Arch. Hydrobiol.
159
1
77–95
Stuttgart, January 2004
Colonisation and temporal dynamics of macrophytes in artificial stream channels with contrasting flow regimes Tenna Riis 1 *, Barry J. F. Biggs 1 and Marty Flanagan1 With 7 figures and 2 tables
Abstract: Colonisation and temporal dynamics of macrophytes were followed over 2 years in 12 artificial channels to determine the effect of contrasting hydrological regimes. The channels had similar baseflow and four different frequencies of flow pulses (six times the baseflow) causing an eight fold increase in current velocity. It took 30 weeks for the initial 1% macrophyte cover to establish in the channels and 83 weeks for 90 % cover to establish. The relatively long colonisation time indicates that the artificial channels were not affected by adjacent vegetation that could grow into the channels, or propagules in the sediment. Propagules were only supplied from upstream reaches. Species composition changed over time. Veronica anagallis-aquatica and Lagarosiphon major were the primary colonists, Potamogeton crispus and Myriophyllum propinquum the secondary colonists, and the tertiary colonists were dominated by amphibious species (V. anagallis-aquatica, Rorippa nasturtium-aquaticum, Mimulus guttatus ). Colonisation of the channels was not affected by local current velocity within the baseflow range (0 – 0.12 m s –1) and plants were not washed out at any velocities up to 0.74 m s –1 at high flows. Flow regime did not show any effect on colonisation or growth of the macrophytes suggesting that high flow conditions did not result in removal of vegetation by increased current velocity. This supports previous studies, which have suggested that removal of vegetation during high flow may be more related to bed sediment destabilisation. Key words: artificial channels, flow pulse, macrophyte community, succession, flow regime.
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Authors’ address: National Institute of Water and Atmospheric Research, Ltd., P.O. Box 8602, Christchurch, New Zealand. * Author for correspondence; present address: Department of Plant Ecology, University of Aarhus, Nordlandsvej 68, DK-8240 Risskov, Denmark; E-mail:
[email protected] DOI: 10.1127/0003-9136/2004/0159-0077
0003-9136/04/0159-0077 $ 4.75
ã 2004 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart
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Introduction The flow regime of streams is considered to be a major factor controlling the distribution and abundance of stream biota (Resh et al. 1988, Power et al. 1995), because it directly or indirectly determines physical habitat, water quality, energy sources and biotic interactions of streams (Poff et al. 1997). The flow regime is often described by the magnitude, frequency, duration, timing and rate of change of flow (e.g., Clausen & Biggs 1997). The magnitude and frequency of high flow events regulate a number of ecological processes (Poff et al. 1997). At high flows, stream sediment is often entrained, and this in combination with the force of the moving water, causes many plants and other organisms to be removed from their habitats (e.g. Biggs et al. 1999). As a consequence, the long-term average composition and relative abundance of species in streams often reflects the frequency and intensity of high flows (Schlosser 1985, Townsend et al. 1997, Biggs et al. 1998). Species with short life cycles and good colonising abilities for example tend to dominate in flood-prone streams. Macrophyte abundance, species diversity and composition have been shown to be directly related to the frequency of high flows in streams (Riis & Biggs 2003). This has important implications for managing stream flows because macrophytes play an important role in the ecology of low-gradient streams, influencing nutrient cycling, light conditions, sediment stability, hydraulic conditions, presence and abundance of microorganisms, invertebrates and fish (Carpenter & Lodge 1986, Sand-Jensen et al. 1989). However, to develop a greater predictive ability in flow management we need to better understand community dynamics of macrophytes under different flow regimes. For example, if species show different colonising abilities following spacecreating floods, we would expect a stream to be dominated by species showing fast colonising traits immediately following a flood. Species traits positively contributing to high rates of colonisation are likely to include large production of dispersal organs, fast establishment of dispersal organs, and high growth rates. Thus, on average, species possessing these traits might be more abundant in highly disturbed streams than in undisturbed streams (Riis & Biggs 2001). Conversely, undisturbed streams might be expected to be dominated by species that are slower to colonise and thus appear in habitats late in the successional sequence, but which have more competitive growth strategies over a longer time span (Grime 1979). Within a stream reach, and over shorter time-scales than are influenced by the flow regime, different macrophyte species can prefer different hydraulic habitat conditions. For example, Riis & Biggs (2003) found that the water velocity preference of four macrophyte species varied between 0 and 0.7 m s –1.
