Picea Chihuahuana Research Articles

Recent evidence of Mexican temperate forest decline and the need for ex situ conservation, assisted migration, and translocation of species ensembles as adaptive management to face projected climatic change impacts in a megadiverse country

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.

Distribution dynamics of Picea chihuahuana Martínez populations under different climate change scenarios in Mexico

ObjectivesSpecies with a restricted distribution in isolated populations are the most threatened under climate change. Chihuahua spruce (Picea chihuahuana Martínez) is one of those species. The analyses of past and present environmental conditions where these species have occurred allow us to understand the dynamics of their distribution. Therefore, projecting the results to the future under different climatic scenarios provides a basis for conservation purposes. The objective of this study was to evaluate the influence of climatic variables on the availability of suitable habitat for P. chihuahuana under paleoclimatic, current and future scenarios. MethodsPaleoclimate data for the Last Glacial Maximum (LGM ∼ 22,000 years BP), Holocene Medium (HM ∼ 6000 years BP) and climatic data from the 1950–2000 periods were derived from CCSM4 and MIROC-ES models. Carbon dioxide concentration for 2050 and 2070 were derived from CCSM4 y HadGEM2-ES models under four levels of forcing. Data for the different time periods was subjected to the MaxEnt program. ResultsThe species distribution models showed a good fit with values of AUC> 0.99 for all time periods. Since the LGM, the potential habitat for P. chihuahuana has been declining, most likely due to global warming. Currently, human activities have exacerbated this decline. Climatic variables that most contributed to the models of suitable habitat were the lowest temperature of the coldest month (Bio6), the precipitation of the coldest quarter (Bio19), the precipitation of the warmest quarter (Bio18), and the annual average temperature (Bio 1). Future scenarios predict a reduction of suitable habitat for P. chihuahuana, or even its disappearance. ConclusionsProjections into the future locate possible micro refuges under some climatic change scenarios; however, under the highest atmospheric greenhouse gases scenario, suitable habitats for this species will disappear. Depending upon the response by humans to mitigate climatic change, P. chihuahuana may persist longer in those micro refuges, either naturally or with assisted transplanting. On the other hand, if there are no actions to reduce GHG, it is futile to develop conservation programs because there will be no suitable habitat for this species, and it will become extinct.

Observed and projected impacts of climate on radial growth of three endangered conifers in northern Mexico indicate high vulnerability of drought-sensitive species from mesic habitats

A decline in productivity and radial growth in conifer forests from mesic areas has been associated with increased drought stress induced by climate warming. Nevertheless, studies showing how vulnerable tree species will be in response to forecasted warming conditions are scarce in such mesic habitats. Here we address this issue by analyzing how growth responds to drought and to observed and projected climate conditions in a conifer forest from northern Mexico, which is a hotspot of conifer diversity. We quantify the trends in radial growth (quantified as basal area increment, BAI) of three species (Abies durangensis, Picea chihuahuana, Cupressus lusitanica) using dendrochronology and a process-based model of tree growth. Growth decreased in A. durangensis and P. chihuahuana from the late 1980s onwards in response to warmer and drier conditions, whereas C. lusitanica growth showed very low sensitivity to precipitation and increased as temperature did. Winter-spring dry conditions adversely affected the growth of A. durangensis and P. chihuahuana. Our modeling approach anticipates growth reductions and an increase in the vulnerability of A. durangensis and the endangered P. chihuahuana against the warmer and more arid conditions predicted after the 2050s. Future warmer and drier climatic conditions could reduce the productivity and lead to growth decline of these mesic conifer forests triggering dieback episodes in highly drought-sensitive species as A. durangensis and P. chihuahuana.

Discrimination of Picea chihuahuana Martinez populations on the basis of climatic, edaphic, dendrometric, genetic and population traits.

