PLANT ECOLOGY
Definition
1.
2.
3.
4.
GENERAL: The study of plants in relation to their environment.
HAEKEL, 1869: The study of the reciprocal relationships between plants and their environment.
ODUM, 1963: The study of the structure and function of nature.
LAMBERT, 1964: What plants live where and why?
 The “what” usually requires at least some element of plant identification, especially where community
studies are concerned
 The “where”, to be useful in a botanical context, generally involves at least a crude assessment of such
physical features of the habitat as might conceivably affect the growth of plants, and
 The “why” is an attempt to erect a hypothesis to explain the facts observed, and to bring them into a single
system.
Brief history
Ecology is derived from Greek OIKOS = “home”, and LOGIA = “the study of”
Autecology and Synecology
Greek AUTOS = self, and SYN = together. Thus autecology is the study of a single species and Synecology is the
study of communities (multi=species mixes).
Introduction
Terrestrial plant –
i) Rooted in one spot
ii) Originating from seed “randomly” distributed
iii) At risk from a whole range of environmental hazards.
From germination the developing plant must cope with –
Soil – chemical and physical conditions
Climatic and microclimatic effects
Pathogens
Herbivores
Seed carries a specific genotype which dictates its responses to the complexity of environmental hazards. If genetic scope for
phenotypic plastic response is exceeded by one or more pressures the germination or establishment will fail. Another seed of
the same species, but with slightly different gene recombinations may survive. So within a short time after the arrival of the
seed (a “random” event) a degree of ecological pattern is generated – i.e. one individual survives, another does not. As the
plants grow, competition begins. By the time the first seeds have developed to adult plants, an interacting web of
environmental and biological limitations has either dictated survival or at least profoundly influenced phenotypic development.
At any time the ecosystem is a legacy of previous events, its commonest species, their morphology and physiology, and the
soil – all these preserve evidence of past development.
As an ecosystem grows and stabilises the wastage rate is enormous – for every plant that survives to reproduce, thousands
die.
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
Failure to germinate
Attack by herbivore or pathogen
Killing by drought or frost
Competitive exclusion – or some other factor ***
It is all about the survival of the fittest, in a complex species mix. This complexity, coupled with the environment interactions
produce the ecological associations so easily recognised by the fieldworker – some as discrete communities, some as a
continuum.
1
How to Recognise Plant Communities
Because groups of plants form communities we can assume that the plants in a community have some influence upon one
another and/or that they have something in common with their environment. This implied INTERDEPENDENCE, that the whole
is greater than the sum of the parts, implies that communities are integrated entities. For practical purposes plant
communities are considered as sub-divisions of vegetation cover. For example wherever the cover shows more-or-less obvious
spatial changes, one may distinguish a different community.
These changes may be due to and/or correspond to spatial changes in the ENVIRONMENT and are manifest as:



Changes in species composition
Changes in spacing and height of plants
Changes in growth form properties
Whatever vegetation parameters are involved in causing such changes – form part of the
1) Definition
2) Description, and
3) Interpretation of a community.
Depending on the properties of the vegetation these changes may be relatively




Abrupt
Transitional
Gradual and/or
Diffuse
Therefore, plant communities may be self-evident or not: and this will depend on the experience of the observer/investigator.
Recognition and definition of plant communities is a skill that can be learned.
The plant communities comprise not only the larger (more obvious) plants (mostly Angiosperms) – but also the

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Mosses
Lichens
Fungi
Algae and
Microflora
However we will stick to angiosperms, gymnosperms and ferns – as specialised, specific knowledge is required to deal with
the lower plants, and the higher plants are the major producers in terrestrial communities.
Some plant ecologists do not believe that plants form discrete units (communities) and that if there is an association this is
due to a chance meeting of species whose tolerance ranges (ecological amplitudes) overlap. Therefore, interdependence is
not assumed – rather it is a continuum.
Today there is still much debate on the utility of the community concept, but this debate rages in those countries where the
flora and the vegetation is already well known.
PLANT COMMUNITY HYPOTHESES
One’s theoretical view of the nature of a plant community influences one’s methods and objectives.
1. Organism Analogies and their Consequences
(i)
The Holistic Viewpoint:
The plant community is viewed as a whole – analogous to an organism. Clements compared succession from bog to forest as
analogous to an insect’s life cycle. Every climax situation can be reproduced by repeating the same pattern of succession.
