Plant strategies

4.1.1 Strategic response to competition, disturbance and stress

The concept that living organisms display ecological 'strategies' has advanced rapidly in recent years. The r-K continuum of MacArthur and Wilson (1967) made an important early contribution in contrasting the opportunistic r-species (with rapid rates of population growth), which exploit temporary habitats, with the equilibrium K-species of stable habitats in which competitive ability and survival of the individual is more important than population growth. In the same way that differing species of organism have gradually evolved over long periods of time (Darwin, 1859), such strategies arose through the exertion of competitive natural selection upon varied populations in which differences from the previous norm constantly arose. The CSR model of Grime (1974,1979) -standing for Competitor-Stress tolerator-Ruderal - made an important further advance in adding the stress-tolerators, organisms capable of exploiting continuously unproductive environments or niches. The competitors are equivalent to the K-species and the ruderals (living on disturbed sites) approximate the r-species. This theory also recognizes that in plants there is a separation of the established (adult) and regenerative (juvenile) strategies and they may respond differently to their environment. This theory has been very thoroughly applied over a long period of time in a number of ecosystems, and with the publication of Plant Strategies, Vegetation Processes and Ecosystem Properties (Grime, 2001), is becoming a major tool in the manipulation of vegetation and ecological prediction.

Stress and disturbance are the two main external factors limiting the amount of living and dead plant material in a habitat. Stress in this context consists of the external constraints limiting the rate of growth (productivity, i.e. dry-matter production) of all or part of the vegetation, including shortages of light, water and mineral nutrients and the influence of suboptimal temperature.

Disturbance limits plant biomass by causing its partial or total destruction through trampling, mowing, ploughing, the felling of trees, and the activities of pathogens and other herbivores. It also results from wind damage, frost, drought, soil erosion and fire.

Though no plants can long survive both high stress and high disturbance, there are three combinations of these factors under which plants continue to exist. These have led to the evolution of the three primary ecological strategies encapsulated in the name CSR. Grime (2001) lists 18 ways in which plants possessing these strategies differ from each other. These involve morphology, physiology, life history, palatability to unspecialized herbivores (Section 5.7) and litter production. If both stress and disturbance are absent then the composition of a plant community is determined by competition between species. i.e. it is made up of competitors (C). Most trees are competitors, exploiting conditions of low stress and low disturbance. Stress-tolerators (S) endure high stress and low disturbance such as dog violet Viola riviniana in the UK, growing in acidic, damp, shady woodlands, while ruderals (R) such as annual meadow-grass Poa annua and Funaria hygrometrica (a moss common in woods, often on recently burnt land) grow under conditions of high stress and low disturbance. Competitors include herbs, shrubs and trees, whereas ruderals are herbs or mosses. Stress-tolerators include various trees, shrubs, herbs and particularly lichens (see Fig. 4.1).

The definition of competition adopted by Grime (1979) in the first edition of his well-known book, is that it is the tendency of neighbouring plants to utilize the same quantum of light, ion of mineral nutrient, molecule of water or volume of space. Competition between plants occurs both above and below ground, competition for light becoming particularly strong as the canopy begins to close so that shoots of one plant are shaded by those of another (Section 1.4.2). Competitive ability is a function of the area, the activity, and the distribution in space and time of the surfaces through which resources are absorbed and as such it depends upon a combination of plant characteristics. Very many plants possessing all the following four features are extremely competitive:

(1) tall stature;

(2) a growth form which allows extensive and intensive exploitation of the environment above and below ground, such as a densely branched rhizome as in stinging nettle Urtica dioica and creeping soft-grass Holcus mollis, or an expanded tussock structure as in tufted hair-grass Deschampsia cespitosa (of which a form is frequent in the heavy soils in the oak woods of southern England);

(3) a high maximum potential relative growth rate (RGR); and

(4) a tendency to deposit a dense layer of litter on the ground surface.

Surface Litter Layer

Figure 4.1 Model describing the various equilibria between competition, stress and disturbance in vegetation and the location of primary and secondary strategies. C, Competitor; S, stress-tolerator; R, ruderal; C-R, competitive-ruderal; S-R, stress-tolerant ruderal; C-S, stress-tolerant competitor; C-S-R,

C-S-R strategist. Ic, relative importance of competition (-); Is, relative importance of stress ( ); Id, relative importance of disturbance

(-----).The strategic range of three life forms is also shown (a) trees and shrubs (b) bryophytes and (c) lichens. (Redrawn from Grime et al., 1988. Comparative Plant Ecology. Unwin Hyman.)

