Comparison of life history characters of broad-leaved species of the genus Potamogeton L. I. General characterization of morphology and reproductive strategies

Aquatic Botany, 39 ( 1991 ) 131-146

131

Elsevier Science Publishers B.V., Amsterdam

Comparison of life history characters of broadleaved species of the genus Potamogeton L. I. General characterization of morphology and reproductive strategies G. Wiegleb and H. Brux* FB 7 Biologie, Universityof Oldenburg, Postfach 2503, D-2900 Oldenburg (F.R.G.) (Accepted for publication 4 July 1990) ABSTRACT Wiegleb, G. and Brux, H., 1991. Comparison of life history characters of broad-leaved species of the genus Potamogeton L. I. General characterization of morphology and reproductive strategies. Aquat. Bot., 39: 131-146. A short historical outline of the development of the study of Potamogeton life history is given. Seven levels of research (population, genet, clone, patch, shoot complex, vertical shoot, modular unit ) are defined and their appropriateness for studying Potamogeton ecology is discussed. The patch, the shoot complex and the vertical shoot are the central research units. A morphological analysis of the shoot complex is given in order to display the special properties of this important entity. Two basic shoot types are distinguished: the vegetative shoot (represented as either the horizontal or vertical shoot) and the sexual shoot. Based on the morphological description, a conceptual growth model is presented comprising the observed transitions between plant parts in the species studied. Five different reproductive strategies can be distinguished, which differ among the species studied. Data on the life span of different entities (genet, shoot complex, vertical shoot) of the species studied are compared. The life span of vertical shoots varies from short-lived (2 months) to biennial. The life span of shoot complexes varies from annual to very long lived. In most species, the genet is potentially immortal owing to extreme fragmentation. No direct correlations are found between reproductive strategies and environmental conditions expressed as stress and disturbance. Similar unfavourable conditions can be overcome by various strategies.

INTRODUCTION

In Europe, the study of the life history of Potamogeton began in the middle of the 19th century, culminating in the works of Sauvageau (1894), Raunkiaer (1896), Glueck ( 1906, 1924), Graebner (1908), and Fryer and Bennett (1915). Fernald (1932) and Miki (1937) contributed to the knowledge of ecological performance in American and Japanese species, respectively. The data available were later compiled by Sculthorpe ( 1967 ), Hutchinson ( 1975 ) and Tomlinson ( 1982 ). Their publications show that research stagnated over a period of several decades. * Present address: Institute for Applied Biology, Landscape Ecology and Landscape Planning, U n t e r m Berg 39, D-2900 Oldenburg, F.R.G. 0304-3770/91/$03.50

© 1991 - - Elsevier Science Publishers B.V.

132

G. WIEGLEBAND H. BRUX

It is only recently that research has been given a fresh impetus. Several papers have been published by different working groups, not only reporting facts, but also outlining conceptual ideas and programs for future research (Verhoeven et al., 1982; Van Wijk, 1983, 1988; Kadono, 1984; Mitchell and Rogers, 1985; Wiegleb and Todeskino, 1985; Brux et al., 1987; Kautsky, 1987, 1988; Spencer and Anderson, 1987; Day et al., 1988; Wiegleb and Kadono, 1989a,b ). The most important new aspects in these approaches are as follows. ( 1 ) Not only the mere occurrence of a character or trait is regarded, but quantitative or at least semi-quantitative data are used for the comparison of species and infra-specific entities. (2) The study of life history in aquatic plants is related to the discussion on strategy concepts (Grime, 1979; Huston, 1979; Holm, 1988) in population ecology of terrestrial plants and animals. (3) As many studies of life history in Potamogeton are based on ill-defined morphological terms, Potamogeton morphology and the functional morphology of rhizomatous and clonal monocotyledons (Harper and Bell, 1979; Bell and Tomlinson, 1980) are combined in order to provide a basis for both description and quantification. The paper by Verhoeven et al. ( 1982 ) showed that in aquatic plants there is a great diversity within and among species in terms of anatomical, morphological, physiological and life history traits, that can be summarized as "strategies". The authors stressed the importance of finding the appropriate level of comparison (and thus generalization). This problem became more obvious when more characters were included in the analysis (see Kautsky, 1988). The aims of the present study were as follows. (1) Pragmatic definitions of levels of integration on which the study of Potamogeton can be carried out. (2) Definition of operational concepts in the functional morphology of Potamogeton. Afterwards, questions such as, "Which character is the property of which operational unit under which circumstances?" can be asked. This approach is necessary since most descriptive work has been carried out on lilies of the valley and oaks, but not on Potamogeton. (3) Qualitative and summarizing descriptions of life cycles as a basis for future quantitative approaches. Knowledge of the life cycle is considered essential for explaining the ecological performance of the species. (4) Comparison of life spans of various entities among species in order to understand the evolutionary and ecological importance of this character. Life span relates the observed patterns in morphology and reproduction to time scale. (5) Investigation of the relationships between the observed traits and environmental conditions. Environmental conditions are conceptualized here

