104 High-Elevation Andean Ecosystems centered on the puna have representatives in pa´ramo and/or southern Andean steppe, among them Werneria, Xenophyllum, Nototriche, Weberbauera Until good phylogenies become available, of course, it cannot be ascertained whether these genera originated in the puna to migrate north and south Humid Puna Figure Puna in Bolivia dominated by Festuca spp 1986) South of 24–251 S the western flanks of the Andes experience a radical shift in climate Here the scarce puna precipitation is derived from the Polar front and falls mostly during the winter months (Arroyo et al., 1988) Compared with pa´ramo and southern Andean steppe, the puna landscape is vegetationally less diverse (Figure 3) and can be monotonous in the drier areas Showy flowers are less frequent than in the pa´ramo and southern Andean steppe While insect pollination occurs, bee pollination is less frequent, and pollination rates are generally lower than in southern Andean steppe (Arroyo and Squeo, 1990) With increasing elevation and aridity wind pollination becomes more important (Arroyo et al., 1983) Three altitudinal vegetation belts are often recognized in puna: (1) the subalpine belt; (2) a low alpine belt, and (3) a high alpine belt where permafrost is usually close to the soil surface and only a few species forming small rosettes and grasses are found (Ruthsatz, 1974; Villagra´n et al., 1981, 1983; Arroyo et al., 1988) Local puna floras, especially in dry and desert puna, are generally more impoverished than in the pa´ramo and in southern Andean steppe (Table 2) Currently there is no published figure for the entire vascular flora of puna An early estimate of the puna flora of northern Chile came up with 865 species (Arroyo et al., 1997) while for equivalent latitudes on the eastern side of the Andes in Argentina the estimate is for 1034 species (Novara, 2003), with Asteraceae, Poaceae, Fabaceae, and Solanaceae dominating, and with many species shared on the two sides of the Andes It has been estimated that the total puna flora, taking its full extent in Peru, Bolivia, Chile, and Argentina into account, could come close to 3000 species (Arroyo, unpublished data) Large plant families include: Asteraceae, Poaceae, Fabaceae, Malvaceae, Solanaceae, Caryophyllaceae, Gentianaceae, Brassicaceae, Valerianaceae, and Calceolariaceae Compared with pa´ramo, it can be seen that there are fewer large families with tropical affinities in puna The authors caution that the puna is by far the most difficult of the three Andean areas to delimit physically, such that the estimate given here is likely to change in either direction on more detailed study Despite the very different climates of pa´ramo, puna, and southern Andean steppe, and some significant biogeographical barriers, several genera Humid puna extends from north central Peru adjacent to the pa´ramos (jalcas) in southeasterly direction to along the eastern margin of the Altiplano and Eastern Cordillera of Bolivia, with a small southward easterly extension into Argentina (Cabrera, 1968) Annual precipitation ranges from 400 to 1500 mm It supports a well-developed plant cover and is often dominated by an uninterrupted carpet of grasses (e.g., Calamagrostis, Festuca, and Stipa) Other common genera of the humid puna include prostrate, rosette, or cushion species of Azorella, Daucus, Baccharis, Werneria, Draba, Echinopsis, Gentiana, Geranium, Hypsela, Isoetes, Lilaeopsis, Lupinus, Nototriche, Ourisia, Oxychloe, Oriastrum, Plantago, and Pycnophyllum, low shrubs in Adesmia, Calceolaria, Mutisia, Satureja, Tetraglochin, and several species of Senecio of diverse habit A good number of species puna species are shared with pa´ramo Dry Puna Dry puna is found in southern Peru just north of Arequipa, from along the western Altiplano of Bolivia, in northernmost Chile to around 201 S and comprises practically all of the puna found in northwestern Argentina, including the fairly moist kind of puna found in Cumbres Calchaquı´es in the Province of Tucuman (Cabrera, 1968) Rainfall ranges from 100 to 400 mm per year and ground cover is usually less than 50% At the lower elevations, dry puna is dominated by extensive shrublands and thickets of the resinous tola shrubs, such as Acantholippia, Parastrephia, Fabiana, Chuquiraga, Junellia, and Chersodoma, Nardophyllum, Ephedra, Acantholippia At higher elevations stiff bunchgrasses of Festuca orthophylla become common, interspersed with species of Nototriche, Werneria, Adesmia, Baccharis, Ephedra, Senecio, Pycnophyllum, and Tetraglochin The hard cushion plant Azorella compacta is commonly found abundantly in dry puna Desert Puna Desert puna is located mostly in the southern part of the Central Andes along the western cordillera of Bolivia and adjacent to the Atacama Desert region of northern Chile It receives o100 mm annual precipitation The vegetation is sparse (o15% cover) and local species richness tends to reach its lowest for puna (Table 2); in contrast with more typical elevational gradients, species richness peaks at mid-elevation Species richness (including the desert edge) drops from 333 species at 181 S in dry puna to 59 at 241 S in desert in the Chilean Andes and is much lower than across the Andes in Argentina (Arroyo et al., 1988) Generally the vegetation is composed of xerophytic plants, including cushion-forming cacti, thorny shrubs, and bunchgrasses such as Festuca