Tenna Riis

Relationships between water level fluctuations and vegetation diversity in shallow water of New Zealand lakes

Hydrological and vegetation data from 21 New Zealand lakes were used in the study, to assess how water level fluctuations affected the diversity of a low-growing species-rich littoral plant community (here LMC). Species diversity of the LMC was affected by the water level range, the frequency of variation and the duration of low water level events. Diversity tended to increase with monthly level range, with a possible saturation at an inter-quartile range (the monthly water level fluctuation occurring between 25 and 75% of the time) of approximately 1 m. The species richness was much lower in lakes with inter-annual level variations than in lakes with intra-annual fluctuations. This indicates a relative long establishment time for a species-rich LMC. Up to a period of 1 month, increasing duration of events below median water level appeared to increase species diversity, though events longer than 2 months reduced it. To determine what hydrological conditions determine the upper depth limit of the LMC we made a series of paired sample tests of the measured upper limit of the LMC and the water levels where the mean length of dry period is 10, 30, 60, 120 and 180 days. We found that the observed upper limit best corresponded to the level where the dry period lasted between 10 and 30 days. Hence, the community was not observed where aerial exposure exceeds 30 days. Amongst the species found in the LMC, Elatine gratioloides, Glossostigmasp., Lilaeopsis ruthiana and Limosella lineataare the commonest species under most conditions, and can be considered as the most competitive species in the community. A generalised optimum habitat for a diverse LMC can thus be defined as occurring in a 1 m monthly water level range, with a mean duration of low level events lasting up to 1 month in lakes with low inter-annual water level fluctuation.

Plant communities in lowland Danish streams: species composition and environmental factors

We studied the quantitative composition of the vegetation at 208 stream sites distributed throughout Denmark with the purpose of identifying distinct plant communities, using cluster analysis, and their relationship to environmental conditions using indirect gradient analysis. Six plant communities were defined in the analysis, although the differences in species composition among them were small. The plant communities were mainly related to differences in water alkalinity with a Potamogeton-community being associated with high alkalinities, Sparganium-, Callitriche- and Batrachium-communities with medium alkalinities and a Myriophyllum alterniflorum-community associated with low alkalinity. Also stream size turned out to be an important variable to the separation of plant communities, with the Callitriche-community mainly found in small streams and the Potamogeton-community and a community of amphibious helophytes mainly found in larger streams. The most common species were Sparganium emersum and Elodea canadensis forming the widespread Sparganium-community at half of the stream sites throughout Denmark, a pattern most likely stimulated by the common practice of weed cutting. Though communities of stream plants could be defined, the overlap of species among them was very substantial. Communities are less distinct in Danish streams compared to streams in other regions including wider environmental gradients of temperature, current velocity, substratum, alkalinity and nutrient richness.

Plant distribution and abundance in relation to physical conditions and location within Danish stream systems

The distribution of obligate submerged plants, amphibious plants and terrestrial plants in streams was examined in relation to water depth, substrate and distance to the banks using univariate and mulitivariate analyses. The analyses were based on recordings in more than 40 000 quadrats (25×25 cm) in 208 unshaded sites in the predominantly small and shallow (<1.6 m) Danish streams. Also, the distribution of plant abundance and richness from the source to the outlet of the stream system was determined.The submerged plants in Danish streams include 87 terrestrial, 22 amphibious and 30 obligate submerged taxa. The distribution of plant types was mainly related to water depth and distance to the bank among the physical conditions, while the type of bottom substrate had no significant influence. Terrestrial plants and amphibious plants (excluding Sparganium emersum) dominated in shallow water near the bank, but declined rapidly with increasing depth and distance to the bank, reflecting the importance of dispersal by ingrowth from populations on the banks to the water among these plants. Accordingly, these two plant groups constituted a higher proportion of total plant abundance in small streams than large streams. S. emersum dominated on great depth and distance to the banks, probably reflecting the lengthwise dispersal of this species from upstream to downstream parts of the stream system, the tolerance of the species to weed cutting and the adaptation to grow at low light intensities. Obligate submerged plants dominated at intermediate depths and at all distances from the bank except from 0 to 50 cm. This distribution reflects the ability of these species to disperse lengthwise in streams and live permanently submerged.The species number of all species and obligate submerged plants was lower in the smallest stream sites compared to larger downstream sites, while there was no difference for terrestrial and amphibious plants. The downstream increase of submerged species can be explained by the increase of habitat area and the dispersal of plants with the current, implying that the species pool accumulates with distance from the source. This result is at variance with a maximum richness of submerged plant species predicted for intermediate-sized streams according to the River Continuum Concept developed for large North American streams having forested shallow upstream sections and unshaded, deep downstream sections both unsuitable to submerged plant growth. The results for Danish streams imply that both the longitudinal connection through the flowing water and the transversal connection to the local species on the banks are important for plant distribution in the streams.

