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Introduction: Communities dominated in the canopy by pinyon pines harbor species richness and have complex physiognomy because they form an ecotone between arid and temperate zones. Objectives: To describe the floristic attributes that distinguish four communities dominated by pinyon pines; define units based on floristic composition and structure; and determine the influence of edaphic factors. Materials and methods: The flora of four communities in the state of Zacatecas was recorded. The arboreal and shrub vegetation was analyzed quantitatively using the point-centered quarter technique. Semi-realistic physiognomic profiles were made for each site. The groups resulting from cluster analysis were ordered with edaphic factors through a canonical correspondence analysis. Results and discussion: Floristic richness was 244 species; each community contains between 60 and 80 species. Structurally, three physiognomic variants were recorded: forests dominated by Pinus cembroides Zucc.; association of P. cembroides - P. cembroides var. bicolor Little; and forest dominated by P. maximartinezii Rzed. The pH, electrical conductivity and soil texture have a great influence on the structure and plant composition of the communities. The P. cembroides - P. cembroides var. bicolor association and its accompanying species develop in nutrient-poor soils, while P. maximartinezii forests prefer shallow soils. Conclusion: The floristic similarity among communities was low. Each community must be managed differently, meeting the particular requirements of the species that develop there.

Pinus maximartinezii is a rare, endemic, threatened species known from a single small population in the state of Zacatecas, Mexico. Among the pine species that produce edible nuts, it produces one of the largest and most nutritious seeds. The seeds of P. maximartinezii have historically been used for human consumption. The cones are harvested directly from the trees, and the seeds are sold illegally in local, national and international markets. However, the effects of seed extraction must be thoroughly evaluated to determine the potential impacts on population stability. To assess the impact of different rates of seed harvesting on the demography of this species, a 2-yr study of population dynamics was conducted in three 0.1-ha plots. A 9 × 9 size-structured matrix model was used to simulate changes in population growth over time in conjunction with increasing stepwise reductions in fecundity. The population growth rate (λ) of P. maximartinezii was 1.1175, with a 95% confidence interval (CI) from 1.1008 to 1.1321, and it was relatively insensitive to changes in fecundity and growth. Under a seed extraction intensity of 99%, λ decreased to 1.0241, with a CI from 1.0177 to 1.0361. Elasticity analysis was then performed to identify the combined effects of proportional changes in fecundities and the largest stasis elements on λ. The results suggest that a sound conservation strategy should focus on improving the survival of juveniles and adults during their first reproductive events and on the largest adults, as well as on protecting the habitat of this threatened endemic species.

Pinus maximartinezii (Rzedowsky) and P. pinceana (Gordon), two endemic species of Pinus from Mexico, were successfully regenerated in vitro by organogenesis starting from mature embryos. In both species, 100% of the explants produced adventitious buds after an 18-day induction period on media containing 6-benzylaminopurine (BA) and a subsequent 8 weeks of incubation on growth regulator-free media. The most successful treatment (13.3 µM BA) resulted in an average of 14.6 and 25.9 buds per explant in P. maximartinezii and P. pinceana, respectively. Rooting was achieved by auxin pulses or by Agrobacterium rhizogenes inoculation. In the first case, roots formed on 13% and 7% of the auxin-treated shoots of P. maximartinezii and P. pinceana, respectively. In the second case, A. rhizogenes induced root formation on 60% and 67% of the inoculated shoots from P. maximartinezii and P. pinceana, respectively. The presence of the foreign genes transferred by A. rhizogenes into these roots was demonstrated by histochemical staining for β-glucuronidase activity, the polymerase chain reaction and Southern blot analysis.

Pinus pinceana) has a restricted and fragmented range, trees are widely scattered within populations, and reproduction is limited. Nevertheless, genetic diversity was high; based on 27 isozyme loci in 18 enzyme systems, unbiased expected heterozygosity averaged 0.174. Differentiation also was high (FST 5 0.152), reflecting isolation between southern, central, and northern fragments of the range. Among populations in the northern fragment, FST was only 0.056, and the number of migrants per generation (Nm) was 4.21, which should preclude fixation. Nm between central and southern populations or between them and populations in the northern fragment was lower, 0.99‐1.66, indicating a degree of genetic isolation. Multilocus outcrossing rates ( tm) ranged from 0.836 in the south to 0.897 in the north. Therefore, selfing is low but statistically significant. The equilibrium inbreeding coefficient

