COMPARISON OF SOIL ROOTING DEPTHS AND SOIL HYDRAULIC PROPERTIES IN BOREAL AND HARDWOOD FORESTS WITH SHALLOW GROUNDWATER TABLES

To better understand biological controls on hydrological processes in forested landscapes, we are investigating root and soil characteristics that influence groundwater table responses. We are comparing two forested regions with shallow water tables. The two study sites have very similar mean annual precipitation values (~1100 mm/year), and precipitation is distributed throughout the year. The sites differ in mean annual temperature, mean annual evapotranspiration (ET), percentage of precipitation as snow, and local topography. The sites are a boreal forest in Newfoundland and a Coastal Plain forest in Maryland. Both sites exhibit seasonality in groundwater elevations. The study goals are to evaluate groundwater responses as driven by the water balance (GW changes = Precipitation-ET) and to determine the influence of root and soil properties on groundwater responses. Data collected at both sites includes: forest characteristics (tree density and root density by soil horizon), soil characteristics (soil organic carbon, soil texture, bulk density, infiltration rate, and soil hydraulic conductivity), and hydrological processes (precipitation, groundwater levels, and GW response rates). Soils in both locations have macropores in surface horizons, but macropore infiltration rates are much higher in Newfoundland soils. The majority of tree rooting depths in Newfoundland are very shallow, with the highest root densities located at the base of the organic horizon, along the organic-mineral soil boundary. Hydraulic conductivity (K) values are highest in this organic horizon, which generates a sharp decrease in hydraulic conductivity below the organic and rooting zones, which facilitates shallow lateral groundwater flow in Newfoundland. In the Coastal Plain forest, rooting depths are distributed over a wider range of soil depths. Effective soil pore size and hydraulic conductivity decrease with depth in the Maryland soil, but an abrupt change in K is not observed. These two forested sites have different root and soil hydraulic properties, even though they have similar water table depths.