Study site:
The Gwynns Falls watershed is a focal research watershed of the
Baltimore Ecosystem Study (BES), a long-term ecological research
project (LTER) (https://baltimoreecosystemstudy.org). The watershed,
is approximately 17,150 ha, spans Baltimore City and Baltimore County,
and drains into the Chesapeake Bay.
The Gwynns Falls watershed was dominated by forests before European
settlement. By the middle of the eighteenth century, 20–30% of the
pre-contact forested land had been cleared for agriculture in the
watershed, and this increased to 50% by the middle of the nineteenth
century. By 1900, forested land had been mostly cleared for
agriculture and urban use, with only about 30% of the secondary forest
remaining. The watershed experienced significant urban development
particularly in the second half of the twentieth century. It is highly
urbanized now, with downtown Baltimore at the base where it meets the
Chesapeake Bay extending out into more suburban land cover dominated
by residential lands with some commercial uses in the upper sections
of the watershed.
Data:
A spatial–temporal database including six time slices of forest
patches—1914, 1938, 1957, 1971, 1999, and 2004—was created from a
historic forest map and aerial photos for the Gwynns Falls watershed
using ArcGISTM 9.3 software. Because of the
differences in the spatial extent captured in each time slice, we only
analyzed areas contained in all six data slices.
Forest patches in 1914 were digitized from a georegistered digital map
scanned from a hard copy of the 1914 forest map created by Fred
Besley, Maryland’s first state forester. Forest patches from 1938 to
2004 were interpreted and digitized from aerial photos based on a
series of rules that were defined for forest patch delineation. The
minimum mapping unit was 0.5 ha (i.e., only forest patches with sizes
larger than 0.5 ha were mapped), smaller than the 2 ha used for the
1914 map. The 1999 and 2004 color-infrared imagery had a spatial
resolution of 0.6 m, and was orthorectified. All panchromatic
(black-and-white) photos collected in 1938, 1957, and 1971 were first
scanned and then individually georectified to the 1999 orthophotos,
using image-to-image georeferencing techniques. A minimum of 15
control points were used to georeference each aerial photo, using
well defined permanent features such as sidewalks and road
intersections as control points. More details about the aerial
photographs are given in
Zhou et al. 2011.
Photo interpretation was conducted by two trained interpreters, and
validated by one quality controller to reduce the inconsistency of the
interpretation between the two interpreters. Patches were redrawn for
improvement when the consistency rate was below 90%.
Zhou, W., G. Huang, S. T. A. Pickett, and M. L. Cadenasso. 2011. 90
Years of Forest Cover Change in an Urbanizing Watershed: Spatial and
Temporal Dynamics. Landscape Ecology 26:645–659.
<ulink url="https://doi.org/10.1007/s10980-011-9589-z">https://doi.org/10.1007/s10980-011-9589-z</ulink>.
