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The contemporary genetic structure of species offers key imprints of how organisms responded to past geological and climatic events, which have played a crucial role in shaping the current geographical distribution of north-temperate organisms. In this study, range-wide patterns of genetic variation were examined in Douglas-fir (Pseudotsuga menziesii), a dominant forest tree species distributed from Mexico to British Columbia in western North America. Two organelle DNA markers with contrasting modes of inheritance were genotyped for 613 individuals from 44 populations. Two mitotypes and 42 chlorotypes were recovered in this survey. Both genomes showed significant population subdivision, indicative of limited gene flow through seeds and pollen. Three distinct cpDNA lineages corresponding to the Pacific Coast, the Rocky Mountains, and Mexico were observed. The split time of the two lineages from the Rockies lineage was dated back to 8.5 million years (Ma). The most recent common ancestors of Mexican and coastal populations were estimated at 3.2 and 4.8 Ma, respectively. The northern populations of once glaciated regions were characterized by a high level of genetic diversity, indicating a large zone of contact between ancestral lineages. A possible northern refugium was also inferred. The Mexican lineage, which appeared established by southward migration from the Rockies lineage, was characterized by the lowest genetic diversity but highest population differentiation. These results suggest that the effects of Quaternary climatic Communicated by S. Aitken Electronic supplementary material The online version of this article (oscillations on the population dynamics and genetic diversity of Douglas-fir varied substantially across the latitudinal section. The results emphasize the pressing need for the conservation of Mexican Douglas-fir.

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The establishment of clonal seed orchards is a viable option for the continuous production of improved seed of desired genotypes. Grafting is the main technique used to establish clonal seed orchards. The objective of this study was to examine how the geographic location and the age class of the donor trees of buds, the phenological status of the buds, and the anatomical characteristics of the scions and the rootstocks affect the survival and growth of Pinus engelmannii Carr. grafts. Scions were collected from two trees in each of three age classes (young, middle-aged, and old). Grafting was performed with buds in two physiological states (end of dormancy and beginning of sprouting). Cross-sections of the grafted organs were obtained for anatomical analysis. A Cox proportional hazards model was used to determine the effects of the variables that were considered. The age class of the scion donor trees, the total area of the cut surface of the scion, and the density of resin channels in the scions significantly affected (p < 0.05) survival of the grafts. By contrast, the physiological state of the buds and the other anatomical characteristics of the grafted organs did not significantly affect graft survival. In P. engelmannii, grafting was most effective when scions from middle-aged trees were used. Graft survival was enhanced by a small total area of the cut surface of the scion and low density of resin channels in the scions. The area of the cambium of the scions directly influenced growth of the grafts.

<p>El objetivo de este trabajo fue conocer la estructura, heterogeneidad de estadios y el patrón de dispersión espacial de diez poblaciones de <em>Dioon holmgrenii </em>De Luca, Sabato &amp; Vázq. Torres, especie en peligro de extinción y endémica al sur de Oaxaca, con fines de conservación. Se establecieron once y nueve unidades de muestreo en sitios sin cambio aparente y con cambio de uso de suelo, respectivamente. Los individuos se clasificaron en reproductivos y no reproductivos, y en cada uno los estadios de desarrollo. La densidad promedio de individuos fue mayor en sitios sin cambio aparente que en sitios con cambio (782 <em>vs.</em> 353 plantas ha^-1). En sitios sin cambio aparente se determinó alta proporción de plántulas y adultos, y escasos juveniles y pre-reproductivos; en sitios con cambio se encontró alto porcentaje de adultos, baja proporción de plántulas, juveniles y pre-reproductivos. La heterogeneidad de estadios fue mayor en sitios sin cambio aparente que sitios con cambio (<em>H´</em> = 1.57 <em>vs</em>. 1.38, <em>P</em> = 0.048). La semejanza dentro de sitios sin cambio aparente (<em>I_M-H</em> = 0.93, <em>IS_ct</em> = 0.81) y dentro de los sitios con cambio (<em>I_M-H</em> = 0.83, <em>IS_ct</em> = 0.61) fue mayor que entre ambas condiciones (<em>I_M-H</em> = 0.70, <em>IS_ct</em> = 0.53). El patrón de dispersión espacial de los individuos fue principalmente agregada (65%). Los individuos reproductivos y no reproductivos se distribuyeron en agregados en 12 y 14 de 20 unidades de muestreo, respectivamente. Los resultados muestran que la agricultura y el pastoreo afectan en la estructura, heterogeneidad de estadios y dispersión espacial; esto debe considerarse en el manejo futuro de las poblaciones.</p>

Symptoms of forest decline, apparently due to climate change, have become evident in the last 10 years on the Trans-Mexican Volcanic Belt and northwestern temperate forest of Mexico, particularly at the xeric (low elevational) limit of several forest tree species. We review and provide recent evidence of massive infestation of timberline Pinus hartwegii Lindl. by the mistletoes Arceuthobium globosum Hawksw. & Wiens and Arceuthobium vaginatum (Humb. & Bonpl. ex Willd.) J.Presl; insufficient Abies religiosa (Kunth) Schltdl. & Cham. seedling recruitment at the Monarch Butterfly Biosphere Reserve; indications of inbreeding and defoliation in endangered Picea chihuahuana Martínez, Picea martinezii T.F. Patt., Picea mexicana Martínez, and extreme southern populations of Pseudotsuga menziesii (Mirb.) Franco; and the incidence of unusual pest and disease outbreaks (e.g., Dendroctonus Erichson, 1836 spp., Neodiprion autumnalis Smith, and Phytophthora cinnamomi Rands) in several conifer and oak species. We also discuss a difficult question: Is natural genetic variation sufficient to provide populations with the adaptive variation necessary to survive the natural selection imposed by projected climate change scenarios, or will phenotypic plasticity be exhausted and populations decline? Controversial ex situ conservation within natural protected areas, assisted migration, and translocation of species ensembles are discussed as options by which to accommodate projected climatic change impacts on the management and conservation practices of the megadiverse Mexican temperate forest.