Influences of a filled valley and deep soil water saturation on tree growth in the Green River floodplain
In January 2006, 10,000 bare-rooted seedling bottomland hardwood trees were planted in a hay meadow in the alluvial floodplain of the Green River. Tree growth of planted and volunteer seedlings is heterogeneous. The apparent patchiness appears to be unrelated to the NRCS soil types and lighting. We hypothesized that characteristics of the substrate well below the depth of the rooting saplings is responsible for variation in growth. Four resistivity transects were studied, each one running perpendicular to the plotted tree rows. There were 29 rows of trees and every row of trees crossing the resistivity transects was analyzed, extending 5 meters on either side of the resistivity transect. For each of the tree transects crossing the resistivity transects, the abundance and height of tree species was recorded. All four resistivity transects had tree transects analyzed in this manner. In order to characterize subsurface conditions, we utilized Electrical Resistivity Tomography (ERT) to map the resistivity of earth minerals to a depth of 20m. In addition to this, we measured root depth for some of the largest planted and unplanted saplings in areas of rapid growth and slow growth. Three soil depths were analyzed: 0-10 cm, 11-19 cm, and 20-30 cm and in both cases, few roots extended below 30cm. Another linear regression analysis was run to determine if root depth and had any correlation to ground-line diameter.
We found that a valley had been buried beneath portions of the relatively flat field, and that patches of tall trees were located over the sediment filled valley. Stunting tree growth was detected away from the valley. The ERT showed that beneath the areas of taller trees there was a low resistivity, while the subsurface of the stunted tree had a high resistivity. Meaning the taller trees sat on an area of higher saturation, while the shorter trees sat on areas of lower saturation. The linear regression analysis was conducted with the x-axis as ground-line diameter, and the y-axis as the number of roots at a given depth category divided by total number of roots observed for each tree. There was no correlation between root and ground-line diameter. Tree height was most closely correlated with soil saturation as determined by resistivity at 1.1m below the surface. We discovered that the trees are obtaining water from below the depth of root penetration via capillary action from the saturated soils in the buried valley.