Is urban old-growth affected by air pollution?

Background/Question/Methods

Urban forest fragments are often small, even-aged, frequently disturbed, and therefore dominated by early successional tree species. In contrast, Fernbank Forest, on the east side of Atlanta has a relatively anomalous history. Despite surrounding urbanization, the inner area of the forest (50 acres) has remained largely intact and the forest contains many large-diameter trees that are thought to be old-growth. In addition to strengthening the conservation value of a forest, old-growth trees can provide a record of historical growth and more recent forest responses to urbanization as indicated by annual growth rings. Previous work has determined that some of Fernbank’s trees are accurate indicators of atmospheric pollutants (e.g. lead), however it is unknown whether changes in air quality are affecting tree growth. The goal of this study was to reconstruct the forest’s growth dynamics using tree-ring data and determine whether a common pattern of growth among trees is related to changes in air quality. Tree cores from 38 trees representing 12 species were collected, processed, aged, and cross-dated using standard dendrochronological techniques. Annual air quality data (e.g. ozone, NO₂) in Atlanta was examined from 1974 to the present.  

Results/Conclusions

Mean tree age was 132 ± 10 years and the oldest tree sampled was a 285 year old Quercus alba. Twenty-eight trees were older than 100 years (74%) and Liriodendron tulipifera tended to be the oldest with a mean age of 186 ± 22 years. Tree age was significantly correlated with tree diameter (r²=0.55, P<0.0001). The oldest species in the forest (present for at least 170 years) are L. tulipifera, Q. alba, Quercus stellata, and Liquidambar styraciflua. Younger species (aged 50 years or less) included Carpinus carolina, Nyssa sylvatica and Oxydendrum arboretum. Preliminary analysis of tree-ring data suggests that interseries correlation among trees was weak, precluding detection of forest-level responses to changes in air quality. A lack of correlation among trees may be explained if factors such as species differences, competition and soil quality are more proximate controls on individual growth than larger scale factors like air quality or climate. Regardless, the uneven age distribution and large number of old trees, suggests that Fernbank represents unique habitat within urban Atlanta and supports the conservation value of the forest.