Introduction to phytocenology

Phytocenosis-plant community (from the Greek φυτóν -. «Plant» and κοινός -

«common») - that exists within the same habitat. It is characterized by the relative
homogeneity of the species composition, structure and certain plant system of
relationships with each other and with the environment. According to N. Barkman
phytocoenosis - the essence of a particular segment of vegetation, in which the
inner floristic differences are smaller than the differences with the surrounding
vegetation. The term was proposed by the Polish botanist Pachosky in 1915.
Phytocenosises is the object of study of science phytocenology (Geobotany).
Phytocoenosis part of the ecological community together with zoocenoses and
microbiocenosis. Biocoenosis, in turn, combined with the conditions of abiotic
environmental (ecotop) forms biogeocenosis. Phytocoenosis is the central leading
element biogeocoenose as transforming primary ecotope in biotope, creating
habitat for other organisms, as well as the first link in the cycle of matter and
energy. From vegetation dependent properties of soils, climate, the composition of
the animal world, such characteristics biogeocoenose as biomass, biological
productivity, etc. In turn, plants coenopopulations (set of individuals of one species
within the boundaries phytocenoses) are the elements of phytocenosis.
Development of views to the nature of phytocenosis.
In the early XX century, it was suggested diametrically opposite point of view that
represents a continuous vegetation cover, and its division into separate elements -
phytocenoses - artificial. The absence of sharp boundaries between plant
communities and the presence of the transition zones between them contributed to
the continuity of vegetation, based on an individualistic concept:
=> Each species of plant has its own individual requirements for environmental
conditions and is characteristic for each environmental factor ecological amplitude
=> Environmental factors vary gradually, both in space and in time
=> The transition from one combination to another coenopopulations carried out
continuously: some species gradually reduced its abundance and go, others -
appear and increase. Extreme supporters of the concept of continuity of vegetation
are not considered phytocoenosis as an object of study geobotany, because of its
artificiality, but separate plant. Extreme supporters of the idea of discreteness is
postulated a clear distinction and demarcation of individual phytocenoses.
On the basis of synthesis of both concepts has been put forward the idea of a
combination of naturally occurring vegetation, privacy, and continuity. This was
seen as a manifestation of the contradictions inherent to the material world as a
whole. According to this idea, vegetation cover has property of continuity, but it is
not absolute but relative. At the same time he has and discrete properties, but it is
not absolute but relative. These properties are organically combined, not excluding,
but complementing each other.

Factors in phytocenosis organization

Factors in the organization of the plant community can be divided into four
groups:
1 the characteristics of the environment (ecotope)
2 the relationship between plants
3 the effect on vegetation of heterotrophic components (animals, fungi,
bacteria)
4 disturbances.
These groups of factors determine the combination and characteristics of
cenopopulations of species in the phytocenosis.

