Elevational Cline Research Articles

An elevational cline in leaf variegation: Testing anti-herbivory and abiotic heterogeneity hypotheses in maintaining a polymorphism.

While some studies have found leaf variegation to reduce photosynthetic capacity, others showed that it can increase photosynthesis. Thus, what maintains variegation remains an open question. Two primary hypotheses-the anti-herbivory and abiotic heterogeneity hypotheses-have been posited, yet little empirical research explicitly investigates the maintenance of naturally occurring variegation. We used field surveys, image analysis, and climatic associations to explore the anti-herbivory and abiotic heterogeneity hypotheses in 21 populations of Hexastylis heterophylla and H. shuttleworthii, both polymorphic for leaf variegation. We measured the frequency of variegated individuals, variegation intensity, and herbivory for each morph, assessed abiotic correlates with variegation, and measured photosynthetic efficiency. We found a strong elevational cline in leaf variegation strongly linked with abiotic heterogeneity; variegation was more common in lower-elevation populations characterized by higher temperatures, UV-B exposure, seasonal light change, and drier, more basic soils. Variegated and nonvariegated individuals experienced similar levels of herbivory. Morphs had similar photosynthetic quantum yields. However, nonvariegated leaves experienced more nonphotochemical quenching, an indication of photoinhibition, and had higher surface temperatures under high light. Our results suggest that variegation may serve as an adaptation to high temperatures and light conditions and can reduce photoinhibition in certain environmental contexts. Thus, abiotic factors can maintain variegation in wild populations and shape geographic clines in variegation.

Co-limitation of resources reveals adaptations of a tropical tree to heterogeneous environments along an elevational cline

IntroductionPlant species often exhibit significant variation in functional traits in populations along elevational gradients to cope with varying stress conditions. While plant development has been assumed to be most limited by a single resource, growing evidence suggests the potential for interactions of co-limiting resources to impact plant performance. Here, we aimed to determine how light, nitrogen, and water availability influence the growth and physiology of different populations of koa (Acacia koa), a tree species of concern that occurs across a large elevational gradient in tropical Hawaii, United States.MethodsPopulations from three seed sources [low (L), mid (M), and high (H) elevation] were grown in a controlled greenhouse experiment and exposed to co-limiting light, water, and nutrient (nitrogen) conditions. Light response, gas exchange, water status, resource use efficiency, nutrients and shoot non-structural carbohydrate concentrations, and growth and biomass allocation responses were quantified.ResultsWe found that resource co-limitation sometimes interacted to determine responses of the measured parameters. In general, the low elevation (L) koa population was more sensitive to conditions where both moisture and nutrients were limiting, while the high elevation (H) koa population was more sensitive to conditions where either light and moisture or light and nitrogen were co-limiting. The M population performed well overall regardless of resource limitation.DiscussionOur findings lend support to the theory that multiple resources limit growth and physiology of populations rather than the traditional view of a single resource limiting performance. Therefore, the possibility that multiple resource limitations drive population differences should be considered when developing population-based guidelines for forest and tree species restoration.

Integrative analysis of physiology and genomics provides insights into freeze tolerance adaptations of Acacia koa along an elevational cline.

Natural selection for plant species in heterogeneous environments creates genetic variation for traits such as cold tolerance. While physiological or molecular analyses have been used to evaluate stress tolerance adaptations, combining these approaches may provide deeper insight. Acacia koa (koa) occurs from sea level to 2300 m in Hawai'i, USA. At high elevations, natural koa populations have declined due to deforestation, and freeze tolerance is a limiting factor for tree regeneration. We used physiology and molecular analyses to evaluate cold tolerance of koa populations from low (300-750 m), middle (750-1500 m), and high elevations (1500-2100 m). Half of the seedlings were cold acclimated by exposure to progressively lowered air temperatures for eight weeks (from 25.6/22.2°C to 8/4°C, day/night). Using the whole plant physiology-freezing test and koa C-repeat Binding Factor CBF genes, our results indicated that koa can be cold-acclimated when exposed to low, non-freezing temperatures. Seedlings from high elevations had consistently higher expression of Koa CBF genes associated with cold tolerance, helping to explain variation in cold-hardy phenotypes. Evaluation of the genetic background of 22 koa families across the elevations with low coverage RNA sequencing indicated that high elevation koa had relatively low values of heterozygosity, suggesting that adaptation is more likely to arise in the middle and low elevation sources. This physiology and molecular data for cold tolerance of koa across the elevation gradient of the Hawaiian Islands provides insights into natural selection processes and may help to support guidelines for conservation and seed transfer in forest restoration efforts.

