COMPARISON OF ORGANIC CARBON CONTENT, ROOT DISTRIBUTION, AND SOIL HYDRAULIC PROPERTIES IN BOREAL AND TEMPERATE FORESTS WITH SIMILAR ANNUAL PRECIPITATION

Tree roots convey water and essential nutrients from the soil into plant tissues via transpiration, which also links roots to hydrological water balances. Decaying roots provide organic carbon to soils. Therefore, soil carbon distributions are influenced by the adaptations of trees and their root structures to available water sources (infiltrating water or groundwater) and evapotranspirative demand. Global warming and increased growing season length and annual evapotranspiration will likely affect tree species, their root structures, and the water sources they access. To evaluate these changes, we examined soil and hydrological characteristics in boreal and temperate forests with shallow water tables and similar mean annual precipitation (~1100 mm/year) that is distributed throughout the year. The sites in Newfoundland and Maryland differ in mean annual temperature, mean annual evapotranspiration (ET), percentage of precipitation as snow, and local topography. Both sites exhibit similar summer water table minimum depths, which are driven by ET demand. Data on forest characteristics (tree density and root density by soil horizon), soil characteristics (soil organic carbon, soil texture, bulk density, infiltration rates, soil hydraulic conductivity), and hydrological processes (precipitation, groundwater levels and response rates) were collected at both sites. Soils in both locations have macropores in surface horizons, but macropore infiltration rates are much higher in Newfoundland soils. Tree rooting depths in Newfoundland are very shallow, with high organic horizon root densities and few roots below 30 cm, suggesting that interception of infiltrating water, not groundwater is the major water source even during summer minima. Hydraulic conductivity (K) values are highest in this organic horizon, which generates a K boundary with the underlying mineral soil. Rooting depths, organic carbon, and high K values are more broadly distributed with depth in the Maryland forest, however few roots reach to the summer minimum water table depth. This suggests that these boreal and temperate trees both use infiltrating water and capillary water sources; the additional temperate forest transpired water is from precipitation during the longer Maryland growing season.