The ecotope is the main factor in the organization of the phytocenosis, although it
can be largely transformed by biotic plant influences or disturbances. To the
abiotic factors influencing the organization of the community can be attributed:
climatic (light, thermal, water regimes, etc.)
edaphic (granulometric and chemical composition, humidity, water regime and
other properties of soils)
topographic (relief characteristics)
The relationship between plants Direct (contact) mutual influence
Appear by contact or intrusion the organisms into each other. They are divided into
physiological (parasitism, symbiosis), when carried out between organisms active
exchange of matter and energy, and mechanical (epiphytes relationship with
phorophytes and lianas with supporting plants) - where it does not exist.
Parasitism
In parasitism one organism (the parasite) uses another (the host) to obtain the
necessary substances and energy to it, with its oppressing. Among higher plants
parasitism found only in angiosperms . Also, all plants are capable to settle
parasitic fungi and bacteria. The degree of plant parasites depends on the
characteristics of the host plant (different types are affected differently), the
conditions of the habitat (almost not affected in terms of salinization plants), on the
presence of evolutionary conjugation of the parasite and the host (if it is not, the
owner is usually there are no mechanisms of protection against the parasite ).
Against parasitic fungi and bacteria plants during evolution formed a complex
defense mechanisms:
 allocation of a plant fungicidal and bactericidal substances that prevent
infestation or inhibiting their development
 availability of powerful cover tissues, preventing the intrusion of parasites
 especially the biochemical composition and metabolism in plant cells,
inhibiting the growth of the parasites.
Despite the existence of such protection mechanisms phytoparasites can cause
numerous diseases of plants, leading to their weakening and destruction. Plants
parasites are not widespread, but also can significantly inhibit coenopopulations
and individual plants.
Symbiotic relations
Symbiotic relationships are appear in the coexistence of plants to fungi and
bacteria (including cyanobacteria). Accordingly distinguish mycosymbiotrophia
and bacteriosymbiotrophia.
Mycosymbiotrophia implemented as mycorrhiza - the interaction of hyphae of
fungi and plant roots. It is assumed that in the early stages of evolution of the plant
world mushrooms were in relation to plants only as parasites, and only in the
course of long-term co-evolution have formed a mutually beneficial relationship.
To date, mycorrhiza is found in more than 80% of vascular plant species: it is
formed by all kinds of gymnosperms, 77-78% of the species of angiosperms and
about 60% of species of vascular spore plants. Ectomycoriza appears when
mycelium envelops the roots of plants and endomycoriza when penetrates into
them.
Epiphytes
Epi- means “upon” and phyte means “plant”, so an epiphyte could be any plant,
lichen, alga, fungus, or bacterium growing on or attached to a living plant.
However, the term usually refers only to seed plants which remain autotrophic and
obtain moisture from the atmosphere. Parasites such as mistletoe (Viscum album)
are not considered “true” epiphytes. By some estimates, about 10% of ferns and
seed plants - and half of all orchid species - are epiphytes. Most epiphytes live
in tropical or temperate rainforests, where humidity and competition for sunlight
are high. What are the benefits of an epiphytic niche which does not allow harm to
the host?
 The host plant provides support.
 Often, support includes elevation toward more sunlight. Only 1-2% of
incident sunlight reaches the rainforest floor.
 Aboveground locations reduce access by some herbivores.
 Elevation may increase the success of wind- or insect-pollination.
Lianas
Lianas - plants using other plants or other objects to preserve the vertical position.
Lianas appeared in the course of evolution as a manifestation of competition for
light. The huge number of species of lianas refers to the angiosperms. Most lianas
rooted in the soil, but there are epiphytic and parasitic forms, such as plants of the
genus Cuscuta (Cuscutaceae) and convolvulus (Convolvulaceae).
Liana, bringing in the growth of their leaves closer to the light obtained from the
co-existence with the host plant benefit, while the latter - mainly harm as a direct -
as a result of mechanical action lianas demolition / destruction of the host plants,
and indirect - as a result of interception liana light , moisture and nutrients.
The greatest diversity of lianas also achieved in tropical rain forests.

Indirect influence
Transabiotic interactions
The effects of plants on each other, mediated by abiotic environmental factors.
Appear due to overlapping phytogenic fields of the neighboring plants. They are
divided into rivalry and allelopathy
Rivalry
Rivalry develops either because the original habitat of limited resources, either by
reducing their share attributable to each plant, as a result of overcrowding. Rivalry
leads to lower consumption of plant resources, and consequently, a decrease in
growth rate and storing substances, and this in turn leads to a decrease in the
quantity and quality diaspore. There are intra- and interspecific rivalry.
Intraspecific rivalry effect on fertility and mortality in populations, identify trends
to maintain its population at a certain level, when both values cancel each other
out. This number is called the marginal density and depends on the amount of
habitat resources. Intraspecific rivalry is asymmetric - uneven effect on different
individuals. Interspecies competition is also widespread in nature, since the vast
majority of phytocenoses (except for some agrocenoses) are multispecies. Multi-
species composition is provided by the fact that each species has a characteristic
for him only an ecological niche that occupies and in the community. At the same
niche that kind could take in the absence of interspecific competition - a
fundamental, it shrinks to the size of sales. In phytocenosis differentiation of
ecological niches is due to:

