FUNDAMENTALS OF PHYTOCENOLOGY AND PLANT GEOGRAPHY - T. P. Larkina - 2017

SECTION 2. FUNDAMENTALS OF PLANT GEOGRAPHY

2.4. Distribution of a Species Within Its Range

Within the geographical range, a species is typically distributed unevenly. The distribution of individuals of a species is tied to specific ecotopes, or habitats; in other words, a species may occur sporadically, patchily, or discontinuously across its range because favorable living conditions are not found everywhere. Consequently, some plants are more densely and frequently distributed within their range—these are Representatives of the plant kingdom with relatively low environmental demands (such as many weeds)—whereas others occur only in isolated localities, representing species with stricter environmental requirements (e.g., the round-leaved sundew, Drosera rotundifolia) (Fig. 17). Ultimately, the range of each species consists of discrete localities (habitats) where the species finds optimal conditions for existence (a specific soil composition and Water regime, microclimate, absence of competition, etc.).

The range of a species may be:

1) continuous, i.e., unbroken; one type of continuous range is the belt-like (or linear) range, where the totality of a species' localities forms a band (e.g., plant species confined to riverbanks);

2) disjunct (discontinuous), i.e., divided into separate, more or less large parts.

Sometimes the number of localities for a species is extremely small, making it impossible to speak of any substantial territory occupied by it—in such cases, we are dealing with a point range (as seen in waterwheel plant, Aldrovanda vesiculosa).

2.5. Methods of Mapping Ranges

There are several Methods for depicting plant ranges on geographical maps:

1) outline method — the most common. In this case, the boundary of the range is drawn on the map, resulting in an outline of a specific shape and size;

2) spot (or dot) method — all known localities of the species are marked on the map with symbols (dots).

3) grid method (raster method) — a grid of squares, the size of which may vary depending on the map scale, is first superimposed on the map; a dot is then placed in each square where a locality of the species occurs.

Sometimes a range is depicted on a map using solid hatching.

2.6. Changes in Ranges Over Time. Range Disjunctions

Species ranges do not remain static over long periods of time. Upon The Emergence of a new species, its range covers a negligible area, which is referred to as the primary range. Gradually, the species spreads as far as soil and climatic conditions and competitive interactions with other plant species permit. The size and shape of the range eventually stabilize. However, ranges can also undergo reverse, regressive changes when their area shrinks. This happens when environmental conditions shift unfavorably for the species, leaving only a small remnant of a once-extensive territory—these are relict ranges.

Over time, not only the area occupied by a species can change, but also the shape of its range itself. Sometimes an initially vast, continuous range splits into several separate, isolated parts. This is frequently characteristic of genus ranges, where certain species of a genus occupy one part of the range while others occupy another: for example, the range of the genus magnolia (Magnolia) consists of two parts—one in North America and the other in East Asia—which is an example of intercontinental disjunction. Within a single continent, the species of the genus Jacksonia, native to Australia, are geographically separated: part of the range is located in the west and part in the east of the continent.

The causes of disjunctions include continental drift (such as in the Proteaceae family, with species distributed across Australia, South America, and South Africa), mountain building, marine transgressions, and glaciations, all of which create impassable barriers. Sometimes migration (the dispersal of plants driven by unfavorable conditions) can also cause disjunction; if dispersal from the original range occurs in multiple directions, the species may subsequently form several new, isolated ranges.

Range disjunctions are frequently caused not by a single factor, but by the simultaneous action of several factors.

2.7. Endemism and Vicariance

Depending on their global distribution, plants are categorized into:

1) cosmopolitans — plants that are widely distributed across all continents, for example, cattails, red clover, etc.;

2) endemics (endemic plants) — plants native exclusively to specific, restricted geographical regions and possessing a very limited, narrow range. Examples include the coast redwood, found only in North America; eucalyptus species, which grow wild exclusively in Australia; the tau-sagiz rubber plant, known only in the small mountainous region of Karatau; ginseng, found solely in the Russian Far East; or the Eldar pine in Georgia, whose range spans a mere 50 ha (Fig. 18).

Endemic species are distributed unevenly across the globe: the highest concentration of endemics occurs on islands and in mountainous regions—areas that differ significantly in ecological conditions from the surrounding zones. When discussing endemic plants, scientists often refer to paleoendemics—very ancient plants that have become extinct almost everywhere else and have survived only in a restricted territory (e.g., the giant sequoia, the bald cypress, etc.). Neoendemics, by contrast, are recently emerged species that have not yet had sufficient time to spread widely across the Earth. Examples of neoendemics include certain "young" species of primrose (Primula), bellflower (Campanula), and whitlow-grass (Draba) growing in the Caucasus.