Some differences: “Death” of a climate cannot be compared to the death of an organism – rather one species population is
replaced by another. Also in growth one population replaces another, they aren’t retained from beginning to end.
Tansley preferred the term quasi-organism and emphasised that communities behave in many respects as wholes, and should
therefore be studied as wholes – led to his ecosystem concept.
2
(ii)
The Systematic Viewpoint:
Braun-Blanquet classified communities in a similar fashion to taxonomic classification of organisms. He compared a plant
community to a species – being the basic unit of classification. He overlooked that species are genetically related - not so for
plant communities – but rather on structural/composition similarity.
(iii)
The Individualistic Viewpoint:
Gleason claimed that while the plant community depends for its existence on the selective forces of its particular environment
and the surrounding vegetation, the environment changes constantly in space and time. No two communities can be
considered alike or closely related.
It is true that this is so in absolute terms. However, on a relative basis one can distinguish between greater and lesser
similarities and differences. No two individuals of a species are exactly identical:
Background to Community Ecology
Environmental Factors
I
Physical or abiotic factors
A
SOIL
a) Provides the medium for seeds to germinate
b) Impacts on the size and erectness of plant
c) Vegetative vigour is influenced by nutrient content, etc.
d) Impacts on the woodiness of stem
e) Affects the depth of root system
f)
Can influence the amount of pubescence
g) Has an impact on the susceptibility to drought, frosts and parasites
h) Affects the number of flowers per plant
i)
Influences the time of flowering, etc.
Soil Defined
A very broad ecologically justifiable definition is:
- “any part of the earth’s crust in which plants are anchored”
or more conservatively is:
- “the weathered superficial layer of the earth’s crust in which is mingled living organisms and products
of their decay”.
Therefore soil = the parent material + the organic material + the water + the gasses
a) Parent material: formed mechanically or chemically. May be residual or transported by gravity (colluvial), by water
(alluvial), by glaciers (glacial), by wind (aeolian), by volcanic, and/or by marine.
b) Textural classification particle size affects

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

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root penetration
fertility
structure (peds)
aeration
temperature
3
c) Organic Increment – radically changes primary framework, derived from dead roots and soil organisms, as well as
being incorporated from above.
d) Soil organisms are principally:
“PLANTS”
ANIMALS
Bacteria
Streptomycetes
Algae
Fungi
Plant parts such as roots, rhizoids and rhizomes
Protozoa
Nematodes
Mites
Insects (esp. ants and beetles)
Earthworms
Burrowing vertebrates
Their chief role is decay and nutrient cycling (e.g. Nitrogen and Carbon cycles), and their secondary role includes soil mixing,
aeration, improving aggregates, etc.
e) Soil Moisture and Air – pore space usually 40-60% of total volume.
f)
Soil Solutes – of the 15 or more elements essential for plant nutrition all but C, H and O are derived exclusively
from the soil.
g) Soil Development – mature soils have/are in dynamic equilibrium, with a good profile (of various horizons).
h) Soil Colour – SA classification.
B. WATER
1.
Importance of water to plants
2.
Atmospheric moisture
It is of paramount importance physiologically – universal solvent, transportation, turgidity, etc. and for
photosynthesis.
a) Humidity: usually expressed as relative humidity – dew point, diurnal fluctuations and gradients.
b) Cloud and fog: consists of minute water droplets. Atmospheric water’s importance is that it affects the intensity of
solar radiation, affects evaporation and transpiration, a source of moisture directly and indirectly.
3.
C
Precipitation: cyclonic or frontal, orographic, convection
The importance of the various forms of precipitation vary from direct to indirect effects – rain, snow, hail ON
plants, soil, seasons, etc.
TEMPERATURE – absolute max & min critical (e.g. frost occurrence)
Little or no biological activity below 0oC or above 50oC – a range bounded at the lower end by the immobilization
of water, and at the upper end by the heat destruction of vital proteins.
1.
Temperature variation
Longitudinally, latitudinal, diurnal, seasonal, colour and composition of surfaces, plant cover, aspect, and angle of
slope.
2.
Effect of temperature
Physiological (e.g. cold injury – chlorosis), transpiration, phenology, mechanical (lesions, frost heaving), etc.
4
D
LIGHT
This is the basic source of energy (except for chemosynthetic bacteria). Intensity, duration and quality variable,
amount varies with atmospheric water, suspended particles, vegetation layers, topography (aspect and slope),
latitude. Light affects physiology, morphology, germination, reproduction, and photoperiodism.
