudes (Rau and Hofmann 1996).
Controls can be obtained by suitable filtering of excess radiation (Tevini et al. 1989) or by comparison
with plant response in lowland areas (Dohring et
al. 1996). These studies have the advantage of realistic spectrum characteristics of solar enhancement, but it is difficult to attribute differences in
plant response at different locations to UV-B radiation enhancement alone, and not genetic, climatic,
or soil differences between sites.
Field manipulations enhance global UV-B radiation by supplementing ambient sunlight using
fluorescent lamps. Conceptually, the use of overhead lamps for UV-B radiation enhancement is
similar to the use of overhead IR lamps for global
warming studies. Determining the proportional
UV -B radiation increase associated with various
decreases in stratospheric column ozone is typically
accomplished with a mathematical radiative transfer model (e.g., Bjorn and Murphy 1985; Green
1983; Green et al. 1980; Rundel1986). Since a reduction of the ozone layer results in a very specific
increase in solar UV-B radiation in a wave band of
only about 30 nm, the fluorescent lamps for UV-B
radiation field manipulations must be filtered in order to screen out wavelengths below 290 nm (e.g.,
Flint and Caldwell 1996; Mackerness et al. 1996).
Different screening techniques and materials can
also be used to obtain different kinds of treatments,
such as to remove or supplement UV-B radiation.
dient (Dunne 1996). A major drawback of this approach is the physical effort needed to deploy the
tarpaulins on rapid response, or shading if they are
left in place. Additional rainfall is, of course, readily simulated by artificial watering. In the Throughfall Displacement Experiment (Hanson and Edwards 1996), soil moisture in an upland oak forest
in eastern Tennessee, U.S.A., is manipulated by intercepting throughfall with subcanopy troughs suspended above the forest floor of the dry plots and
transferring it by gravity flow across an ambient
plot for subsequent distribution onto the wet treatment plots.
Field Methods:
Enhancing UV-B Radiation
Separate from the issue of global warming, stratospheric ozone depletion from emissions of chlorofluorocarbons and other anthropogenic compounds
also poses a threat to ecosystems. The natural
stratospheric ozone layer attenuates solar UV-B radiation (290 to 320 nm), and therefore depleting the
ozone layer increases penetration ofUV-B radiation
to the earth's surface. UV-B radiation affects plant
chemistry and physiology (Caldwell et al. 1989), as
well as plant morphology and competition (Barnes
et al. 1995). Terrestrial plants are especially vulnerable due to their obligatory requirement for sunlight for photosynthesis (Greenberg et al. 1996).
There have been hundreds of studies of the impacts of elevated solar UV-B radiation on plants
(for a summary, see Caldwell and Flint 1994). Very
few have dealt with multiple species, much less
plant communities or whole ecosystems. Most have
been short-term studies in greenhouses or growth
chambers, but these often overestimate the UV-B
radiation effects (Mark et al. 1996). Results of several studies suggest that different species of plants
show large differences in sensitivity to UV-B radiation (Rau and Hofmann 1996), making it difficult to extrapolate results from chamber studies of
single-species response to UV-B radiation alteration to whole-ecosystem response. As is the case
with ecosystem manipulations of climate, there is a
relative scarcity of whole-ecosystem, long-term
field manipulations of UV-B radiation (Barnes et
al. 1995; Caldwell and Flint 1994; Flint and Caldwell 1996).
363
Field studies of UV-B radiation have taken one
of three approaches: excluding UV-B radiation by
filtering sunlight, transplanting to high altitudes
(where UV-B radiation intensity is greater due to
reduced screening from natural ozone), or enhancing ambient UV-B radiation artificially. Excluding
UV-B radiation in the field is done with a shade or
greenhouse. Exclusion experiments are simpler and
cheaper than enhancement manipUlations, and if
carefully conducted, can suggest whether ambient
UV-B radiation is influential (Caldwell and Flint
1994). They cannot be used to realistically simulate
ozone depletion, however, since extrapolating results from UV-B radiation exclusion experiments
to enhanced levels of UV-B radiation is not necessarily justified. The effects of elevated UV-B radiation can also be examined by studying plant responses at high elevation or in low latitudes, since
the amount of UV-B radiation reaching the earth's
surface increases with altitude above sea level and
with lower latitudes (Rau and Hofmann 1996).
Controls can be obtained by suitable filtering of excess radiation (Tevini et al. 1989) or by comparison
with plant response in lowland areas (Dohring et
al. 1996). These studies have the advantage of realistic spectrum characteristics of solar enhancement, but it is difficult to attribute differences in
plant response at different locations to UV-B radiation enhancement alone, and not genetic, climatic,
or soil differences between sites.
Field manipulations enhance global UV-B radiation by supplementing ambient sunlight using
fluorescent lamps. Conceptually, the use of overhead lamps for UV-B radiation enhancement is
similar to the use of overhead IR lamps for global
warming studies. Determining the proportional
UV -B radiation increase associated with various
decreases in stratospheric column ozone is typically
accomplished with a mathematical radiative transfer model (e.g., Bjorn and Murphy 1985; Green
1983; Green et al. 1980; Rundel1986). Since a reduction of the ozone layer results in a very specific
increase in solar UV-B radiation in a wave band of
only about 30 nm, the fluorescent lamps for UV-B
radiation field manipulations must be filtered in order to screen out wavelengths below 290 nm (e.g.,
Flint and Caldwell 1996; Mackerness et al. 1996).
Different screening techniques and materials can
also be used to obtain different kinds of treatments,
such as to remove or supplement UV-B radiation.