LI FE HISTORY OF BROAD-LEAVED POTAMOGETON

13 3

as predictability and severeness of the habitat, and not as single physicochemical factors. MATERIALS AND METHODS

Between 1985 and 1988, three different research projects were carried out in order to study the life cycle of Potamogeton. ( 1 ) Two species, Potamogeton distinctus A. Bennett and Potamogeton malaianus auct. ( = P o t a m o g e t o n wrightii Morong), were studied in Japan in 1985 / 86 (see Wiegleb and Kadono, 1989a,b ). (2) Research on Potamogeton alpinus Balbis in Northwest Germany (see Wiegleb and Todeskino, 1983, 1985; Wiegleb, 1984; Brux et al., 1987) was continued. ( 3 ) All broad-leaved species (Potamogeton lucens L., Potamogeton perfol-

iatus L., Potamogeton natans L., Potamogeton gramineus L., Potamogeton polygonifolius Pourret, Potamogeton nodosus Poiret) and hybrids (Potamogeton X sterilis Hagstroem, Potamogeton × spathulatus Schrader, Potamogeton X decipiens Nolte ) occurring within a radius of 100 km around Oldenburg (F.R.G.) were included in the study from 1986 onwards (R. Heim and G. Wiegleb, unpublished results, 1988; H. Brux and G. Wiegleb, unpublished results, 1988). Thus, comparable data are available for nine species and three hybrids. The sampling methods and study areas are described in detail in Brux et al. ( 1987 ) and Wiegleb and Kadono (1989a). Additionally, transplantation experiments were carried out with P. alpinus, P. gramineus, P. natans, P. perfoliatus, P. polygonifolius and P. X sterilis. Plants from 5-10 different sites were planted in an artificial clear-water lake near Oldenburg and their morphological response was observed for at least 2 years. Clonal plants of all German species were grown in plastic tanks in a greenhouse at 18 ° C. RESULTS

Levels of study When studying the ecology of Potamogeton, several possible levels of research can be distinguished which differ in their degree of "tangibility" (Allen et al., 1984; Wiegleb, 1989). In Table 1, seven levels are listed and set theoretical definitions are given. The entities of all levels are considered to be composed of lower level units. There is more than one sequence of levels. In the context of a particular research program, some levels may be skipped while other levels may become identical under certain conditions. The approach has to be pragmatic and special properties of the research object have to be considered. In the study of Potamogeton, the population, the genet and the clone are

134

G. WIEGLEBAND H. BRUX

TABLE 1 Levels of study in population ecology and demography of Potarnogeton Level

Definition

Tangibility

Population

Set of units (genets, shoot complexes, etc. ) of a certain species in a certain area (river, lake, physiographical region)

Low

Genet

Set of shoot complexes with (supposed) genetic identity

Clone

Set of shoot complexes that can be identified as belonging to one genet despite a lack of morphological integration

Patch

Set of shoot complexes in a certain arbitrarily defined place ( = sampling site), either belonging to one genet or not

Shoot complex (ramet)

Set of modular units comprising horizontal and vertical, sexual and asexual modules (operational unit = individual)

Vertical shoot (ramet)

Set of modular units comprising at least one vertical shoot module with the potential of producing a flowering module (operational unit)

Modular unit

Set of functionally integrated cells produced by one meristem

High

levels of a low degree of tangibility and thus cannot be expected to yield operational research units. The modular unit (regarded here as identical with the meristem) has not yet been an object of intensive research. The patch, shoot complex and vertical shoot are the most important levels. Observable and measurable characters can be assigned to each of these levels. The concept of "ramet" is not used here since it is likewise applied to potentially independent units (the vertical shoot) and actually independent units (the shoot complex) (see Silvertown, 1987; Begon and Mortimer, 1986 ).

General structure of Potamogeton The general structure of a broad-leaved Potamogeton is displayed in Fig. 1. The abbreviations used in the text are defined in the caption. The shoot complex is chosen here because it is the basic unit of growth. In stoloniferous species (like P. alpinus), it is the most tangible entity ( = the individual plant ). This kind of tangibility is lost in species with rhizomes creeping deep in the sediment. In those species (e.g.P. natans), the vertical shoot is in itself a composed unit and may become an appropriate unit of observation. Tangibility of the shoot complex is also low in many narrow-leaved species where it is impossible to disentangle any individual unit. The general scheme includes all possible morphological properties of broad-leaved Potamogeton. In

135

LIFE H I S T O R Y O F B R O A D - L E A V E D POTAMOGETON

\ FAL:

ALl

VS

AL t

,\

L L K K

-

AR

HI"