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The hydraulic habitat of a species is likely to be determined in the early stage of the individual’s establishment when the plant is most susceptible to uprooting and stem breakage, rather than when the individual is well established. However, little is known of the relationship between establishment and local hydraulic conditions, and thus of the processes leading to niche separation of macrophytes along hydraulic gradients. The objectives of our study were to examine experimentally: 1) how different hydrological regimes (measured in terms of varying frequency of high flow pulses) influence colonisation time and rates of different macrophytes and might control the temporal abundance, diversity and species composition of macrophytes in streams; 2) the time required for macrophyte establishment in streams following space clearing disturbances; and 3) whether the early establishment of macrophyte species is related to spatial variations in velocity.
Methods Experimental set-up The experiment was carried out in a low-gradient, spring-fed stream (mean annual flow 0.46 m3 s –1) in Canterbury, New Zealand. The upstream landuse was dominated by moderate intensity sheep grazing. Twelve wooden channels (4 m long, 0.5 m wide, 32.5 cm high) were placed in the river downstream of a weir. The channels were levelled so no gradient was present and 7 cm deep substrate (pre-mixed 50 % sand and 50 % soil, topped with gravel) was placed in each channel. The substrate came from a nearby gravel-pit and did not comprise propagules (i.e. stem fragments, seeds, turions) from aquatic plants. The gravel covered the soil/sand completely to avoid the fine substrate being washed out during flow pulses. Water from the upstream weir ran into each channel through a pipe (diameter 15 cm). Water flow was regulated between a low (4 L s –1) and a high flow (23 L s –1) by changing the diameter of the intake hole. Three concrete blocks (15 ´ 15 cm) were put in each channel (1.5, 2.3 and 3.2 m from the upstream end) on alternate sides of the channels to provide a variety of current velocities. The mean velocity in the channels was 0.04 m s –1 (range 0 – 0.12 m s –1 depending on location) during low flows and 0.33 m s –1 (range 0 – 0.73 m s –1) during high flows. During high flows, water depth was also reduced from a mean of 0.22 m (low flow) to a mean of 0.12 m by removing a board at the tail end of the channel. The upstream 1/3 of each channel was characterised by strong turbulence resulting from discharge into the channel, and was excluded from our analyses. Bed sediments were not mobilised during the high flow events. The experiment lasted 24 months from July 2000 to July 2002. The twelve channels were exposed to four different frequencies of high flow events: 3 channels with 17 high flows per year, 3 channels with 11 per year, 3 channels with 5 per year and 3 control channels with no high flows. Each channel was exposed to its high flow treatment for 48 hours at a time. The treatments continued throughout the experiment, but high
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flow velocities declined from around week 70 because of the large volume of vegetation in the channels. For recording vegetation the downstream 2/3 of each channel was divided into 140 grid cells each 10 ´ 10 cm. Cover and length of species present in each of the 140 cells were recorded every 2 weeks. Cover was recorded as the percentage cover within each grid cell, and length was recorded as an estimated average length of each species in each grid cell. We also recorded the approximate mean diameter of the plant bed of each species in each grid cell. Stem fragments function as vegetative dispersal organs for many species (Sculthorpe 1967). To assess the magnitude of dispersal by stem fragments from upstream reaches into the channels, stem fragments were collected from the drift for 48 hours each month for the first twelve months using drift nets (mesh size < 0.5 mm). For practical reasons, the fragments were collected downstream of the channels and we assumed that only an insignificant number settled in the channel during the 48 hours of collection. The collected fragments of each species were counted and their dry weight was determined (drying at 120 ƒC over 48 hours).
