Multi-scaled heterogeneity of a riparian patch mosaic following a large infrequent flood disturbance in the Sabie River, South Africa.
Parsons, Melissa*,1, McLoughlin, Craig1, Rogers, Kevin1, 1 Centre for Water in the Environment, Johannesburg, South Africa
ABSTRACT- Disturbances initiated by floods of different magnitudes reorganize the river template and create heterogeneity in river landscapes at multiple spatial and temporal scales. Catastrophic floods sit at one end of a continuum of flow variability. Hypotheses of large infrequent disturbances (LIDs) suggest that catastrophic floods do not wipe the slate clean, but rather, leave a heterogeneous, patchy legacy that becomes the template upon which subsequent ecological processes take place. Models of ecological heterogeneity and patch dynamics further suggest that responses to heterogeneity, or spatial pattern, are scale-specific, so that some scales of heterogeneity may be irrelevant for particular organisms or processes. A 1 in 100 year flood occurred in the Sabie River in 2000, providing an opportunity to examine a) the effects of LIDs on the heterogeneity of riparian patch mosaics at multiple spatial scales and b) the subsequent relationship between multi-scaled heterogeneity of patch mosaics and riparian vegetation response. Using a GIS we delineated pre and post-flood mosaics of tree, shrub, reed, herbaceous, sand, bedrock, debris and water patches in four geomorphological channel types. Residuals of every pre-flood patch type remain in the river landscape after the flood, but the proportion and aggregation of patches differed between channel types. The pattern of conversion from vegetated to unvegetated patches differed between the bank and floor areas of the channel. Vegetation community and population responses in relation to patch changes are expected to vary among channel types, at different elevations, and among different types of patch conversions. However, riparian patch mosaics are maintained by the flow regime and in the longer-term, inundation can not be decoupled from mosaic heterogeneity as a fundamental influence on vegetation response to LIDs in river systems.
Key words: rivers, riparian-landscapes, patch-dynamics, flooding
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