TB

RZ Fig. 1. Morphological analysis o f broad-leaved Potamogeton. AL = aerial leaf ( o f land f o r m ) ; A L S = a x i l l a r y leafy shoot (non-flowering); A R ---a d v e n t i t i o u s root; F A L S = f l o w e r i n g axillary leafy shoot; F L = floating leaf; G S = g e r m i n a t i n g seed; G T = " g r e e n " turion; I = internode; LHS = lower horizontal shoot ( i n general); M T = multiple turion; P = petiole; P D = peduncle; P H = phyllodium; R = ( p r i m a r y ) root; R L = reduced leaf; R Z = rhizome ( r h i z o m a t o u s LH S ); S = spike; S C L = scaly leaf; S L = s u b m e r s e d leaf; SLHS = stoloniferous LHS; S T L = stipulate leaf; T = turion; TB = tuber; U H S = u p p e r horizontal shoot; VS = vertical shoot.

136

G. WlEGLEB AND H. BRUX

principle, all combinations of characters observed can be deduced from this scheme. The shoot complex can be interpreted as a sympodial construction of serial shoot modules (White, 1979) or shoot generations (Sauvageau, 1894). This concept was applied, for example, by Wiegleb and Kadono (1989a,b) for describing biomass allocation among shoot generations. The shoot complex can also be conceptualized as being composed of different module types (see Harper and Bell, 1979 ). Most authors apply this concept for describing the allocation of biomass among plant parts. Both concepts can be regarded as complementary and are equally useful for describing complex morphological structures. A strict distinction between the concepts of sympodial and monopodial branching (cf. Tomlinson, 1982) is regarded as irrelevant to the description of Potamogeton in the population ecological context. The main substructures are defined as follows. (A) Vegetative shoot modules (VSM). VSM are represented by both vertical (VS) and horizontal shoots (HS). They are characterized by a common stem anatomical structure with a central cylinder surrounded by an endodermis. Distinction between HS and VS is more or less deterministic in some species, opportunistic in others, or meaningless, as in some populations of P. polygonifolius. The VSM form a kind of mycelium to which the sexual shoot modules (SSM) are attached in specific positions. (AI) Horizontal shoots ( H S = m o d u l e type 2, Harper and Bell, 1979). HS are characterized by a special sequence of internodes with one internode bearing only roots, followed by an internode bearing vertical shoots, proliferate HS and roots. The growth of HS is usually infinite. ( A l l ) Lower horizontal shoots (LHS). LHS are either stoloniferous or rhizomatous. In stoloniferous species, they allow for rapid spread in vegetation gaps. In rhizomatous species, they can be considered as a bud bank for vertical shoot production. LHS produce (besides scaly leaves and roots) turions, tubers, LHS of second and higher order (by proliferate branching), as well as various generations of vertical shoots. (AI2) Upper horizontal shoots (UHS). UHS are stoloniferous and may bear VS, turions and roots. They serve as a means of vegetative reproduction because they are easily detached. Distinction between UHS and LHS is only spatial in some species, but morphological differences can be found in others (Wiegleb and Kadono, 1989a). (A2) Vertical shoots (VS = module type 1, Harper and Bell, 1979 ). VS differ from horizontal shoots by their great number of different leaf types (scaly leaves, stipulate leaves, phyllodia, submersed leaves, floating leaves) and sexual shoot modules. VS are rooted at the base. Growth is either infinite or finite in the case of a high degree of differentiation between HS and VS. (A21 ) Main shoots (MS). MS are characterized by the production of LHS

LIFE HISTORY OF BROAD-LEAVED POTAMOGETON

137

on several lower nodes. Their basic structure is the same, regardless of their origin. (A211 ) Main shoots originating from seeds (not displayed). In some species, MS from seeds do not produce sexual shoot modules. (A212 ) Main shoots borne on turions or tubers. Main shoots from turions and tubers bear LHS, UHS, axillary leafy shoots (ALS) and SSM (see Fig. 1 ). LHS are produced deterministically at a few lower nodes and inserted either alternately or oppositely. In some species, UHS are produced as a reaction to environmental conditions (disturbance). In other species, this kind of branching occurs regularly. The number, nodes of insertion and type of ALS are characteristic of each species. Production of SSM can be opportunistic or deterministic. (A22) Vertical shoots from LHS. These shoots are usually completely equivalent to A212. They do not produce LHS as long as the integrity of the first order LHS is given. (A23) Vertical shoots in the upper regions of A21 and A22. (A231 ) Axillary leafy shoots (ALS). The structure of the inflorescence depends on whether the ALS are flowering (see flowering axillary leafy shoot (FALS)) or not (Hagstroem, 1916). Most ALS are found in the axils of alternate leaves. ALS can also be found in the axils of subopposite leaves instead of sexual shoot modules. Short ALS may serve as unspecialized winterbuds. (A232) Vertical shoots from UHS. Variable unit, either flowering and more or less equivalent to a main shoot, or non-flowering and functioning as a means of dispersal together with UHS. (B) Sexual shoot modules (SSM). Differ from VSM by their different stem anatomy (usually five main vascular bundles without a distinct endodermis ). Growth is finite, resulting in the production of 12-120, 1-4-four carpellate flowers. In most cases, SSM are inserted in the axils of subopposite leaves, rarely opposed to an alternate leaf directly below the zone of the subopposite leaves.

