Quantifying macrophyte colonisation strategies—A field experiment in a shallow lake (Lake Balaton, Hungary)
Introduction
Plants, thus submerged aquatic macrophytes too, can propagate by both vegetative and sexual means. Macrophytes usually use a wider range of possible modes for establishing in new areas and in newly emerging patches than terrestrial plants do, thereby being able to colonize a diversity of habitats (Barrat-Segretain, 1996, Sculthorpe, 1967). This multiplicity of colonisation modes makes it possible to investigate the relationship between life-history traits (like different ways of reproduction) and colonisation success.
Re-colonisation can occur on different spatial scales: locally, following either minor disturbance (such as strong currents or waves, which can dislocate bundles of plants, or feeding, trampling, boat movements), or major disturbance events, which eradicate greater areas of aquatic vegetation (such as ice-scouring, drought or floods (Barrat-Segretain and Amoros, 1996, Kautsky, 1988), or at the whole lake-level, when a lake returns (through re-oligotrophication) from algae-dominated turbid state that resulted from previous eutrophication (Scheffer, 1990, Scheffer and Van Nes, 2007). Re-establishment of macrophytic vegetation after return from high eutrophication levels is a common issue, especially in shallow lakes world-wide (excellent review by Bakker et al., 2012, Galanti et al., 1990, Hilt et al., 2010, Hobbs et al., 2012, Jeppesen et al., 2005, Lauridsen et al., 2003, Lauridsen et al., 1994, Ozimek, 2006), as it is also in the study area Lake Balaton (Herodek et al., 1988, Istvánovics et al., 2007). This article argues that looking into details at the small scale might elucidate the potentials and limitations of processes on the large scale, whole-lake level.
Colonisation by plants can be regarded as conquering new areas, establishing from propagules which arrived from other areas to that patch. Regeneration is seen as the process by which plants re-grow from some part of the whole plant, which might be below-ground or not (e.g. Barrat-Segretain et al., 1998, Umetsu et al., 2012). For the purpose of this study we defined (similarly to Capers, 2003 and to Henry et al., 1996) all mechanisms of establishing new, aboveground canopy as colonisation, as they are all ways by which previously non-visible plants appear in the standing vegetation and can participate in further life-history events.
Life history is composed of survival probabilities and rates of reproduction (Partridge and Harvey, 1988), including patterns of development, growth, reproduction and lifespan (Fabian and Flatt, 2012). The development of both vegetative and generative reproductive structures is determined by different life history traits and phenology, which in turn act on potential speed and timing of colonisation. Within the diversity of reproductive options open to a macrophyte, there are two major ways of reproduction, depending on whether a species overwinters solely as sexual propagules (as annuals do) or in some vegetative shape (like perennials). Overwintering in perennials can take place by receeding to underground plant parts, while some aquatic perennials remain partly green above-ground. The latter start in spring with a head start in developing biomass.
Colonisation of newly available areas depends, apart from patch suitability, also on the surrounding vegetation within the waterbody, the effect of which is mostly detectable at the boundaries of gaps (e.g. Capers, 2003, Henry et al., 1996). Testing the effect of gap edges throws light on the importance of already existant macrophytic vegetation as kernels of (re)establishment.
The set of reproductive options a species can resort to is delimited and timed by its life-history, whereas the relative importance with which the different colonisation modes are used also depends on the local environment (Capers, 2003, Wiegleb and Brux, 1991). However, it is expected that there are species-specific propagation strategies combining the available options in a characteristic way, but in lake ecosystems this has never before been quantified in-situ. Differences in the preference of colonisation modes might explain spatial as well as temporal patterns of macrophyte occurrence. Therefore, we wanted to investigate, a) whether different macrophyte species use different strategies in colonising and to quantify the propagation modes in relation to each other. In order to test the source of colonisation, we hypothesized that b) edge-effects would be strong, and c) there would be differences between the months in summer and autumn in colonisation intensities, relatable to the species life history traits.
Section snippets
Material and methods
Ten plots of 1 m × 1 m were randomly designated in 0.6–1.0 m deep water in Lake Balaton (coordinates: 46°54′50.26″N 17°53′36.38″) on 4th June 2009. Plots were arranged on four points (sub-sites), within a range of 5–70 m apart.
The site represented the typical submerged vegetation of Lake Balaton with Potamogeton perfoliatus L., Stuckenia pectinata (L.) Böerner, Myriophyllum spicatum L., Ceratophyllum demersum L. and Najas marina L., belonging to the category of “Euhydrophyte vegetation of naturally
Results
A total of 1822 plants established during the whole research period from 7th June till 8th October (126 days), resulting in an overall colonisation rate of 1.4 plants per day per m2. Most intense colonisation took place during August (peak value from harvesting on 7th September) with 85.1 ± 15.7 colonisation events per m2 on average (± SE), while it was lowest during the following month, harvested at the beginning of October (19.8 events ± 2.8).
First it was M. spicatum and C. demersum which colonized
Discussion
The noted species proved to be highly successful in colonising newly created gaps in the vegetation. Characteristic strategies of the main macrophytic species in Lake Balaton could be identified in the presented field experiment, built of a species-specific set of several modes of reproduction and establishment. Life-history traits were often reflected in the single species‘ strategies.
The observed establishment modes were almost exclusively based on fragment rooting and rhizomatic growth. Even
Acknowledgements
The authors would like to thank András Zlinszky for help with the field work, Timothy Hollo for English linguistic editing and Ágnes I. György for fruitful discussions on the manuscript. The authors are grateful for valueable comments of E. Gross and Jan E. Vermaat, as well as two anonymous reviewers on a previous version of the manuscript. The study was supported by TÁMOP-4.2.2.A-11/1/KONV-2012-0038.
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