Growth form parameters of clonal herbs
Kalevi Kull
Publication data:
Kull, Kalevi 1995. Growth form parameters of clonal herbs. In: Aaviksoo, K.;
Kull, K.; Paal, J.; Trass, H. (eds.), Consortium Masingii: A Festschrift for Viktor
Masing. (Scripta Botanica 9.) Tartu: Tartu University,
106-115.
The main aim of this paper is to provide methods for the inclusion of vegetative reproduction characteristics into standardised autecological accounts of the biology and ecology of clonal plants. We started the extensive measurement of the ecophysiological parameters of local plant species from leaf characteristics (Niinemets & Kull 1994). The current project concentrates on the rhizome parameters of local perennial herbs.
A large collection of autecological accounts of common vascular plants of the British flora was provided by Grime et al. (1988). However, the quantitative data about vegetative reproduction are absent in that collection, and methods for collecting them are not included in contemporary manuals (Hendry & Grime 1993). Several other accounts contain data about the environmental characteristics connected to the species (Hundt 1966; Landolt 1977; Ellenberg 1978), but do not include ecomorphological traits of the organisms.
1. Terminology: life forms, growth forms, and ecomorphological patterns
‘Life forms are considered types of plants having the same kind of morphological and/or physiological adaptation to a certain ecological factor’ (Barkman 1988, p. 11). The term ecomorph has also been proposed, having a similar meaning (Aleyev 1986).
‘Growth forms are types of plants with the same gross morphology (architecture). The concept is therefore free of any hypotheses about adaptation... I agree with Du Rietz (1931) that growth forms should not be based on (established or supposed) adaptations to the environment’ (Barkman 1988, p. 11).
Several large-scale classifications of life forms and growth forms have been worked out in recent decades (Barkman 1988; Orshan 1986; Aleyev 1986), in addition to the classical systems (Raunkiaer 1934; Serebryakov 1962). These classifications, however, rarely use quantitative characteristics.
For comparative autecology, universal quantitative parameters enabling the description of the morphological patterns of different types of growth forms are needed. The quantitative analysis of the distribution and coherence of the morphological patterns of many species in different communities will provide a new level in the classification of life forms, but presumes voluminous measuring work. This approach is in concordance with the views of T.Lippmaa, who emphasised the importance of building the life form system inductively, beginning from the smaller taxa, in contrast to the classical deductive life form systems (Masing 1961). V.Masing (1958) has also stressed the importance to achieve a dynamic understanding of life form types.
In the case of clonal plants, the phalanx and guerilla growth forms have been defined (Harper 1985; Lovell & Lovell 1985).
2. Growth form parameters and the central problem of community dynamics
The three main approaches in plant community research - classification(ism), ordination(ism), and construction(ism) - have all applied a different emphasis in growth form analysis.
The classification paradigm has put the main emphasis on the structure of vegetation, and the elaborated life form typologies help considerably in this task.
The ordination paradigm stresses the continuous character of vegetation variation, the existence of intermediate forms. Raunkiaer’s (1934) life form system corresponds to the ordination approach. Also, the primary life strategies and the corresponding types of life histories according to J.P.Grime (1979) serve as an example of the ordination approach.
The construction paradigm tries to model the structure of vegetation on the basis of the mechanisms of its dynamics. It is also a step towards a quantitative characterisation of growth forms.
From the point of view of the constructionistic approach, the central problem of phytocenology could be formulated as follows:
let there, on a plot of size S at time t, live n known species of plants with ramet numbers a1,...,an and age state r spectrums P1(r),...,Pn(r). It is required to predict the ramet numbers of all species on this plot at time t+T, in a given climate, soil, and with a given invasion of diaspores from the species pool.
Attempts to find solutions to this problem in some concrete cases (building models of community dynamics), have raised the necessity to possess data on species invariants (Kull 1987; 1988), i.e. the values of eco-morpho-physiological traits which characterize species and determine the behaviour of the species in a community.
These species invariants could be the attributes that have been demonstrated to confer competitive ability in plants (Epp & Aarssen 1988; Caldwell 1987). Nevertheless, the resultant dynamics of a certain population on a plot could be influenced by individual traits of immigration, extinction and density regulation, which may not have direct relevance to competition.
The spatial behaviour, and corresponding traits which are responsible for the spatial dynamics, are among the most important characteristics which could be used in the deterministic models of community dynamics.
In grasslands, the majority of species are clonal, which may have a considerable impact on the mechanisms of species coexistence (Herben et al. 1994).
3. Clonal herbs: the archetype and its parameters
Comparing different taxonomically close clonal species we may see sequences of growth forms, e.g. from cushion-plants and tussock herbs to rhizomatous species which form sparse stands and smooth mats. These types, which differ considerably in their external appearance, may be a result of only one variable parameter, namely the length of rhizome belonging to a ramet. For example, the genus
Carex has species which grow as tussocks (C. caespitosa), as dense clumps
(C. ornithopoda), or as sparse mats
(C. disticha). The main morphological difference between these species is the length of spacers, which may vary from 0.2 cm to 40 cm in different species of
Carex. The length of spacers is simply the parameter, which places the species in the continuum between two extreme growth forms, phalanx and guerilla
(Soukupova 1990).
