In the territory of the Central Pampa which includes this formation and partly in the province of San Luis there are large extensions covered by caldenes, good looking trees, with large rough trunk, of great height, very branchy and which are actively exploited for wood and to pave the streets of Buenos Aires. The taxonomy of these caldenes is somewhat debated; it is attributed to a variety of Prosopis algarrobilla Gris.
The tree that mixes with the previous ones is the quebracho blanco, apocinacea of the genus Aspidosperma, whose bark is rich in alkaloids, in relation to the chemical constitution of the soil. In the Sierra de Córdoba there is also a tree with leaves similar to the avellana, the Ruprechtia corylifolia of the Polygonaceae, together with another one called coco, which is the Fagara coco, whose bark is covered with large spines and the leaves are rich in glands containing an essential oil.
Cactaceae abound in the woods, very curious plants that give the landscape a special aspect that draws the attention of the traveler. These species of tunas, quiscos, pencas, or cardones have been well studied by Dr. Spegazzini in several works, recently summarized in the family monograph by the North American botanists Britton and Rose.
Among the numerous plants of this family, two species are particularly interesting: Trichocereus pasacana (Web.) Britt. and Rose and Trichocereus Terschecki (Parm.) Britt. and Rose. The first is common in all the plateaus and mountain slopes of the provinces of Catamarca, Salta, Jujuy and Tucumán in shady and stony places; it is an exclusively alpine plant since it never lives at a height less than 1000 meters and reaches up to 3500 meters; the stature of the individuals varies with the height above sea level, reaching 5 meters in the lower places, where it also appears branched; the natives use its woody skeleton all reticulated and perforated for the construction of houses. The other species is what they call cardón del valle, which never exceeds a thousand meters above sea level; with a quicker habit than the previous one, it reaches almost 12 meters in height, giving beautiful large flowers; in the Calchaquí valleys, women use thorns to weave stockings. Given the nature of the soil of the environment, the grasses of the forest are characterized by hard, narrow and rolled leaves. They belong to the genera Stipa, Aristida, Boutelona, Pappophorum, Chloris, Trichloris, Poa etc.
Composites of almost always woody consistency also abound, especially of the genera Baccharis, Senecio, Eupatorium, Vernonia, Tessaria, Flourensia, etc. Of the first genus there are toxic species such as romerillo (Baccharis coridifolia DC) which owes its deadly effects to a resinous substance.
Finally, we will note the existence in this formation of a palm tree, the Trithrinax campestris (Burm.) Drude and Gìris., Found in certain places in the sierras of San Luis and Córdoba, in the provinces of Santiago del Estero, Tucumán and Salta.. It is the same species as Entre Ríos.
According to justinshoes.net, the Andean region corresponding to the forest is characterized by a harsh and harsh climate and, so strong is the drought that reigns in those desolate districts, that a little rain constitutes an extraordinary event. It reaches a height that oscillates on 4000 meters; the water is very scarce and the vegetation is reduced to stunted, hard plants with deep woody roots, without leaves or with small, leathery or intricate leaves; characteristic is the presence of the yareta or llareta (Azorella yaretaHaum., Of the Umbelliferae) vegetable pillow that constitutes a real underground tree in which a mass of resin, earth and leaves is formed between branch and branch, which gives the plant a very unique aspect. Other plants found in this region are those called tola, which belong to various species of the genera Lepidophyllum, Baccharis etc .; there are also Leguminosae of the genera Astragalus and Adesmia, Violacee of the genus Viola, Rosaceae of the genus Acaenia, Cruciferae of the genera Draba and Hexaptera, Portulacaceae of the genus Calandrinia, Hard grasses of the genera Poa, Stipa, Melica, Elymus etc. It is the vegetation that precedes the perpetual snows.
Besides the mentioned Azorella, there are other species of the same genus and of the genus Bolax which are used as fuel in the local borax furnaces.
Man, in his eagerness to populate and civilize, has unconsciously introduced into this formation exotic plants which are of great benefit to pastoralism; such as the cosmopolitan species Erodium cicutarium L’Herit., commonly known as alfirelillo, and the legumes of the genus Medicago, called trébol de carretilla, which are excellent forage in spring and winter.
The immense phytogeographic region of forests and sub-tropical savannahs includes the whole north of the country, from the Sierra del Aconquija, the easternmost mountains that detach from the Andes, to a line that links the south of Tucumán with the center in the province of Corrientes, a line that is rather difficult to specify due to its excessive tortuosity.
In this formation we observe all the transitions between the woods with forest characteristics, clearly hygrophilous, such as those found in Misiones, and the dry woods of Gran Chaco, which include the north of the province of Santa Fe, part of the province of Santiago del Abroad, the eastern part of the province of Salta and the territories of Formosa and Chaco. And alternating with the woods we have savannas, that is, places that resemble grasslands, but with tall and hard plants, scattered with shrubs and palms.
The climatic characteristics are as follows:
The amount of rain precipitation reaches up to mm. 1500; the minimum quantity is 600 mm. and the number of precipitations is about 100, according to the places.
The part of this formation where there are true tropical forests is relatively small; this occurs in part of the territory of Misiones and in the provinces of Tucumán, Salta and Jujuy, where heat and humidity come together in such a way as to originate this exuberant vegetation, rich in trees that reach great heights and under which large quantities of herbaceous shrubs, ferns, mosses and epiphytes such as Orchids and Bromeliads. The presence of Triuridaceae and Burmanniacee, families whose species have a saprophytic life, has been reported in the forests of Misiones.