Healthy riparian vegetation has a positive impact on the adjacent river. Unfortunately,
riparian vegetation is often threatened by human impacts such as dam construction and
clearing. To gain the knowledge underlying the effects of such impacts and to aid riparian
rehabilitation, the objective of this thesis was: to determine riparian vegetation association
with, and response to, variation in fluvial geomorphology over several scales and
consequently to fluvial disturbance. Only woody riparian plant species were considered.
Flood disturbance was the unifying theme of this thesis. Linked to this theme and arising
from the main objective was the supposition that plant interactions with the abiotic
environment, but not biotic interactions between species, control riparian species distribution
because of frequent fluvial disturbances.
Woody riparian vegetation and riverine environmental variables were recorded along the
upper Murrumbidgee River at three spatial scales based on a geomorphic hierarchy for
Chapter 2. Multivariate analysis was used to group species and to associate environmental
variables with vegetation at the three spatial scales. Observations at the two larger scales, of
river segment (site) and riparian reach (transect), identified a river-longitudinal speciescomposition
gradient associated with geology, river width and stream channel slope.
Observations at the smallest scale of geomorphic units (plot) identified a lateral riparian
gradient and also the longitudinal gradient; these gradients were associated with geomorphic
variation, land use, plot elevation and also river longitudinal variables.
Using the same data set, but varying the spatial scale of analysis caused the species
composition pattern to change between scales. Increase in scale of observation, that is from
geomorphic unit to reach and segment scales, resulted in disproportionate importance of rarer
species and decreased importance of some key riparian species at the larger scales. It would
appear that in this instance the geomorphic unit scale best described patches of different
species composition because this scale had high spatial resolution and was also able to
identify multiple gradients of environmental variation. It was recommended that riparian
sampling take place at scales that represent dominant gradients in the riparian zone. These
gradients are represented by geomorphic scales, indicating the appropriateness of using
geomorphic based scales for observation of riparian vegetation. Chapter 3 considered whether there is a geomorphic template upon which riparian vegetation
is patterned and whether it is associated with process variables, such as flooding and soil type.
This question was investigated at different spatial scales in three ways: i) by an experiment to
determine whether soil nutrient condition affects plant growth; ii) by graphical analysis of
trends between geomorphic units, species and process variables; and iii) by analysis of
vegetation distribution data.
The smallest scale (meso) found experimental differences in plant growth because of soil
type. Plants growing in sand had the lowest performance, with an average plant Relative
Growth Rate (RGR) of 0.01, compared to plants growing in soils with small amounts of silt or
clay particles, with an average plant RGR of 0.04. This pattern was attributed to differences
in nutrients. Clear relationships were demonstrated at the larger geomorphic unit scale
between species distribution and process variables. For example, hydrology and substratum
type were found to be associated with geomorphic units and species. The largest scale
considered in Chapter 3 was the riparian reach scale. At this scale species were clearly
grouped around reach type. Therefore, geomorphology was considered to be a template for
riparian species distribution. Findings in this chapter suggested that geomorphic variables
should be good predictors of riparian species distribution. This hypothesis was tested and
supported in Chapter 6.
The experiments reported in Chapter 4 aimed to determine whether inundation depth and
duration affected plant performance and survival for five common riparian zone species.
Riparian seedling patterns in the field were also compared with experimental results to test
whether species performance was reflected by field distribution. The experiments that were
conducted included an inundation period and depth experiment, and a survival period test
whilst under complete inundation. Biomass and height relative growth rates were determined,
and the results were analysed using factorial Analysis of Variance. Obligate riparian species
(Callistemon sieberi, Casuarina Cunninghamiana, Leptospermum obovatum) were found to
be tolerant of inundation duration and depth, to the point where inundation provided a growth
subsidy. On the other hand, non-obligate riparian species (Acacia dealbata,
Kunzea ericoides) were either just tolerant of inundation or showed a negative growth
response. For instance, C. sieberi demonstrated an average height RGR of 0.04 after
complete inundation and 0.007 when not inundated, while A. dealbata had an average height
RGR of 0.001 after complete inundation and 0.01 when not inundated. These experimental
findings were found to closely reflect both seedling and adult plant distribution in the field
such that inundation tolerant species were found close to the river and intolerant species
further away. Thus, the conclusion was drawn that riparian species establishment and
distribution is affected by inundation and that change to the flood regime could have serious
impacts on riparian zone plant composition.