E
WIND
Air composition, except for water vapour, fairly constant at 79% N 2, 21% 02 and 0,03% CO2 (and going up!), plus
various quantities of particulate matter (e.g. quartz granules as little chisels, sodium chloride, etc.) and other
gasses. Pollution and temperature inversions. Wind as a mechanical agent – erosion, corrosion, breakage,
dispersal, pollination.
Wind as a physiological agent – evaporation, salt spray, etc.
II Biotic factors
A
HERBIVORY
Grazing and browsing: many ecological problems are involved with management of natural or artificial plant
growth – food preference, impact (direct and indirect).
B
SEED AND SEEDLING DAMAGE
C
POLLINATION AND ENERGY
D
DISPERSAL
E
GROWTH FORMS/STRUCTURE AND FUNCTION
F
NITROGEN FIXATION
Nodules (Leguminosae, Ericaceae, Myrica, Podocarpus, Eucephlartos, etc.).
G
ALLELOPATHY
Chemical toxins (phenolic compounds, aldehydes, glycosides and turpenes).
H
COMPETITION
I
REPRODUCTIVE ABILITY (vegetative and sexual).
J
FIRE
Except in very wet, very cold and very dry regions; fire has always been an important factor in the terrestrial
environment. Charred fossils show that lightning-started fires have periodically ravaged land vegetation, and
modern observations support this. However, primitive man also used fire, and as people became more numerous
fires became more unseasonal. Today the role of fire is complex and comparatively little studied and understood.
 Kinds of fire: ground fires (lethal), surface fires (rapid, sub-lethal), crown fires (lethal to sub-lethal).
 Fire adaptations: particularly apparent where climate is seasonal. Herbaceous perennial, germination, rapid
growth and development, fire resistance (foliage, bark), adventitious roots and lignotubers, serotinous “cones”.
 Indirect effects of fire: removal of competition, injury and parasitism, environmental change, stimulation.
 Value of burning: so many variables that generalisations are hazardous – some hold that they are devastatingly
detrimental, others that they are beneficial.
Fire can favour economically important plants, can improve “quality” of vegetation, facilitate the removal of debris.
CONCLUSIONS
When ecology was in its infancy as a science, a disgruntled wag characterised it as “the painful elaboration of the obvious”.
However, what seems obvious superficially, involves complexity. For example, cessation of cambial activity in temperate zone
trees is not only related to temperature, but also to a cycle of physiological activity. Thus environmental factors result in
adaptation and evolution.
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Community Structure and Composition
The first objective of the plant synecologist is to analyse and record the STRUCTURE, COMPOSITION and DISTRIBUTION of
plant communities. Then follows a search for causes, experimentation and interpretations based upon a firm foundation.
Historically plant ecological studies were simple observation and description, today it is essentially quantitative. Basic
information to be recorded is:
1. Environmental – macro- and micro2. Floristic composition
3. Physiognomy
Impossible to record all the details so sample. Random and/or non-random.
PHYSIOGNOMY
This is the study of vegetation form and structure. The very existence of any plant species is governed by the adaptation of its
form to function efficiently in response to its total environment. Thus all plant species are functionally adapted, and some
morphological features are more obvious than others (features such as periodicity, seed dispersal, etc. are not always readily
observable as may only be seasonal).
Where to draw the line, to demarcate functional characteristics which greatly influence the physiognomic appearance and
those adaptations that do not make a considerable contribution to the success of the species, is sometimes a matter of
opinion. Thus the solution to the problem is far from obvious and there have been many attempts to create universal systems,
that of Raunkiaer is by far the best and most widely used (though it was evolved for the Temperate Zone of the Northern
Hemisphere where it works extremely well, but it needs to be adapted for South Africa conditions).
1. Life Form – best known and most useful is that of Raunkiaer; based on height of perenniating buds, as an indication of
total climate:
a)
Phanerophytes: buds on aerial shoots at least 0.25 to 0 50 m above the ground. Evergreen/deciduous.
Nanophanerophytes (less than 2 m high)
shrubs/bushes
Microphanerophytes (2 to 8 m high)
bushes/trees
Mesophanerophytes (8 to 30 m high)
trees
Megaphanerophytes: (more than 30 m high)
trees
b)
Chamaephytes: buds close to the ground.
Suffruticose – more-or-less erect, die-back in non-growing season
Passive – bud from horizontal stems that were upright
Active – all vegetative buds from horizontal stems
Cushion plants – compact “active”.
c)
Hemicryptophytes: buds at ground level, seasonal die-back of above ground parts may occur
d)
Cryptophytes or Geophytes: buds below ground level
e)
Therophytes: annuals.
