The role of floods in particulate organic matter dynamics of a southern Appalachian river/floodplain ecosystem.

We investigated the role of a flood in particulate organic matter (POM) dynamics of the Little Tennessee river/floodplain ecosystem in western North Carolina, USA. We measured litterfall, leaf breakdown, and floodplain litter at 12 sites. Annual litterfall (274-625 g m-2 y-1) was typical of a temperate deciduous forest but lower than lowland floodplain forests in the eastern United States. Leaf breakdown rates of 4 tree species ranged from 0.001-0.010 d-1. Following a flood on 8 January 1998, the 12 sites were separated into 3 classes of inundation: inundated, partially-inundated, and non-inundated. Sites inundated by the flood had significantly less floodplain leaf litter and large woody material after the flood. There was no significant change at the partially-inundated or non-inundated sites. In addition, there was no significant difference in herbaceous litter between pre- and post-flood collections at any site. The flood also increased leaf breakdown rates of red maple (Acer rubrum) and black walnut (Juglans nigra) but had no effect on American hornbeam (Carpinus caroliniana) or sycamore (Platanus occidentalis). Our measurements of litterfall, leaf breakdown, and floodplain litter suggest that flood entrainment of POM from the floodplain of Little Tennessee River is a source of POM to the active channel. However, floodplain POM inputs were a small annual source of POM for the river compared to direct litterfall. Litter inputs to the floodplain | Ten litter traps were placed at each of the 12 sites at 2-m intervals along a transect parallel to the river channel located approximately 2 m from the river bank. Litter traps consisted of a 0.25-m2 frame fitted with 1-mm mesh fiberglass screen and supported by three wooden stakes. Litter was removed from the traps twice monthly during times of peak leaffall (October-November) and monthly at other times from September 1997 until September 1998. Litter was dried (50 degrees C, 2 d) and separated into leaves, wood, and miscellaneous litter (e.g., flowers, seeds). Leaves were sorted to species, weighed, recombined, ground, and subsampled. Wood and miscellaneous litter were separately weighed, ground, and subsampled. Subsamples were weighed, ashed (550 degrees C, 45 min), and reweighed to determine ash free dry mass (AFDM). | Leaf breakdown | Fifteen leaf bags of the 2 most common floodplain species, sycamore and American hornbeam, were placed on the floodplain at 8 of the 12 sites, and 15 leaf bags of 2 other common floodplain species, red maple and black walnut, were placed at 3 sites. Leaf bags of a given species were placed at sites where that species was common. Each leaf bag initially contained 8 g of leaf material. Three bags of American hornbeam, black walnut, and red maple were retrieved after 1, 2, 4, 6, and 7 mo at each site, and 3 leaf bags of sycamore were collected after 1, 2, 4, 7, and 9 mo at each site. All leaves were washed, dried, weighed, ground, and subsampled. Subsamples were weighed, ashed, and reweighed to determine AFDM. Species-specific exponential rates of breakdown were determined from the negative slope of the log percentage AFDM remaining (y-axis) vs. the days of exposure (x-axis) (e.g. Benfield 1996). | Floodplain litter standing crop | Floodplain litter standing crop (FPL) was measured by removing all coarse litter from 20 0.05-m2 plots at each site (240 total plots) in December 1997 (pre-flood) and January 1998 (post-flood). Litter was dried and separated into leaves, wood, and herbaceous material. Sorted materials were then weighed, ground, and subsampled to determine AFDM.