The other aim of this chapter was to determine whether optimum germination temperatures
were associated with flood or rainfall. Growth chamber germination trials were conducted at
air temperatures of 15�C, 20�C and 25�C to determine the 'best' germination temperature.
These germination patterns at different temperatures were then related to annual variation in
field temperature, flooding period and rainfall. No evidence was found to suggest a
relationship between ideal germination temperature and flood season, rather it was suggested
that germination was patchy through time and may simply reflect recent rainfall.
Investigations that were reported in Chapter 5 aimed to elucidate relationships between
species and flow velocity variables. Two experiments were conducted: i) a flume experiment
to determine the effect of flow velocity on plant growth; and ii) an experiment to observe the
response of plants to damage (imitating flood damage) and inundation. Field observations of
species distribution and flow velocity related variables were also conducted to put the flume
results into a real-world context.
Treatments for the flume experiment were fast flow velocity (0.74 m s-1), slow velocity
(0.22 m s-1) and no velocity (control) but still inundated. All treatments were flooded
completely for four days. Subsequent biomass and height relative growth rates were
determined, and the results were analysed using factorial Analysis of Variance. Results were
unexpected, given that obligate species exposed to the fastest velocity had the highest growth
rate with an average height RGR of 0.046, compared to plants in still water, which grew the
least with an average height RGR of 0.013. It was hypothesised that this response was
because relatively greater carbon dioxide and oxygen levels were available in the moving
water compared to the still water. With regard to shoot damage, the species that were nonobligate
riparian species lost more leaves from velocity treatment than the obligate riparian
species. The cut and flood experiment found growth of the obligate species
(Casuarina cunninghamiana) to be greater after cutting than the non-obligate species.
Flooding was not found to have an effect in the cut and flood experiment, probably because
the period to sampling after flood treatment was longer (4 weeks) than other flooding
experiments (3 weeks).
Field observations were found to support the experimental findings, with a gradient of species
across the riparian zone that reflected potential flood velocities. Therefore, velocity is one of
a suite of riparian hydrological factors that are partially responsible for the gradient of species
across the riparian zone. Potentially the absence of flooding could result in a homogeneous
mix of species, rather than a gradient, except on the very edge of the river.
The study that was reported in Chapter 6 investigated a technique for predicting riparian
vegetation distribution. One of the aims of this investigation was to address a current riparian
rehabilitation shortfall, which was how to objectively select species to plant for rehabilitation.
Field data were collected from three confined river valleys in south-eastern New South Wales.
Using data on plant species occurrence and site and plot measures of soils, hydrology and
climate, an AUSRIVAS-style statistical model, based on cluster and discriminant analysis,
was developed to predict the probability of species occurrence. The prediction accuracy was
85 % when tested with a separate set of plots not used in model construction. Problems were
encountered with the prediction of rarer species, but if the probability of selection was varied
according to the frequency of species occurrence then rarer species would be predicted more
often. Various models were tested for accuracy including three rivers combined at the
geomorphic unit (plot) scale and riparian reach (transect) scale in addition to a Murrumbidgee
River plot scale model. Surprisingly, the predictive accuracy of the all rivers and single river
models were approximately the same. However, the difference between the large scale and
small scale models pointed to the importance of including small scale flood-related
parameters to predict riparian vegetation.
When these riparian predictions were compared to predictive outcomes from a hill slope
model, which was assumed to be affected by fewer disturbances (i.e. flooding), predictive
accuracies were not very different. Overall though, predictive accuracy for riparian
vegetation was high, but not good enough to support the supposition that riparian vegetation is
abiotically controlled because of frequent flood disturbance. Nevertheless, geomorphology
and consequently flood effects are still important for the determination of the riparian
community composition.
Overall, riparian vegetation was found to be closely linked to its environment (evidenced in
Chapters 2, 3, 4, 5) in a predictable manner (Chapter 6). Species pattern relied on flood
disturbance affecting species distribution. Some riparian species were found to be highly tolerant of flooding and gained a growth advantage after flooding (Chapters 4 and 5).
Therefore, flood tolerance was important for the formation of a species gradient across the
riparian zone. These species tolerances and growth requirements reflect riparian geomorphic
pattern (Chapter 3), which was suggested to form a template on which riparian vegetation is
structured.
| Identifer | oai:union.ndltd.org:ADTP/218590 |
| Date | January 2003 |
| Creators | Evans, Lisa J, n/a |
| Publisher | University of Canberra. School of Resources Environmental and Heritage Sciences |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Lisa J Evans |
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