Time, space and causality in floodplain palaeoecology

The nature and distribution of palaeoecological entities in floodplain deposits is controlled by many factors in addition to the absence/abundance of the organisms. These factors, which include rates and styles of channel change, watertable dynamics and sedimentation rates and calibre, are all scale-dependent aspects of the fluvial dynamics of the system. This paper discusses the scale dependency of ecological processes, palaeoecological data and causal inference in relation to floodplain biogeomorphology. Ecological processes operate at a variety of temporal and spatial scales determined by hydrogeomorphic processes and this determines whether they are dependent or independent factors in relation to landform evolution ( Schumm & Lichty 1965 ). The temporal and spatial scale sensitivity of palaeoecological data must be appreciated prior to the inference of causality. A multi-proxy approach has advantages here, being able to operate at a variety of spatial scales and in relation to different physical components of the fluvial system. An example is the complimentary information that can be provided by plant macrofossil, pollen and coleopteran data. The greater the recognition of biogeomorphological feedback processes on channel/floodplain evolution, the greater the value of palaeoecological data. Palaeoecological data can also be used to test some of the current ecological models of floodplain/riparian ecology as it can provide data on adaptation, succession, vegetational equilibria and disturbance. The value of a multi-proxy approach combined with a geomorphological model of channel behavior should allow the causality of palaeoecological change to be determined. However, the value of the data will depend upon the temporal resolution of the system; a function of geomorphological processes. It will also depend upon the spatial resolution of the system and this is a function of both the size and density of palaeochannels. It follows that an optimum floodplain would be relatively small with a high number of short-lived channels and high watertables.