To represent these strategies, Grime et al. (1988) have developed a triangular diagram whose points are the maxima of competition, stress and disturbance (Fig. 4.1). It is here that the three primary strategies are situated. The position of a particular species is determined in relation to the indices of competition (Ic), stress

(Is) and disturbance (7d). The four main types of secondary strategy (C-R, C-S, S-R and the central C-S-R) occur in intermediate positions. Grime et al. (1988) derive the position of a particular species by using a dichotomous key involving various morphological, behavioural and reproductive characteristics. Each strategy (also called a functional type) can be represented within the triangular diagram by a set of C, S and R co-ordinates. These co-ordinates relate to a whole set of attributes that contribute to a species' ability to survive under given conditions of productivity and disturbance.

Figure 4.2 illustrates the strategies employed by some common species of the UK woodland field layer. Woodlands are usually stable; indeed the only ruderal present in the main figure is common chickweed Stellaria media which is occasionally encountered at disturbed woodland margins. Like other vegetation dominants, the stinging nettle Urtica dioica can generate high shoot thrust, pushing aside foliage of other species and resisting physical displacement. The leaves of such perennial competitors, capable of rapid growth, also have high nitrogen contents; this coincides with high concentrations of the enzyme 1,5-biphosphate carboxylase/oxygenase (Rubisco) which appear to facilitate rapid rates of photosynthesis in shady conditions. This latter feature is also aided by foliar phosphorus concentrations that are much higher than in slow-growing species. Shoots of stinging nettle die back completely in winter so associates such as the annual competitive-ruderal goosegrass Galium aparine, low-growing rough meadow-grass Poa trivialis (Fig. 4.2) and the moss Brachythecium rutabulum, which climbs stems of other plants, are able to exploit relatively high light intensities in spring and autumn.

Though the main focus of attention in earlier studies of plant strategies, and indeed of Grime (1979), was that of life-history traits (i.e. the strategies of life-history events such as germination, early survival, seed production and longevity), it is now apparent that the primary strategies outlined above also concern other attributes (resource capture and utilization, tissue chemistry and life-span, anti-herbivore defence, rates of decomposition). As Grime (2001) remarks these 'have obvious and direct connections to the functioning of ecosystems'; much of his second edition is designed to establish the nature and usefulness of these connections.

4.1.2 Influence of forest clearance in Prince Edward Island, Canada

The extensive survey made by Sobey (1995a, b) of the vegetation of Prince Edward Island in eastern Canada is of particular interest in its demonstration of the very long period required for a succession to culminate in climax hardwood forest here. Some reproductive strategies are exceedingly long term! The

Witchcraft Triangle Hebrew

Figure 4.2 C-S-R ordination diagram of woodland field layer species in the UK. This describes the various equilibria between competition, stress and disturbance in vegetation. The point shown for each species indicates the position of its maximum percentage occurrence in a matrix of vegetation types classified according to the strategies of the component species in Grime et al. (1988). Where points are not provided the data for the species concerned are insufficiently clear. The small diagram shows the strategic range of herbs. (From Packham and Cohn, 1990. Arboricultural Journal 14.)

Figure 4.2 C-S-R ordination diagram of woodland field layer species in the UK. This describes the various equilibria between competition, stress and disturbance in vegetation. The point shown for each species indicates the position of its maximum percentage occurrence in a matrix of vegetation types classified according to the strategies of the component species in Grime et al. (1988). Where points are not provided the data for the species concerned are insufficiently clear. The small diagram shows the strategic range of herbs. (From Packham and Cohn, 1990. Arboricultural Journal 14.)

S-C

Cv

Calluna vulgaris

Heather

C

Ca

Chamaenerion angustifolium Rosebay willowherb

(C-R)

Circaea lutetiana

Enchanter's nightshade

(C-R to C-S-R)

Digitalis purpurea

Foxglove

S-C

Df

Dryopteris filix-mas

Male fern

C

Hm

Holcus mollis

Creeping soft-grass

S to C-S-R

Hn

Hyacinthoides non-scripta

Bluebell

S to S-C

Lg

Lamiastrum galeobdolon

Yellow archangel

R to C-R

V

Lc

Lapsana communis

Nipplewort

S to S-C

Mu

Melica uniflora

Wood melick

S-C

Mp

Mercurialis perennis

Dog's mercury

S to C-S-R

Oa

Oxalis acetosella

Wood-sorrel

C-R to C-S-R

o

Pt

Poa trivialis

Rough meadow-grass

C

Pa

Pteridium aquilinum

Bracken

R to S-R

Rf

Ranunculus ficaria

Lesser celandine

C-R

o

Rr

Ranunculus repens

Creeping buttercup

S-C

Rfr

Rubus fruticosus agg.