Ramet is the principal unit to be used in descriptions and, if limited to vascular plants in meadows, almost creates no problems in practical identification. Genet is synonymous with clone, and may consist of one or more ramets. Polycormon is a part of the genet which is (still) connected through living tissues (Penzes 1960;
Falinska 1985).
The general structure, or archetype of a clonal plant could thus be viewed as being rather simple, and the measures to be used in species comparisons are applicable to the majority of plants having a modular pattern of growth. The main traits whose values could be used as species invariants are listed
below.
A. Ramet life span (duration of ramet) is a principal trait which determines the replacement rate and speed of spatial dynamics. Ramet life span means the duration of the apical meristem. For example, in
Carex panicea it has the value of 3-4 years - during the first year the rhizome starts to grow from a bud (situated laterally at the base of the shoot of a mother ramet), forming a horizontal rhizome which then turns up and starts to produce leaves; during the second year the apical meristem lives close to ground level and produces several leaves; in the third or fourth year the meristem grows upwards and differentiates into flowers. In some species the bud from which a ramet starts to grow may also be situated on the rhizome or on a root. To give some examples, duration of ramets in
Dactylis glomerata, Cirsium acaule, Asperula tinctoria and Cypripedium
calceolus is one year (one season), whereas in Sesleria
caerulea it may be 2-7 years and in
Convallaria majalis 8-12 years. For the majority of meadow perennials, the data about ramet life span are not available from existing literature.
B. Spacer length, or mean annual extension of rhizome, stolon, runner, corm, or bulb (Bell 1991), determines the speed of vegetative mobility. To be exact, two parameters should be defined - (1) the length of the spacer, from the mother shoot to the point where the ramet is situated on the ground, and (2) the yearly increment of the spacer. In the majority of cases, these two parameters coincide for temperate grassland herbs, but there are exceptions which should not be overlooked. For example,
Sesleria caerulea has rhizome annual increments of 1-3 cm, but since the same ramet (the same apical meristem) is shifting horizontally for several years, the length of the rhizome may be 2-14 cm.
Quite often, there may be different types of spacers within the same species, e.g. intravaginal and extravaginal tillers, which may have very different lengths. Therefore, the measure to be used should not be the mean, but the frequency distribution of the spacer lengths. In cases of large variabilities in the lengths of stolons or rhizome branches, several scales of morphological pattern may be evident in that species (Kershaw 1964).
Spacer length is a parameter used in several models of population dynamics (Oborny 1994). The parameter can be very different in different species, but there is some evidence showing that stolon length may not be dependent on growth conditions (Eriksson 1986).
C. Frequency of rhizome branching, measured as ramets per ramet per year, is an indicator of the intensity of vegetative reproduction. This parameter, however important, is rather variable as it depends on the space available (Waller & Steingraeber 1985). The dynamics of a population is very much dependent on the potential speed of vegetative reproduction, therefore the estimations of its maximum value (e.g. the higher quartile) should be used.
D. Branching angle is frequently used in computer simulations of clonal growth (Bell 1985), but there is not much data concerning this parameter for the majority of species. Usually, the rules of branching can not be represented by one single angle, since there is often more than one bud in the regular position at the basement of a ramet, and they may have a certain fixed order of starting growth. As a rule,
Monocotyledoneae have a more regular geometry in rhizome growth than
Dicotyledoneae.
E. Ramet density. There is some evidence showing that the ramet density of perennial herbs in a monospecies community has a relatively stable value despite differences in above-ground biomass and ramet size (Cook 1985).
F. Relative frequency of ramet types. Generally, there are two types of ramets - generative and vegetative ones. However, this discrimination is rarely used correctly - considering that the perennial generative ramets may have flowers, for example, only in the last year of their lives (as usual in monocarpic ramets, e.g.
Sesleria and Carex), or at some intermediate age (as in polycarpic ramets, e.g.
Convallaria majalis). Therefore, the discrimination into generative and vegetative ramets has three different meanings, which may concern:
(a) the whole life span of a ramet,
(b) a particular year during the life of a ramet, or
(c) the particular moment of observation.
If the meaning (b) is applied (which we find most frequently in population studies), then the division the types speaks more about the age distribution of ramets than about the ability of ramets to flower.
G. Ramet size (shoot height, shoot weight) is a highly variable measure, but it could be used as additional data about the conditions in a specific site where the other measurements have been taken. It is important to notice, that the ramet sizes of different species from the same layer are usually highly correlated.
4. Quantitative characterization of clonal herbs: examples
L. Klimes (1992) uses the following parameters describing rhizomes of Rumex
alpinus: segment length, segment mortality, branching probability, branching angles. For
Carex bigelowii, B.C.Carlsson & T.V.Callaghan (1990) have measured shoot density, rhizome length, sizes of floral and vegetative shoots. Modeling growth of
Glechoma hederacea, Carex flacca and Brachypodium pinnatum, H.de Kroon & F.Schieving (1990) used branching intensity, runner length, specific runner weight and ramet dry weight as model input parameters.
Distribution of annual extension of rhizome in Asperula
tinctoria and Sesleria caerulea, together with data on rhizome branching, is given on fig. 1.
Acknowledgements
To A.Sellin for discussions in some terminological issues. To M.Sammul for doing measurements and help in fieldwork. The project is partly supported by grant no. 859 of the Estonian Science Foundation.
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