Dat a analysis Cover and volume of each plant species were calculated as the percentage of total possible plant cover and total plant volume in the channels. For the calculation of plant volume of each species in the grid cells we assumed that plants were cylindrical beds and used the radius (r) and length (l) of the plants to calculate plant volume by p r2 l in each cell. Cover and volume estimates for each cell were then summed and divided by the maximum possible cover/volume in the channel. Maximum percentage plant cover was calculated as the number of grid cells (140) times 100 % = 14,000 %, whereas maximum volume was calculated as the length of channel used in the analyses (2.8 m) times channel width (0.5 m) times channel depth (0.3 m) = 0.42 m3. Two 1-way ANOVAs were performed on the cover and volume data, one for each of the 1 % and 90 % thresholds. The two thresholds were chosen to describe the first establishment (1 %) and the almost maximum cover/volume (90 %). Species richness and Shannon diversity were calculated for each survey date and channel. Shannon diversity was calculated as: S
Shannon diversity = –
S P ln P
i=1
i
i
where Pi is proportion of species i in the channels. Bray-Curtis similarity measures based on volume data were calculated for each survey date, first between species assemblages in each combination of control channels and channels with 3 disturbances per year, and, second between control channels and channels with 10 disturbances per year. A mean of the 9 combinations for each survey date was calculated and used to test for differences between treatments over time using 2-way ANOVA (no transformation needed). Species succession was analysed by tracking the relative volume of the dominant amphibious and aquatic species in the channels over time. The relative volume of the dominant species on each survey date was calculated as:
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relative species volume (%) =
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species volume · 100 total vegetation volume in channel
The time of establishment of plants in a channel was taken to be the date on which any plant was first seen rooted in the channel. For amphibious and aquatic plants this date is represented by the first symbol in the diagrams in Figs. 3 and 4. The rate of plant cover and volume development was analysed in three periods: 0 – 30 weeks, 30 – 60 weeks and 60 – 90 weeks after start. The rates of development were the slope of the graphs of cover and volume in Figure 1 but on data without log transformation. We compared 1) the rates of development for each flow treatment between the three periods and 2) the rates of development between the four flow treatments within each period, by comparing linear regression lines. Relationships between the locations within channels where vegetation established and current velocity were examined after 40 – 42 weeks when vegetation was well established. The relative frequency of total observations present in different current velocity intervals present during baseflow and high flow, respectively, were compared with the relative frequency of vegetation established in each velocity interval (based on number of observation with plants within each velocity intervals). Relative frequencies of flow and vegetation were based on observations from all channels. Differences between total observed frequencies and plant frequencies within flow treatments were tested using the non-parametric Mann-Whitney U-test, and differences between flow and vegetation distribution among flow treatments were tested using the non-parametric Kruskal-Wallis test, since the data were not normally distributed.
Results Development of macrophytes
The channels were colonised by a mixture of fully aquatic and amphibious plant species. The aquatic species included Callitriche stagnalis Scop., Elodea canadensis Michaux, Lagarosiphon major (Ridl.) Wag., Myriophyllum propinquum Cunn., Potamogeton cheesemanii A. Bennett, Potamogeton crispus L. and Ranunculus trichophyllus Chaix. Dominant amphibious species were Mimulus guttatus. DC., Myosotis laxa Lehm., Rorippa nasturtium-aquaticum (L.) Hayek and Veronica anagallis-aquatica L. It took an average of 30 weeks before 1 % and 83 weeks before 90 % of the total possible cover of vegetation had developed in the channels (Fig. 1). Flow regime had no effect on the number of weeks it took the vegetation to develop 1 % cover (ANOVA, p > 0.05) or 90 % cover (ANOVA, p > 0.05; Table 1), or on the variance in the number of weeks it took to develop 1 % and 90 % cover (Bartlett’s test; p > 0.05; Table 1). The development of vegetation volume showed the same trends as for vegetation cover (Fig. 1). On average it took 33 weeks before 1 %, and 69 weeks before 90 % of the total possible vegetation volume had developed in the chan-
Tenna Riis, Barry J. F. Biggs and Marty Flanagan
Fig. 1. Vegetation cover and plant volume as percentage of maximum possible cover and volume in the channels over two years. Species richness and the Shannon diversity index are also shown. Means ( ± SD) for three channels with similar flow variability treatments are shown. m marks high flow treatments.
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nels. Flow regime had no effect on the time required for volume development (Table 1). Volumes > 100 % derive from the presence of emergent vegetation in the channels. The rate of development in cover and volume for each flow treatment was lowest in the first 30 weeks compared to the last two periods (Table 2). Moreover, development rates within each period varied between flow treatments.
