MEDICAL BOTANY - A.H. Serbin - 2003

CHAPTER 4. ECOLOGY, PHYTOCENOLOGY, PHYTOGEOGRAPHY

ELEMENTS OF PHYTOGEOGRAPHY

Phytogeography, or botanical geography, studies the plant cover of the globe and establishes the patterns of distribution of vegetation types and plant communities across geographical zones.

Ranges (Areals)

Each plant species is distributed over a specific territory, large or small, which is referred to as the species range (areal). One can also speak of the range of a genus, family, and so forth. The following types of ranges are distinguished: continuous ranges, where the plants of a given species occupy the entire range without interruption (or except for isolated unoccupied spots); disjunct ranges, where the plants occupy not a continuous area, but separate, spatially disconnected patches; ribbon-like (linear) ranges, where the territory occupied by a plant species is stretched out in a ribbon-like shape, for instance, along a river. There are plant species distributed across almost the entire surface of the globe, or at least across multiple continents. Such plant species are called cosmopolites (reed, dandelion, stinging nettle, jimsonweed, sow thistle). Species that grow only in a single, specific region of the globe are called endemics, or endemic species (for example, wormseed is found only in the Chimkent region of Kazakhstan, and tausasag lives exclusively on the Karatau Ridge). The ranges of certain endemic relict species are remnants of an extensive range that occupied a vast territory in the geological past (sequoia, which occupies only a small area in California; Eldar pine, found only in eastern Georgia). The flora of certain Regions of the globe is extremely rich in endemics (Australia, the Cape Province in South Africa, New Zealand).

Of particular interest are disjunct ranges, such as the arctic (north-latitudinal) and alpine (altitudinal-zonal) ones. For example, certain species of the genus Vaccinium, matgrass (Nardus stricta), and others occur simultaneously in the Arctic, the high-altitude Caucasus, or the Swiss Alps.

So-called vicariant, or replacement, ranges are widespread, represented by very closely related species that differ only in minor traits. For example, the lily of the valley (Convallaria majalis) is distributed in the forests of Eastern Europe, whereas in the mountain forests of Transcaucasia it is replaced by a very closely related species—Transcaucasian lily of the valley (C. transcaucasica), which has a cylindrical corolla and finer staminal filaments; lily of the valley is absent in Siberia, while in the Far East we find the Manchurian lily of the valley (C. manshurica), distinguished by wider flower corollas and bluish-green leaves, and a fourth vicariant species grows in North America. The broad concept of a range should not be confused with the habitat of a particular plant species. Within its range, a species and even plant communities may not be found everywhere, but rather in specific habitats characteristic solely of them.

The formation of individual species ranges is determined, firstly, by the plant's ability to disperse across a territory and, secondly, by the availability of conditions suitable for the growth of that species. Seeds and fruits possess various adaptations (spores, hairs, pappi, sticky glands, etc.) that facilitate plant dispersal. With the help of wind, Water, animals, birds, and humans, fruits and seeds are carried over very long distances, germinate, and take ROOT, which leads to an increase and expansion of the species range. The spread of many plants is also facilitated by their high seed productivity. Some plants produce a colossal number of seeds (rye brome produces 1,500 seeds per plant, corn cockle produces 2,500, shepherd's purse 75,000, and so on). Thus, migrating over many millennia, plants gradually form their species ranges. Naturally, a range does not remain constant; its area and boundaries can change: under a favorable combination of environmental conditions it expands, and conversely, under unfavorable conditions it shrinks, sometimes leading to the extinction of the plants altogether. A major influence on the expansion or reduction of individual plant species ranges is exerted by human activity, which creates favorable conditions for the growth of some plants and unfavorable ones for others.

Flora and Vegetation

Flora refers to the historically established totality of plant species growing in a given territory. Vegetation, or plant cover, is the totality of plant communities growing in a specific territory. Landscape, or scenery, is the general appearance of an area characterized by its vegetation, relief, soil, and other features.

