Random sampling of vegetation in the Gwynns Falls uplands in Baltimore City, a National Science Foundation Long Term Ecological Research urban site, shows that water availability is the strongest determinant of tree species distributions.  While some species grow on a variety of soil types, a number of species are restricted to areas of high or low water availability.  Based on regional pollen profiles, the present species composition is similar to that occurring on this  landscape over the past 6000 years.  However, land use beginning with European settlement in the late 17^th century, has altered the disturbance regime, affecting the landscape successional mosaic.  We propose that as agricultural activity declined in the Baltimore region, beginning in 1900, the first fields abandoned were marginal lands on steep, erodible slopes.  In contrast, the flatter, less erodible fields remained in agriculture for longer periods, with afforestation of these areas if it occurred, taking place later.  The successional stage of species is related to erodibility (the product of slope and soil k factor) and 1992 forest cover within a 0.5 km radius.

Results/Conclusions

On average, late successional upland species occur in plots with a mean erodibility of 2 (0.9-4.4) and mean forest cover of 0.5 (0.1-0.8), whereas early successional upland species occur in plots with a mean erodibility of 1.05 (0.4-2.1) and mean forest cover of 0.3 (0.2-0.4).  These results indicate that upland areas abandoned earlier are characterized by a greater percentage of late successional upland species than those areas where afforestation began following delayed agricultural abandonment.  These results contrast with results for riparian areas of the Gwynns Falls watershed, where the hydrologic regime has changed, and wetland species are being replaced by upland species in many areas.