Macroclimate associated with urbanization increases the rate of secondary succession from fallow soil

To examine the impact of projected climate changes on secondary succession, we exposed the same fallow soil with a common seed bank to an in situ gradient of urban to rural macroenvironments that differed in temperature and CO2 concentration ([CO2]). This gradient was established at three locations: Baltimore city center (urban), a city park on the outskirts of Baltimore (suburban), and an organic farm 87 km from the Baltimore city center site (rural). Over a five-year period, the urban site averaged 2.1 degrees C warmer and had a [CO2] that was ~20% higher than at the rural location, indicating that this gradient was a reasonable surrogate for projected changes in those variables for this century. Previous work had demonstrated that other abiotic variables measured across the transect, including tropospheric ozone and nitrogen deposition, did not differ consistently. The first year of exposure resulted in (two- to threefold) greater aboveground biomass in the urban relative to the rural site, but with uniform species composition across sites. Simple regression of abiotic variables indicated that temperature and vapor pressure deficit (VPD) were the best predictors of plant biomass among locations. Stepwise multiple regressions were also performed to analyze the effect of more than one macroenvironmental variable on total plant biomass. The combination of daily CO2 concentration and nighttime temperature explained 87% (P < 0.01) of the variability in total biomass between sites. After five years, the species demography of the plant communities had changed significantly, with a greater ratio of perennials to annuals for the urban relative to the rural location. Greater first-year biomass and litter accumulation at the urban site may have suppressed the subsequent seed germination of annual species, accelerating changes in species composition. If urban macroenvironments reflect future global change conditions, these data suggest a faster rate of secondary succession in a warmer, higher [CO2] world.