Using life forms it is possible to compare the vegetation of one part of the world with another, as the life form is assumed to
have evolved in direct response to climate/total environmental pressures of the site. The proportion of the various life forms in
a geographical region should give a good indication of its climatic/environmental regime.
2. Leaf size – this also follows Raunkiaer who argued that leaf size is primarily a measure of environmental response. This
is complicated locally by sclerophylly (it has also been demonstrated that leaf size is a function of soil phosphorus levels).
a)
Leptophyll (up to 25mm2)
b)
Nanophyll (25-225 mm2)
c)
Microphyll (225-2,025 mm2)
d)
Mesophyll (2,025-18,225 mm2)
e)
Macrophyll (18,225-164,025 mm2)
f)
Magaphyll (>?164,025 mm2)
To suit the conditions of the CFR I have introduced an additional size class that is much smaller than Leptophyll:
Picophyll (<10mm2) thus Leptophyll has to be adjusted to 10-25 mm2.
The final physiognomic factors that need to be considered are:
STRATIFICATION where I suggest you follow the “Structural formations in the Fynbos Biome” (practical handout), and
COVER (where I suggest you use the BB/ZM scale).
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BIOTIC STRUCTURE IN SPACE – PLANT FACTORS
The structural features of vegetation are basically those characteristics related to the spatial distribution of phytomass.
Structure may be defined by three components:
1. Vertical structure – stratification (most work in forest), profiles useful across gradients and to illustrate
structure pictorially, etc.
2. Horizontal structure – the spatial distribution of individuals of the species i.e. pattern. Scale depends on overall
morphology and size of plants:
a) closed vegetation – canopies touch or overlap
b) open vegetation – canopies do not form a closed layer but are not more than two diameters apart
c) sparse vegetation – canopies are more than two diameters apart and the substrate dominates the
landscape.
PATTERN and types of pattern
1) random – almost theoretical
2) non-random or patchy distribution (what happens in Nature)
Causal factors of pattern
1) environmental requirements
2) environmental modification
3) plant “toxins”
4) competition
5) morphological.
More-or-less all vegetation contains pattern at one or more scales. Perhaps the random colonisation of a homogenous bare
area is an exception, but morphological pattern will become evident as the plants grow, i.e. the intrinsic properties of the
plants interact to produce pattern. Accordingly we are forced to the conclusion that pattern will always be present in
vegetation except in a few rare and extreme cases.
I
Subjective methods of vegetation data capture
COVER-ABUNDANCE - scale of Braun-Blanquet (BB)
r
+
1
2
3
4
5
(+)
very rare with a negligible cover (usually just a single individual)
present but not abundant and with a small cover value (less than 1% of the quadrat area)
numerous but covering less than 1% of the quadrat area or not so abundant but covering between 1
and 5%
very numerous and covering less than 5%, or covering between 5 and 25% of the quadrat area
independent of abundance
covering between 25 and 50% of the quadrat area independent of abundance
covering between 50 and 75% of the quadrat area independent of abundance
covering between 75 and 100% of the quadrat area independent of abundance
outside the plot but in the stand
Disadvantages



some species in different strata lumped
differences between observers
difficulty in deciding in marginal cases
Advantages


quick and accurate (experience)
biological importance built-in
As shown above 3, 4 and 5 only refer to cover, whereas the other units can refer to cover and abundance.
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A modification of unit 2 which has been generally accepted is:
 2m very numerous, covering less than 5%
 2a covering between 5 and 12%, independent of abundance
 2b covering between 13 and 25%, independent of abundance.
II Objective assessment of vegetation data capture
(i)
DENSITY – the number of individuals per unit area
Disadvantages

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

what is an individual?
time consuming
influenced by pattern
equal weight to biologically different species
Advantages:


accurate count (especially animals)
allows direct comparisons in space and time
(ii)
COVER – that portion of the ground occupied by the perpendicular projection on to it of the aerial
part(s) of the plant(s) under consideration. Can be obtained visually, or can be measured (point, line,
optical). Cover may exceed 100% in stratified communities.
(iii)
BASAL AREA (COVER) – similar measure to cover but more specifically “the area occupied by a
species at ground level” and used in tussocked grassland, or in forest measured as diameter at breast
height, i. e. dbh at about 1.5 m.
