by
Marie-Françoise Passini
Laboratoire de Botanique tropicale
Université Pierre & Marie Curie
1, rue Guy de la Brosse - 75005 Paris, France
INTRODUCTION
The American pinyons belong to the subgenus Strobus Lemm (Haploxylon) (Shaw 1914, Critchfield and Little 1969), section Parrya (needles 1 to 5 with dorsal umbo), sub-section cembroides Engelm. characterised by pines with fascicles of 1 to 5 needles, usually whole, 2 to 9 cm long, and large wingless seeds.
Three species belonging to the above, viz. Pinus maximartinezii Rzed., P. nelsonii Shaw and P. pinceana Gord., have well defined characteristics and a limited natural range in Mexico. The others, viz. P. cembroides Zucc., P. catarinae Robert-Passini, P. culminicola Andr. and Beaman, P. discolor Bailey and Hawk, P. edulis Engelm., P. johannis Robert, P. monophylla Torr., P. quadrifolia Sudw. and P. remota (Little) Bailey and Hawk, are difficult to distinguish and therefore they are generally combined under the name P. cembroides, “the cembroides group” (Lanner 1974 and Robert-Passini 1981).
Natural range of “the cembroides group” of pines in Mexico
The natural range of P. cembroides lies between latitudes 18° and 32° N. The species is found bordering the arid areas of the Central Plateau and in isolated occurences on the massifs scattered throughout this Plateau. It has a more or less continuous distribution range along the eastern slopes of the Sierra Madre Occidental of the Nuevo Casas Grandes mountains in the south of the State of Durango, extending into the State of Zacatecas, and becoming fragmented as it curves towards the east. To the east of the Central Plateau, only scattered stands are found (Passini, in press).
Climatic conditions
Mean rainfall varies from 266 mm (Ramos Arizpe, Coah.) to 780 mm (Mazapil, Zac.). The rains fall in summer or at the beginning of autumn, except in northern Baja California, which receives more than 36 percent of its total annual rainfall during winter.
The woody formations of P. cembroides are thus found in a climate of a semi-humid, transitional or semi-arid type. However, north of the Tropic of the Cancer, the annual temperature variation is more than 10° C (States of Coahuila, Chihuahua, Durango, southern Baja California), while to the south of this line the annual temperature variation is below 10° C (San Luis Potosí, Guanajuato, Queretaro, Hidalgo, Veracruz, Puebla).
Characteristics of “the cembroides group” of pines
Of all the characteristics which distinguish the various species of “the cembroides group” from each other, the most immediately identifiable one is the number of needles per fascicle. If one considers this characteristic alone, the species in “the cembroides group” appear to follow a continuous, evolutionary cline, in which P. monophylla and P. culminicola represent the two extremes.
The taxonomic characteristics which distinguish the species within “the cembroides group” from each other are summarized below:
| 1. | Shrubby form, stem ramified from the base | ||
| 2. | Needles in clusters of 5; 1 dorsal resin canal; seeds with white kernel; average number of cotyledons 9 | P. culminicola | |
| 2.1 | Needles in groups of 3, sometimes 2, occasionally 4, bicoloured; 2 resin canals; no stomata on dorsal side; seeds with white kernel; average number of cotyledons 8.7 | P. johannis | |
| 2.2 | Needles in groups of 2 (sometimes 3); 2–4 resin canals; seeds with white kernel | P. catarinae | |
| 1.1 | Tree-like habitat, clearly developed trunk | ||
| 3. | Needles large and hard | ||
| 4. | Single needles; 1–17 resin canals; seeds with white kernel; average number of cotyledons 7 | P. monophylla | |
| 4.1 | Needles in groups of 2; 2 resin canals; seeds with white kernel; average number of cotyledons 8.2 | P. edulis | |
| 4.2 | Needles in groups of 4; 2 resin canals; seeds with white kernel | P. quadrifolia | |
| 3.1 | Flexible needles | ||
| 5. | Needles in groups of 3, bicoloured; 2 resin canals; no stomata on dorsal side; seeds with thick husk 0.5 – 1 mm | P. discolor | |
| 5.1 | Needles in groups of 2–3; 2 resin canals; seeds with thick husk 0.2 – 0.5 mm, with pink kernel; average number of cotyledons 10.5 | P. cembroides var. cembroides | |
| 5.2 | Needles in groups of 2–3 (more often 3); 2 resin canals; seeds with thin husk 0.1 – 0.3 mm, with pink kernel; average number of cotyledons 12.6 | P. cembroides var. lagunae | |
P. cembroides var. cembroides grows at altitudes of 1 500 to 2 800 m. In the Sierra Madre Occidental and within the massifs in the south of the State of San Luis Potosí, it is sympatric with P. discolor (Bailey and Hawksworth 1979).
P. cembroides var. lagunae is found only in the south of the State of Baja California, in the Sierra de la Laguna, at 1 500 to 2 200 m altitude. This variety is faster growing than P. cembroides var. cembroides and is therefore more suitable for reforestation work in dry zones. The plantations carried out by the Agricultural University “A. Narro” of Saltillo show that it can withstand a pH of 7–8, in spite of the fact that the pH of the soils of the Sierra de la Laguna is between 4 and 5.
The shrubby forms of the species group have limited distribution ranges. P. catarinae grows at an altitude of 1 100 to 1 600 m in the mountainous area of Monterrey. Two other shrubby forms are found at altitudes of over 2 500 m: P. johannis, which grows between Mazapil and Concepcion del Oro (Zac.) at an altitude of 2 500 to 2 800 m; and P. culminicola, strictly confined to the Cerro Potosí and the Sierra de la Marta, at altitudes of 3 200 m or more. The stands of P. culminicola are threatened both by domestic animals and by attacks of pests and fungi. Collection of seeds will be necessary to safeguard the species from extinction.
As evidenced above, “the cembroides group” of pines represents a complex, genetic group of species. Apart from the actual commercial value of the seeds of these species, they have a potential value in soil conservation and environmental amelioration. It is therefore of great importance that the research presently underway be continued.
BIBLIOGRAPHY
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Manuscript received in March 1982.
