Temperature and pollutant influences on tree growth

Trees in urban areas are exposed to higher temperatures, enrichment of carbon dioxide (CO2) and nitrogen as well as greater levels of other pollutants than those in rural ecosystems. Each of these factors exerts a negative or positive influence on trees (see also Section 11.4.2); the important question is to ask what their net effect is in different parts of a large region. Studies by Gregg et al. (2003) used soil transplants, nutrient budgets, chamber

vj 11iiy

vj 11iiy

Aquatic community Water level

Plant communities

Bog mat

Aquatic community Water level

Gyttja

HI rr~n

Clay Open Gyttja water macrofossils

Swamp forest

Plant communities

Bog mat

Wood peat

Shrub peat

Figure 3.14 A model for forested peatland development in central New England. Gyttja (a Swedish word pronounced 'yut-tya') is a nutrient-rich organic deposit. Events in the two basins are not assumed to be synchronous. (From Anderson et al., 2003. Journal of Ecology 91, Blackwell Publishing.)

experiments and multiple regression analyses to investigate this. They also used an inherently fast-growing clone of eastern cottonwood Populus deltoides as a 'phytometer' to measure overall growth responses to multiple pollution sources both in urban New York City and in surrounding rural areas finding, surprisingly, that urban plant biomass accumulation so measured was double that of rural areas. Soils, CO2 concentration, nutrient deposition, urban air pollutants and microclimatic variables could not account for increased growth in the city.

Their results demonstrate the overriding importance of ozone (O3) as a gaseous pollutant, one which - contrary to expectation - was at a higher concentration and did more to reduce productivity in rural than in urban areas. These urban trees also benefited from higher temperatures and higher rates of nutrient and base-cation deposition. Though urban precursors fuel the reactions of O3 formation, nitrous oxide (NOx) scavenging reactions resulted in lower cumulative O3 levels in urban sites than in agricultural and forested areas throughout north-eastern USA. This made a vital difference despite the considerable amounts of many gaseous, particulate and photochemical pollutants (including NOx, nitric acid - HNO3, sulphur dioxide - SO2, sulphuric acid - H2SO4, O3 and volatile organic compounds) in the city, where heavy metal pollution of the soil was also much greater.

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