A conceptual model of Potamogeton growth and reproduction On the basis of the morphological definitions, a conceptual growth model can be constructed (Fig. 2) in which all processes observed so far in the species studied are summarized. The frames show functionally distinct parts of the plant. Transitions between parts (processes) are indicated by arrows. In the conceptual growth model the shoot complex, composed of a vertical shoot and some kind of horizontal shoots, is also of main interest. Constant production of vertical shoots from stoloniferous horizontal shoots can be observed in all species. Moreover, there are five modes of reproduction, by which the species differ. ( 1 ) In some species (P. natans, P. lucens, P. gramineus), stoloniferous hor-

138

G. WlEGLEB AND H. BRUX

seeds I flowering

fructification sexual shoot

branching branchin9

_J

branchinq l upper I_ F vertica[ horizontal shoot shoo, I verticot growth -]

I

horizontal

stolon t-growth, -j branching ( lower horizontel~-verticat growth

I

shoot )

storage

germination

/

rome,

production

I

fro( mentotion II

fragmen,a,ion

Ifragments I L turions I

L__

seed bank

v;rr~iwCta/

\

bud I (incl. r,izo° bank)

i

Fig. 2. A conceptual model of the reproductive strategies of broad-leaved Potamogeton. The numerals refer to the reproductive strategies listed in the text and Table 3. izontal shoots are converted into rhizomes with shorter internodes and storage function. Both horizontal growth of rhizomes and growth of vertical shoots from a rhizomatous bud bank result in expansion and enlargement of the shoot complex. The fragmentation rate is usually low in these species. This repro-

LIFE HISTORY OF BROAD-LEAVED POTAMOGETON

139

ductive mode corresponds to the classical model of vegetative reproduction ("short life cycle" of rhizomatous plants; Soukupova, 1988 ). In fact, this kind of "reproduction" can be summarized under the concept of "growth" (see discussion of this term in Watkinson, 1988 ). In some species, perennial parts of stolons may have the same function as the rhizomes. (2) In all species, sexual shoots are produced in specific positions on the vertical shoots. After flowering and fructification, numerous seeds are produced and buried in the seed bank, but among the broad-leaved Potamogeton species only in P. distinctus does sexual reproduction regularly lead to the formation of new genets by seed germination. ( 3 ) There is a vegetative life cycle starting from a turion-tuber, producing vertical shoots, producing LHS, producing turions-tubers, etc. This cycle is seasonal with propagules being produced mainly at the end of the vegetative period (October/November) and vertical shoots starting to grow again in spring (March/April). There is, however, some variation in the exact timing, depending on species, latitude and even water quality (see also Spencer and Anderson, 1987). This process leads to the formation of new shoot complexes. It is realized in the stoloniferous species P. alpinus, P. distinctus and

P. wrightii. (4) In all species, there is a compartment "fragments", which is supplied by UHS, LHS, ALS and VS, and which after re-rooting may be the source of new vertical shoot formation. This is an aseasonal life cycle, leading likewise to the production of new shoot complexes. Unspecialized "winter buds" may also be involved in this process. Both seasonal and aseasonal vegetative life cycles cannot be summarized under the concept of growth because they actually have a similar dispersal function as sexual reproduction. (5) In P. polygonifolius, the vertical shoot has the potential for infinite growth. Thus, an apical vegetative renewal shoot is produced after flowering. This kind of continuous growth is frequent in other aquatic plants such as Ranunculus peltatus Schrank (Zander and Wiegleb, 1987 ), but rare in broadleaved Potamogeton.

Comparison of species and units with respect to life span Table 2 shows the life spans of different entities of the species studied most intensively. There are still many unanswered questions, resulting from methodological difficulties. At the level of the genet, there are species with infinite life expectancy as well as species with short life expectancy (annual to a few years). In some species (P. distinctus), both cases occur in different habitats. At the level of the shoot complex, there are most probably three groups: long-lived rhizomatous perennials, shorter-lived stoloniferous perennials and annuals. The actual life span of the shoot complexes of species growing deep in the sediment is still unknown. Three groups can also be distinguished at

140

G. WlEGLEBAND H. BRUX

TABLE 2 Comparison of life spans of several species of Potamogeton based on personal observations of the authors in northern Germany and Japan Genet

Shoot complex

Vertical shoot

P. alpinus

? Infinite (at least 12 years )