Types of Plant Cover

A great diversity of climates, soils, and reliefs, as well as their various combinations, significantly affects the plant cover across all latitudes and longitudes. The primary role in the Formation of Plant cover belongs to climate. In this regard, the following biomes, or types of plant cover, are distinguished: forests (moist and deciduous tropical, moist subtropical, sclerophyllous, deciduous, coniferous), steppes, deserts, tundras, alpine vegetation, and sphagnum bogs.

Moist tropical forests. The founder of botanical geography, A. Humboldt, called these forests hylaea (from the Greek "hyle" meaning forest). They are distributed in equatorial regions, in South America (Amazon river basin), India, Indochina, Indonesia, equatorial Africa (Congo river basin), etc. The maximum annual precipitation falls evenly throughout the year, ranging from 300 to 14,000 mm. Temperatures favorable for plant growth (20–35 °C) also vary little throughout the year. These forests are characterized by a multitude of species and multiple canopy layers (in Java—165 tree species, 360 shrub and herb species, 65 liana species). Plants are evergreen, flowering and fruiting at various times throughout the year. There are numerous lianas and epiphytes, palms, giant tree ferns, bamboos, trees belonging to the families Myrtaceae, Fabaceae, Rutaceae, Rubiaceae, Lauraceae, etc., as well as herbaceous Araceae, Orchidaceae, and others.

Along ocean shores subject to tidal flooding, a special so-called mangrove vegetation develops. It consists of shrubs and low-growing trees that have developed tall aerial and supportive (stilt) roots As a result of periodic inundation.

Deciduous tropical forests. These are distributed in arid tropical regions—in Central and South America, East and South Africa, Indochina, and central India—where precipitation falls unevenly and the wet period is followed by a prolonged dry season. Plants shed their leaves during the dry period. Here there are fewer lianas and epiphytes. The species composition of tree species is less rich than in moist tropical forests (Cassia, Dalbergia, teak, sal, Ficus, Acacia, Albizia). Very frequently, such a forest transitions into a savanna—a tropical or subtropical steppe with highly distinctive herbaceous vegetation and wildlife. Here the grass stand is represented by tall grasses that vegetate vigorously during the short rainy period. Xerophytic trees and shrubs are restricted to places where moisture penetrates deeply and is protected from rapid evaporation. African savannas are characterized by flat-topped shrubby acacias and huge baobabs with short, thick trunks and broad crowns. Savannas also occur in South America and Central Asia.

Moist subtropical forests. These occupy small areas in the Canary Islands, southern China and Japan, New Zealand, Florida, Chile, etc. The climate is milder and less humid than that of tropical forests. The forests are characterized by evergreen broad-leaved and coniferous trees (Canary and camphor laurels, oaks, beeches, magnolias, sequoia, bald cypress, arborvitae, etc.). Lianas and epiphytes are abundant.

Sclerophyllous forests. These are distributed in the arid climate of the Mediterranean region, Australia, Mexico, South Africa, etc. They are characterized by trees and shrubs with stiff, evergreen, small leaves, whose blades are usually positioned obliquely to the sun's rays to reduce overheating. Many species are thorny. The most typical are: dwarf evergreen oak, myrtle, laurel, eucalyptus, oleander, wild olive, junipers, and others.

Steppes. These are distributed in regions with insufficient precipitation (in the interior PARTS OF THE continents of Europe, Asia, and North America). The steppe is characterized by the predominance of herbaceous xerophytic plants (feather grass, sheep fescue, junegrass, bluegrass). In spring, ephemerals, bulbous, rhizomatous, and tuberous plants bloom in the steppe (tulips, wild onion, bulbous bluegrass, desert sedge, etc.). The steppe has a more or less evenly distributed plant cover that can serve as pasture year-round (in the absence of snow). Pampas in South America and prairies in North America belong to the steppe vegetation type. Meadows consisting of perennial herbaceous mesophytes can also be included in this category. Meadows represent an azonal phenomenon and occur within other zones, such as the forest zone. Sometimes meadows are bordered by riparian forests. They are dominated by perennial grasses and legumes. In moist meadows, the proportion of legumes is high, the overall grass stand is taller, and the species composition is more diverse due to the appearance of many species from other families. Dry meadows are close to steppes and occupy slopes and terraces.

