apparently it's a name applied to various species of Genista, i.e. broom - this can be prickly as well as not so; gorse/whin/furze are slightly different and more prickly

VEGETA PIORNALES (PIORNAIS, PIORNEDOS)
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Los piornales son formaciones caracterizadas por la presencia de Genista florida (=Genista polygaliphylla), conocida en gran parte de Galicia y regiones ...
www.xunta.es/conselle/cma/CMA05e/CMA05enHumidais/cast/intro... -

Herbario Virtual
(Familia Leguminosae) (n.v.: “piorno”) (Piornales) [Vertiente norte del Puerto de Ventana (Asturias)]. Genista occidentalis (Rouy) H.J. Coste (Familia ...
www.uniovi.es/bos/Asignaturas/Herbario Virtual/Herbario Vir... - 50k - Cached - Similar pages

JSTOR: (1262) Proposal to Conserve the Name Genista purgans ...
1984) pointed out that the Linnaean type of Genista purgans L., ... "genet griot" or "genet purgatif" and in Spain by the vernacular name "piorno serrano", ...
links.jstor.org/sici?sici=0040-0262(199611)45%3A4%3C699%3A(PTCTN%3E2.0.CO%3B2-E - Similar pages

The northern slope supports a sub-alpine flora, with great stands of pine, large tracts of Genista purgans -Piorno serrano- and rich meadows. ...
www.equiberia.com/nature.asp - 11k - Cached - Simil

genista anglica - definition by dict.die.net
Genista anglica n : prickly yellow-flowered shrub of the moors of New England and Europe ... a low prickly shrub (Genista Anglica) common in Western Europe. ...
dict.die.net/genista%20anglica/ - 8k

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Note added at 1 hr (2007-02-13 13:47:42 GMT)
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or tracts

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Note added at 1 hr (2007-02-13 13:55:18 GMT)
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or the Cytisus family, v. closely related to Genista

Piorno.- Bot. Arbusto muy ramificado, de 30-100 cm de alto, con forma normalmente hemisférica para protegerse de los fuertes vientos que azotan las cumbres en las que vive. Las hojas superiores tienen un folio y las inferiores tres, siendo estos de forma lanceolada y con envés pubescente. Flores olorosas, con corola de color amarillo-vivo y cáliz muy piloso. Fruto en legumbre negruzca en la madurez.

El piorno serrano pertenece a la familia de las leguminosas y durante mucho tiempo ha sido denominado Cytisus purgans, aunque actualmente se le conoce como Cytisus oromediterraneus. Aparece en las partes altas de las sierras, colonizando laderas secas y pedregosas, preferentemente silíceas.

http://www.piornal.net/cajondesastre/piorno.htm

Brooms are a group of evergreen, semi-evergreen, and deciduous shrubs in the subfamily Faboideae of the legume family Fabaceae, mainly in the three genera Chamaecytisus, Cytisus and Genista, but also in five other small genera (see box, right). All genera in this group are from the tribe Genisteae (syn. Cytiseae). These genera are all closely related and share similar characters of dense, slender green stems and very small leaves, adaptations to dry growing conditions. Most of the species have yellow flowers, but a few have white, orange, red, pink or purple flowers.

http://en.wikipedia.org/wiki/Broom_(shrub)