BackgroundPicea chihuahuana, which is endemic to Mexico, is currently listed as “Endangered” on the Red List. Chihuahua spruce is only found in the Sierra Madre Occidental (SMO), Mexico. About 42,600 individuals are distributed in forty populations. These populations are fragmented and can be classified into three geographically distinct clusters in the SMO. The total area covered by P. chihuahuana populations is less than 300 ha. A recent study suggested assisted migration as an alternative to the ex situ conservation of P. chihuahuana, taking into consideration the genetic structure and diversity of the populations and the predictions regarding the future climate of the habitat. However, detailed background information is required to enable development of plans for protecting and conserving species and for successful assisted migration. Thus, it is important to identify differences between populations in relation to environmental conditions. The genetic diversity of populations, which affect vigor, evolution and adaptability of the species, must also be considered. In this study, we examined 14 populations of P. chihuahuana, with the overall aim of discriminating the populations and form clusters of this species.MethodsEach population was represented by one 50 × 50 m plot established in the center of its respective location. Climate, soil, dasometric, density variables and genetic and species diversities were assessed in these plots for further analyses. The putatively neutral and adaptive AFLP markers were used to calculate genetic diversity. Affinity Propagation (AP) clustering technique and k-means clustering algorithm were used to classify the populations in the optimal number of clusters. Later stepwise binomial logistic regression was applied to test for significant differences in variables of the southern and northern P. chihuahuana populations. Spearman’s correlation test was used to analyze the relationships among all variables studied.ResultsThe binomial logistic regression analysis revealed that seven climate variables, the geographical longitude and sand proportion in the soil separated the southern from northern populations. The northern populations grow in more arid and continental conditions and on soils with lower sand proportion. The mean genetic diversity using all AFLP studied of P. chihuahuana was significantly correlated with the mean temperature in the warmest month, where warmer temperatures are associated to larger genetic diversity. Genetic diversity of P. chihuahuana calculated with putatively adaptive AFLP was not statistically significantly correlated with any environmental factor.DiscussionFuture reforestation programs should take into account that at least two different groups (the northern and southern cluster) of P. chihuahuana exist, as local adaptation takes place because of different environmental conditions.

Earlywood and Latewood Widths of Picea chihuahuana Show Contrasting Sensitivity to Seasonal Climate

The existence of endangered tree species in Mexico necessitates an understanding of their vulnerability to the predicted climate changes (warming and drying trends). In this study, the sensitivity to climate of earlywood (EW) and latewood (LW) widths of the threatened Picea chihuahuana was determined. The response of EW and LW to climate variables (maximum temperature, minimum temperature, precipitation, evaporation, and a drought index) was analyzed by means of correlation analysis using dendrochronology over the period of 1950–2015. EW and LW production were enhanced by cool and wet conditions during winter prior to the start of growing season. During the growing season, EW and LW production increased in response to cool spring and summer conditions, respectively; temperatures and year-round evaporation, excluding summer and the previous drought in the period prior to the growing season. EW was sensitive to seasonal drought, which is a concern considering the predicted aridification trends for the study area. These results provide further knowledge on the dendroecological potential of Picea chihuahuana.

Ruling out genetic erosion in Picea chihuahuana Martínez

Genetic erosion has been variously defined as the loss of particular alleles, the reduction in richness of the total number of alleles, and the reduction in evenness of the frequencies of alleles in a given place within populations or across species. Picea chihuahuana Martinez is an endangered endemic species known to occur in at least 40 locations along the Sierra Madre Occidental, in the states of Durango and Chihuahua (Mexico), but covering a total area of no more than 300 ha. The total number of individuals has been estimated to be around 42,600, and the number of mature individuals may be <2500. The populations (including mature trees, saplings and seedlings) vary in size from 21 to 5546 individuals. It has been suggested that small populations may be more susceptible to loss of genetic variability via genetic erosion caused by genetic drift, endogamy depression and strong unidirectional selection. The predicted reduction and eventual disappearance of a suitable habitat for P. chihuahuana due to climatic change imposes an additional risk of extinction. The principal aim of this study was therefore to estimate genetic erosion in 14 populations of P. chihuahuana Martinez by comparing genetic diversity across diameter at breast height classes (as a proxy for age classes) by using AFLP markers and four indices of genetic diversity that are commonly applied in combination with dominant genetic markers. No evidence of genetic erosion was found in the 14 P. chihuahuana populations analyzed.

