Vegetation and hydrologic characteristics of an urban riparian forest

Background/Question/Methods

With channelization of streams and rivers across the Midwest United States during the early 1900’s, riparian forests have likely undergone dramatic changes in vegetation corresponding to hydrogeomorphological disturbance. Modifications to waterways provide opportunities to study the resulting distribution and structure of riparian forests. A riparian forest along Hinkson Creek (38° 55' N, -92° 20' W), a tributary of the Missouri River, in central Missouri was selected to examine long term changes in forest structure and composition related to water availability. Aerial photographs show that the study area was a mixture of widely spaced trees and grasses in 1939. The meandering creek was channelized by the mid 1940’s. Currently, the area is dominated by silver maple (Acer saccharinum) and eastern cottonwood (Populus deltoides), two of the most common species found in riparian forests in the upper Midwest.  This project was undertaken with the following objectives: 1) characterize current forest structure, tree species composition, and spatial distribution; 2) determine temporal patterns of tree establishment using dendrochronology; 3) quantify the difference between a forested and agricultural floodplain in terms of soil water content to better understand riparian forest establishment and growth.  We developed stem maps to represent all trees >1.3 meters tall in four adjacent 1600 m² plots.  Diameter of each tree was measured and increment cores were taken from representative individuals of four species in each plot. Leaf area index was estimated using the ceptometer method. Soil water status was quantified using the soil core method at depths of 0, 15, 30, 50, 75, and 100cm (n=150/plot).

Results/Conclusions

Total basal area of the sampled forest was 142 m²/ha. Average leaf area index (LAI) was 2.98 (SD=0.7). Populus deltoides dominated in terms of basal area with 56.3 m²/ha.  Tree increment cores collected from plots indicated only slight variation in the year of establishment for four species. Across all species, establishment dates ranged from 1939 to 1962.  Average establishment dates were: A. saccharinum-1942, A. negundo-1953, Ulmus americana -1957, and Juglans nigra -1954.  A. saccharinum, therefore, was the first to establish after the channelization, and the most recent colonizer was Ulmus americana.  Average volumetric water content was 0.48, and 0.52 in agricultural and forested sites respectively, a 9% difference in soil water. These results hold important implications for soil infiltration, transpiration and riparian forest establishment, growth and resilience.