Average 60 days, maximum 122 days

P. distinctus P. gramineus P. lucens P. natans P. nodosus P. perfoliatus P. polygonifolius

Annual to few years ? Infinite ? Infinite ? Infinite ? Infinite ? Infinite ? Infinite

P. wrightii

? Infinite

P. X decipiens P. × spathulatus P. xsterilis

? Infinite ? Infinite ? Infinite

Annual ( 120 days to a maximum of 270) Annual ( 120240 days) ? Perennial ? Perennial ? Perennial ? Perennial ? Perennial Perennial or annual At least 250 days partly perennial ? Perennial ? Perennial ? Perennial

Annual (90-120 days) ? Annual to biennial ? Annual to biennial Annual ( 100 days) to biennial ~ 90 days (short) Annual Perennial with infinite growth Annual (80-120 days), rarely biennial Annual Perennial with infinite growth Annual (240 days)

the level of the vertical shoot: short-lived annuals (60-90 days), longer-lived annuals with shoots living for > 100 days and biennials whose shoot bases persist during the cold season. Thus a term like "wintergreen" has no ecological meaning because it remains unclear whether there are hibernating biennial vertical shoots or short-lived vertical shoots that are replaced by a new vertical shoot generation at the beginning of the warm season. An increasing degree of immortality can be observed proceeding from the vertical shoot to the genet. An almost unlimited potential for regeneration from components can already be found in the shoot complex of most species. The destruction of a whole genet is only probable in the case of a large-scale catastrophic event affecting a whole lake or river section. This seems to be the major advantage of disintegration and fragmentation (see also Cook, 1986; Sebens and Thorne, 1986).

Relationship of reproductive strategies to environmental factors In Table 3, the reproductive strategies observed are ordered in relation to the observed stress and disturbance. In the eutrophicated waters studied, stress is mainly caused by turbidity (limiting light availability) and toxic substances. Disturbance can be subdivided into predictable (continuous and

141

LIFE HISTORY OF BROAD-LEAVED POTAMOGETON

TABLE 3

Reproductive strategies of Potamogeton species in relation to observed habitat conditions ( 1,2 indicates that two strategies contribute to a similar degree to the maintenance of a population, 1 > 2 indicates that the first named strategy contributes more and 1 > > 2 indicates that the first named strategy contributes considerably more ). Strategies: 1a = rhizomes; 1b = perennial stolons; 2 = seeds; 3 = turions and tubers; 4 = fragments; 5 = continuously growing vertical shoots Disturbance

Low

Stress

Low

Medium

High

gramineus

alpinus

natans

3>1b>4

3>>4 > 2

polygonifolius

1a>>4 > 2

perfoliatus

5>>4

lb>4

perfoliatus

lucens

lb>4>3

la

× decipiens la>4 High,

wrightii

lucens

alpinus

cyclic or continuous

3 > 4 > lb

la>4

3>4

gramineus

X sterilis

natans

lb>3>4

la,4

la>4

× spathulatus

x spathulatus

nodosus

lb

lb

3 > lb

High,

alpinus

erratic or catastrophic

natans

3,4

1a>>4

distinctus 2,3>4

cyclic ) and unpredictable (erratic) disturbance (Holm, 1988 ). Its main constituents are mechanical disturbance (both by cutting/dredging and current velocity/wave action) and drought. All combinations of this habitat templet matrix exist in natural waters. Furthermore, all combinations of habitat conditions are colonized by Potamogeton species. Some combinations are obviously colonized by narrow-leaved species only (see Kautsky, 1988 ), which are excluded from the present study. There is no one-to-one relationship between certain habitat conditions and reproductive cycle options. Performance of the species is relatively uniform in different habitat types, only the relative importance of certain options varies. The importance of Option 4 (fragmentation) increases in all species with increasing disturbance. Sexual reproduction seems to be essential only in water subject to complete desiccation (see also Mitchell and Rogers, 1985 ). Heavily and erratically disturbed sites can be colonized by three different strategies: one immobile strategy with rhizomes and perennial stolons growing ~ 40

142

G. WlEGLEB AND H. BRUX

cm deep in the sediment (in P. natans) and two mobile strategies, one based on fragments and at least partly mobile turions in combination with a high growth rate (in P. alpinus), and the other based on sexual reproduction (in P. distinctus). None of these strategies fits into the classical dichotomy of tolerance and avoidance. DISCUSSION

