Vegetation Research Projects
|Effects of a Changing Land Ecosystem on Terrestrial-Estuarine Ecology: History and Paleoecology|
|Gwynns Falls Vegetation|
Different transformations of the landscape are overlain over time, resulting in changes that can affect processes important for nutrient cycling, species diversity, etc., such as increased runoff related to the building of roads and other impervious surfaces. Regenerating vegetation would be expected to respond to these changes, some of which are outlined in Table 1
Table 1, Land use history in the Gwynns Falls watershed and its effect on the landscape and on streams and the riparian zone.
|The Upland Vegetation|
Tree distributions in the uplands are closely related to geologic substrates. Tulip poplar and red maple are the dominant trees on schist, gneiss ands marble – substrates with the highest water-holding capacity; beech and green ash on the less acidic amphibolite but with a lower water-holding capacity than gneiss and schist; red oak and black gum on the dry mafic substrates, and American elm and black locust on the Coastal Plain sand/clay/fill substrate. Below are histograms of species associated with one or two substrates. The following table is a list of species that occur on only one geologic substrate
In order to analyze the afforestation patterns taking place in an area with a progression of land uses beginning with agriculture and culminating with urbanization, the successional status of the species in the watershed was examined.
Each species was assigned a mean erodibility index number and mean percent forest cover based on the erodibility index number and percent forest cover of the plots in which they occurred. Each plot was then assigned to one of five successional stages (early, early intermediate, intermediate, intermediate-late and late) based on the species in the plot. As agricultural activity declined in the Baltimore region beginning about 1900 the first fields abandoned would most likely be marginal lands. These areas would undergo afforestation earlier and would at present be expected to have the greatest percent forest cover, and the largest trees.
Occurrence of different successional groups in plots arranged by average erodibility and percent forest cover.
Early successional species occur more frequently on average in plots with a low erodibility and low percent forest cover than late successional species, although the range for late successional species is quite high. Succession after disturbance on the uplands of the watershed follows a normal successional pattern that is unchanged by a history of agricultural and urban disturbance.
|The Riparian Vegetation|
Riparian data were collected in 110 100 m2 (10 m x 10 m) plots located along 56 randomly selected transects that crossed 1 st to 4 th order streams. Transects were located on Maryland Geological Survey topographic maps and digital ortho-quarter quadrats, and were laid out perpendicular to the stream channel. The width of the floodplain was defined as the distance from the stream channel to a terrace or the change from natural vegetation cover to managed land cover (lawn, parking lot, etc.). For each transect, one plot was sampled on each side of the channel and every 30 m for the extent of the floodplain. Mean riparian width varied from 79.7±11.5 m in the upper part of the watershed to 46.2±10.2 m in the middle zone and 31.8±4.2 m in the lower zone. Relative elevations of each of the 45 transects were measured using CST/Berger 20x construction grade level surveying equipment. Relative elevations were measured approximately every 0.5 m or less where the elevation change was significant such as from the stream edge to the bank edge or old cut-off channels. The bank edge was defined as a significant break in slope of the cross-sectional profile from the stream channel to the floodplain. Elevation measurements were taken across the floodplain at 5 m intervals where the elevation change was not noticeable. The limits of the floodplain were determined by a significant change in elevation and vegetation. The depth of the stream was taken as the zero point in the cross-section.
Of 147 plant species sampled in the Gwynns Falls riparian corridor, 55 were exotics. Of those 55 exotics, 10 were trees and 45 were herbs. Diversity was similar throughout the corridor for trees and herbs, but higher for herbs.
Comparison of Basal Areas
Structurally, the lower section differed from the middle and upper sections. The lower section had the smallest mean diameter at breast height (DBH) (9.5 cm), the highest mean stem density (1585 stems/ha), and the lowest mean basal area (20.1 m2/ha). In contrast, the middle and upper sections had similar means for DBH (11.6 and 10.6 cm), total density (1235 and 1288 stems/ha), and basal area (27.7 and 25.2 m2/ha), respectively.
Differences in composition throughout the watershed are shown by a comparison of basal area of the most important species. Among native species, Fraxinus pennsylvanica, Acer negundo, Acer saccharinum, Liriodendron tulipifera and Robinia pseudo-acacia are restricted almost entirely to the middle and lower watersheds, while Acer rubrum and Juglans nigra occur predominantly in the upper and middle sections. Quercus bicolor, Quercus palustris, Acer saccharum, Salix nigra and Platanus occidentalis are predominantly in the upper watershed.
Size distributions of trees and percent cover of herbs show that species have discrete distributions throughout the watershed with small trees of some species occurring in the upper and middle sections and large trees occurring in the lower watershed, and vice versa . Below are plotted histograms of Acer negundo (a typically riparian species) and Acer rubrum (found both in riparian and upland areas).
Non-North American exotic trees are concentrated in the lower watershed. Of the exotic species only 3 (below) are distributed throughout the watershed. The others have very discrete distributions but are abundant where they occur.
Species distributions with regard to elevations are shown below.. Land surfaces <1 meter above the stream channel elevation support mainly species of trees that normally occupy dry habitats. Red maple is the most abundant species at <1 meter elevation, while box elder is most abundant at elevations >1 meter. The majority of wetland species occurs as large trees on surfaces where elevations are >1 meter, suggesting that these areas represent former floodplains that no longer function as riparian areas. The distribution of species appears to be related to superimposed patterns of stream flooding, bank aggradation and stream incision related to the long history of variable land use as outlined in Table 1. The heterogeneity of past land use is reflected in a heterogeneous riparian area. The occurrence of mainly upland species at the lower elevations suggest a dry conditions related to hydrologic rather than climate change, which is interpreted as a “hydrologic drought”.
|The Oregon Ridge Vegetation|
Several tree ring cores were obtained from Liriodendron tulipifera and Fagus grandifolia, the two most important species in the park in order to analyze the history of tree growth and water availability. These data were compared with climate records.
History of Oregon Ridge
Although Oregon Ridge is largely forested today, it has a long history of different land uses. In the mid 19th century, an iron ore and marble mining operation took place within the park, where an iron smelting furnace was built along the Oregon Branch stream. The furnace was in operation from 1844 until 1858. As a result of these operations, an industrial village developed which remained until the 1930s. In addition to the large scale mining and smelting operations, farmlands surrounded the region. Agriculture included both crop production and grazing. Eventually farming was abandonedDuring this time the park was mostly deforested. The trees that are presently there are no more than 70 to 80 years in age.
Using these data, histograms of the size distributions of the more common species show graphically the growth patterns of trees in the park.
The tree ring part of this study focused on tulip poplar (Liriodendron tulipifera) and beech (Fagus grandifolia), the histograms for which are shown below.
By measuring the distance between the rings, the age of each tree and the amount of growth in individual years could be determined.
Tulip poplar and beech were analyzed with respect to the pattern of succession in the park, since these are the two most numerous trees in the study area. With respect to tulip poplar, no tulip poplar saplings were found in any of the 30 plots sampled. All of the tulip poplar trees are in the larger diameter classes, and presumably older. On the other hand, most of the beech trees were in the sapling category with very few larger (older) trees. This would seem to indicate that tulip poplar is being replaced by beech. Analysis of the tree rings would indicate that hydrologic change is not the reason for the difference between the two species because the growh rates of both species appear similar. However, unusually dry years occurred between 1995 and 2000, during which time tulip poplar growth rings were smaller than before and after this period, whereas beech growth rates are greater during this period. The results, though preliminary, indicate that these species are sensitive to dry and wet climate periods, but whether they are responding to changes in hydrology is not evident.