Species composition and phytosociology of xerophytic plant communities after extreme rainfall in South Peru

We present a phytosociological overview of the arid and semi-arid montane vegetation of the province of Arequipa in southern Peru. The xerophytic vegetation was studied after extreme rainfall had promoted exceptionally lush vegetation and a high aboveground floristic diversity. We used TWINSPAN for classification and Detrended Correspondence Analysis for gradient analysis of our relevés. PC-ORD was used to show the hierarchical similarity structure of the syntaxa, and to compare them with related communities in Peru and surrounding countries from literature. We present a synoptic table, and describe the physiognomy, floristic composition, ecology and spatial distribution of the plant communities. In total, we recorded 187 plant species, including 50 endemics, in 196 phytosociological relevés distributed over 2030 km2 at an elevation between 2020 and 3260 m a. s. l. The relevés were assigned to three alliances in the class Opuntietea sphaericae. The vegetation consists mainly of native species of trees, shrubs, grasses, succulents, annual herbs, and ferns. The most diverse families were Asteraceae, Cactaceae, Solanaceae, Malvaceae, Boraginaceae, Fabaceae, Poaceae, Amaranthaceae and Pteridaceae. Within the class Opuntietea sphaericae, three alliances have been distinguished of which two are new. The Ambrosio artemisioidis-Weberbauerocerion weberbaueri comprising six associations was recorded on barren hillsides between 2000 and 2900 m a. s. l. in the Arequipa city boundary zones. The Corryocaction brevistyli defines xerophytic scrub between 2700 and 3200 m a. s. l. in semi-dry regions bordering the puna grasslands. It contains the Balbisio weberbaueri-Ambrosietum artemisioidis and the Aloysio spathulatae-Corryocactetum brevistyli, all in need of further investigation as they lack diagnostic species. A unit clearly distinguished by Weberbauerocereus rauhii and Neoraimondia arequipensis is here described as a new alliance, Neoraimondio arequipensis-Weberbauerocerion rauhii. It grows in inter-Andean valleys in dry regions (1100– 2200 m a. s. l.), with abundant cacti accompanied by few xerophytes