In the context of a particular study, the question of which level of integration a measured character or trait belongs to has to be addressed. For example, some ambiguities of the life form concept have already been discussed by van Wijk ( 1983, 1988 ) and Brux et al. ( 1987 ). In most Potamogeton species, the (mainly hypothetical ) genet is in fact "perennial" or potentially so, while in P. alpinus and Potamogeton pectinatus L. the shoot complex (the observable unit) is annual. From the hierarchical point of view, the old knowledge is not inconsistent with the results of the researchers named above. Circumstances may be different in different species or in the same species in different sites. Terms like "annual" or "perennial" can only be used contextually and cannot be generalized for ideal constructs like species. "Canopy height" is a character of a patch that can be related to the length of a vertical shoot or the height of a shoot complex. "Lateral spread", however, or particularly "morphology index" (Kautsky, 1988 ) are concepts that can only be applied to shoot complexes (or to genets in an early developmental stage). "Primary production" may have different operational meanings according to the method used for estimation. In chemical methods, the unit of study is a vertical shoot or even a subunit. In harvest methods, the unit of study is inevitably the patch. Generalization is usually sought for populations of species. "Density" is a character of a patch. The basic units are the vertical shoots. It is obvious that density and primary production can be easily related by means of set theoretical or algebraic operations, as both refer to patches. Nonetheless, one must not expect an ecologically meaningful result from such computation as branched and unbranched vertical shoots are treated equal, and the level of the shoot complex is skipped, thus neglecting the possible effects of within-shoot complex competition and among-shoot complex competition. On the other hand, it is difficult to relate life form, morphology index and primary production among one another and with environmental variables because these concepts refer to different levels of study. The higher levels of organization cannot simply be reduced to the lower levels. All levels have to be studied and connections between the levels have to be sought (Allen et al., 1984 ). In the first instance, this procedure leads to an increase in the number of characters. It may be predicted that in future research the number of variables can be minimized when general knowledge increases. In any case, a con-

LIFE HISTORY OF BROAD-LEAVED POTAMOGETON

14 3

textual and operational definition of the units of study would prevent many students of aquatic ecology from the uncritical adoption of concepts developed in terrestrial ecology and dealing with different kinds of vegetation (like biomass-density relationships, above-ground-below-ground biomass relationships, etc.). Just as in a terrestrial habitat, the occurrence of a particular trait (tactic, character, option) in a particular site is not predictable. The same environmental factor can be answered by different traits (see above, the case of heavy, erratic disturbance ). Such differentiation occurs even within one species (see drought resistance in P. alpinus: land forms, emergency turions, or seeds, Brux et al., 1987 ). Traits (options, etc. ) cannot be interpreted as "adaptations" to present-day habitat conditions. They are possibilities evolved in the past. Nevertheless, varying degrees of adaptedness to various habitat conditions can be found and traits may be considered vital attributes (Noble and Slatyer, 1980), allowing predictions on the future persistence of the plants. Persistence can be regarded as the ultimate criterion of fitness (Bazzaz and Sipe, 1987). "Life history theory" (Stearns, 1976) was usually more interested in the predictability of combinations of characters: strategies or patterns of traits (Chapleau et al., 1988). The concept of a limited number of strategies has been adopted by many botanists in order to find generalizations in the obvious multiformity of ecological answers. The number three, particularly has a certain magic appeal. Thus the problem of idealization and reification also arises in strategy thinking, even though strategies were originally introduced in plant ecology as anti-taxonomist and anti-essentialist tools. Like single traits, strategies are not strictly correlated to habitat conditions either. The reasons are the same as discussed above. There is a certain hierarchy in strategies. Low-level strategies are based on a few mainly qualitative characters. The old concepts of "growth form" and "life form" are such low-level strategies. The "reproductive patterns" described above can be regarded as higher-level strategies. At the highest level, a strategy should include all possible, measurable and observable, qualitative and quantitative characters. High-level strategies can only be defined as fuzzy sets because most traits occur relatively independently and complete correspondence is rare. Obviously, the Potamogeton species have a wide ecological flexibility (Heathcote et al., 1987 ), which is reached by means of a wide range of optional low-level strategies. This has also been shown by Kautsky ( 1987 ) and van Wijk ( 1988 ) for P. pectinatus. Despite a high degree of plasticity, certain biological constraints still operate. In P. natans, Reproductive Option 3 will never contribute much to the maintenance of a stand. However, having developed Options 1, 2 and 4, the species will only be excluded from sites where Option 3 is essential. Seed formation in P. distinctus, to overcome the long-

144

G. WIEGLEB A N D H. BRUX

lasting drought in irrigation reservoirs under warm-temperate conditions (Wiegleb and Kadono, 1989a), represents the only case observed in which a certain option is really essential. Desiccation in the Mediterranean climate does not necessarily require this option (van Wijk, 1988), even though the relatively highest n u m b e r o f germinations were observed in the case reported. Van Wijk (1988) pointed out that the concepts of strategy theory developed in terrestrial plants (Grime, 1979) cannot be applied to the study of Potamogeton without modification. This is also true of concepts of animal life history theory (Murphy, 1968; Stearns, 1976; G r a h a m e and Branch, 1985 ). This finding is not surprising taking into account the special properties of Potamogeton as a modular organism with predominantly vegetative reproduction, which has developed various entities o f growth that guarantee persistence even under the most adverse conditions. ACKNOWLEDGEMENTS We thank O. Hostrup, Y. K a d o n o ( K o b e ) , P. Moog and G.P. Zauke for discussions on life history theory in general and apomictic monocotyledons in particular. D. Todeskino and R. H e i m provided unpublished data and assisted in the field work. G. Lampen provided technical assistance and R. Stoschek produced the figures. Financial support was given by the Deutsche Forschungsgemeinschaft (Az Wi 647/3-1,2).

