Hydrologic Influences on Leaf Decomposition in a Channel and Adjacent Bank of a Gallery Forest Stream

Abstract

Hydrologic extremes of flooding and drought typically occur each year in prairie streams. Two experiments were conducted in a fifth-order, gallery forest reach of Kings Creek, Kansas, to assess the effect of hydrologic conditions on decomposition of leaves in the stream channel and on the adjacent floodplain. Temporal patterns of weight loss were examined in the first experiment. Leaves of bur oak and hackberry decomposed more rapidly in the channel than on the adjacent bank. A sharp drop in percent remaining for hackberry in the 2nd month (mid-December to mid-January) coincided with a period of high shredder densities on hackberry leaves. On the bank, decomposition of hackberry leaves was fastest during intervals that included one or more inundations of the leaf packs. Bur oak leaves decomposed more slowly than hackberry leaves and were influenced less by the hydrologic history. In the second experiment, spatial variation in decomposition rate of hackberry leaves was examined by placing 20 pairs of leaf packs in a transect extending from the center of the stream channel to the top of the upper bank. Position on the transect affected the frequency and duration of inundations on the floodplain, which ranged from once (minimum of 0.2 hr) to 17 times (272 cumulative hr under water), while three pairs of packs in the channel were always under water. The logarithm of percent remaining after 274 days was significantly correlated with number of hours inundated and number of times inundated, although other factors such as soil moisture or amount of flood-deposited silt may have influenced differences in decomposition rates along the transect. We concluded that flood frequency, duration and timing affected both spatial and temporal patterns of decomposition, especially of a fast-decomposing species, in the riparian forest of an intermittent prairie stream. INTRODUCTION Hydrologic extremes of flooding and drought typically occur each year in prairie streams. Periodic flooding may cause increases in decomposition rates in channels of prairie streams, while desiccation slows decomposition (Tate and Gurtz, 1986). Storm flows can dislodge stored organic materials from the stream bottom and either export them to downstream reaches or deposit them laterally on the stream bank. Organic matter deposited on the floodplain remains there until it is decomposed, returned to the stream by lateral movement, or resuspended by another storm flow. Residence time of organic matter in the channel and on the floodplain is therefore affected by hydrologic conditions, which in turn may influence decomposition rates. Flooding in seasonally inundated wetlands or floodplain forests generally promotes faster decomposition, whereas decomposition in freshwater systems is faster than in terrestrial systems (Webster and Benfield, 1986). Cellulose sheets placed in a North Carolina alluvial swamp forest decomposed in the sequence (fast to slow) of river > swamp > levee environments, probably due to differences in moisture conditions (Brinson, 1977). In that wetland system, increased frequency or duration of flooding did not always lead to faster decomposition rates for all species. Rather, the fastest loss rates may occur under aerobic conditions and some optimum regime of wetting and drying (Brinson et al., 1981). Day (1983) found that faster decomposition rates were directly related to flooding in microcosms that remained aerobic. In a floodplain hardwood forest along the Appalachicola River, Florida, rates of leaf decomposition were much slower in dry 'Present address: P.O. Box 2857, Raleigh, N. Carolina 27602.