EXPLORING THE PRECIPITATION-GROWTH RELATIONSHIP FOR TREES GROWING IN PERENNIALLY SATURATED SOILS

Bald cypress (Taxodium distichum) are known to respond to increases in precipitation with increased radial growth even when rooted in perennially saturated sediments. When water is not a growth-limiting factor, other factors related to precipitation must cause changes that facilitate tree growth. Precipitation may introduce nutrients either attached to sediments in runoff, or pore water in the root zone may be flushed with water rich in nutrients and oxygen. The later possibility was explored in Sky Lake, an oxbow-lake wetland in northern Mississippi, where a correlation has been previously observed between precipitation and tree-ring widths. Measurements of δ¹⁸O in precipitation, and δ¹⁸O, Clˉ, ³H, redox potential and hydraulic head in surface water and shallow groundwater over the course of a year show that rapid downward flow of surface water into the root zone is initiated only after precipitation-induced increases in surface water depth exceed 0.5 to 1.0 m (maximum depths of over 4 m observed). Transport through the clay-rich sediment is slow, but numerous preferential flow pathways are created by the ubiquitous presence of partially decayed roots and fallen tree limbs and trunks. Hydraulic conductivity measured in 9 piezometers in the upper 3 m varied by over 5 orders of magnitude. Rapid downward flow increases the flux of oxygenate water, and should increase the flux of nutrients dissolved in surface waters.