REFERENCES Allen, T.F.H., O'Neill, R.V. and Hoekstra, T.W., 1984. Interlevel relations in ecological research and management: some working principles from hierarchy theory. USDA For. Serv. Gen. Tech. Rep. RM-110, 11 pp. Bazzaz, F.A. and Sipe, T.W., 1987. Physiologicalecology,disturbance, and ecosystem recovery. In: E.D. Schulze and H. Zw61fer (Editors), Potentials and Limitations of Ecosystem Analysis, Springer, New York, pp. 203-227. Begon, M. and Mortimer, M., 1986. Population Ecology.A Unified Study of Animals and Plants. Blackwell, Oxford, 220 pp. Bell, A. and Tomlinson, P.B., 1980. Adaptive architecture in rhizomatous plants. Bot. J. Linn. Soc., 80: 125-160. Brux, H., Todeskino, D. and Wiegleb, G., 1987. Growth and reproduction of Potamogeton alpinus Balbis growing in disturbed habitats. Arch. Hydrobiol. Beih., 27:115-127. Chapleau, F., Johansen, P.H. and Williamson, M., 1988. The distinction between pattern and process in evolutionary biology: the use and abuse of the term "strategy". Oikos, 53:136138. Cook, R.E., 1986. Growth and demography in clonal plants. In: J. Jackson, L. Buss and R.E. Cook (Editors), Population Biologyand Evolution of Clonal Organisms, Yale University Press, New Haven, pp. 259-296. Day, R.T., Keddy, P.A., McNeill, J. and Carleton, T., 1988. Fertility and disturbance gradients: a summary model for riverine marsh vegetation. Ecology, 69: 1044-1054.

LIFE HISTORY OF BROAD-LEAVED POTAMOGETON

145

Fernald, M.L., 1932. The linear-leaved North American species of Potamogeton section Axillares. Mem. Am. Acad. Arts Sci., 17: 1-183. Fryer, A. and Bennett, A., 1915. The Potamogetons (pondweeds) of the British Isles. Reeve & Co., London, 94 pp. Glueck, H., 1906. Biologische und morphologische Untersuchungen fiber Wasser- and Sumpfgew~ichse, Teil II: Untersuchungen fiber die mitteleurop~iischen Utricularia-Arten, fiber Turi onenbildung bei Wasserpflanzen sowie fiber Ceratophyllum. Fischer, Jena, 255 pp. Glueck, H., 1924. Biologische und morphologische Untersuchungen fiber Wasser- und Sumpfgew~ichse, Tell IV: Untergetauchte und Schwimmblattflora. Fischer, Jena, 746 pp. Graebner, P., 1908. Potamogeton. In: O. Kirchner, E. von Loew and C. Schroeter (Editors), Lebensgeschichte der Blfitenpflanzen Mitteleuropas, I, 1, Stuttgart, pp. 400-503. Grahame, J. and Branch, G.M., 1985. Reproductive patterns of marine invertebrates. Oceanogr. Mar. Biol. Annu. Rev., 23: 373-398. Grime, J.P., 1979. Plant Strategies and Vegetation Processes. Wiley, Chichester, 222 pp. Hagstroem, J.O., 1916. Critical researches on the Potamogetons. K. Sven. Vetenskapsakad. Handl,, 55(5): 1-281. Harper, J.L. and Bell, A.D., 1979. The population dynamics of growth form in organisms with modular construction. In: R.M. Anderson, B.D. Turner and L.R. Taylor (Editors), Population Dynamics. Blackwell, Oxford, pp. 29-52. Heathcote, C.A., Davies, M.S. and Etherington, J.R., 1987. Phenotypic flexibility of Carexflacca Schreb. Tolerance of soil flooding by populations from contrasting habitats. New Phytol., 105: 381-391. Holm, E., 1988. Environmental restraints and life strategies: a habitat templet matrix. Oecologia (Berlin), 75: 141-145. Huston, M., 1979. A general hypothesis of species diversity. Am. Nat., 113: 81-101. Hutchinson, G.E., 1975. A Treatise on Limnology. Vol. 3: Limnological Botany. Wiley, New York, 660 pp. Kadono, Y., 1984. Comparative ecology of Japanese Potarnogeton: An extensive survey with special reference to growth form and life cycle. Jpn. J. Ecol., 34:161-172. Kautsky, L., 1987. Life-cycles of three populations of Potamogeton pectinatus L. at different degrees of wave exposure in the Ask6 area, northern Baltic proper. Aquat. Bot., 27:177-186. Kautsky, L., 1988. Life strategies of aquatic soft bottom macrophytes. Oikos, 53:126-135. Miki, S., 1937. The water phanerogams in Japan with special reference to those of the province Yamashiro. Rep. Hist. Remains, Scenic Places Nat. Monuments Kyoto Pref., 18:1-127 (in Japanese). Mitchell, D.S. and Rogers, K.H., 1985. Seasonality/aseasonality of aquatic macrophytes in Southern Hemisphere inland waters. Hydrobiologia, 125: 137-150. Murphy, G.I., 1968. Pattern in life history and the environment. Am. Nat., 102: 391-403. Noble, I.R. and Slatyer, R.O., 1980. The use of vital attributes to predict successional changes in plant communities subjected to recurrent disturbance. Vegetatio, 43:5-21. Raunkiaer, C., 1896. De Danske Blomsterplanters Naturhistorie. I. Helobiae. Gyldendahl, Copenhagen, 724 pp. Sauvageau, C., 1894. Notes biologiques sur les Potamogetons. J. Bot. (Paris), 8: 1-9, 21-43, 45-58, 98-106, 112-123, 140-148, 165-172. Sculthorpe, C.D., 1967. The Biology of Aquatic Vascular Plants. Arnold, London, 610 pp. Sebens, K.P. and Thorne, B.L., 1986. Coexistence of clones, clonal diversity, and the effects of disturbance. In: J. Jackson, L. Buss and R.E. Cook (Editors), Population Biology and Evolution of Clonal Organisms. Yale University Press, New Haven, pp. 357-398. Silvertown, J.W., 1987. Introduction to Plant Population Ecology. 2nd Edn, Longman, New York, 219 pp. Soukupova, L., 1988. Short life-cycles in two wetland sedges. Aquat. Bot., 30: 49-62. Spencer, D.F. and Anderson, L.W.J., 1987. Influence of photoperiod on growth, pigment and