Mexican conifers differ in their capacity to face climate change

The recent massive dieback of forest trees due to drought stress makes assessment of the variability of physiological traits that might be critical for predicting forest response and adaptation to climate change even more urgent. We investigated xylem vulnerability to cavitation and xylem specific hydraulic conductivity in seven species of three principal conifer genera (Juniperus monticola, Juniperus deppeana, Juniperus flaccida, Pinus pseudostrobus, Pinus leiophylla, Pinus devoniana, and the endangered Picea chihuahuana) of the Mexican mountains in order to identify the species most vulnerable to future warmer and drier climates. Hydraulic traits were examined using the in situ flow centrifuge technique (Cavitron) on branches collected from adult trees of natural populations and seedlings growing in a common garden. We found evidence of significant differences in xylem safety between genera (P50: pressure inducing 50% loss of hydraulic conductance): the three juniper species exhibited low P50 values (ranging from -9.9 to -10.4 MPa), relative to the much more vulnerable pine and spruce species (P50 ranging between - 2.9 to - 3.3 MPa). Our findings also revealed no variation in P50 between adult trees assessed in the field and seedlings growing in a common garden. We therefore propose that if, as projected, climate change makes their natural habitats much warmer and drier, populations of Mexican pines and the studied spruce will be likely to decline severely as a result of drought-stress induced cavitation, while the juniper species will survive.

Spatial Distribution Patterns in the Very Rare and Species-Rich Picea chihuahuana Tree Community (Mexico).

The very rare Mexican Picea chihuahuana tree community covers an area of no more than 300 ha in the Sierra Madre Occidental. This special tree community has been the subject of several studies aimed at learning more about the genetic structure and ecology of the species and the potential effects of climate change. The spatial distribution of trees is a result of many ecological processes and can affect the degree of competition between neighbouring trees, tree density, variability in size and distribution, regeneration, survival, growth, mortality, crown formation and the biological diversity within forest communities. Numerous scale-dependent measures have been established in order to describe spatial forest structure. The overall aim of most of these studies has been to obtain data to help design preservation and conservation strategies. In this study, we examined the spatial distribution pattern of trees in the P. chihuahuana tree community in 12 localities, in relation to i) tree stand density, ii) diameter distribution (vertical structure), iii) tree species diversity, iv) geographical latitude and v) tree dominance at a fine scale (in 0.25 ha plots), with the aim of obtaining a better understanding of the complex ecosystem processes and biological diversity. Because of the strongly mixed nature of this tree community, which often produces low population densities of each tree species and random tree fall gaps caused by tree death, we expect aggregated patterns in individual Picea chihuahuana trees and in the P. chihuahuana tree community, repulsive Picea patterns to other tree species and repulsive patterns of young to adult trees. Each location was represented by one plot of 50 x 50 m (0.25 ha) established in the centre of the tree community. The findings demonstrate that the hypothesis of aggregated tree pattern is not applicable to the mean pattern measured by Clark-Evans index, Uniform Angle index and Mean Directional index of the uneven-aged P. chihuahuana trees and P. chihuahuana tree community and but to specific spatial scales measured by the univariate L-function. The spatial distribution pattern of P. chihuahuana trees was found to be independent of patches of other tree species measured by the bivariate L-function. The spatial distribution was not significantly related to tree density, diameter distribution or tree species diversity. The index of Clark and Evans decreased significantly from the southern to northern plots containing all tree species. Self-thinning due to intra and inter-specific competition-induced mortality is probably the main cause of the decrease in aggregation intensity during the course of population development in this tree community. We recommend the use of larger sampling plots (> 0.25 ha) in uneven-aged and species-rich forest ecosystems to detect less obvious, but important, relationships between spatial tree pattern and functioning and diversity in these forests.

Modeling the Potential Distribution of Picea chihuahuana Martínez, an Endangered Species at the Sierra Madre Occidental, Mexico

Species distribution models (SDMs) help identify areas for the development of populations or communities to prevent extinctions, especially in the face of the global environmental change. This study modeled the potential distribution of the tree Picea chihuahuana Martínez, a species in danger of extinction, using the maximum entropy modeling method (MaxEnt) at three scales: local, state and national. We used a total of 38 presence data from the Sierra Madre Occidental. At the local scale, we compared MaxEnt with the reclassification and overlay method integrated in a geographic information system. MaxEnt generated maps with a high predictive capability (AUC &gt; 0.97). The distribution of P. chihuahuana is defined by vegetation type and minimum temperature at national and state scales. At the local scale, both models calculated similar areas for the potential distribution of the species; the variables that better defined the species distribution were vegetation type, aspect and distance to water flows. Populations of P. chihuahuana have always been small, but our results show potential habitat greater than the area of the actual distribution. These results provide an insight into the availability of areas suitable for the species’ regeneration, possibly through assisted colonization.