(iv)
YIELD – the determination of the quantity of material produced per unit area. Mainly used in applied
ecology for determination of:
 standing crop
 net production
 sustainable yield
(iv)
FREQUENCY – is a measure of the chance of finding a species with one throw of a quadrat in a given
area. Can record either SHOOT FREQUENCY or ROOT FREQUENCY. Main advantage of frequency is
the ease and rapidity with which an area can be sampled. The figure obtained finally for frequency
depends on a number of very important factors:



quadrat size
plant size
pattern
DATA CAPTURE
Sampling aims
1. to gain an overall impression of the vegetation
2. to investigate the variation within an area
3. to correlate vegetation differences with habitat
Some problems
a) choice of site
b) choice of method
c) choice of parameter
d) distribution of samples (random, regular or stratified)
e) number of samples
f) size of samples
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BACKGROUND INFORMATION
Need to have some knowledge of the history, physiography, climate and vegetation of the region as a whole. Need to study
available literature, maps and aerial photographs.
PLANNING TABLE
What are the most important factors determining the vegetation/pattern?
Geology
Granite
Sandstone
Restio
Erica
Restio
Erica
Protea
Restio
Erica
Restio
Quartzite
Climate
Oceanic
Inland
Protea
Restio
Erica
Protea
Protea
METHODS
1. Phytosociological: essentially that of BB/ZM
a) subjective/objective choice of stands
b) size of sample
c) environmental data
d) floristic list
e) cover-abundance
2. Quadrats
There are various types of quadrates:
a) chart quadrats
b) point quadrats
c) square quadrats
d) rectangular quadrats
e) circular quadrats
3. Transects and profiles
4. Line intercept
Problems (and solutions)
Size
Distribution (patterns)
Gradients
ABIOTIC STRUCTURE IN SPACE - COMMUNITIES & ENVIRONMENTS
Recognition of communities in the field is generally easy, but to define specific and general characteristics to study and
understanding is not always that simple.
Definition: a community is an aggregation of living organisms having mutual relationships among themselves and to their
environment. A STAND is a concrete example of a community which is usually an abstract, synthesized concept. Stand can be
big or small, and can have stands within stands.
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Mutual relationships among organisms: two basically:
Competition: two basic manifestations; for light = stratification (vertical pattern), for food = horizontal pattern.
Dependence: subordinate species dependant on dominants; consumers or producers.
Mutual relationship to environments: plants adapted therefore successful. Vegetation of a site a produce of the TOTAL
ENVIRONMENT (macro-, micro-climate + biotics) so plants basis for site evaluation.
Environmental gradients: regional or local – due to species response to environment. In practise observation agree with the
principle of species individualism, with gradation down the environmental gradient (if such exists?). further reading in
Whittaker p. 40 to 51.
STRUCTURE IN TIME - SUCCESSION & CLIMAX
Communities modify, change and regulate their physical environment. An important consequence of this biological regulation
is ecological succession, which is:
1. directional and therefore predictable
2. a result of modification of the physical environment by the community
3. culminates in a stable ecosystem
The physical environment determines the pattern of succession, but does not cause it – succession is community controlled.
Some basic terms
SERAL = developmental stages
CLIMAX = steady state
SERE = entire gradient of communities
Primary and Secondary Succession – Types include HYDROSERE (= in water), XEROSERE (= dry sites), PSAMMOSERE (= on
sand), and LITHOSERE (= on rock).
Stability and climax: can communities be complete self-perpetuated and permanent, even assuming no change in regional
climate?
The significance of ecological succession: the mature community with its greater diversity, larger organic structure, and
balanced energy flow, is able to be a better buffer to the physical environment than a younger community, which may be
more productive (concept of HOMEOSTASIS). Hence the survival value in evolution (?).
Climax
1.
2.
3.
4.
concepts: static or dynamic.
Monoclimax (Clements) – complex terminology
Polyclimax (Tansley)
Continuum
Cyclical climax
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VEGETATION SAMPLING
Any detailed vegetation study is based on the description and investigation of plant communities – that must first be
recognised in the field. The communities are then sampled through analysis of representative stands. So one must:
1.
2.
3.
4.
Recognise stands
Select samples within the stand
Decide on shape and size (type) of sample
Decide what parameter to record.
This will depend on viewpoint, character of vegetation, objectives of the study and time available.
The importance of vegetation sampling is that all subsequent treatment of the data and
conclusions that may be drawn, depend upon the initial selection and characteristics of
the samples.
