Plant communities linked with cryogenic processes in the Peruvian Andes

The Andean Cordillera is the second highest mountain range in the world after the Himalayas and therefore, one of the places where the cryogenic manifestations are more prominent. Tropical Andean glaciers usually present an ice-cap form, and various geomorphological forms -rock glaciers, block streams, morainic deposits, cryoplanation surfaces (sometimes mixed with volcanic pumices) with polygon soils, and solifluction terraces -can be distinguished in the surroundings. The study was carried out in the main glacial zones of Peru: Cordillera Blanca, Cordillera Central, Department of Puno (Allincapac and Yuracjasa), Department of Arequipa (Coropuna, Huarancante, Ampato and Imata plains), and Department of Tacna near the Tutupaca volcano. Above 4000 m (oro-and cryorotropical bioclimatic belts) we documented 152 plots using the Braun-Blanquet method, adding 287 relevés from other authors from Peru, and also from Venezuela to southern Argentina and Chile. To interpret the variability, geographical distribution and vertical continuum of the associations, the concepts of basal community (BC), derived community (DC), altitudinal form and geographic race were used. Field and bibliographic tables were synthesized, and arranged using two dendrograms as a result of applying the Sørensen index to compare glacial vegetation between Peru and other regions of South America. Rock glaciers support a rupicolous vegetation dominated by Valeriana nivalis, and Saxifraga magellanica on the more humid rocks. Block streams contain specific plant communities with Xenophyllum species (X. ciliolatum, X. dactylophyllum, X. decorum, X. digitatum and X. poposum), but Chaetanthera is also very important in these biotopes across the Andes. Cryoplanation surfaces, with more stable and deep soils, present a greater diversity of plants, such as Anthochloa lepidula, Dielsiochloa floribunda, Lachemilla frigida, Mniodes coarctata, Nototriche obcuneata, N. pedicularifolia or N. turritella. On solifluction terraces and flood surfaces, communities with Festuca rigescens and Trichophorum rigidum can be distinguished respectively. Deep clayey soils, support small pasturages of Deyeuxia minima and Aciachne pulvinata sometimes grazed, while the cushion vegetation caused by snowbreak streams is represented by Deyeuxia ovata and Werneria aretioides. From a syntaxonomical point of view, 32 Peruvian plant communities were recognized. Rock communities are the Senecio bolivarianus community-mono-specific plant community on humid rocks distributed form Huancayo to Cusco, the Asplenio triphylli-Melpomenetum moniliformis ass. novaa rupiculous association installed on granitic rocks of the Cordillera Blanca, the Senecio algens community associated with the basal part of the rocks of the humid puna, and the Senecioni culcitioidis-Valerianetum nivalisa characteristic rock community usually present on andesites and basalts from Lima to Cordillera El Barroso (Tacna) [this association includes the subassociation saxifragetosum magellanicae, found on semi-permanent humid rocks, the geographic race with Draba cryptantha (Cordillera Central), the geographic race with Draba brackenridgei (near Cotahuasi Canyon, Arequipa), the geographic race with Draba cuzcoensis (near Colca Canyon, Arequipa), and the thermic altitudinal form with Woodsia montevidensis (Callalli, Arequipa)]. The Xenophyllo-Englerocharion peruvianae alliance is represented by the following communities: Xenophyllo ciliolati-Plettkeetum cryptanthae-a humid puna association present on block streams and morainic deposits with superficial stones from the Cordillera Blanca to Allincapac (Puno) [this association includes an altitudinal form with Anticona glareophila, from the limits of the vegetation of the Cordillera Central, a variant of semi-fixed blocks with Xenophyllum digitatum, a variant of mobile blocks with Xenophyllum ciliolatum, a derived community (DC) with Chaetanthera cochlearifolia from Central Peru, found on clayey places that will evolve to the polygon soils colonized by the Stangeo rhizanthae-Weberbaueretum rosulantis association, and a DC with Valeriana globularis and Anthochloa lepidula on the same environments from southern Peru], and the Poa gymnantha-Cerastium peruvianum community, documented on volcanic conglomerates from Callalli (Arequipa). Nototricho obcuneatae-Xenophylletum poposi-installed on semi-fixed blocks of the altiplano of Peru and Bolivia in drysubhumid climate (its variability presents the sub-associations valerianetosum nivalis as a rupiculous aspect, and mniodetosum coarctatae on lightly sloping polygon soils), Nototricho-Mniodetum coarctatae ass. nova-cryorotropical vegetation on flat polygon soils enriched with the volcanic pumices of the altiplano, and the Belloo piptolepis-Dissanthelietum calycini-that indicates wetter soils without volcanic pumices in the altiplano-belong to the Nototrichion obcuneatae alliance. The Deyeuxion minimae alliance indicates deeper and more humid soils, where we can differentiate five associations: Nototricho pinnatae-Lachemilletum frigidae-present on the rock cornices and polygon soils coming from intrusive geologic materials of the Cordillera Blanca, Pycnophyllo mollis-Festucetum rigescentis-very typical on solifluction terraces of the humid puna of Peru and Bolivia, Deyeuxio minimae-Trichophoretum rigidae-on flooded surfaces of the humid Peruvian Andes, Azorello diapensioidis-Deyeuxietum minimae-on humid, deep and clayey cryogenic soils, sometimes with very little superficial stones [this association includes an altitudinal form with Deyeuxia rigida, an altitudinal form with Pycnophyllum molle, a variant on incipient solifluction terraces with Dissanthelium macusaniense, and another variant on deep and humid soils with Werneria nubigena], and Gnaphalio badii-Aciachnetum pulvinatae grazed vegetation in the orotropical belt. Finally, Deyeuxio ovatae-Wernerietum aretioidis ass. nova is a cushion association belonging to the Plantagini-Distichietea class occurring between 4800 and 5000 m a.s.l. To study the relationships between plant communities and some selected climatic parameters (T, M, m, It, P, Pm and Hm-see abbreviations of the Table 6), we have made a biplot from a Principal Component Analyses for each plant community group (rock communities, Xenophyllo-Englerocharion, Nototrichion obcuneatae, and Deyeuxion minimae and other syntaxa. Plant communities placed at high altitude or in dry puna (Oruro-Arequipeña biogeographic province) are linked with the smaller values of the lowest mean temperature of the coldest month (m), while those placed in the humid puna (Ancashino-Paceña biogeographic province) are linked with the highest values of the highest mean temperature of the coldest month (M). Finally, the syntaxa Empetro rubrum-Balecetea gummiferae, Hamadryo kingii-Oreopolion glacialis, Leucherio hahnii-Nassauvietum juniperinae and Empetro rubrum-Oreopoletum glacialis, described earlier from Southern Patagonia, are typified.