Results
All topographic sites exhibited the typical pattern of a vegetation phenological cycle (Figure 2). Low NDVI values were observed during the early part of the growing season (May) and achieved full-growth potential in July/August, prior to subsiding in late September through mid-October. The marsh and wet meadow areas had the highest NDVI values (e.g., 0.615 for marshland in 1987), when compared to the dune slope and dune top. Marshes and wet meadow are covered with lush vegetation cover, and exist in sharp contrast to the sparsely vegetated uplands (dunes). This is the result of a “dynamic interaction between climatic, hydrologic, chemical, and biological processes” (Gosselin et al., in press). In many interdune valleys the water table intersects with the valley floor thus resulting in the formation of inland fresh marshes and wet meadows. Conversely, in upland areas, depth to groundwater is greatly increased, and the sandy soils cause rapid infiltration of any precipitation. This results in approximately 30-40% vegetation cover on the dune slopes and dune tops, dominated mainly by mixed prairie grasses.
Figure 2. NDVI patterns and trends observed after analyzing 57 images during the growing seasons from 1979 to 1989. The x-axis represent Julian day for a given year.
Note the seasonal growth pattern for each year.
The mean NDVI values allow an inter-annual comparison of variations in NDVI (Figure 3). For the most part, the mean NDVI fluctuates less than 0.100 from year-to-year for each topographic feature. Notable exceptions include the marsh in 1981 and a dramatic increase in NDVI observations in 1988 for all sites. The potential reason for the marsh mean NDVI for 1981 plummeting to 0.200 was high quantities of rainfall received during the peak of the growing season. For example, precipitation recorded during the 15-day period prior to image acquisition on 30 July 1981 was 3-inches, and the Standard Precipitation Index was 2.20. Excessive rainfall quantities would temporarily raise water levels, thus concealing the wetland vegetation, which in turn would lower the NDVI values.
Figure 3. Mean NDVI values recorded for various terrain features within the study area
Increases in mean NDVI for all terrain features was attributed to the fact that only two images were acquired for 1988. Both images were acquired in August, which is generally considered the peak of the growing season. The images were spaced 16-days apart (August 16 and 29), and high NDVI values were recorded for both dates (0.590 and 0.515 respectively). Consequently, the mean NDVI was comparatively high. The paucity of data for 1988 was also reflected in the standard deviation observed for that year (Figure 4). In most other years, the variation in NDVI for marshland was the highest amongst all other topographic features. This is attributed to the seasonal changes in canopy cover in a densely vegetated area. During the early part of the growing season, more water is visible, thus lowering the NDVI. As vegetation growth accelerates, and canopy closure occurs, the increased near-infrared reflectance would lead to higher NDVI values.
Figure 4. Variations in seasonal NDVI values for each of the four topographic locations.
Conclusion
Fifty-seven Landsat MSS images spanning an 11-year period were analyzed for trends and patterns in a wetland area and its surrounding terrain. The NDVI was computed for each image and fundamental statistical procedures were implemented on the observations. Because climatologic and hydrologic parameters have an impact on this environmentally unique area, location can be a key factor in defining the variability in the vegetation health and vigor.

References

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