Bramble

(S)

Sanicula europaea

Sanicle

(C-S-R)

Stellaria holostea

Greater stitchwort

R

Sm

Stellaria media

Common chickweed

C-S-R

n

Ts

Teucrium scorodonia

Wood sage

C

Ud

Urtica dioica

Common nettle

S

Vr

Viola riviniana

Common dog violet

1200 plots sampled in 1991 were divided into 930 that were bearing forest when the first aerial survey of Prince Edward Island was made in 1935, and 270 that were not. Those plots which had been clear of trees in 1935 were dominated by softwoods, primarily white spruce Piceaglauca. These were in the regeneration stage, carried a relatively low amount of timber, and were typical of an 'old-field' origin. Other characteristic species were early successional trees and shrubs, 'weedy' herbs and shade-tolerant mosses. In contrast, plots which were afforested in 1935 were typically dominated by hardwoods and positively associated with partial-cutting/thinning, though there was a marked gradation towards the softwood-dominated group in those plots which had by 1935 regenerated after previous clearance. Thus, the forest stands divide neatly into softwoods (conifers) and hardwoods, dependent upon their recent history.

Conifers encountered in the survey included balsam fir Abies balsamea, tamarack Larix laricina, white, black, and red spruce (Picea glauca, P. mariana and P. rubens), jack, red and white pine (Pinus banksiana, P. resinosa and P. strobus), eastern white cedar Thuja occidentalis and eastern hemlock Tsuga canadensis. There were 15 broadleaved hardwoods: red, striped and sugar maple (Acer rubrum, A. pensylvanicum and A. saccharum), yellow, white and grey birch (Betula alleghaniensis, B. papyrifera and B. populifolia), American beech Fagus grandifolia, white and black ash (Fraxinus americana and F. nigra), ironwood Ostrya virginiana, largetooth and trembling aspen (Populus grandidentata and P. tremuloides), red oak Quercus rubra, American mountain ash Sorbus americana and American elm Ulmus americana.

Sobey (1995a, b) employed two statistical methods based on reciprocal averaging (TWINSPAN and DECORANA, both devised by Hill, 1979a, b), to handle the very large amounts of multivariate data generated by this survey, which provided much useful information regarding successional processes and the influence of environmental variation. Figure 4.3 is a TWINSPAN classification of 1127 plots recorded in 1991. The first division was almost equal. Group 0, on the left side consisted of plots whose species were associated with broadleaved woodlands (hardwoods), those of Group 1 on the right were mainly characteristic of woodlands dominated by conifers (softwoods). Wood fern Dryopteris spinulosa was an indicator for the negative Group 0, while Schreber's moss Pleurozium schreberi, wild raisin Vibernum cassinoides, and blueberry Vaccinium angustifolium acted as indicators for the positive Group 1. An indicator species is characteristic of a particular habitat and in this case can be reliably used to decide or indicate which type of forest is being considered (see also Hill et al., 1975).

The 595 hardwood plots of Group 0 then split unequally into the 165 of Group 00, that included the 156 plots belonging to the more nutrient-rich and wetter broadleaved woodlands of Group 0011 (GP3 in Fig. 4.3) (delineated at the fourth division), and the 430 of Group 01. This was then split into the 79 plots of Groups 0100 (4) and 0101 (5), and the 351 plots of Groups 0110 (6) and 0111 (7). Group 1 was similarly divided into eight smaller groups as a result of three further divisions. Terms for the Stand Groups used in the figure follow the classical notation, those in bold type were used for the 15 end groups which Sobey finally employed. Of these, Groups 1, 2, 4 and 15 were so small as to be insignificant, the 21 plots of the rather oddly placed Group 11 were of a species-poor hardwood-related type, and the 12 plots of Group 12 were wet transition plot types. After a detailed review of the floristic and environmental features of all the plots involved, Sobey considered that major floral variations