Pinus maximartinezii Rzedowski is a rare pinyon pine known to occur naturally on only one site on the Cerro de Pinones, in the Sierra de Morones, near the town ofJuchipila, in southern Zacatecas, Mexico(21° 21' N, 103° 13' W). P. maximartinezii has been considered one ofthe most rare and endangered ofall pine species(Farjon et a1.1993,Perry 1991, Dvorak et al. 2000). The entire distribution of the species is reported as approximately 400-1000 hectares and the population size about 2000 to 10,000 mature individuals (Donahue and Mar-Lopez 1995, Arteaga et al. 2000, Dvorak et al. 2000). Although trees will grow from 1450-m to 2550-m elevation, most of the population at Cerrode Pinones occurs in a band from 2100 to 2300 m. This species has attracted much attention in recent years due to its limited natural distribution, endangered status, hugeconesand edible nuts, and its potential as an ornamental.

Maxipinon (Pinus maximartinezii Rzedowski), which is confined to a single population of approximately 2000 to 2500 mature trees, covers about 400 ha in southern Zacatecas, Mexico. Genetic diversity measured by expected heterozygosity was 0.122, which is moderate for pines. However, percentage polymorphic loci was low, 30.3%. The fixation index (F) of 0.081 indicated only slight heterozygote deficiency. Mating system analysis indicated a significant but low level of selling; the multilocus outcrossing rate, t,,, was 0.816. The mean of single locus estimates, tS, was smaller (0.761), perhaps suggesting mating among relatives, although the difference between t,.. and ts was not sta- tistically significant. The most striking features of maxipifion's genetic structure were that no polymorphic locus had more than two alleles and most alleles at polymorphic loci were at intermediate frequencies. This is in contrast to other pines, which often have three to five or more alleles at some loci and in which the distribution of allele frequencies is U-shaped, most alleles being present at frequencies less than 10% or greater than 90%. A population with only two alleles per locus and at intermediate frequencies could occur if the population had been reduced to an extreme bottleneck and then expanded rapidly before random drift modified allele frequencies. A novel origin from a hybridization event would also explain the results. Significant gametic disequilibrium was detected at several pairs of loci in both maternal and paternal gametes. The presence of disequilibrium is in agreement with an origin from an extreme bottleneck, perhaps even a single seed. Furthermore, it demands that the event be relatively recent. The number of generations, as calculated from the observed mean disequilibrium, suggested that maxipinion derived from an extreme bottleneck four to five generations ago, which is less than 1000 years in this species.

The response of Pinus maximartinezii Rzedowski to photochemical oxidant air pollution was examined using 100 trees, during a 1 year cycle, at Vivero de Coyoacan, a tree nursery located in the south central part of Mexico City, where exposure to polluted air masses has been continuous. The tree response assessment method was based upon documented symptoms of pollutant injury on the foliage. The results showed a homogeneous pattern of health and vigor and only medium sensitivity (based on foliar injury) to photochemical oxidants, although the trees maintained their needles through a three and not the normal 5 year period as at its geographic location of origin (different habitat or environment). Nevertheless, these data suggest that this species of pine could be proposed as good planting material for reforestation in the urban metropolitan area of Mexico city.

Wood from 92 trees of Pinus culminicola, Pinus nelsonii, Pinus pinceana and Pinus maximartinezii from nine localities and oleoresin from several trees of Pinus nelsonii from one Iocality were analyzed for monoterpenes by gas liquid chromatography. Monoterpenes of Pinus nelsonii were composed mainly of α- and β-pinenes, while the main constituent of the monoterpenes of Pinus pinceana and Pinus maximartinezii was limonene, present in amounts of about 90 percent. The similarity in composition of the monoterpenes of Pinus pinceana and Pinus maximartinezii strongly supports the contention of Rzedowski [12] of the close evolutionary relation of the two pines. Pinus culminicola wood produced monoterpenes composed mainly of α-pinene, sabinene, terpinolene and limonene, with other monoterpenes occurring sporadically in sizeabie amounts. The variability between individual populations was relatively high, probably due to their long-time isolation by the intervening low-elevation barriers. Appreciable differences were found in compositions of monoterpenes from stem and from twigs of Pinus culrninicola.

ResumeLa formation ligneuse a Pin us maximartinezii s'etend de 1800 a 2300 m. Les plus vieux arbres, producteurs de cones, ont de 150 a 200 ans. Le pin se regenere vigoureusement dans les espaces verts.