THE VECETATION OF THE JUAN FERNANDEZ ISLANDS 887 



escarpment; the westernmost forest patches are found here. The strata dip southeast, 

 a direction consequently taken by the valleys, which run down to the south coast 

 without reaching sea-level, as abrasion has been more effective than erosion. The 

 reason for this is that the ridge west of Cerro Tres Puntas rapidly loses height, 

 reaching its lowest point between the Padre and Carbajal coves, separated by the 

 so-called Puente. only about 50 m high. Thus, the long western promontory does 

 not force the winds up to an altitude sufficient to cool the air and cause the moisture 

 to condense; rainfall is small, and the shallow gullies are dry most of the year. 

 This is the barren, treeless section of Masatierra. In the eastern half of the island 

 the lower slopes and the distal part of the valley bottoms also are barren; down 

 toward the sea they may have been treeless always — this was, as we have seen, 

 Johow's opinion — but their poverty is otherwise, as shown by historical documents, 

 due to man's destructive influence (Figs. 32 — 34, Pis. 86, 90: i). 



General Distribution of the Plant Communities. 



The principal plant communities may be grouped under three headings, ever- 

 green dicotyledonous forest ("rain forest"), evergreen brushwood and scrub, and 

 natural grass-land. To these we shall add the vegetation along the streams and 

 waterfalls, the communities on rock faces, and the halophytic vegetation of the 

 shore. Communities of hydrophytes are absent; there is no lake or pond in either 

 island, and hardly a true water-plant in the streams, all of them small; there is 

 nothing that could be called a river. 



Where the forest has been cleared, it has been replaced by secondary types 

 of vegetation, the macal, formed by almost pure stands o{ Aristotelia }naqui, and 

 the weed-fields, formed by a mixture of indigenous and adventitious herbs and 

 grasses, or lacking all native species. 



No climatic upper timber-line exists in Masatierra; even the summit of El 

 Yunque harbours a small forest grove. Nevertheless, one easily gets the impression 

 that a timber-line is more or less distinctly developed at the foot of the backbone 

 of east and central IMasatierra, but it is a topographical or, as it were, edaphic 

 line, following the foot of the perpendicular rock wall where the forest is forced 

 to dissolve — see e.g. Figs. 31 and 3; and Pl. 88. The position of this line varies; 

 500 — 600 m is perhaps a common le\el, but it lies lower or higher according to 

 the circumstances. 



Nobody who has followed the trail from the settlement up the wooded spur 

 in the background of the valley to Portezuelo de Villagra has failed to observe 

 that the composition of the forest changes with increasing altitude and that there 

 is a sequence of belts, between which, however, no sharp limits can be drawn. 



\' alleys and ridges from Puerto Frances to Pangal. Starting 

 from the small cove up the valley, we cross the macal at about 200 m above 

 sea-level; it is not pure, as some members of the original luma {Xothoniyrcia) 

 forest are still left. As we climb Cordon Chifladores, the maqui falls back, and 