Spatial genetic structure in the very rare and species-rich Picea chihuahuana tree community (Mexico)

AbstractIn natural plant populations, the spatial genetic structure (SGS) is occasionally associated with evolutionary and ecological features such as the mating system, individual fitness, inbreeding depression and natural selection of the species of interest. The very rare Mexican P. chihuahuana tree community covers an area no more than 300 ha and has been the subject of several studies concerning its ecology and population genetics. The overall aim of most of these studies has been to obtain data to help design preservation and conservation strategies. However, analysis of the fine-scale SGS in this special forest tree community has not yet been conducted, which might help enrich the above mentioned conservation programs. In this study, we examined the SGS of this community, mostly formed by P. chihuahuana Martínez, Pinus strobiformis Ehrenberg ex Schlechtendah, Pseudotsuga menziesii (Mirb.) Franco, and Populus tremuloides Michx, in 14 localities at both the fine and large scales, with the aim of obtaining a better understanding of evolutionary processes. We observed a non-significant autocorrelation in fine-scale SGS, suggesting that the genetic variants of all four tree species are randomly distributed in space within each sampled plot of 50 x 50 m. At the larger scale, the autocorrelation was highly significant for P. chihuahuana and P. menziesii, probably as a result of insufficient gene flow due to the extreme population isolation and small sizes. For these two species our results provided strong support for the theory of isolation by distance.

The relationship between species diversity and genetic structure in the rare Picea chihuahuana tree species community, Mexico.

Species diversity and genetic diversity, the most basic elements of biodiversity, have long been treated as separate topics, although populations evolve within a community context. Recent studies on community genetics and ecology have suggested that genetic diversity is not completely independent of species diversity. The Mexican Picea chihuahuana Martínez is an endemic species listed as “Endangered” on the Red List. Forty populations of Chihuahua spruce have been identified. This species is often associated with tree species of eight genera in gallery forests. This rare Picea chihuahuana tree community covers an area no more than 300 ha and has been subject of several studies involving different topics such as ecology, genetic structure and climate change. The overall aim of these studies was to obtain a dataset for developing management tools to help decision makers implement preservation and conservation strategies. However, this unique forest tree community may also represent an excellent subject for helping us to understand the interplay between ecological and evolutionary processes in determining community structure and dynamics. The AFLP technique and species composition data were used together to test the hypothesis that species diversity is related to the adaptive genetic structure of some dominant tree species (Picea chihuahuana, Pinus strobiformis, Pseudotsuga menziesii and Populus tremuloides) of the Picea chihuahuana tree community at fourteen locations. The Hill numbers were used as a diversity measure. The results revealed a significant correlation between tree species diversity and genetic structure in Populus tremuloides. Because the relationship between the two levels of diversity was found to be positive for the putative adaptive AFLP detected, genetic and species structures of the tree community were possibly simultaneously adapted to a combination of ecological or environmental factors. The present findings indicate that interactions between genetic variants and species diversity may be crucial in shaping tree communities.

English

The conservation of genetic diversity in tree populations is an essential component of sustainable forest management. Picea chihuahuana Mart&iacute;nez is an endemic conifer species in Mexico and is considered to be endangered. P. chihuahuana covers a total area of no more than 300 ha at the Sierra Madre Occidental, a mountain range that harbor a high diversity of tree species. There are 40 populations of the species that have been identified in the region, and it cannot be found elsewhere. These populations form clusters within gallery forests and are usually associated with eight other tree genera. The P. chihuahuana community is mostly well preserved. Owing to its remarkable characteristics and high conservation value, P. chihuahuana has been the subject of several studies aimed at learning more about the genetic structure, ecology and potential effects of climate change. However, the overall applicability of such studies is to confirm a dataset to develop management tools to help decision makers and to implement preservation and conservation strategies using genetic diversity. In this review, we summarize the studies carried out to date, emphasizing those concerning the most important aspects of the genetic diversity of the species. Although, genetic diversity in Chihuahua spruce is mostly moderate compared with other Picea, this species is unlikely to survive without help due to its small and isolated populations. Efforts should focus on the protection of populations displaying the highest degree of genetic variation because these populations have the greatest potential for adaptive evolution. Finally, continuous monitoring of size and genetic diversity of the current populations in situ is essential. &nbsp; Key words: Pinabete, spruce, endangered species, heterozygosity, genetic variability, Sierra Madre Occidental.