Relicts — ancient plants that have survived in their original habitats from more or less distant geological epochs. The presence of relicts within a specific flora indicates its antiquity and implies that the climate of the region has remained relatively stable throughout the existence of these relict plants. The relict status of a plant is determined using available paleobotanical data: fossil plant discoveries in specific geological strata make it possible to establish the plant's age while simultaneously revealing its former distribution across the Earth's surface.

Relicts vary in age, and are accordingly classified into the following types:

1) Mesozoic relicts – the most ancient relicts in the global flora. Examples include ginkgo (Ginkgo biloba), one of the oldest gymnosperms on Earth, and the giant sequoia (Sequoiadendron giganteum) (Figs. 16, 19). The flora of Russia contains no Mesozoic relicts;

2) Tertiary relicts, which are considerably younger. These are thermophilic forms that were widely distributed across the Earth during the Tertiary period (especially in Eurasia and North America), but died out in many regions with the onset of the ice age and general cooling. In areas where the climate changed relatively little, they managed to survive—such locations are known as refugia. The primary refugia of the Northern Hemisphere's Tertiary flora have survived in the southeastern North America (relicts include the tulip tree, bald cypress, and certain magnolias, among others), as well as in China and Japan (relicts include various species of oak, beech, chestnut, and magnolia);

The territory of the former USSR contains quite a few Tertiary relicts concentrated in several refugia. One of these is Western Transcaucasia (south of Tuapse), which boasts a distinctly subtropical climate. Here, surviving relicts include the sweet chestnut (Castanea sativa) (Fig. 20), Caucasian wingnut (Pterocarya pterocarpa), pontic rhododendron (Rhododendron ponticum), Colchis boxwood (Buxus colchica), and several others.

A considerable number of Tertiary relicts are also found in the Far East refugium (Primorye), including Amur cork tree (Phellodendron amurense), Asian ginseng (Panax ginseng) (Fig. 21), the aquatic plant water shield (Brasenia schreberi) (Fig. 22), and others.

Tertiary relicts are likewise present in the flora of the Southern Coast of the Crimea.

3) Ice age (glacial) relicts – relatively cold-tolerant plants that survived the glaciation in areas that were not directly ice-covered but lay close to the ice sheets. Following the retreat of the glaciers, these plants persisted in their former habitats. Examples include cranberry, marsh Labrador tea (Ledum palustre) (Fig. 23), and lingonberry (Vaccinium vitis-idaea), which grow in certain areas of the Central Russian Upland. While these plants are fairly widespread overall, they are considered glacial relicts exclusively within the Central Russian Upland.

4) Xerothermic period relicts – remnants of southern, frequently steppe plants that migrated far to the north during the warm and dry post-glacial period and, following the subsequent cooling, survived in isolated pockets—even far to the north—up to the present day. These are more recent than glacial relicts.

The Kungur forest-steppe, located in the Western Cis-Urals (Perm Krai), is rich in xerothermic relicts. About 30 relict species are found here, including feather grass (Stipa pennata), steppe cherry, hairy oxytropis (Oxytropis pilosa), sheep fescue (Festuca valesiaca), dyer's greenweed (Genista tinctoria), and others (Figs. 24, 25, 26, 27, 28).

Xerothermic relicts grow only in a limited number of locations—on very dry sandy patches, limestone and gypsum outcrops—under highly specific soil conditions, i.e., wherever they could not be displaced by the moisture-loving local flora.

The presence of endemic and relict plants in any given flora provides insight into its age, origin, genetic connections with other floras, and so forth.

Vicariance – the existence of species or other taxa that exhibit minimal morphological differences and are closely related, yet are geographically segregated, with each taxon (species, genus, etc.) occupying its own distinct territory. The cause of this phenomenon may lie in the geological METABOLISM/13.html">History of the region where vicariant (substitute) taxa are found: an originally continuous range becomes fragmented by marine transgressions, mountain building (orogeny), glaciation, etc. Furthermore, the older the geological event that fragmented the initial range, the more pronounced the differences between the vicariant taxa will be, and the higher the taxonomic rank. Example: two pine species found in North America (Pinus strobus and Pinus monticola) are morphologically similar; the former is distributed in the eastern part of the continent and the latter in the western part. This is explained by the Cretaceous fragmentation of their ancestral range due to a marine transgression, followed by the Evolution of the ancestral species, which ultimately resulted in two distinct, related species.