A common garden experiment: tree physiological performance across a suburban to urban gradient

Background/Question/Methods

As urbanization increases globally, the proportion of land area subject to higher atmospheric CO₂ concentrations and temperatures in urban centers also rises.  Studies of plant physiology along suburban to urban gradients yield important insights into the effects of both urbanization and climate change on plant performance.  Such research also allows for the investigation of interacting environmental drivers of physiological change.  Toward this end, we studied two common garden sites, one located in an urban area at the Maryland Science Center in Baltimore, MD and the other located 16.5 km away in suburban Parkville, MD.  Both sites were planted with hybrid poplar (Populus sp.) and northern red oak (Quercus rubra) saplings in 2006.  Gas exchange measurements were made on canopy leaves from three Populus and two to three red Q. rubra trees per site on each of two measurement dates.  Mid-day water potential was also measured daily on six canopy leaves per tree.  Leaf tissue from the upper third of the tree canopies was collected for specific leaf area (SLA) determination and percent nitrogen analysis.  Analysis of variance was used to evaluate measured parameters for each species across sites.

Results/Conclusions

Maximum photosynthetic rate (A_max) did not differ for Populus sp. across the urban to suburban gradient (p > 0.05), though A_max was significantly greater for Q. rubra trees in the suburban environment (p = 0.0076).  Stomatal conductance and mid-day leaf water potential were significantly lower for both species at the urban site (p < 0.01).  Significantly higher SLA was observed at the suburban site relative to the urban site for both species (p < 0.01), while leaf nitrogen content did not vary across sites for either species (p > 0.05).  These findings provide no evidence of an urban CO₂ fertilization effect, but are indicative of greater water stress at the urban site.  Vapor pressure deficit was ~0.5 kPa greater at the urban site, perhaps leading to more conservative water use in both species and reduced A_max for Q. rubra.  Additionally, the urban site is adjacent to the brackish Inner Harbor, so tree roots may have reached this water source and been subject to lower osmotic potential.  Differences in SLA and leaf nitrogen content are unlikely to account for the observed differences in A_max observed for Q. rubra.  These findings highlight the importance of moisture stress in mitigating CO₂ fertilization effects.