SUBJECTIVE VERSUS OBJECTIVE SELECTION OF SAMPLES
1. Subjective with preconceived bias – reflects what one wishes
2. Subjective without preconceived bias
3. Objective, according to chance
a. stratified
b. random
RECOGNITION OF ENTITIES OR COMMUNITIES
1) General (Gleason) – forest, scrub, grassland: requires little reconnaissance/familiarisation
2) Specific (B.B.) – recurring similar plant assemblages grouped and then sampled – this requires thorough familiarisation
SAMPLE APPROACHES
The reasons for using a particular sampling method are primarily a matter of viewpoint and purpose. In aiming for a detailed
floristic vegetation description, two major approaches can be distinguished:
1.Relevé analysis for classification – aimed at grouping individual stands into categories. Stands closely similar to one another
will form one class separated from others.
2. Ordination analysis for continuum studies – this portrays the individuality of each stand. Achieved by demonstrating the
similarity or dissimilarity of all stands with one another in the form of geometric models.
CLASSIFICATION
Typified by relevé method and complete plant lists.
Homogenous stands/sub-sampled (both in spp., structure
and environment).
Description through classification the aim – communities
usually visually distinguished.
High sampling intensity looking at recurring patterns which
involves replicates.
Species presence or absence considered more important
than minor variations in quantity. In this respect method
largely qualitative.
ORDINATION
Some sampling method and incomplete plant lists or other
quantitative data.
Homogenous with respect to dominants only – communities
usually much large and heterogeneous.
Description through ordination. Sampling randomly,
systematically, subjectively along gradients or continua.
Sampling intensity may be low. Replicate sampling not
emphasised, and demonstration or recurring patterns largely
a matter of chance.
Species presence or absence considered of less importance
than minor variations in quantity. Method largely
quantitative.
Yet, both methods can be complementary in their treatment of sample data. Stands sampled for classification can be
subjected to Ordination and vice versa (as long as floristically complete).
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THE BRAUN-BLANQUET METHOD SUMMARISED
OUTDOORS
(sampling & recording)
INDOORS
(synthetic)
ABSTRACTION
&
FIELD CHECK
STAND A
( = phytocoensis)
FINAL
PHYTOSOCIOLIGICAL
TABLE
STAND B
STAND C
CLASSIFICATION
□
ecological
□ □
SAMPLE
confirmation
Floristics Cover-ab.
Environ.
RELEVE
Structure
etc.
VEGETATION
FIRST
PHYTOSOCIOLOGICAL
TABLE
Re-arrangement
RAW TABLE
TABULATION
B.B. BASED ON THE COMMUNITY UNIT THEORY – WHICH POSTULATES THAT VEGETATION CONSISTS OF NATURAL
ENTITIES THAT GENERALLY CONTACT EACH OTHER ALONG NARROW BOUNDARIES.
COMMUNITY SAMPLING
1.
THE RELEVÉ METHOD
Following a thorough reconnaissance and familiarisation of the vegetation – which is so essential before sampling –
communities are established (recognised). Subsequent sampling and data collection will merely add detail, assist in greater
understanding, give quantitative data and allow hierarchical classification (all this irrespective of one’s choice of quantitative
methods). Remember that some changes or additional information may be necessary as one increases knowledge during the
investigation. Therefore, one’s approach should be flexible enough to allow for such changes.
Homogeneity requirements: A relevé should be:
(i)
large enough to contain all species belonging to the community
(ii)
of uniform habitat (as far as can be determined)
(iii)
the plant cover should be as homogeneous as possible.
The requirement for homogeneity of the relevé rests on the premise that the vegetation parameters recorded should result in
a meaningful average (edge effect not important).
In the field one is first guided by physiognomy and habitat, and then floristics.
Minimal sample area – determination of optimum sample size: the “minimum area” is defined as the smallest area on which
the species composition of the community is adequately represented. In many South African situations this area has been
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impossible to define because sufficiently large homogeneous stands are somewhat rare. This has let Werger and others to
propose the use of an optimum sample size – which is one not too big or too small but sufficient to capture 70-80% of the
diversity of the community. Some ideas of sample size for S.A. vegetation using optimum sample sizes are:
Forest
Fynbos
Grassveld
Bushveld
Karoo
Desert
_____________
_____________
_____________
_____________
_____________
_____________
Cape 1002m
Dry
Short
Closed
Natal and Transvaal
Wet
Tall
2002m to 4002m
Minimal area can only be determined in a community that is relatively homogeneous. The usual method is nested or randomnested quadrates and the subsequent construction of a species area curve (such graphical presentation is influenced by the
ratio of the X and Y axes).