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Table 1. Mean number ( ± SD, n = 3) of weeks for vegetation to develop to 1 % and 90 % cover and volume in the four high flow frequencies in the channels. There were no significant differences among mean values (ANOVA p > 0.05) or standard deviation (Bartletts tests, p > 0.05) among treatments. No disturbances 3 high flows y – 1 5 high flows y – 1 10 high flows y – 1 1 % cover 90 % cover 1 % volume 90 % volume 1
32.3 ± 7.4 75.3 ± 13.6 34.3 ± 10.0 71.7 ± 4.0
34.0 ± 5.3 86.7 ± 17.0 37.3 ± 8.5 68 ± 1.0
25.0 ± 10.1 90.7 ± 12.7 31.3 ± 8.1 69.0 ± 2.0
27.7 ± 9.5 981 29.6 ± 7.4 671
Only occurring in one channel.
Table 2. Comparison of linear regression lines for the mean rate (n = 3) of cover and volume development in four flow treatments. The rates were determined for three periods: 0 – 30 weeks, 30 – 60 weeks and 60 – 90 weeks after start. We made two comparisons: 1) the rates of development for each flow treatment between periods (capital letters), and 2) the rates of development between flow treatments within each period (small letters). Slope ± SD is given. For each slope the p-value and grouping letter is indicated for both comparisons. In groups 30 – 60 and 60 – 90 weeks p-values for between period comparisons are omitted. *: p < 0.10; **: p < 0.05; ***: p < 0.01. Rate of cover development
Rate of volume development
0 – 30 weeks 0 high flows y – 1 3 high flows y – 1 5 high flows y – 1 10 high flows y – 1
0.023 ± 0.004*** A -*b 0.008 ± 0.001*** A -*a 0.043 ± 0.009*** A -*d 0.028 ± 0.004*** A -*c
0.048 ± 0.014*** A-*** b 0.005 ± 0.001*** A-*** a 0.047 ± 0.013*** A-*** b 0.066 ± 0.015*** A-*** c
30 – 60 weeks 0 high flows y – 1 3 high flows y – 1 5 high flows y – 1 10 high flows y – 1
2.174 ± 0.191 C-*** c 2.414 ± 0.165 C-*** d 0.849 ± 0.091 C-*** a 1.088 ± 0.077 C-*** b
1.312 ± 0.176 B-*** c 1.793 ± 0.160 B-*** d 1.036 ± 0.153 B-*** b 0.813 ± 0.098 B-*** a
60 – 90 weeks 0 high flows y – 1 3 high flows y – 1 5 high flows y – 1 10 high flows y – 1
1.789 ± 0.156 B-**d 0.640 ± 0.305 B-**a 1.348 ± 0.442 B-**c 0.684 ± 0.216 B-**b
3.284 ± 0.914 C-*d 1.700 ± 1.691 B-*b 3.039 ± 1.780 C-*c 0.643 ± 0.656 B-*a
From 0 to 30 weeks the highest development rates were in the two treatments with high frequency of high flow. Conversely, from 30 to 60 weeks the highest development rates were in the channels with 0 and 3 high flows per year, and highest from 60 to 90 weeks in channels with 0 and 5 high flows per year. In most channels there was a gap in observations around week 20 caused by extensive filamentous algal growth on the substrate and established plants. This had an adverse effect on the growth of small seedlings and might also
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have affected the recording, because of difficulties observing plants below the algal cover. Species richness, diversity and composition over time
Species richness reached a peak between weeks 40 and 60 in most channels (Fig. 1). This peak was followed either by a decrease in richness (the channels with 5 high flows per year), or no change in richness. Shannon diversity varied extensively within the channels over time and showed no overall trend within flow regimes (Fig. 1). Bray-Curtis similarity between species assemblages in control channels and channels with 3 high flows per year was not significantly different to the similarity between control channels and channels with 10 high flows per year (Fig. 2; 2-way ANOVA, both main effects and interaction effect, p > 0.05). This result suggests that the flow regime had no effect on species composition in the channels over time. To further investigate species composition in the channels we analysed establishment of amphibious versus aquatic species. The initial establishment of amphibious species occurred, on average, in week 5 but the variation among
Fig. 2. Mean Bray-Curtis similarity ( ± SD) in plant volume between control channels and channels with 3 high flow treatments per year (A), and between control channels and channels with 10 high flow treatments per year (B). Values are means of 9 values (see text for further explanation).
Fig. 3. Mean volume ( + SD) in three channels of the four dominant amphibious species relative to maximum possible volume over 2 years. Values are a running average of three values. s : Mimulus guttatus; d : Myosotis laxa; , : Rorippa nasturtium-aquaticum; . : Veronica anagallis-aquatica. m marks high flow treatments.