Influence of Neighbouring Tree Species on AFLP Variants of EndangeredPicea chihuahuanaMartínez Populations on the Sierra Madre Occidental, North-Western Mexico

ABSTRACT: Plant-plant interactions, which are defined as the effect of one plant on another of the same or different species, are inherently local in nature. Plants interact in both positive and negative ways, which are governed by the same underlying ecological dynamic processes. The Mexican endemic Picea chihuahuana Martinez is a largely untouched tree species and it is therefore an excellent model for estimating genetic potential of plant-plant interactions under natural conditions. Because of the scarce research examining plant-plant interactions at the genetic level, we tested the hypothesis that specific genetic variants of P. chihuahuana are associated with (and likely selected by) the presence of particular neighbouring tree species. To explore this question, we applied AFLP genotyping to five P. chihuahuana populations in the State of Durango, North-western Mexico. Under the assumption that the variables genetic variant of P. chihuahuana and nearest-neighbour tree species are directly related to ...

Origin and demographic history of the endemic Taiwan spruce (Picea morrisonicola)

Taiwan spruce (Picea morrisonicola) is a vulnerable conifer species endemic to the island of Taiwan. A warming climate and competition from subtropical tree species has limited the range of Taiwan spruce to the higher altitudes of the island. Using seeds sampled from an area in the central mountain range of Taiwan, 15 nuclear loci were sequenced in order to measure genetic variation and to assess the long-term genetic stability of the species. Genetic diversity is low and comparable to other spruce species with limited ranges such as Picea breweriana, Picea chihuahuana, and Picea schrenkiana. Importantly, analysis using approximate Bayesian computation (ABC) provides evidence for a drastic decline in the effective population size approximately 0.3–0.5 million years ago (mya). We used simulations to show that this is unlikely to be a false-positive result due to the limited sample used here. To investigate the phylogenetic origin of Taiwan spruce, additional sequencing was performed in the Chinese spruce Picea wilsonii and combined with previously published data for three other mainland China species, Picea purpurea, Picea likiangensis, and P. schrenkiana. Analysis of population structure revealed that P. morrisonicola clusters most closely with P. wilsonii, and coalescent analyses using the program MIMAR dated the split to 4–8 mya, coincidental to the formation of Taiwan. Considering the population decrease that occurred after the split, however, led to a much more recent origin.

Ecology of the Mexican Abies durangensis Martínez

The genus Abies has about 40 species, which are mainly distributed in boreal or subalpine areas. In Mexico as a diversity center for this genus, all eight species of Abies are protected. Six of them are endemic and mainly distributed in the Sierra Madre Occidental and Oriental. The often isolated relict Abies durangensis Martinez, also known as Durango Fir, is located in the states of Durango, Chihuahua, Sinaloa, and Jalisco and usually grows on slopes with well-drained and shallow lithosols, where the climate is humid and cool. A. durangensis is specifically associated with species such as Pseudotsuga menziesii, Pseudotsuga lindleyana, Pinus strobiformis, Picea chihuahuana, Pinus durangensis, Cupressus lusitanica (lindleyi), and Juniperus deppeana. In the state of Durango, the species has a restricted geographic distribution between 23.2° and 25.7°N and 105° to 107°W and elevations varying from 2,195 m to 2,955 m (mean 2,690 m). The species’ habitat has a mean annual temperature that ranges from 8.6 °C to 12.2 °C (mean 10.6 °C) and a mean annual rainfall of 1,088 mm to 1,395 mm (mean 1,224 mm). According to the Red List of Threatened Species of the International Union for Conservation of Nature and Natural Resources, the species is in low risk and does not depend on conservation efforts.

