Physical factors affecting the distribution of leaf litter patches in streams: comparison of green and senescent leaves in a step-pool streambed

Abstract

We investigated the distribution patterns of senescent and green leaf litter patches on a streambed to evaluate the hypothesis that the different immersion times of senescent leaves in long-term benthic storage and newly retained green leaves affect streambed distribution patterns in summer (June, July, and August). We counted all the leaf litter patches in the streambed of the study reach, comparing the physical condition of patches and classifying the trapping obstacles associated with each patch type. The distribution patterns of senescent and green leaf litter patches differed. Green leaf litter patches were more numerous at every sampling date, with most trapped by cobbles, whereas senescent leaf litter patches were retained by twig obstacles, backwaters, and cobbles. In June and July, senescent leaf litter patches were located in significantly deeper stream areas than were green leaf litter patches. The distribution of senescent leaves would be primarily determined during spring snowmelt-driven floods. We speculate that senescent leaves were originally located at the edges of pools in the main flow pathway of the channel, which overflowed in the floods. We conducted flume experiments to clarify the transport characteristics of senescent and green painted maple and manchurian alder leaves in the water column. Our hypothesis was that the transport characteristics of each leaf type differ when they first enter the water, because of differing leaf properties. The flume experiments showed that duration of surface flotation differed for senescent and green leaves and for the two tree species. These differences in the duration of leaves on the streambed and in the floating time of green leaves of different trees ensure varied food resources for macroinvertebrates in various physical conditions.

Biogeomorphological effects of leaf accumulations in stepped-bed channels: Exploratory study, Moravskoslezské Beskydy Mountains, Czech Republic

“…Although the field of fluvial geomorphology has expanded in the past two decades to include greater knowledge of channel-reach morphology and processes related to high gradient streams, the influence of leaf litter is poorly characterised. Interdisciplinary research on the effects of leaf litter has usually described results from a hydrobiological point of view or it has focused on transport dynamics; however, the distribution of leaf patches is one of the main aims of leaf research (Richardson, 1992;Pretty and Dobson, 2004;Hoover et al, 2006;Kochi et al, 2009). Terrestrially-derived leaf litter is a critical resource in many small streams, providing food and habitat for decomposers, detritivores and macroinvertebrates (Richardson, 1992).…”

“…Leaf transport distances were 2.6 times greater in riffle units than in pool channel units; the leaves also had a higher transport velocity. Kochi et al (2009) investigated the differences between green leaves (from the current year) and senescent leaves (from previous years). Senescent leaves were retained in greater numbers on large wood pieces and in backwaters than were green leaves, which were usually trapped by cobbles.…”

“…Larger leaves, however, have higher potential to be deposited on obstacles in the stream channel (Kochi et al, 2009). Once leaves reach the channel, they become an integral part of the fluvial system and they are stored or transported downstream by flowing water (Hoover et al, 2006).…”

“…Thus, leaves are transported during much lower flow events, meaning that the morphology of foliaged forms has high temporal dynamics. During transport, organic matter may be retained by structures in the streambed such as woody debris and cobbles, or by backwaters behind obstacles (Kochi et al, 2009). Deposition of leaves is usually realised in contact zones with obstacles, and typically occurs on larger boulders or LW (Hoover et al, 2006).…”

“…This layer is practically waterproof and concentrates the direction of water flow into single points on a step (Přibyla and Hradecký, 2009). Leaves accumulated over the long-term gradually subside; the total height of steps decreases and leaves lose their isolation characteristics as anaerobic biologic decomposition occurs (Kochi et al, 2009). A higher amount of leaves is observed in the steps of steppool systems, whereas in pool-riffle reaches, leaves tend to be stored in pools (Hoover et al, 2006).…”