Tuatara: Volume 22, Issue 1, February 1976
The Ecology of Nothofagus and Associated Vegetation in South America — Introduction
The Ecology of Nothofagus and Associated Vegetation in South America
This paper reports on observations and research during a visit to Chile and Argentina in 1971-72, against the background of some of the extensive literature on the ecology of Nothofagus, the southern beech, in these two countries.
The 10 species of Nothofagus in South America are variously distributed from latitude 56° S. to latitude 33° N. The southern limit is due only to lack of land, the northern limit to the aridity that accompanies the Mediterranean climate of central Chile.
In comparing South America's Nothofagus with those of New Zealand, there are several biological differences, some shared with the other temperate Nothofagus of Australia and Tasmania.
These characters are summarised on Table 1; the most striking is the lack of relation between the pollen group/genetic separation and biological behaviour. Deciduousness, not found in New Zealand Nothofagus, and vegetative regeneration, normally lacking in New Zealand Nothofagus, are indiscriminately scattered among the two genetically separated groups, named on the pollen types, ‘N. menziesii’ and ‘N. fusca’ (Cranwell, 1939) and the corresponding ‘N. obliqua’ and ‘N. dombeyi’ types of Heusser (1971).
Likewise the leaf size, small as in N. menziesii, medium as in N. fusca or large and Fagus-like as in N. moorei and N. obliqua, show no grouping corresponding to the pollen genetic groups.
The character of deciduousness, classically, is associated with climatic extremes, of cold or of drought. The Tasmanian deciduous N. gunnii, a high altitude small tree fits this hypothesis. Seven of the South American species are deciduous but show direct relation to climate only in the far south, where the evergreen species N. betuloides occupies the wetter and more isothermal western end of a precipitation gradient from west to east (Fig. 2).
Comparisons of pollen grouping (Cranwell, 1939; Heusser, 1971), and biological characters of Nothofagus in New Zealand, South America and Australia. (Data from various sources; see references.)
Summarised forest distribution, across Chile and Argentina at about latitude 40 S. to show distribution of deciduous and evergreen trees. Altitudes on vertical scale are approximate. All species of Valdivian rainforest are evergreen, except N. obliqua. (From Weinberger. 1973 (coast range). Dimitri (1962) and personal observations)
On ascending the Andes, or the inland flank of the Cordillera de la Costa, N. obliqua is replaced by another deciduous tree, N. alpina, in climates still of continental tendency. The next species upwards is the page 41 evergreen N. dombeyi, localised in a shallow altitudinal zone in the Cordillera de la Costa, and a deeper altitudinal zone on the west and east flanks of the Andes. Weinberger's characteristation of the climate of N. dombeyi stands is of sufficient humidity and limited temperature range to be oceanic. His data is, however, only from Chile; on the eastern flank of the Andes, in Argentina, N. dombeyi forms a continuous forest out to the semi-arid and deciduous N. antarctica forest of the Patagonian plateau.
Above the evergreen N. dombeyi is the deciduous N. pumilio; it is the treeline species, in a high altitude, in a cool climate of oceanic tendency. The very plastic N. antarctica, also deciduous, forms a subalpine scrub, and occupies frost hollows, both areas characterised by Weinberger as having wide diurnal temperature ranges.
There remains the apparent anomaly of the evergreen N. dombeyi sandwiched, on both sides of the Andes, by deciduous Nothofagus; in subcontinental climates below, and humid mountain climates above in Chile; but extending into the subcontinental climate of Argentinian Patagonia.
The other deciduous species of wide distribution, N. glauca, not studied by Weinberger, has its range within the range of N. obliqua, and in similar climates. N. alessandri and N. leoni, both deciduous, are of very restricted occurrence; and in climates at the more arid range of the more common N. obliqua and N. glauca.
The dispersonal of Nothofagus in South America shows a major difference from the pattern in New Zealand. Forest type maps, both published (1964) and seen in draft, show no major gaps in the distribution of Nothofagus in South America such as are found in New Zealand. Where there is enough moisture, and summer temperature, Nothofagus will grow except on the very cloudy oceanic flanks of the Cordillera de la Costa (Weinberger, 1973). This continuous distribution in its northern range exists despite a history of vulcanism that extends into the present day, as forest destroying lava flows, and volcanic ash showers up to 60 cm deep in the last 50 years. The coppicing ability of the widely distributed N. obliqua, N. glauca, N. alpina and N. pumilio in its more northern distribution may be connected with their survival after deep ash showers.
N. pumilio, which coppices at medium latitudes (37°-48° S.) (F. Schlegel pers. comm.) but not in the south of its range to 55° S., displays a parallel to the behaviour of N. cunninghamii; with only limited coppicing in Tasmania, but vigorous regeneration by this means in Victoria (Howard, 1973).
In the southern part of its range, Nothofagus is represented by three species, only one of which reproduces vegetatively there (N. antarctica). Yet this southern range of the three species includes ice sheets today reaching sea level, and still Nothofagus has made its way on to all the glacier fragmented landscape of the Southern Chilean ‘Canales’.page 42
Comparing this over-all distribution with the fragmentation of New Zealand's Nothofagus by Pleistocene ice, and by recent vulcanism (Wardle, 1964), one associated factor differing is the presence of terrestrial mammals indigenous to South America; and among these, notably the great variety of rodents. Little seems known of their food preferences but a parallel with the European squirrel as a seed eater, transporter and forgetful storer is apt, as one possible explanation of seed transport of Nothofagus in South America.
The isolation of the large land mass of South America from the north, from Cretaceous to Plio-Pleistocene times (Graham, 1972), has apparently been allowed sufficient genetic diversity to persist in Nothofagus on a large land mass. This diversity is expressed by the vegetative reproduction of four species and the deciduous character of six species, both characters absent from New Zealand Nothofagus. Australia, with a larger land mass, has likewise among its three species, the characters of vegetative reproduction and deciduousness.
This paper is in two parts, the first dealing with Nothofagus and the vegetation surrounding its forests in the Magellanic areas of Chile and Argentina. The second part, to appear in a subsequent issue, deals with forest distribution in the more temperate regions, around latitude 41° S., and more comparable climatically with New Zealand.