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Fig. 4. Mean volume ( + SD) in three channels of the seven dominant aquatic species relative to maximum possible volume over 2 years. Values are a running average of three values. For clarity three species are shown in the upper panel (d : Callitriche stagnalis; s : Elodea canadensis; . Lagarosiphon major) and four species are shown in the lower panel (, : Myriophyllum propinquum; j : Potamogeton cheesemanii; h : Potamogeton crispus; r : Ranunculus trichophyllus). m marks high flow treatments.
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Fig. 5. Mean plant volume ( + SD, n = 4 sampling occasions) of dominant species relative to total plant volume. d : Mimulus guttatus; s : Rorippa nasturtium-aquaticum; . Veronica anagallis-aquatica; , : Callitriche stagnalis; j : Elodea canadensis; h : Lagarosiphon major; r : Myriophyllum triphyllum; h : Potamogeton cheesemanii; m : Potamogeton crispus; n : Ranunculus trichophyllus; m marks high flow treatments. Each column of three diagrams represents a flow treatment.
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channels was large ( ± 6 weeks; Fig. 3). No difference in the time of amphibious plant development was found between flow regime treatments (ANOVA, p > 0.05). Veronica anagallis-aquatica was the dominant amphibious species during the first 60 – 80 weeks (Fig. 3). Other amphibious species (M. guttatus, R. nasturtium-aquaticum and to a lesser extent M. laxa) became common during the second spring (c. week 70). The initial establishment of aquatic species occurred later than that of amphibious species (mean = week 12). However, the variation among channels was also high for aquatic plants ( ± 9 weeks; Fig. 4), and no difference in initial establishment time was found between flow regime treatments (ANOVA, p > 0.05). Furthermore, there was no difference in the initial time of overall amphibious and aquatic plant establishment overall in the channels (t-test, p > 0.05). P. crispus and L. major were the most common aquatic species in the channels, but M. propinquum and E. canadensis were also fairly common (Fig. 4). The relative proportions of dominant species over time are presented in Figure 5. No clear trends within treatments or differences between treatments were apparent. V. anagallis-aquatica and L. major were dominant in most channels during the first spring. During the first fall and winter (weeks 40 – 60) P. crispus became dominant in five of the channels (4, 8, 1, 9 and 7). In the channels where V. anagallis-aquatica was already dominant during the first spring and summer, it maintained its dominance through to the second spring. During the second spring and summer (weeks 60 – 80) V. anagallis-aquatica was dominant in all channels, but in late summer and fall the vegetation was dominated by R. nasturtium-aquaticum and M. guttatus. Vegetation establishment and current velocity
No relationship was found between baseflow velocity and vegetation distribution after 40 – 42 weeks (Fig. 6). Percentage frequency of vegetation paralleled distribution of current velocities, indicating no preference for a particular current velocity within the range of baseflow velocities (0– 0.14 m s –1; MannWhitney U-test, p > 0.05). At high flow velocities (0– 0.74 m s – 1) plant frequency also followed velocity frequency (Fig. 6) and plants remained where velocity reached the maximum of 0.74 m s – 1. We found no difference in the frequency distributions of plants among high flow treatments (Kruskal-Wallis test, p > 0.05). Similar results were obtained when we repeated the analysis for the two most common species: V. anagallis-aquatica and P. crispus (Fig. 6). These species were expected to show different adaptation to high flow, since V. anagallis-aquatica is a rigid-stemmed plant, whereas P. crispus is more flexible. However, within the range of velocities at baseflow and high flow, there was
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Fig. 6. Frequency (%) of observations in velocity intervals (bars) and the frequency of occurrence (%) of all vegetation, V. anagallis-aquatica , and P. crispus established after 40 – 42 weeks (d ) in each velocity class during baseflow and high flow events. Values are based on observations from all channels.
no difference in the distribution of either within available habitats with different current velocities (Fig. 6). Stem fragm ent survey
The stem fragment survey showed that the upstream individuals of L. major and P. crispus produced by far the highest amount of stem fragments during the first twelve months of the experiment (21 and 47 g DW in total in the 12 channels compared to < 3 g DW for each of the other species, Fig. 7). Both species had maximum stem fragment production during the three summer months (December– February) with 58 % and 82 % of total annual stem fragment production, respectively.