Estimating genetic erosion using the example of Picea chihuahuana Martínez

Genetic erosion can be defined as a permanent reduction in richness or evenness of common local alleles or the loss of combination of alleles over time in a defined area. This process can be detrimental to the short-term viability of individuals and populations, the evolutionary potential of populations and species, and the direct use of genetic resources. Various international and intergovernmental organizations and networks have therefore recognized the need to assess and monitor plant genetic erosion in order to prevent such effects. The rare tree species Picea chihuahuana Martinez, which is endemic to Mexico, is an excellent model for estimating potential genetic erosion. The species occurs in about 40 often isolated relict populations in the Sierra Madre Occidental. In this study, the degree of genetic erosion was evaluated in five populations of P. chihuahuana M. in the State of Durango (Mexico), by comparing the genetic diversity in diameter classes (as a surrogate variable for age classes). The results of the study demonstrate a moderate loss of genetic diversity at some amplified fragment length polymorphism (AFLP) loci from the older trees to saplings and to young seedlings in the two largest populations. Significant genetic erosion was only detected in a very small population, named San Jose de las Causas (SJ). Hence, if genetic diversity at AFLP loci reflects diversity in the genome on the whole, genetic erosion per se does not explain the relict status of Chihuahua spruce, except in very small populations, such as SJ, which comprised of ca. 120 individuals.

First Report of White Fir Dwarf Mistletoe (Arceuthobium abietinum f. sp. concoloris) on Mexican Spruce (Picea mexicana) in Northern Mexico.

White fir dwarf mistletoe (Arceuthobium abietinum Engelmann ex Munz f. sp. concoloris Hawksw. & Wiens, Viscaceae) severely parasitizes true firs (Abies spp.) from southern Washington to southern California (1). It also occurs in widely isolated populations on white fir (Abies concolor (Gord. & Glend.) Hildebr.) in Nevada, Utah, and Arizona (1). In addition, the two known populations of dwarf mistletoe on Durango fir (Abies durangensis Martínez) in Chihuahua, Mexico (near Yahuirachi and on Cerro Mohinora) have been classified as white fir dwarf mistletoe (1). Although a subspecies of fir dwarf mistletoe (A. abietinum Engelm. ex Munz subsp. wiensii Mathiasen & C. Daugherty) severely parasitizes Brewer spruce (Picea breweriana S. Watson) in northern California and southern Oregon (2), Engelmann spruce (P. engelmannii (Parry) Engelmann), blue spruce (P. pungens Engelm.), and Chihuahua spruce (P. chihuahuana Mart.) have been reported to be immune to infection by white fir dwarf mistletoe in the southwest and Mexico (1). However, in September 2009, white fir dwarf mistletoe was found to be infecting the rare Mexican spruce (P. mexicana Mart.) on Cerro Mohinora in southern Chihuahua, Mexico (25°57'42″N, 107°02'28″W, elevation 3,040 m). Infected Mexican spruces were growing among severely infected Durango firs in a mixed conifer forest of Durango fir, Mexican spruce, Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), and Mexican white pine (Pinus ayacahuite Ehrenb.). White fir dwarf mistletoe was the only mistletoe present in the forest with the infected Mexican spruces. Only five infected spruces were observed, but some trees had as many as 20 infections. No mortality of Mexican spruce associated with mistletoe infection was observed. Some of the infected spruce branches were producing mature male and female plants with flowers and fruits, respectively. Mistletoe plants collected from Durango fir and Mexican spruce were identical when compared using morphological characters such as plant height (mean 8.4 cm), plant color (yellow-green, green, green-brown, and rarely red-brown), mean diameter of flowers (2.8 mm), and fruit dimensions (5.0 × 3.0 mm). It should be noted that although the dwarf mistletoe parasitizing Durango fir on Cerro Mohinora had been classified as white fir dwarf mistletoe (1), the morphological characters above are slightly different than those reported previously for this mistletoe (1,2). On the basis of the number of infected trees and the light to moderate level of infection observed, Mexican spruce should be tentatively classified as an occasional host of white fir dwarf mistletoe using the host susceptibility classification system proposed by Hawksworth and Wiens (1). Specimens of white fir dwarf mistletoe on Mexican spruce were collected and deposited at the Deaver Herbarium (ASC), Northern Arizona University, Flagstaff (Accession No. 93827). To my knowledge, this is the first report of white fir dwarf mistletoe parasitizing Mexican spruce and the only know instance of white fir dwarf mistletoe parasitizing a species of spruce found in Mexico (1).

