ROLE OF WETLAND SEDIMENTATION, PRECIPITATION, AGRICULTURAL RUNOFF, AND SUBSURFACE FLOW ON BALDCYPRESS GROWTH

A correlation has been frequently observed between annual or seasonal precipitation and the growth rate of baldcypress (Taxodium distichum) even when the trees grow in continuously saturated soils. Where the availability of water is not a limiting factor, accelerated growth following periods of higher precipitation must be caused by a secondary phenomenon such as an increase in the availability of nutrients created by flushing surface or ground water through the root zone, or by an influx of nutrient-rich sediments associated with precipitation events. The possible link between tree growth and sediment influx was investigated by comparing the growth rate of two populations of baldcypress at Sky Lake in the agricultural Delta region of Mississippi. The two populations are similar in mean tree age, number of trees per unit area, and elevation, but differ substantially in the local rate of sediment accumulation. Sedimentation rates were determined using ²¹⁰Pb and ¹³⁷Cs analyses of 1-m sediment cores. The western population grows near the mouth of a small perennial stream and receives sediment-laden water during storm events. Sediment accumulation rate in this area has been approximately 1.8 cm/yr during the previous century, with a possible recent reduction due to alteration of an adjacent road in the 1960’s. The eastern population receives water only by overland flow or from the lake when water levels rise. Sediment accumulation rate in this area has averaged approximately 0.25 cm/yr during the previous century. All mature trees within a 20-m radius of the sediment sampling sites were cored to measure ring-increment width. The mean annual ring-width for trees in the western area from 1895 to 1998 was 17% (0.05 cm/yr) higher than for trees in the eastern area, supporting the hypothesis that tree growth is at least partially linked to nutrient-rich sediment deposition. Additional observations of long term increases and decreases in growth rate in both populations suggest that the application of specific classes of agrochemicals in the surrounding fields may also have influenced growth.