Sample size ultimately depends on species diversity and size of plants . in the filed when collecting relevé data one records
those species present in the stand but outside the sample as (+). A rule of thumb method is that if one has a large number of
species recorded as (+), then your sample size is too small. Samples are kept as small as possible to keep the workload to a
minimum.
After establishing how large a relevé should be the next step is to list all the species present (a good floristic knowledge and
careful collection of species is the answer). In S.A. most workers have been content to record only those species that are
perennially recognisable (practical but with disadvantages). Following the floristic data one collects various habitat data, notes
the date and exact location of the relevé for future field checks….for further details see sample check sheets.
With the floristic data cover abundance, sociability, phonological, structural and another data is/may be collected. (Life form,
leaf size, branching, habitat, etc.). The amount of data collected depends on the type of survey.
Estimating species quantities:
In earlier times each worker had his/her own scale – so results were difficult to compare. Braun-Blanquet made a major
contribution in selecting, simplifying, and modifying a system that is convincingly simple and almost universally acceptable. For
reasons of comparability one should not unnecessarily deviate from this method even if one does not wish to process the data
further according to the B.B. table technique.
The B.B. Cover-abundance scale is:
5 4 3 2 1 + r.
Values 5 4 3 and 2 refer to cove r only (P.C.C.), while 1 + and r are primarily estimate of abundance.
Unequal class intervals biologically weighted: Brian Walker’s…????
2. QUANTITATIVE VEGETATION PARAMETERS
The more important measurable quantities are:
(i)
Density
(ii)
Frequency
(iii)
Cover
(iv)
Biomass
3. PLOTLESS SAMPLING TECHNIQUES
For
(i)
Frequency (wheel point for example)
(ii)
Cover
( “
“
“
“
and line intercept)
(iii)
Density – by measuring the distance between individuals
VEGETATION CLASSIFICATION BY TABULAR METHODS
In classifications based on floristic criteria (species composition and their quantitative variations) the problem of separating
vegetation into units can be studied and resolved after the species lists of all sample stands are transferred onto a single
table. Such a table is conveniently referred to as a synthesis table. This synthesis table, in addition to being an aid to
classification, often reveals information that was not realised during the field work. Tabulation can be done in a number of
ways – all lead to an organisation of tabular data to aid interpretation. In doing this we isolate groups of species that show
similar distributions and place similar relevés side by side. This is conveniently done in a number of phases:
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1. CONSTRUCTION OF A RAW TABLE
Comparing data by merely placing filed data forms side by side becomes impracticable when one has more than 5 or 6.
Therefore practical to assemble all relevés into a table (conveniently done on graph paper). Before transferring data onto a
table the individual relevés may be sorted according to any viewpoint: e.g. increasing altitude or some other site factor,
presence or absence of species, number of species per relevé, etc. in the raw table one vertical column is allotted for each
relevé. The relevé number and total number of species per relevé is entered first. As additional relevés are added the number
of new species increases until all in the community are entered.
2. CALCULATING DEGREE OF CONSTANCY
On first impression the raw table is difficult to comprehend. Frequently and rarely occurring species are in irregular sequence
(because the species were written down in order of their chance appearance on the stand analysis sheets).
The next step involves sorting of species according to their degree of constancy – by calculating constancy and then rearranging (transcribing) a new “constancy” table which greatly facilitates the comparison of individual relevés and it enables
one to more easily distinguish the similar from the similar relevés.
3 RECOGNISING DIFFERENTIAL SPECIES
For grouping the relevé series into vegetation classes neither the species with a high constancy nor those with a low
constancy are useful. (High = more-or-less characteristic of whole type. Low = more-or-less accidental). The species with an
intermediate range of constancy are usually the most useful to distinguish groups – so in the next step we tend to ignore
constant and rare species and concentrate on the intermediates: (Present in-between 10-60% of releves).
What must be looked for are species that occur together in several relevé – as one wants to distinguish between such
mutually associated species and others that seem to avoid association. If one finds two or more such groups one has to
determine differential species groups. The association tendency of certain species may have already been noted in the field.
Moreover, if releves were selected for recurring combinations of species, these become the most likely candidates for
differential species groups. But the tabular comparison usually brings out additional ones. Species that appear to be useful as
differential species can be underlined in the constancy table. These underline species are scattered throughout the table body
– so the table must be re-arranged. Experience has shown that this is best done in three steps:
1) PARTIAL TABLE – extracting only those species that are possible differentials and shuffle.