=> Different plant height

=> Different penetration depth of the root system
=> Contagious distribution of individuals in a population (in separate groups /
patches)
=> Different periods of vegetation, flowering and fruiting
=> Unequal efficiency of use by plants habitat resources
With a weak overlapping ecological niches can be observed the coexistence of two
cenopopulations, with a strong competitive as a kind of habitat displace less
competitive. The co-existence of two strongly competing species is also possible
due to the dynamic environment, when one or the other kind of gets a temporary
advantage.
Allelopathy is a biological phenomenon by which an organism produces one or
more biochemicals that influence the germination, growth, survival, and
reproduction of other organisms. These biochemicals are known
as allelochemicals and can have beneficial (positive allelopathy) or detrimental
(negative allelopathy) effects on the target organisms and the community.
Allelochemicals are a subset of secondary metabolites, which are not required for
metabolism (i.e. growth, development and reproduction) of the allelopathic
organism. Allelochemicals with negative allelopathic effects are an important part
of plant defense against herbivory.
Antibiotics - are extracted by microorganisms are used to suppress other
microorganisms;
Marazminy - extracted by microorganisms are used to suppress the higher plant
life;
Phytoncides- are extracted by higher plants are used to suppress the
microorganisms;
Kolyns - are extracted by higher plants are used for the suppression of vital
functions of other higher plants.
Allelopathy can be regarded as a form of ecological competition between
organisms in ecosystems.
The production of allelochemicals are affected by biotic factors such as nutrients
available, and abiotic factors such as temperature and pH.
Allelopathy is characteristic of certain plants, algae, bacteria, coral, and fungi.
Allelopathic interactions are an important factor in determining species
distribution and abundance within plant communities, and are also thought to be
important in the success of many invasive plants.
Allelopathy has been shown to play a crucial role in forests, influencing the
composition of the vegetation growth, and also provides an explanation for the
patterns of forest regeneration. The black walnut (Juglans nigra) produces the
allelochemical juglone, which affects some species greatly while others not at all.
The leaf litter and root exudates of some Eucalyptus species are allelopathic for
certain soil microbes and plant species. The tree of heaven, Ailanthus altissima,
produces allelochemicals in its roots that inhibit the growth of many plants.
Transbiotic interactions. The indirect effect of some plants to other organisms
through the third (other plants, animals or fungi). Exposure may occur at the level
of the individual organism, and at the level of the whole coenopopulations.
Transbiotic interference may be:negative: the interaction of the fungus and the
plant mikosimbiotropha improves its competitiveness in relation to the plant
having no mycorrhiza.
positive:cattle on pasture eating some plants and not eating weeds, reduces the
abundance of the first, and, as a consequence, the spread of the second.
The influence of phytocenosis on the environment.
When forming the phytocenosis, the primary abiotic environment of the ecotope is
transformed into a phytosphere, and the ecotope itself is converted into a biotope.
Phytocenosis thus affects almost all abiotic factors, changing them in one direction
or another.
Light regime of phytocenosis
Inside any phytocenosis, the light mode will be different from the light mode of the
open area not occupied by vegetation. Such differences arise due to the fact that the
light in the phytocenosis is definitely redistributed and the following processes
occur:
=> reflection of a part of the light outside the phytocenosis
=> absorption of part of the light by plants (including in the process of
photosynthesis)
=> penetration of light into the phytocenosis
Due to the reflection and absorption of light by plants, only a small part of it
reaches the soil level, which is especially clearly seen in multi-tiered forest
phytocenoses: under the canopy of pine forest, the illumination is, on average, 25-
30%, oak - about 3%, and humid tropical forest - about 0.2% of total illumination
(on an open surface in the same geographical conditions).
The illumination in the phytocenosis is not uniform: it varies in both the vertical
and horizontal directions. When moving from the upper boundary of the
phytocenosis to the level of the soil, the illumination decreases spasmodically,
primarily due to the peculiarities of lining (thickness and arrangement of leaves in
space) on each tier.
In herbal phytocenoses, there are two types of illumination: cereal and
dicotyledonous. The grass type is characteristic for communities with a
predominance of plants with vertically oriented leaves (cereals, sedges) and is
characterized by a gradual decrease in illumination from top to bottom.
Dicotyledonous type - for communities with horizontally oriented leaves; it is
characterized by sharp irregularities in light and is similar in this respect to
changes in light intensity in forest communities.
Absorption of visible light of different wavelengths by photosynthetic
pigments.
In aquatic ecosystems, in addition to plants, absorption and reflection of light also
involves water and particles suspended in it, as a result of which the existence of
plants at a great depth becomes impossible. In transparent freshwater reservoirs,
the illumination becomes less than 1% at depths of more than 5-10 meters, as a
result of which higher plants in such conditions are found at depths of not more
than 5, and algae - no more than 20 meters, but in clear waters of seas and oceans
certain types of red algae penetrate to a depth of several hundred meters.
Plants in phytocenoses alter the qualitative composition of the spectrum,
selectively absorbing and reflecting light with a specific wavelength. The hard UV
radiation (λ <280 nm), harmful to the living structures of the plant cell, is almost
completely (up to 95-98%) absorbed by the epidermis of the leaves and other
surface tissues. The visible part of the solar spectrum (physiologically active
radiation) is absorbed up to 70% by photosynthetic pigments, while the blue-violet
and red parts of the spectrum are absorbed more intensely, and the green part is
