User:DmitrySA/Drafts/Tussocks and bunch grasses

Tussocks and bunch grasses – is a form of an herb that usually grows as singular plants in clumps, tufts, hummocks, or bunches, rather than forming a sod or lawn. Mature plants usually have no primary roots since they die off early, so only highly developed secondary roots left. Annually these plants produced many reproductive buds that form new sprouts. As a result, a tussock or bunch forms – a bundle of shoots that grow very close to each other. On one plant there could be dozens or hundreds of shoots. New sprouts on the periphery of a tussock make it wider.

Tussocks and bunch grasses are mostly perennial and polycarpous plants. In wild nature, some of them can reach age of dozens and even hundred years (up to 140 years for Filifolium sibiricum), the others rarely outlive 10-15 years (for instance - Poa botryoides).

According to Raunkiær plant life-form system tussocks and bunch grasses are chamaephytes or hemicryptophytes.

Elevated parts of the bunch grasses remained in the tussock after they die off forming a brush together with the living sprouts. The height of such brush varies from several cm to more them one meter height (depending on the size of the tussock itself). The brush keeps and absorbs water during the warm period of year (atmospheric and snow water, and dew), and keeps snow during winter. In the underground part, tussocks have dense and ramified fibrous root system. The total root surface area per unit volume of soil is colossal; therefore, tussocks can extract water from the ground with very high efficiency even under the water shortage conditions. Usually, the most of the roots located in the surface layer of soil that allows tussock to catch percolated water of precipitations.

Leaves of the steppe bunch grasses and tussock-forming sedges are narrow and could roll into a tube or conduplicate to reduce evaporation from the surface. Usually, leaves change shape to diminish surface area during the hot time of day.

There are firm- and loose-bunch grasses, as well as large- and small-tussock grasses. Additionally, there is a group of giant tussock grasses, which tussock can reach more than 1 m diameter and more than 2 m height. Among the last ones, there are Achnatherum splendens, Erianthus spp., Cortaderia selloana and the others.

Systematic structure
The most commonly tussocks are formed by plants from Poaceae and Carex families, but seldom tussocks-form plants could be found among other families. In temperate zone firm-bunch grasses are represented by the most species of genera Stipa, Koeleria, some species of Festuca, Deschampsia cespitosa and other cereals, some species of Carex (Carex cespitosa, Kobresia spp.), and also some Alliums (Allium polyrhizum, Allium bidentatum). Some authors also attribute several species of Asteraceae family to the firm-bunch plants like Edelweiss (Leontopodium leontopodioides), Arctogeron gramineum, but it is uncommon. Loose-bunch grasses are represented by Argopyron (for instance, Agropyron cristatum), Helicototrichon, Psathyrostachys, Poa (Poa stepposa, Poa botryoides), Phleum (Phleum phleoides, Phleum pratense), Achnatherum and some other genus from Poaceae family, and by some Asteraceae from genera Galatella (Galatella tatarica, Galatella villosa), Tanacetum (Tanacetum achilleifolium), Filifolium (Filifolium sibiricum) and others.

However, rating Asteraceae as a tussocks is not sequential since most of this species remains the main root throughout life period, so according to the structure of the root system this plants could be not treated as tussocks.

Role in the plant community
Bunch grasses and tussocks are the most typical type of vegetation and predominant life-form of plants in the steppes of Eurasia and similar grass ecosystems of other continents. Most dominant species in the steppe grass communities are tussocks.

Tussocks play an important role in some types of tundra (like grass-sedge tundra) and meadows also. They also dominate in several types of grassy highlands. Outside temperate climatic zone, tussocks are very important in vegetation of tropical savannah where giant bunch grasses are typical.

Ecological features
Bunch grasses got several advantages from the tussock that help them to survive in severe conditions of water shortage during hot season and very low temperatures and little snow in winter, to stand against constant pasturing and trampling down by herds of ungulates, as well as stand against fast low-temperature fires. That exact conditions are the most typical for the steppe ecosystems.


 * 1) A very special microclimate forms in the inner part of the tussock that is favorable for the vital functions of a plant. Inside the tussock, temperature drops are not as pronounces as they are in the external environment. It is cooler inside during heat and warmer when it is cold outside. This feature is most noticeable for smoothing of diurnal temperature differences. For instance, inside the tussock of Helictotrichon altaicum temperature maximum could be 8-16ºС lower then the outside temperature during the hot time of day.
 * 2) In the hot time of year, tussock accumulates moisture from atmosphere like a sponge and prevents it from evaporation. In winter tussock fills with snow and keeps it from blowing away by wind that is very important for plant water loading in spring. This quality of the tussock is most significant in steppe areas that have low amount of snow and strong wind that rapidly blows snow away from plains and relief elevations. In total, structure of tussock promotes moisture transfer from atmosphere to the very places of soil where it could be assimilated by the dense rootage of the plant with the highest efficiency.
 * 3) A sharp contrast of abiotic conditions is a characteristic feature of steppe zone. Despite steppes are droughty places through most time of year, during summer showers and rapid spring snowmelt (especially in western part of the steppe zone of Eurasia) an excess of water could emerge on the ground for short period. At that time, the tussock appeared to be very useful too – since tussock towers above the ground and got a dense “brush” of died stems and leaves surrounded fresh sprouts, it preserves tillering zone of xerophytic sedges and grasses from pernicious water flood. At the same time, a shallow ring-shape depression forms around the tussock that collects precious moisture for the future use. (Хорошо бы тут иллюстративную картинку).
 * 4) Remains of died-off sprouts in the tussock preserve fresh reproductive buds of a plant from grazing and trampling down by ungulates, as well as from nibble by other plant feeder.
 * 5) Tussock increases chances for survival of a plant during frequent fires in steppe. Low temperature and fast fire speed – are characteristic features of fires in steppe areas. Under such conditions dense mass of entangled sprouts (both dead and alive) could withstand fire and do not inflame. Tussock could be burnt on the edges but preserve reproductive buds in the inner part. Tussocks and bunch grasses could resume their growth in a few days after fire. However, overground part of the tussock could not survive slow and high-temperature fire, but underground part of a plant with reproductive buds still have a chance to survive.
 * 6) Root system of tussocks and bunch grasses is mostly located in the surface layer of soil (from 5-10 cm to half meter depth) that is most abundant with humus, best aerated and contains the highest concentration of biologically active soil microbes, fungi and invertebrate saprophages. In dry climate ecosystems like steppe biome it is essential that densely networked roots in the surface layer of soil could absorb maximum amount of atmospheric moisture concentrated on ground with rains and snow slush . Some roots of tussock plants could reach underlying soils, which enable them to extract water more effectively, especially during drought. Thus, tussocks form of plants benefit over rhizome plants and over taproot plants as well.

Structural origin of tussock
Cereal steam could branch only in its lower part named tillering zone that located either directly on the surface of the soil or rather superficially underground. It consists of many buds, covered with cylindrical leaf boot. Those plants which sprouts break through a leaf boot form no tussocks, but rhizomes – creeping underground stems. But cereals whose sprouts grow upright inside a leaf boot close to and in parallel to the maternal stem form tussocks.