Decoupled mitochondrial and chloroplast DNA population structure reveals Holocene collapse and population isolation in a threatened Mexican‐endemic conifer

Chihuahua spruce (Picea chihuahuana Martínez) is a montane subtropical conifer endemic to the Sierra Madre Occidental in northwestern México. Range-wide variation was investigated using maternally inherited mitochondrial (mtDNA) and paternally inherited chloroplast (cpDNA) DNA markers. Among the 16 mtDNA regions analysed, only two mitotypes were detected, while the study of six cpDNA microsatellite markers revealed eight different chlorotypes. The average cpDNA diversity (H = 0.415) was low but much higher than that for mtDNA (H = 0). The distribution of mitotypes revealed two clear nonoverlapping areas (G(ST) = N(ST) = 1), one including northern populations and the second one including the southern and central stands, suggesting that these two regions may represent different ancestral populations. The cpDNA markers showed lower population differentiation (G(ST) = 0.362; R(ST) = 0.230), implying that the two ancestral populations continued to exchange pollen after their initial geographic separation. A lack of a phylogeographic structure was revealed by different spatial analyses of cpDNA (G(ST) > R(ST); and samova), and reduced cpDNA gene flow was noted among populations (Nm = 0.873). Some stands deviated significantly from the mutation-drift equilibrium, suggesting recent bottlenecks. Altogether, these various trends are consistent with the hypothesis of a population collapse during the Holocene warming and suggest that most of the modern P. chihuahuana populations are now effectively isolated with their genetic diversity essentially modelled by genetic drift. The conservation efforts should focus on most southern populations and on the northern and central stands exhibiting high levels of genetic diversity. Additional mtDNA sequence analysis confirmed that P. martinezii (Patterson) is not conspecific with P. chihuahuana, and thus deserves separate conservation efforts.

Fire regime in a conservation reserve in Chihuahua, Mexico

Fire regime characteristics were reconstructed from fire-scarred trees in the Tutuaca reserve, a newly designated protected area in the Sierra Madre Occidental of western Chihuahua. The reserve was created to protect thick-billed parrot nesting habitat (large snags) and a relict forest of Chihuahua spruce (Picea chihuahuana Martínez). We collected fire-scarred samples from conifers (Pinus ayacahuite Ehrenb., Pinus durangensis Martínez, and Pseudotsuga menziesii (Mirb.) Franco) in three 25-ha sites arrayed at different watershed positions, from a low site adjacent to the spruce trees up to the watershed divide. Fire analysis periods began in 1702, 1704, or 1761 and continued through the final fire in 1955 (two sites) or 1995. All sites had frequent fire regimes (mean fire interval (MFI) 3.9–5.2 years; MFI for years in which 25% or more of the samples were scarred: 6.9–8.4 years). Almost all fires occurred before cambial growth began or early during the season of cambial growth. Fire years were significantly dry, and the years immediately preceding fire were significantly wet. After 1955, no further fires occurred at two of the three study sites, a pattern similar to that observed elsewhere in northern Mexico. The third site had fires in 1987 and 1995. The extended fire-free period in portions of the Tutuaca landscape may result in fuel accumulation and eventually in severe wildfire. For effective conservation of fire-susceptible habitat features, managers should seek to incorporate surface fire as a management tool.

Relationships among the Spruces (Picea, Pinaceae) of Southwestern North America

Abstract Numerous populations from six spruce taxa, including four relict endemics, Picea chihuahuana (Chihuahua spruce), P. martinezii (Martinez spruce), P. mexicana (Mexican spruce), and P. breweriana (Brewer spruce), and two widespread species, P. engelmannii (Engelmann spruce) and P. pungens (blue spruce), were compared at homologous isozyme loci to test various hypotheses about their affinities and origins. Each of the species was clearly separated, and Neighbor-Joining and Unweighted Pair Group analyses of Nei's genetic distance grouped all populations within a taxon into their own clusters. Spruces from Flys Peak, Chiricahua Mountains, Arizona, joined a P. engelmannii cluster and were not a bridge to P. mexicana as previously believed. Spruces from Cerro Mohinora, Chihuahua, were clearly P. mexicana, not phantom hybrids of P. chihuahuana and P. pungens. Nuclear random amplified polymorphic DNA and chloroplast simple sequence repeat and cleaved amplified polymorphic genetic markers were compared in ...