Water sources in riparian tree species of the southeastern United States

Background/Question/Methods

Identifying water sources of riparian, stream-side plants is critical for evaluating impacts of environmental variables such as precipitation and water pollution. Previous studies in the arid southwestern US have shown that mature individuals, growing along snow-fed streams, take up groundwater and little or no surface water. In comparison, water sources of riparian species in the southeastern United States are largely unstudied. The purpose of the present study was to determine the source of water uptake for two common riparian species found in the foothills of the southern Appalachians, Acer negundo (Boxelder) and Betula nigra (River Birch), using signatures of the stable isotopes found naturally in water, ²H and ¹⁸O. The study site (Newton, North Carolina) is located along the Jacob Fork River within a riparian zone typical of this region. Water samples were taken from surface and groundwater sources along with soil samples from shallow and intermediate depths. Woody tissue samples were also taken from systematically-selected, mature individuals of each species. Samples and measurements were collected once each month for the length of a growing season (June-November).

Results/Conclusions

Preliminary results suggest both species have a consistent reliance on a deep, underground water source protected from evaporation effects. Results also demonstrate a strong correlation between isotopic values of shallow soil moisture (30cm) and woody tissue of both Acer negundo and Betula nigra over time (r²= 0.97 and 0.97, respectively), with growing season average δD values of -41.6±9.5‰ and -39.7±4.6‰. These findings suggest that both species are dependent on groundwater, but also utilize shallow soil moisture to supplement total plant water uptake. These results are supported by the corresponding rooting habits of these species-- lateral roots within 20cm of the soil surface, along with development of tap roots >50cm in deeper soils. These results are also in agreement with similar studies performed in the southwestern United States, in that the species appear to have little reliance on surface water.