Polylepis
| Polylepis | |
|---|---|
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| Polylepis rugulosa | |
| Scientific classification | |
| Kingdom: | Plantae |
| (unranked): | Angiosperms |
| (unranked): | Eudicots |
| (unranked): | Rosids |
| Order: | Rosales |
| Family: | Rosaceae |
| Subfamily: | Rosoideae |
| Tribe: | Sanguisorbeae |
| Genus: |
Polylepis Ruiz & Pav. |
| Species | |
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Polylepis (pronounced pah-lee-LEE-piss or pah-lee-LEH-piss) is a genus comprising twenty eight 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 arborescent angiosperm genus in the world.
The genus Polylepis is very complicated taxonomically. It 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 Erickssonn et al. (2003) showed evidence of a close relationship between Polylepis and Acaena, 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, (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 vegetation, caused by the 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.
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.
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