Deserts, as a rule, are covered with vegetation only in spring; in summer, a limited number of perennials remain. Certain parts of large deserts (the Sahara in Africa, the Arabian Desert, the Gobi Desert in Asia) are entirely devoid of higher plants. The most barren desert areas are the Namib (South Africa), Atacama (Chile), and Taklamakan (Central Asia). Central Asian deserts are distinctive in that their plant cover includes many shrubs and subshrubs. These deserts serve as year-round pastures. In deserts, precipitation is extremely scarce—100–200 mm.

Clay, solonchak (saline), stony, and sandy semideserts and deserts are distinguished. The plant cover of dry deserts is extremely sparse and consists mainly of wormwoods, saltworts, various densely pubescent xerophytes, and, in spring, ephemeroids. Sandy deserts feature their own special psammophytic vegetation. In addition to dry deserts, There are also cold arctic and high-mountain deserts, where the plant growing season is very short. Plants grow flat against the ground, confined to sheltered spots where snow accumulates or the soil warms up better. Algae, Lichens, mosses, and a few flowering plants—such as saxifrages and the polar poppy—are characteristic.

Summer-green mixed forests. Trees shed their leaves for the winter. These forests are distributed predominantly in temperate regions, i.e., the middle belt of Eastern Europe, the mountainous Crimea, the Caucasus, the mountains of Central Asia, the Far East, Western and partially Southern Europe, North America, etc. The dominant broad-leaved species are oaks, beeches, lindens, birches, aspens, alders, and others, while conifers include spruce and pine. An understory of shrubs and lianas develops beneath the trees—including hops, ivy, grapes, schisandra, actinidia, ampelopsis, and others.

Winter-green coniferous forests. These are distributed in temperate regions, extending northwards to the forest-tundra. Coniferous forests are evergreen and make up the main forest area of the temperate latitudes in the Northern Hemisphere. In the south, they occupy the mountainous regions of Crimea, the Caucasus, Central Asia, etc. The dominant species are spruces, firs, and larches. Mosses and Fungi are abundant in these forests.

Tundras and alpine vegetation. Tundras are distributed in subarctic and arctic regions, while alpine vegetation occurs in high-altitude mountain areas. In these regions, the climate is harsh, and permafrost is present almost everywhere, which is why the growing season lasts only about 2 months. Tundra vegetation consists of dwarf trees (birches, willows) and shrubs, mosses, and lichens, while the grass stand is composed of grasses, saxifrages, buttercups, cudweeds, cinquefoils, and others. Alpine vegetation is characterized by grasses, sedges, legumes, gentians, anemones, and others.

The listed types of plant cover are complemented by the so-called wandering vegetation type, consisting of lower aquatic and terrestrial organisms that are not attached to a substrate (phytoplankton of water bodies, soil algae, fungi, lichens, Bacteria).

Class="center">Fig. 4.5. Global distribution of major biome types

Sphagnum bog vegetation can also be distinguished. The formation of bogs is preceded by The Development of a thick moss cover, caused by an imbalance between moisture input and loss. Excessive moisture leads to the growth of hypnaceous and, in particular, Sphagnum mosses. The latter act like Sponges, absorbing and retaining vast amounts of water. The Sphagnum layer tends to grow upward through its upper strata each year. This growth is especially pronounced in the central part of the cover, which is why Sphagnum bogs feature a convex surface. Flowering plants also stretch intensively upward, but typically fail to keep pace with the growth of the Sphagnum carpet. They are drawn downward, buried within the Sphagnum mass, and gradually die off, transforming into peat. Depending on the buried species of flowering plants, cotton-grass, sedge, heather peat, and other types are distinguished. Sphagnum bogs, which are particularly abundant in the forest zone, are inhabited by small shrubs such as cranberries, lingonberries, and wild rosemary, dwarf trees like birches and willows, as well as sedges, cotton-grass, and sundews.