2) TOTAL TABLE - enter all data except rare
3) PHYTOSOCIOLOGICAL TABLE - enter environmental data.
INTRODUCTION TO ORDINATION
The mathematical approach to grouping and ordering vegetation samples has received a great deal of attention. The basic
supposition being that this is more objective (i.e. will permit exact repetition) therefore have special value for the novice or for
people with limited knowledge or familiarity with the data – gives a project creditability:
ORDINATION
The primary aim of ordination is the reduction of the multi-dimensional structure of vegetation to a 1-, 2-, or 3- dimensional
model – WITH the least distortion of stand position.
A number of similarity coefficients have been tried but in each case the principles are much the same:
1.
2.
3.
4.
5.
Relevé/samples are compared each with every other
Data matrix is constructed (similarity or dissimilarity)
Axes are extracted
Ordination diagrams are constructed
Data is plotted and interpretations made.
UNI-DIMENSIONAL ORDINATION: simply a linear ranking of values from one extreme to another e.g. moisture gradient in
montane forest on T.M. (continuum – if such exists it should be possible to demonstrate th is in uni-dimensional form with the
right samples).
MULTI-DIMENSIONAL ORDINATION: similarity relationships can be demonstrated geometrically – making use of multidimensional hyperspace. This arrangement can show clusters but the interpretation of these clusters or groups remains a
matter of personal judgement. In addition to cluster recognition (almost classification), ordinations can serve to identify trends
in vegetation variation, and can and do lead to explanations in terms of environmental gradients.
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Most ordinations are two dimensional (most practical), but the spacing of species or stands (plots – relevé) to one another is
not perfectly geometrical.
The first step in multi-dimensional stand ordination is the calculation of the similarity index.
C = ___2w_____ x 100 %
a+b
C = coefficient of similarity
w = the sum of the smaller of the two quantitative values of the species that are common to two samples
a = the sum of all quantitative values of one stand
b = the sum of all quantitative values of the other stand.
Hypothetical matrix of three stands containing varying densities of three species:
Matrix of similarity and dissimilarity coefficients (%) for six stands (A to F) sum of dissimilarity coefficients for each stand
(SUM) and the standard deviations (s.d.) of their means.
When considering ordination data and plotting it, it is usual to work with the dissimilarity matrix – so best to calculate:
C
dis
= 100 - _ 2w__ x 100
a+b
The selection of end stands to construct the ordination diagram can be done:
1. OBJECTIVELY: By totalling the coefficients of dissimilarity for each stand with every other stand – the one with the highest
sum being considered most dissimilar (A in above),. This most dissimilar stand becomes one end stand, the other being the
most dissimilar with it (in our example it is D). in our example stands A and D become the end stands (Beal’s method of end
stand selection) of the first axis. The length of an ordination axis is made proportional to the dissimilarity coefficient between
the two end stands (e.g. 94 cm.). the other stands are located between the end stands by drawing arcs proportional to their
dissimilarity values from each of the end stands. Arc intersections above and below the line between the end stands are
projected on to the line to give the positions of the other stands.
2. SUBJECTIVELY: By looking for stands that show a good range if dissimilarity with most others. The assumption being that
this stand contains much ecologically meaningful data.
In practice both methods are used – but computationally Beal’s method is far quicker and is thus most often used (perhaps
not the most effective either).
When all stands are located along the first axis there may be stands placed together (as near the centre as possible or in a
cluster towards the centre) which are actually very dissimilar from one another. A second, or Y, axis is then constructed to
separate these stands. Further axes may also be extracted from the X, Y or Z axes (depending on the nature of the data). (In
practice it is usual to have only two – perhaps three axes.).
In the example above “A” is a bad end stand as it does not give a good scatter of points in the multi-dimensional hyperspace.
From the matrix of dissimilarity values we can see that the s.d. of “A” is low.
This is an example of how Beal’s method does not work (in a matrix where one or more stands are very dissimilar from the
rest – a very heterogeneous array of data).
So an improvement suggested by Morris (1967) is that one must consider not only a high sum of dissimilarity values but also
the spread of individual values (expressed as s.d.).
C is chosen as an end stand (SUM 294 and s.d. 20.6) as E is most dissimilar to C (64%) and has a high s.d. (19.7) it is second
end stand.
Although the C-E axis is shorter than the A-D axis there is a better spread.
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