Enhanced tree growth within an urbanizing landscape offsetting carbon losses associated with land cover change?
Urban and developed lands cover 6% of the contiguous U.S. and are rapidly expanding, changing the structure and productivity of landscapes. The process of urban expansion often fragments ecosystems into a complex patchwork of urban /developed land uses and native vegetation, resulting in a reduction in biomass at the landscape level. However, urbanization can also enhance growing conditions of remnant vegetation through increased availability of light, nitrogen and carbon dioxide as well as warmer temperatures and longer growing seasons associated with the urban heat island effect. While high biomass has been observed in some urban areas, the overall productivity of these systems is largely unknown and can have important implications for the terrestrial carbon cycle. Using Greater Boston as a case study, we quantified biomass and productivity within an urban landscape using FIA data, field measurements of urban biomass, dendrochronology of tree cores collected along light availability gradients and remotely sensed land cover data. These data were then coupled with the EPA Integrated Climate and Land Use Scenarios (ICLUS) model to project the effects of urbanization on biomass and productivity between 2010 and 2050 for three different population growth scenarios.
Early results suggest biomass, averaged across development intensities within Greater Boston, is 33.8 ± 6.7 Mg C ha-1, which is less than half of the biomass in adjacent forests (e.g., FIA: 78.6 Mg C ha-1). However, the productivity of trees growing in high light environments is nearly twice that of forest grown trees, with basal area increments of 35.2 ± 0.13 and 17.6 ± 0.06 cm2 yr-1, respectively. ICLUS modeling results project up to a ~40% reduction in forest area by 2050 in the most rapidly developing counties within the Greater Boston area. If growth rates of all remnant vegetation double with urbanization, productivity in these rapidly developing counties would decline by only 20% by 2050 despite much larger declines in forest extent and biomass. Taken as a whole, these early results highlight the importance of understanding the productivity of vegetation within urbanizing landscapes and may indicate the presence of a compensatory response of landscape productivity within an urbanizing area whereby productivity reductions associated with reduced biomass are at least partially offset by more rapid growth of remnant vegetation.