much weaker. IR radiation with λ> 7000 nm is absorbed up to 97%, and with λ
<2000 nm it is absorbed very weakly.
Leaf permeability to light is also unequal and depends primarily on the thickness
and structure of the leaf, as well as the length of the light wave. Thus, the leaves of
medium thickness allow up to 10-20% of light, very thin - up to 40%, and thick,
hard, wax-coated or pubescent leaves may not transmit light at all. IR and green
light have the greatest penetrating power, the rest of the spectrum penetrate
through the leaves significantly less.
The light regime also changes during the day, during the year, and depending on
the age composition of the plant phytocenosis.
Thus, the phytocenoses change the conditions of illumination of an ecotope,
forming a special, inhomogeneous light regime in space. The specificity of this
mode in each specific phytocenosis determines the set of species, their distribution
and the structure of the phytocenosis as a whole.
Thermal regime of phytocenosis
A portion of the Earth’s surface devoid of vegetation receives heat both directly
from the Sun and indirectly through the diffused light of the sky and the return
radiation of the heated atmosphere. The incoming heat is partially reflected back
into the atmosphere, partially absorbed and drained into the deeper layers of the
Earth, and is partially emitted by the heated soil back into the atmosphere. All
processes of heat input and discharge form a heat balance.
The heat balance changes during the day: in the afternoon, in the insolation phase,
it turns out to be positive, at night - negative. The heat balance is also influenced
by weather conditions, terrain relief, season and geographical location of the
ecotope.
Phytocenoses significantly change the thermal regime of an ecotope, as the plants:
=> reflect part of the sunlight back into the atmosphere, reducing the flow of heat
into the phytocenosis
=> absorb sunlight and spend it on physiological processes
=> release some heat during breathing
=> transpiration and guttation
=> absorb part of the heat emitted by the soil
=> reduce evaporation from the soil surface
=> slow down the movement of air masses
Also, the change in the thermal regime of an ecotope occurs as a result of
condensation and physical evaporation of moisture from the surface of plants.
The main energy exchange in the phytocenosis is not at the surface of the soil, as
in a plot devoid of vegetation, but in the upper closed phytocenter zone, which is
heated most of all during the daytime and cools the most at night.
In general, the thermal regime of phytocenosis has the following features in
comparison with that of a plot lacking vegetation:
=> maximum temperatures decrease, and minimum temperatures increase
=> daily and seasonal temperature amplitudes are smoother
=> average annual temperature below
Air regime of phytocenosis
The influence of phytocenosis on the air regime is manifested in changes in the
speed of movement and composition of the air. Wind speed in phytocenoses falls
from the top down and from the more open area to the less open. The dynamics of
the air composition is determined mainly by the change in the concentrations of
oxygen and carbon dioxide in the process of photosynthesis and respiration. The
content of CO2 is subject to more significant fluctuations: in the first half of the
day it decreases, which is connected with the intensification of photosynthesis, in
the second half of the day it rises and reaches a maximum at night. The change in
the oxygen content in the air occurs synchronously, but in the opposite direction.
There is a certain variability in the content of CO2 and O2 in the seasons of the
year: for example, in the forests of the temperate zone, the lowest concentration of
CO2 is observed in spring, when the leaves bloom and the vital processes in plants
are intensified.
Moisture regime of phytocenosis
In plant communities a special phytoclimate is created. Thus, in forest
phytocenoses, atmospheric precipitation is significantly redistributed. In summer,
they often do not reach the surface of the soil, since a significant part of them is
held up by crowns (up to 75%) and then evaporates. The thicker the forest, the
more atmospheric moisture is spent on wetting plants. Very hygroscopic forest
litter, absorbing a large amount of melt water. About 85% of winter precipitation,
retained by the forest, falls on the soil and percolates, replenishing groundwater.
The forest releases a large amount of water vapor as a result of transpiration, which
increases the humidity of forest air and the temperature drops. When air is
saturated with moisture, steam condenses on the branches. High humidity
conditions the existence of hygrophyte forest herbs.
Forests affect the climate of vast areas. Due to the redistribution of moisture within
the forest zone, afforested areas are always relatively richer in rainfall. Convection
currents from the forest carry the air colder and more humid compared to the
treeless area, and there is more precipitation over forest areas than above treeless
areas. By contributing to the slow melting of snow, the forest greatly reduces
ground flow, preventing soil erosion. When it is reduced over large areas, rapid
snow melting occurs and, as a result, strong and short floods of rivers with an early
onset of low water (low water levels in the rivers). The destruction of natural
vegetation cover and the increase in surface run-off cause problems associated with
soil erosion. Thus, according to research in the Vologda Oblast, old arable soil on
the slopes has already lost 50-60 cm of the upper layer. Soil erosion contributes to
the ravine formation, erosion of river banks, increased sand and silt removal into
the deltas, expansion and shallowing of the channels. Deforestation in the
mountains leads not only to an increase in surface runoff, but also to climate
change in the valleys, drying up of torus sources, and an increase in the avalanche
danger.
In the meadow herbage also forms its own microclimate. For some meadows, the
only source of moisture is precipitation, for others it is still groundwater and (or)
hollow water. The more close herbage and the more its mass, the more it can delay
precipitation and the more humidity inside and above it. This creates favorable
conditions.
for parasitic fungi (rust, etc.), as well as for fillosferic nitrogen-fixing bacteria.
Dynamics of phytocenosis.
• Disturbances
• Succession