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Fig. 7. Production of stem fragments (g DW) of species during 18 months of the experiment as indicated by 48 hours drift net collections each month. For clarity three species are shown in the upper panel (d : Elodea canadensis; s : Lagarosiphon major; m : Potamogeton crispus) and four species in the lower panel (D: Ranunculus trichophyllus; j : Myriophyllum propinquum; h : Potamogeton cheesemanii; r : Rorippa nasturtium-aquaticum ; e : Veronica anagallis-aquatica ).
Discussion Colonisation and high flow events
The flow regime treatments had no effect on the time required for different species to colonise the artificial channels, the temporal dynamics of vegetation abundance and species composition during the study period, or the establishment and growth of plants in relation to current velocity. Only two effects of flow regime were found. First, there was a decline in species richness during the second spring in channels with 5 high flows per year, compared to no decline during this period in the other channels. Second, we found that the rate of plant development was highest in the most disturbed channels in the first 30 weeks but highest in the least disturbed channels in the following 60 weeks. This was opposite of what we expected to find. We expected that initially while there was still plenty of empty space in the channels, biomass accumulation would be fastest in the channels without disturbance. As the channels would be filled with vegetation, we expected the channels with more frequent disturbance to have slightly more space to cause a delayed lack of space in these channels and thereby a higher rate of accumulation later on. However, as
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stated above, there was no difference in vegetation abundance between flow treatments and consequently our hypothesis on development rates could neither be verified in this study. The general lack of effects was unexpected and may have occurred because the high flow regime did not remove substantial parts of vegetation from the channels. The high flow treatments applied six times higher flow and eight times higher current velocity compared to baseflow conditions. We assume that stem breakage due to current velocity or uprooting due to sediment mobility are the only mechanisms that remove rooted macrophytes during high flow events. As the increase in flow in our study only increased current velocity, we suggest that plant removal may be caused mainly by sediment mobility and not by increased current velocity alone. This suggestion is strongly supported by a previous study in which there was no significant macrophyte loss by stem breakage in current velocities up to 1.5 m s – 1, when no sediment mobility occurred (Riis & Biggs 2003). Our results strongly suggest that high flow events, even at high frequencies, do not affect macrophyte colonisation and temporal dynamics as long as the velocity threshold for mobilising sediment is not exceeded. A previous experiment also showed that the duration of high flow (up to 2 hours) only had very little effect on the vegetation during a high flow event (Riis & Biggs 2003) and flow duration was therefore not considered in the present experiment. However, factors such as water depth, emergent/submerged vegetation and seasonal timing might influence the effects of high flows, but have yet to be tested.
Colonisation time and succession
The nature of the propagule source exerts a strong influence over the time plants require to colonise open stream habitats. In our study, initial plant colonisation required about 30 weeks to reach 1 % cover and volume in the channels. This is much more than the three weeks found in 2 m 2 bare plots in a French stream (Barrat-Segretain & Amoros 1996). However, there were fundamental differences in propagule sources in the two experiments. In our experiment, the channels were bare at the start of the experiment, had no seed bank, subsurface rhizomes or connections to adjacent vegetation. Therefore, colonisation could occur only through dispersal of propagules from upstream reaches. In the French experiment, the bare habitats were surrounded by vegetation, which immediately grew into the bare plots (i.e., “gap-replacement”), and seed and rhizomes were present in the streambed. Thus, the studies suggest that if colonisation occurs only from drifting propagules it can take up to 10 times longer to establish the initial 1 % cover than if colonisation occurs by gap-replacement from adjacent vegetation or propagules in the sediments.