0010

0011

156 GP 3

0100

0101

77 GP 5

0110 0111

196 155 GP 6 GP 7

1000

165 GP 8

1010

135 GP 10

1011

21 GP 11

1100

12 GP 12

1101

29 GP 13

1110

51 GP 14

1111

GP 15

1001

GP 9

0010

0011

156 GP 3

0100

0101

77 GP 5

0110 0111

196 155 GP 6 GP 7

1000

165 GP 8

1010

135 GP 10

1011

21 GP 11

1100

12 GP 12

1101

29 GP 13

1110

51 GP 14

1111

GP 15

1001

GP 9

156 428 283 135 80

Figure 4.3 Two-way indicator species analysis (TWINSPAN) of 1127 of the circular ground flora plots (each with an area of 4 m2) assessed in the 1991 Prince Edward Island Inventory. This dendrogram shows the classical notation to the third level of division; plot allocations for the fourth order division are shown together with the final Group Numbers used by the original author, who considered the major floral variations to be best considered under the five categories a, b, c, d and e. (After Sobey, 1995a. Report to the Prince Edward Island, Forestry Division.)

Scientific names of indicator species are given below.

within the remaining 1082 quadrats were best considered under the five categories of:

(a) Wet rich deciduous woodland (Group Number 3, of which 3B - not distinguished in Fig. 4.3 - had a ground flora associated with wetter and richer soils than 3A). Indicator species for the 71 quadrats of 3 A were starflower Trientalis borealis and lady fern Athyrium filix-femina: those for the 85 of 3B were sensitive fern Onoclea sensibilis, the moss Mnium sp., cinnamon fern Osmunda cinnamomea and jewelweed Impatiens capensis.

Caption for Figure 4.3 (cont.)

The five-letter short forms on the figure consist of the first three letters of the generic name and the first two letters of the specific name. Numbers following an indicator imply that a certain percentage cover has been exceeded.

ACHMI Achillea millefolium Yarrow

ARANU Aralia nudicaulis Wild sarsaparilla ASTAC Aster acuminatus Whorled wood aster CLIBO Clintonia borealis Bluebead lily

CORCA Cornus canadensis

Bunchberry DENPU Dennstaedtia punctiloba

Hay-scented fern DICSC Dicranum scoparium Broom moss

DRYSP Dryopteris spinulosa Wood fern

FRAVI Fragaria virginiana

Strawberry HIESC Hieracium scabrum Rough hawkweed HYLSP Hylocomium splendens

Mountain fern-moss IMPCA Impatiens capensis

Jewelweed KALAN Kalmia angustifolia Sheep-laurel

LEDGR Ledum groenlandicum

Labrador-tea LYCAN Lycopodium annotinum

Bristly clubmoss LYCOB Lycopodium obscurum

Ground pine MAICA Maianthemum canadense Wild lily-of-the-valley

MYRPE Myrica pensylvanica

Bayberry NEMMU Nemopanthus mucronata

False holly ONOSE Onoclea sensibilis Sensitive fern

OSMCI Osmunda cinnamomea

Cinnamon fern PLESC Pleurozium schreberi

Schreber's moss PTEAQ Pteridium aquilinum Bracken

PTIPU Ptilidium pulcherrimum

Ptilidium moss RUBPU Rubus pubescens Dewberry

SMITR Smilacina trifolia Three-leaved false solomon's seal SPHAG Sphagnum Bogmoss species

TAXCA Taxus canadensis Yew

TRIBO Trientalis borealis Starflower

TRIUN Trillium undulatum Painted trillium

VACAN Vaccinium angustifolium

Blueberry VIBCA Viburnum cassinoides Wild raisin

VICCR Vicia cracca Tufted vetch

VIOPA Viola pallens Small white violet

(b) Three different ground flora community variants associated with tolerant hardwood forests on dry upland soils (Groups 5, 6 and 7).

(c) Cut-over or disturbed conifer-dominated forest on dryish soils (Group 8 with a tree canopy dominated by softwoods, especially balsam fir, and Group 9 which is another similar successional type but on drier soils).

(d) Old field white spruce woods (Group 10).

(e) Black spruce 'bog' (Group 13) and 'heath' (Group 14) forests.

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Responses

  • eugenio sal
    Why does the creeping buttercup show little csr strategy?
    7 years ago
  • Furuta
    How stress tolerator plants longer their life span?
    3 months ago
  • SIRJA
    How stress tolerators are characterized by slow growth?
    3 months ago

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