Abstract ID
biodiv39
Type
Oral
Theme
biodiv
Full title
Riparian vegetation response to vertical template heterogeneity following a large infrequent flood disturbance in
the Sabie River, RSA.
Abstract text
Riparian vegetation distribution patterns are determined by the interaction of biotic and abiotic factors, usually
depicted as occurring spatially along longitudinal, lateral and vertical gradients. Many alluvial floodplain rivers
in temperate climates, and wetland systems, exhibit a close association between lateral distance from the main
channel, height above the channel, flooding frequency and vegetation distribution. In these rivers, plotting
vegetation assemblages on cross-sections is effectual for describing lateral gradients of riparian vegetation
distribution and associating this with flood pulse frequencies in the vertical dimension. However, in
geomorphologically complex systems, such as the mixed bedrock/alluvial rivers of the Kruger National Park,
arrangement of vertical heterogeneity may be associated with unique hydrogeomorphic patterns that influence
the distribution of riparian vegetation, but not easily demonstrated with a two-dimensional cross-sectional
approach. This study employs a three-dimensional, spatially explicit technique to quantify the vertical
complexity of the mixed bedrock/alluvial Sabie River in the Kruger National Park. A geo-referenced vegetation
survey was conducted four years after a large infrequent flood disturbance, to link spatial distribution of
vegetation with elevation information, and the large flood allowed separation of past from immediate vegetation
response patterns. We tested the relationship between vertical template heterogeneity and the distribution of
newly recruited woody vegetation by measuring species abundance and complementarity between different
elevation positions. The Sabie River exhibits a complex pattern of juxtaposed elevation patches along the
macro-channel floor, with no generalised relationship between lateral distance from the channel and height
above the channel. The large infrequent flood disturbance did not ‘wipe the slate clean’, rather generated a
complex biotic-abiotic patch mosaic in the post-flood river landscape. Although abundance within woody
species was changed drastically, overall the large flood did not change species composition within the river
system, although newly recruited individuals accounted for 70% of woody species abundance post-flood.
Results suggest that woody vegetation assemblages do not recruit and establish randomly across the vertical
dimension, even prior to direct flooding disturbance impacts. In addition to the more well known (direct)
flooding impacts associated with aquatic systems, considering differential adaptations of vegetation assemblages
to disturbance impacts, such as flood shear stresses and anaerobic conditions via inundation, conditions
generated by vertical template heterogeneity in the rivers of the Kruger National Park bestows differential
habitat niches for species recruitment and survival. In these river systems, large sections of the riparian zone are
only inundated sporadically when larger floods do occur, these being key events linked to ecological processes,
such as germination. However, the findings of this paper suggest that vertical template heterogeneity, per se,
plays an important role in the high biodiversity observed in these rivers. Further research is required
investigating what mechanisms are governing spatial patterns of riparian vegetation, and response to this vertical
heterogeneity, including distance to water table and the role of longer-term flow variability occuring in these
rivers. Increasing anthropogenic altering of the natural flow regime, diminishing flow variability, impacting
hydro-geomorphological processes occurring within the river systems of the Kruger National Park, may
undermine conservation objectives aimed at maintaining riparian biodiversity.
Submission date
2009-10-15
Keywords
flooding river riparian vegetation heterogeneity vertical template
Will be submitting paper?
No