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''Polylepis'' is a genus comprising 28 recognised shrub and tree species, that are endemic to the mid- and high-elevation regions of the tropical Andes. This group is unique in the rose family in that it is predominantly wind-pollinated. They are usually gnarled in shape, but in certain areas some trees are 15–20 m tall and have 2 m-thick trunks. The foliage is evergreen, with dense small leaves, and often having large amounts of dead twigs hanging down from the underside of the canopy. The name ''Polylepis'' is, in fact, derived from the Greek words poly (many) plus letis (layers), referring to the shredding, multi-layered bark that is common to all species of the genus. The bark is thick and rough and densely layered for protection against low temperatures. Some species of ''Polylepis'' form woodlands growing well above normal tree line within grass and scrub associations at elevations over 5000 m; which makes ''Polylepis'' appear to be the highest naturally occurring
arboraceous A woody plant is a plant that produces wood as its structural tissue and thus has a hard stem. In cold climates, woody plants further survive winter or dry season above ground, as opposite to herbaceous plants that die back to the ground until s ...
angiosperm Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants t ...
genus in the world.


Classification/taxonomy

The genus ''Polylepis'' contains about twenty species that are distributed across the Andes. It is in the rose family, Rosaceae. The genus belongs to the tribe Sanguisorbeae, which mainly comprises herbs and small shrubs. Although the relationship of ''Polylepis'' to other genera of Sanguisorbeae is largely unknown, the analysis of Torsten Eriksson et al. (2003) showed evidence of a close relationship between ''Polylepis'' and
Acaena ''Acaena'' is a genus of about 60 species of mainly evergreen, creeping herbaceous perennial plants and subshrubs in the family Rosaceae, native mainly to the Southern Hemisphere, notably New Zealand, Australia and South America, but with a few ...
, which shows tendencies towards having fused stipular sheaths, reddish, flaking-off bark, and axillary, somewhat pendant inflorescences, features otherwise characteristic of ''Polylepis''. There are several characteristics that are important taxonomically to distinguish between species of ''Polylepis'', for example: 1) The amount of leaf congestion, 2) presence or absence of spurs and their size and vestiture, 3) presence or absence and type of
trichomes Trichomes (); ) are fine outgrowths or appendages on plants, algae, lichens, and certain protists. They are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae. A covering of any kind of hair on a plan ...
, (4) size, shape, thickness and vestiture of leaflets. The most important taxonomic character, however, is the leaflets. Studies suggest that repeated fragmentation and reconnection of
páramo Páramo () can refer to a variety of alpine tundra ecosystems located in the Andes Mountain Range, South America. Some ecologists describe the páramo broadly as "all high, tropical, montane vegetation above the continuous timberline". A narrower ...
vegetation, caused by the
Pleistocene The Pleistocene ( , often referred to as the '' Ice age'') is the geological epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was finally confirmed i ...
climatic fluctuations, had a strong influence on the evolution and speed of speciation in the genus ''Polylepis'' as well as the páramo biota as a whole.


Species


Habitat and distribution

Tree species in the genus ''Polylepis'' are confined to the high tropical South American Andes Mountains, with the most abundant concentrations of ''Polylepis'' ranging from northern Venezuela to northern Chile and adjacent Argentina. One known group of extra-tropical populations of ''Polylepis'' is distributed in the mountains of Northwestern Argentina. Most species of ''Polylepis'' grow best at high elevations between 3500 and 5000 meters. However, there are occurrences of species at altitudes as low as 1800 meters. These low altitude species are mixed with montane forest which indicates that components of the genus could have been present in western South America during the Miocene Period or even earlier. It is extremely rare for tree species to live at such altitudes, making ''Polylepis'' one of the highest naturally occurring trees along with the conifers of the Himalayan Mountains. ''Polylepis'' racemosa grows as shrubby trees on steep, rocky slopes above cloud forest. ''Polylepis'' tarapacana is one that reaches 4,800 m; the highest elevation of tree growth in the world. There is much debate on whether ''Polylepis'' was forced to exhibit such extreme elevation habitats due to habitat destruction by human interference. Physiological tolerances for growth at these elevations are subject to considerable debate among scientists, but evidence indicates that even before severe decimation by man, high elevation trees were limited in their distribution by the presence of specialized microhabitats. Due to the harsh environment in which many species of ''Polylepis'' grow the growth of the tree's stems and branches are generally contorted. This abnormal growth is often associated with windy, cold or arid habitats. The climate of the South American Andes changes drastically throughout the region creating many microhabitats. Overall, the climate consists of short southern summers when temperatures are warm and rainfall is high and long winters when temperatures are low and rainfall is limited. The temperature and amount of rainfall also depend on which side of the mountain (eastern or western side), elevation and latitude.


Morphological characteristics

Bark: The bark of ''Polylepis'' consists of numerous layers of thin, dark red exfoliating sheets. In some cases, the layered bark can be more than an inch thick. A majority of the larger branches have similar shredding bark. It would seem that the bark serves as insulation from both the nightly frosts and the intense daytime irradiation. The thick bark of Polylepis also serves an important function as protection against fire. It is thought to originally have been a protection against epiphytic mosses, whose thick masses may damage trees by adding weight to the branches and providing a suitable environment for fungi which attack the trees. Branching pattern and leaf arrangement: ''Polylepis'' trees tend to have twisted, crooked stems and branches with repeated
sympodial Sympodial growth is a bifurcating branching pattern where one branch develops more strongly than the other, resulting in the stronger branches forming the primary shoot and the weaker branches appearing laterally. A sympodium, also referred to a ...
branching. Contorted growth is often associated with windy, cold, or arid habitats. The leaves are generally congested along the branch tips often at the end of long, naked branch segments.
Stipule In botany, a stipule is an outgrowth typically borne on both sides (sometimes on just one side) of the base of a leafstalk (the petiole). Stipules are considered part of the anatomy of the leaf of a typical flowering plant, although in many speci ...
sheath: Each leaf has a pair of stipules fused around the branch forming a sheath. The crowding of the leaves results in a pattern of stacked, inverted cones due to the overlapping of the stipule sheaths. On the top of the sheaths on either side of the petiole there are often projections, or spurs. The presence or absence of these spurs and their size are important taxonomic characteristics. Leaves and leaflets: All species of ''Polylepis'' have compound,
imparipinnate Pinnation (also called pennation) is the arrangement of feather-like or multi-divided features arising from both sides of a common axis. Pinnation occurs in biological morphology, in crystals, such as some forms of ice or metal crystals, and i ...
leaves, but the number of pairs of leaflets varies within and between species. The arrangement of the leaflets and the position from the terminal leaflet of the largest pair of leaflets determine the shape of the leaf. The outline of the leaf is usually rhombic in species with one pair of leaflets. Depending on the position of the largest pair, the leaf can be trullate to obtrullate in taxa with more than one leaflet pair. Leaf anatomy: The leaves of all species are built on a
dorsiventral A dorsiventral (Lat. ''dorsum'', "the back", ''venter'', "the belly") organ is one that has two surfaces differing from each other in appearance and structure, as an ordinary leaf. This term has also been used as a synonym for dorsoventral organs, ...
arrangement of cells, with the
epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water relea ...
and
palisade A palisade, sometimes called a stakewall or a paling, is typically a fence or defensive wall made from iron or wooden stakes, or tree trunks, and used as a defensive structure or enclosure. Palisades can form a stockade. Etymology ''Palisade'' ...
layer on the
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