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QUESTIONS AND ANSWERS
The role of plant ecology
in urban land-atmosphere interactions.
Diane
Pataki, Univ. of California, Irvine
Q: You stated that urban ecosystems can be
considered a proxy for global climate change. What subtle differences might you
expect, if any, between the isotope signatures of "natural" forests versus
"urban" forests? Would you expect atmospheric pollutants to be the main factor
affecting responses?A: This is an
issue we are actively working on at the moment. The stable carbon isotope ratio
of vegetation in urban areas has two major influences--plant physiological
processes and the isotope ratio of the CO2 used for
photosynthesis, which tends to be diluted in heavy carbon relative to CO2
in natural ecosystems due to fossil fuel combustion. In natural ecosystems we
generally disregard the effects of the isotope ratio of atmospheric CO2
because it does not vary much. If we account for the atmospheric effect (which
is non-biological) in urban systems, we can then use isotopes to evaluate
differences in physiology between urban and natural vegetation. We expect
differences due to a number of factors including pollutants, differences in
temperature and relative humidity, soil compaction, species composition, etc. We
will need several isotope tracers, physiological measurements, and observations
of environmental conditions to understand these differences.
Q: How does humidity in the city compare to what ambient would be in
natural environment? Are there data to compare the native grassland to the urban
forest now covering the Salt Lake basin? Are there measurable impacts on
surrounding native vegetation resulting from higher atmospheric moisture and CO2
in SLC?
A: The first European settlers to reach the Salt
Lake Valley reported seeing abundant perennial grasses and very little woody
vegetation; however, much of the non-urbanized region of northern Utah is now
dominated by the shrub Artemesia tridentata and the exotic invasive
annual grass Bromus tectorum. So we have little information about the
native grassland that once covered the valley. We are now analyzing the humidity
observations in the non-urbanized shrubland to the west of SLC to compare with
observations within the city, although we will not be able to distinguish
between effects of the Great Salt Lake and urban irrigation, since the lake lies
immediately adjacent to the city and air masses pass from the desert to the lake
and then to the city. The effect of the altered urban atmosphere on peri-urban
vegetation, especially the native subalpine forests to the east, is a very
interesting issue. We don't expect CO2 to have a large
effect because the largest concentrations are observed in the winter when plants
are not very active, but the influence of other pollutants such as ozone may be
significant.Q: It has been suggested that
the largest "ecological footprint" comes from suburban areas. Can you
distinguish between agricultural, suburban, and urban landscapes?
A: Our original intention was to distinguish
between different land cover types with atmospheric measurements. However so
far, the influence of atmospheric transport has been the largest effect on our
atmospheric observations. Atmospheric transport and meteorology of mountain
basins is very complex, and our measurements are actually providing some insight
into transport and mixing of air in the basin. CO2 is a
stable, non-reactive gas that is actually quite a good tracer of atmospheric
transport, especially in the winter when there are no CO2
sinks, only sources. Although we haven't been able to make large distinctions
between surburban and other land cover types, we've been able to learn more
about atmospheric mixing during unusual meteorological events like persistent
temperature inversions by analyzing the spatial variability in CO2
concentrations around the valley.
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