Oak forest characteristics and functioning along an urban-rural gradient
The oak-dominated forests common in central Maryland and the Baltimore metropolitan region allow us to examine two different but perhaps interacting ecological questions. First, as is typical for eastern deciduous forests, there is a general lack of oak regeneration in these forests despite abundance in the canopy. Second, we can explore the potential effects of the urban environment on the forest ecosystem. To examine these two patterns, we identified six intact, mature forests along an urban-rural gradient extending northwest from Baltimore City for approximately 30 miles. All forests were predominantly oak though other canopy species and the understory varied. Additionally, stands contained relatively large trees and did not demonstrate obvious disturbances. We collected data on forest structure, composition, mature tree growth rates, seedling photosynthetic rates, canopy cover, forest floor, soil organic matter, soil nutrient availability, microclimate, and CO2 concentration along transects or in random plots. We expected the urban environmental to have a negative impact on forest patterns and processes.
Overall, variation in measured traits could largely not be linked to the urban-rural gradient. For instance, each of the six sites demonstrated similar community structure (tree density and basal area), ecosystem patterns (forest floor mass, soil organic matter), and species responses (tree growth rates). There were subtle shifts in microclimate (warmer in the urban sites) though no consistent impacts on other environmental variables. Maximum seedling photosynthesis rates differed mostly by species (white < red oak) and did not show strong connection to the urban-rural gradient though the red oak seedlings did have higher rates in the rural forests. Oak seedlings were in low abundance across the gradient but this pattern was broader in that the understories were generally sparse, regardless of species. We hypothesize that other factors may be limiting forest regeneration (e.g. white-tailed deer abundance). In conclusion, our results suggest two possible scenarios: 1) the urban environment does not have a negative impact on these forest communities, or 2) the effect of the urban environment is so vast that it exceeds our 30 mile gradient.