Dispersal and colonisation of plants in lowland streams: success rates and bottlenecks

Plant dispersal and colonisation, including rates of dispersal, retention, colonisation and survival of dispersed propagules (shoots and seeds), were studied in a 300-m stream reach in a macrophyte-rich lowland stream during one growing season. Relationships between colonisation processes and simple flow parameters were tested. Each fortnight during a growing season, the number of dispersed plant propagules and the number of new and lost plant colonisations since the last sampling day were recorded. The retention of dispersing shoots was tested on two occasions during the growing season by releasing plant shoots and subsequently re-collecting the retained shoots in the study reach. The main bottleneck for plant colonisation in macrophyte-rich lowland streams is the primary colonisation (development of attached roots in the sediment from vegetative propagules or seedlings) of retained shoots and seeds, due in part to low retention success (1% of the dispersed shoots per 100-m reach) and to unsuccessful colonisation of retained shoots (3.4% of retained shoots colonised). The number of drifting shoots and seeds per day during the growing season were 650–6,950 and 2,970–62,780, respectively, and caused no constraint to colonisation. The survival rate of primary colonists was high with 80% surviving during the first growing season and about 50% surviving the first winter. There was no relationship between number of drifting shoots and flow, but the number of drifting seeds increased with flow. The number of colonisations between two consecutive sampling days correlated to the extent of low flow in the period. The loss rate of colonisations correlated to high flow events, but was low overall. My study strongly indicates that the number of propagules is not a constraint to colonisation in macrophyte-rich lowland streams. Rather, I conclude that primary colonisation is the main constraint to regaining vegetation in lowland streams in general and in vegetation-free rehabilitated streams in particular. Therefore, if plant colonisation is a target for stream rehabilitation, it is important to enhance retention and colonisation of propagules by creating areas of low flow and by providing physical obstacles to work as retention agents in the stream.

Invasion strategies in clonal aquatic plants: are phenotypic differences caused by phenotypic plasticity or local adaptation?

BACKGROUND AND AIMS The successful spread of invasive plants in new environments is often linked to multiple introductions and a diverse gene pool that facilitates local adaptation to variable environmental conditions. For clonal plants, however, phenotypic plasticity may be equally important. Here the primary adaptive strategy in three non-native, clonally reproducing macrophytes (Egeria densa, Elodea canadensis and Lagarosiphon major) in New Zealand freshwaters were examined and an attempt was made to link observed differences in plant morphology to local variation in habitat conditions. METHODS Field populations with a large phenotypic variety were sampled in a range of lakes and streams with different chemical and physical properties. The phenotypic plasticity of the species before and after cultivation was studied in a common garden growth experiment, and the genetic diversity of these same populations was also quantified. KEY RESULTS For all three species, greater variation in plant characteristics was found before they were grown in standardized conditions. Moreover, field populations displayed remarkably little genetic variation and there was little interaction between habitat conditions and plant morphological characteristics. CONCLUSIONS The results indicate that at the current stage of spread into New Zealand, the primary adaptive strategy of these three invasive macrophytes is phenotypic plasticity. However, while limited, the possibility that genetic diversity between populations may facilitate ecotypic differentiation in the future cannot be excluded. These results thus indicate that invasive clonal aquatic plants adapt to new introduced areas by phenotypic plasticity. Inorganic carbon, nitrogen and phosphorous were important in controlling plant size of E. canadensis and L. major, but no other relationships between plant characteristics and habitat conditions were apparent. This implies that within-species differences in plant size can be explained by local nutrient conditions. All together this strongly suggests that invasive clonal aquatic plants adapt to a wide range of habitats in introduced areas by phenotypic plasticity rather than local adaptation.