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Colonisation time found in our study also could have been prolonged by the experiment beginning in winter, and because filamentous algae were abundant in the first spring (around 20 weeks after the commencement of the study). The winter start could have meant that the initial source of propagules was low compared to that in summer. Stem fragment flux recordings showed that the two species producing most fragments did so in summer. Also, the high biomass of filamentous algae in spring could have stopped, or even reversed macrophyte colonisation and growth for some weeks due to shading. We found that the rate of plant development was lower in the first 30 weeks compared to the last 60 weeks. This probably reflects the difficulties of plant fragments and seeds to establish on a bare stream bed. Plant establishment is most likely favoured by the presence of obstacles in the stream such as other plants that will catch the propagules as they drift down through the channel. Species succession in the channels reflected the ability of the species to colonise new habitats. Overall, the dominant primary colonists in the channels were L. major, V. anagallis-aquatica and C. stagnalis suggesting that these species have high rate of biomass production, strong dispersal and/or efficient establishment of the propagules. In accordance with these proposed traits we found that L. major produced a high number of stem fragments during the first year. It has previously been suggested that L. major would be a successful primary colonist based on its species traits (Riis & Biggs 2001), especially its ability to produce many propagules (Willby et al. 2000). For V. anagallisaquatica we observed a high density of seedlings after flowering in the channels, suggesting a high degree of establishment from seeds. Grime et al. (1988) suggested that V. anagallis-aquatica is both a good colonist and a good competitor, as found in our study. In fact, we found that in the channels where V. anagallis-aquatica was dominant from the start, the proportion of other species was modest compared to their representation in channels where species other than V. anagallis-aquatica were dominant, initially. This suggests that if V. anagallis-aquatica can colonise early, it will potentially out-compete other colonist and climax taxa. A group of secondary colonists appeared in the channels around weeks 34 – 36. These were E. canadensis, M. propinquum, N. hookerii, P. crispus, and R. trichophyllus. These are all aquatic species and three of the four species produced few fragments at any time, suggesting a lesser ability to colonise than the primary colonist species. P. crispus, however, had low production of fragments during much of the year, but high production from December to April (summer-fall) when it became more abundant in the channels. This suggests that P. crispus in the channels successfully established from stem fragments as demonstrated also by Barrat-Segret ain et al. (1998, 1999).
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During the last three months (after around week 72), the secondary species along with L. major generally disappeared again and R. nasturtium-aquaticum, M. guttatus and M. laxa were dominant. These three species are amphibious and developed emergent shoots, which caused shading of the aquatic species and consequently a decline in their abundance. The emergent shoots probably developed abundantly at this time because the current velocity in the channels had declined due to hydraulic resistance of the vegetation. Temporal patterns of species richness in channels with 5 high flow events per year peaked at around week 50 and declined thereafter. According to the intermediate disturbance hypothesis this could be due to the fact that maximum species richness is reached after an intermediate time interval since the last disturbance, before the competitive species exclude the less competitive species (Connell 1978, Huston 1994). However, as this pattern of species richness was not found in the channels with fewer high flow events per year, applicability of the intermediate disturbance hypothesis cannot be verified from our results. Superimposed on the successional pattern of species is a possible seasonal effect of growth of different species. However, as there were no repeating patterns in species changes during the two years, and most aquatic species disappeared in the second summer-fall as amphibious species became dominant, we considered seasonal change to be unimportant as a factor influencing temporal dynamics of vegetation in our channels. Colonisation and current velocity
Different macrophyte species show different preferences to current velocity in the range 0 – 0.7 m s –1 in natural streams (Riis & Biggs 2003). Our experimental study, however, showed no patterns in colonisation of different species at the lower end of this range (0 – 0.12 m s – 1 at baseflow). Had a greater range of baseflow velocities been present in the channels, then greater velocity related niche separation may have been apparent. In the channels with high flow events, high water velocities during high flows were expected to remove plants and thus vegetation would be most abundant in areas with low velocities. However, we found plants were not susceptible to high flow velocities up to 0.74 m s – 1, the highest water velocity in the channels during a disturbance. This finding is consistent with results of our previous experiments, in which plant species did not lose a significant amount of biomass at velocities up to 1.5 m s – 1 (Riis & Biggs 2003). In summary, our results showed that high flow events, resulting in increased current velocity but no sediment mobility, had no effect on the colonisation and temporal dynamics of macrophytes in artificial stream channels. This result indicates that higher velocities and/or sediment motion is probably
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required to disturb stream vegetation. The study has provided new information about potential colonisation time, development rates and species successions in streams where no propagules occur in the stream sediments, and where colonisation is entirely from upstream sources. Our results indicate that propagule source has important implications for establishment time and colonisation. Moreover, our results indicate that development rate of vegetation is slowest initially when fewer obstacles are present to catch seeds and fragments drifting down through the channel. The ability to predict abundance and distribution of different plant species in streams would benefit from an increased knowledge of dispersal characteristics of different species and of the importance of dispersal processes compared to local hydraulic processes in a stream reach. Acknow ledgem ents This work was supported by the grant CO1X0023 (“Environmental Hydrology and Habitat Hydraulics” program) from the New Zealand Foundation for Research, Science and Technology. We thank Michael Reid and two anonymous referees for useful comments on the paper.
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