The Isotopic Composition of Urban Plant Biomass

We have discussed the influence of plant and soil processes on the urban atmosphere, but the reverse effects are also an essential part of urban ecology. The highly modified urban atmosphere may influence urban forest physiology and growth, and the services that these provide to urban residents (carbon storage, transpirational cooling effects, removal of pollutants, etc.). Stable isotopes also provide a useful tool in this regard. The carbon isotope composition of c3 plant biomass is a function of the intercellular to ambient ratio of CO2 partial pressures (ci/ca) and the isotopic composition of the atmosphere (Farquhar et al, 1989). Carbon isotopes have been used to infer historical changes in atmospheric CO2 concentrations (Marino and McElroy, 1991) as well as CO2 exposure of plants grown in elevated C02 experiments (Pepin and Körner, 2002; Pataki et al, 2003c). In the urban environment, the extent to which elevated levels of CO2 and other products of combustion affect plant physiological processes and biogeochemistry has not been well investigated, such that stable isotope analyses can provide important new information.

c4 plants provide a convenient tool for assessing CO2 exposure, as their isotopic composition is relatively insensitive to changes in Ci/CA (Marino and McElroy, 1991) if bundle sheath 'leakiness' can be neglected (Farquhar, 1983; Buchmann et al, 1996). However, c4 plants may not be common in all areas where an assessment of urban plant exposure to high CO2 is desirable. If the variety of environmental conditions that affect ci/ca are similar at a given urban location and a rural area that serves as a control, then analysis of c3 biomass may also be informative. Lichtfouse et al. (2003) compared ¿13C of grasses growing near a busy roadway in Paris, France to i513C of grasses growing in a rural area. Urban grasses were depleted in 13C

Day of year

Figure 12.3 Proportional contributions of gasoline combustion, natural gas combustion, and biogenic respiration to total CO2 source in Salt Lake City, USA determined from Eqs 12.6-12.8. Modified from Pataki et al. (2003a).

Day of year

Figure 12.3 Proportional contributions of gasoline combustion, natural gas combustion, and biogenic respiration to total CO2 source in Salt Lake City, USA determined from Eqs 12.6-12.8. Modified from Pataki et al. (2003a).

Paris, France

May 1996

Urban

Rural

Figure 12.4 The carbon isotope composition (S C) of grasses growing near an urban Paris roadway and a rural area. The shaded area encompasses 50% of the data points, and the line shows the median value. The bars show the upper and lower 25% of the data. Modified from Lichtfouse et al. (2003).

by 4.5%o on average (Fig. 12.4). These data were used to calculate the proportion of fossil-fuel derived CO2 incorporated into urban grass biomass, which ranged from 20.8 to 29.1% (Lichtfouse et al, 2003). Such calculations assume that elevated CO2 does not in and of itself affect c,/ca, which has been supported by many instantaneous measurements (Morison, 1985; Drake et al., 1997) but not all isotope measurements (Pataki et al., 2003c).

Tree rings provide a longer-term record of plant exposure to increasing carbon dioxide. Dongarra and Varrica (2002) measured <S13C in tree rings of Platanus hybrida individuals in Palermo, Italy and found progressive depletion of 3.6%o on average during the period 1880-1998 (Fig. 12.5). This is considerably larger than the global background depletion of approximately 1.5%o during the same period, and indicates a large local effect of urban, 13C depleted CO9 emissions on plant biomass. These data may be used to reconstruct the historical record of CO2 concentrations during urban development. Even taking into account long-term changes in c -Jc^ during the tree ring record, Dongarra and Varrica (2002) inferred that local CO2 concentrations in Palermo had increased by about 90 ppm on average since the 1950s, or 30 ppm above the global trend. Elevated atmospheric CO2 has been shown experimentally to affect water use, growth, allocation, and other processes, regardless of changes in c2/c^ (Drake et al, 1997). Isotopic evidence suggests that elevated CO2 may play a role in urban plant and soil processes.

Palermo, Italy Platanus hybrida Brot.

Figure 12.5 The carbon isotope composition of tree rings of Platanus hybrida Brot.

grown in Palermo, Italy. Modified from Dongarrà and Varrica (2002).

Elevated concentrations of CO2 in urban areas are often associated with other products of combustion such as so2, NO*, and ozone (Douglas, 1983). While elevated CO2 tends to increase plant growth, high concentrations of atmospheric pollutants such as sulfur dioxide and ozone may cause tissue damage and stomatal closure in many plants (Beckerson and Hofstra, 1979; Darrall, 1989; Skarby et al, 1998). These effects may reduce or offset the effects of elevated CO2 on growth (Fiscus et al, 1977; Karnosky et al, 1999; Olszyk et al, 2001; Karnosky et al, 2003). Decreases in Ci/CA resulting from stomatal closure causes carbon isotope enrichment in c3 plants, an effect which has been documented in both controlled environment studies of plant growth in response to elevated SO2 and ozone (Douglas, 1983; Martin et al, 1988), and in field studies comparing tree ring carbon isotope composition near point sources of pollution to unpolluted controls (Martin and Sutherland, 1990; Savard et al, 2002). Isotopes have also been used to document plant uptake of nitrogenous products of pollution such as NH3/NH4 and N02 (Ammann et al, 1999; Stewart et al, 2002) that may act as a fertilizer at low concentrations or may be toxic at high concentrations (Krupa, 2003). Because there are a variety of possible opposing effects of atmospheric pollutants that may enhance or reduce plant gas exchange and growth, the application of multiple isotope tracers to identifying the role of individual trace gases is an important direction of future research.

Was this article helpful?

0 0

Post a comment