146

G. WIEGLEBAND H. BRUX

vegetative propagule formation in Potamogeton nodosus Poir. and Potamogeton pectinatus L. Aquat. Bot., 28:103-112. Stearns, S.C., 1976. Life-history tactics: a review of the ideas. Q. Rev. Biol., 51: 3-47. Tomlinson, P.B., 1982. Anatomy of the Monocotyledons. VII. Helobiae (Alismatidae). Clarendon Press, Oxford. Van Wijk, R.J., 1983. Life-cycles and reproductive strategies of Potamogeton pectinatus L. in The Netherlands and the Camargue (France). In: Proceedings of an International Symposium on Aquatic Macrophytes, at Nijmegen, pp. 317-321. Van Wijk, R.J., 1988. Ecological studies on Potamogeton pectinatus L. I. General characteristics, biomass production and life cycles under field conditions. Aquat. Bot., 31: 211-258. Verhoeven, J.T.A., Jacobs, R.P.W.M. and Van Vierssen, W., 1982. Life strategies of aquatic plants: some critical notes and recommendations for further research. In: J.J. Symoens, D. Hooper and P. Compbre (Editors), Studies on Aquatic Vascular Plants. Bruxelles, pp. 158164. Watkinson, A.R., 1988. On the growth and reproductive schedules of plants: a modular viewpoint. Acta Oecol. (Oecol. Plant.), 9:67-81. White, J., 1979. The plant as a metapopulation. Annu. Rev. Ecol. Syst., 10: 105-149. Wiegleb, G., 1984. Bibliography on Potamogeton alpinus Balbis. Part 2. Excerpta Bot. Sect. B, 23: 145-153. Wiegleb, G., 1989. Explanation and prediction in vegetation science. Vegetatio, 83:17-34. Wiegleb, G. and Kadono, Y., 1989a. Growth and development of Potamogeton distinctus in an irrigation pond in SW Japan. Nordic J. Bot., 9: 241-249. Wiegleb, G. and Kadono, Y., 1989b. Growth and development of Potamogeton malaianus in SW Japan. Nordic J. Bot., 9:167-178. Wiegleb, G. and Todeskino, D., 1983. Habitat conditions of Potarnogeton alpinus Balbis stands and relations to the plants biological characters. Proceedings of an International Symposium on Aquatic Macrophytes, at Nijmegen, pp. 311-316. Wiegleb, G. and Todeskino, D., 1985. Der biologische Lebenszyklus von Potamogeton alpinus und dessen Bedeutung f'tir das Verkommen der Art. Verh. Ges. Okol., 13:191-198. Zander, B. and Wiegleb, G., 1987. Biosystematische Untersuchungen an Populationen von Ranunculus subgenus Batrachium in Nord-westdeutschland. Bot. Jahrb. Syst., 109:81-130.