The search for reference conditions for stream vegetation in northern Europe

Summary 1. The European Water Framework Directive provides a framework for improving the ecological quality of stream ecosystems, with deviation from reference used as a measure of ecological status. 2. Here we examine the possibility of using less impacted stream sites from Latvia, Lithuania and Poland to establish a Danish reference network for macrophyte assemblages, and as a guiding image for identification of possible references sites within Denmark. Both approaches were evaluated using historical Danish records. 3. Four different macrophyte assemblages were identified for mid‐sized streams in the Central and Eastern Lowland ecoregions. Macrophyte assemblages could not be delineated using physical stream site characteristics; however a gradual change in assemblage composition was attributed to differences in alkalinity and human impact. 4. Assemblages of contemporary vegetation in Denmark were quite similar to those found in Polish, Latvian and Lithuanian streams (26–35%). However, more importantly, from species‐based predictions we noted higher similarity, particularly with Latvian and Lithuanian streams, before intensive land use commenced in Denmark (c. 1900). These results show that stream sites from these three countries can be considered in a Danish reference network. 5. Two of the four macrophyte assemblages comprised species such as Fontinalis antipyretica, Myriophyllum spicatum, Nuphar lutea, Potamogeton alpinus and P. perfoliatus that have a very scattered occurrence in the contemporary vegetation in Denmark. These groups were closely associated with the predictions from historic records, thereby lending support the conjecture that these assemblages could be part of the guiding image for the identification of potential reference sites within Denmark.

Distribution of macrophytes in New Zealand streams and lakes in relation to disturbance frequency and resource supply—a synthesis and conceptual model

Abstract A conceptual model for the occurrence of common macrophyte species in different freshwater habitats in New Zealand is presented. The model is developed in two steps. First, a classification of freshwater habitats based on the level of disturbance frequency and resource supply in the habitats is performed. Second, a classification of common New Zealand freshwater macrophytes is made based on the appearance of relevant competitive species traits in different combinations of disturbance frequency and resource supply. From this we predict in which habitat each species is most competitive. We discuss how to test and refine the model, especially by gaining more knowledge about the presence and distribution of the plant species in relation to disturbance frequency, based on field and experimental studies. Benefits of a thorough predictive model in relation to the management practices and maintenance of macrophyte vegetation in lakes and lowland streams are discussed.

Exploring the borders of European Phragmites within a cosmopolitan genus

European Phragmites australis is one of four main cp-DNA haplotype clusters present worldwide. The European gene pool extends from North America to Far East Asia and South Africa. Extensive gene flow occurs only within the temperate region of Europe.

Long-term effects of stream management on plant communities in two Danish lowland streams

Submerged macrophytes grow abundantly in most shallow streams common in the cultivated lowlands of northwestern Europe. Weed-cutting has been practised for years in many of these streams to reduce the risk of flooding of adjacent land. Our objective was to quantify long-term impacts of weed-cutting on macrophyte communities in two Danish rivers. We found that the total macrophyte coverage was similar in the weed-cut and uncut reaches in the two rivers, but species richness, diversity and patch complexity were higher in the uncut reaches. The spatial distribution of macrophytes on the stream bottom was also more heterogeneous in the uncut stream reaches. We also found evidence of a strong effect of weed-cutting on macrophyte species composition. P. natans was abundant in the uncut reaches in both streams but practically eliminated in the cut reaches, despite the fact that its basic habitat requirements were met. Also, the established phase strategy of the macrophyte community was affected by weed-cutting. Species displaying characteristically ruderal traits were more abundant in the cut reaches and species with competitive abilities were only abundant in the uncut stream reaches. We suggest that important species traits in streams, where the weed is cut regularly, are associated with rapid growth and high dispersal-capacity. Our results indicate that weed-cutting can contribute significantly to a decline in species diversity in streams. To provide optimal conditions for diverse stream macrophyte communities, we therefore suggest that weed-cutting should be minimised.

Genetic diversity in three invasive clonal aquatic species in New Zealand

BackgroundElodea canadensis, Egeria densa and Lagarosiphon major are dioecious clonal species which are invasive in New Zealand and other regions. Unlike many other invasive species, the genetic variation in New Zealand is very limited. Clonal reproduction is often considered an evolutionary dead end, even though a certain amount of genetic divergence may arise due to somatic mutations. The successful growth and establishment of invasive clonal species may be explained not by adaptability but by pre-existing ecological traits that prove advantageous in the new environment. We studied the genetic diversity and population structure in the North Island of New Zealand using AFLPs and related the findings to the number of introductions and the evolution that has occurred in the introduced area.ResultsLow levels of genetic diversity were found in all three species and appeared to be due to highly homogeneous founding gene pools. Elodea canadensis was introduced in 1868, and its populations showed more genetic structure than those of the more recently introduced of E. densa (1946) and L. major (1950). Elodea canadensis and L. major, however, had similar phylogeographic patterns, in spite of the difference in time since introduction.ConclusionsThe presence of a certain level of geographically correlated genetic structure in the absence of sexual reproduction, and in spite of random human dispersal of vegetative propagules, can be reasonably attributed to post-dispersal somatic mutations. Direct evidence of such evolutionary events is, however, still insufficient.