Trends to adaptation of the Sahara frog (Pelophylax saharicus) larvae across an environmental gradient

Species adjust their behavior and life-history to adapt to local environmental conditions. Species with a broad ecological niche often show signatures of local adaptations to different environment, particularly in extreme ones. Here, we investigate local adaptation in different populations of the North African Sahara frog (Pelophylax saharicus) living in various environmental conditions that vary mostly in temperature, precipitation, and elevation by mean of common garden experiment aiming to estimate the growth rate under two predation treatments (absence or presence of non-lethal cues of dragonfly larvae). First, we found an elevational cline in the reproductive phenology where, from low to high elevation, the reproductive season shifts to later dates, whereas that in arid environment was later than all other populations. We suggest that geographic differences in temperature and rainfall (in arid areas) explain this phenological pattern. Second, hatching success was overall high but showed a slight decline across elevation. Third, growth rate was generally faster in low and intermediate elevation populations, but slower in high elevation and arid environment populations. Populations in low and intermediate elevation responded to predation by reducing growth rate and the size at metamorphosis, but no predatory responses were recorded in high elevation and arid environment populations. Our study shows some life history signatures of local adaptation of P. saharicus in Northeast Algeria, which does not go in line with recent genetic analysis showing low population differentiation in the region.

Ecophysiology of a small ectotherm tracks environmental variation along an elevational cline

AbstractAimPhysiological responses to climate can be used to quantify the environmental limits that a species can tolerate and are, therefore, key to biogeographical studies. Several ecophysiological responses to climatic factors may shape the distribution of species, but our knowledge is mostly centred in thermal ecophysiology. We applied an integrative approach to investigate how ecophysiological responses of a small ectotherm are modulated by climatic variation across an elevational gradient.LocationVictoria, Australia.TaxonLampropholis guichenoti (Scincidae, Squamata).MethodsWe examined metabolic rate, evaporative water loss, thermal physiology and locomotor performance, of four populations of the skink Lampropholis guichenoti across an elevational gradient in Australia (17–1546 m). We applied biophysical modelling to test how behaviour impacts the survival and activity of these skinks.ResultsPopulations from the lowlands had lower metabolic rates and evaporative water loss, lower thermal preferences, higher optimal temperatures and increased locomotor performance compared populations from high elevations. Biophysical models revealed that behaviour allows skinks from high elevation to maintain their body temperatures above freezing and is key to determining activity patterns.Main conclusionsOur holistic study shows that a combination of physiological and behavioural responses allows ectotherms to survive in different climatic environments. The interplay between physiology and behaviour can help to explain how climate sets limits for the geographical range of ectotherms.

Selective sorting of ancestral introgression in maize and teosinte along an elevational cline.

While often deleterious, hybridization can also be a key source of genetic variation and pre-adapted haplotypes, enabling rapid evolution and niche expansion. Here we evaluate these opposing selection forces on introgressed ancestry between maize (Zea mays ssp. mays) and its wild teosinte relative, mexicana (Zea mays ssp. mexicana). Introgression from ecologically diverse teosinte may have facilitated maize's global range expansion, in particular to challenging high elevation regions (> 1500 m). We generated low-coverage genome sequencing data for 348 maize and mexicana individuals to evaluate patterns of introgression in 14 sympatric population pairs, spanning the elevational range of mexicana, a teosinte endemic to the mountains of Mexico. While recent hybrids are commonly observed in sympatric populations and mexicana demonstrates fine-scale local adaptation, we find that the majority of mexicana ancestry tracts introgressed into maize over 1000 generations ago. This mexicana ancestry seems to have maintained much of its diversity and likely came from a common ancestral source, rather than contemporary sympatric populations, resulting in relatively low FST between mexicana ancestry tracts sampled from geographically distant maize populations. Introgressed mexicana ancestry in maize is reduced in lower-recombination rate quintiles of the genome and around domestication genes, consistent with pervasive selection against introgression. However, we also find mexicana ancestry increases across the sampled elevational gradient and that high introgression peaks are most commonly shared among high-elevation maize populations, consistent with introgression from mexicana facilitating adaptation to the highland environment. In the other direction, we find patterns consistent with adaptive and clinal introgression of maize ancestry into sympatric mexicana at many loci across the genome, suggesting that maize also contributes to adaptation in mexicana, especially at the lower end of its elevational range. In sympatric maize, in addition to high introgression regions we find many genomic regions where selection for local adaptation maintains steep gradients in introgressed mexicana ancestry across elevation, including at least two inversions: the well-characterized 14 Mb Inv4m on chromosome 4 and a novel 3 Mb inversion Inv9f surrounding the macrohairless1 locus on chromosome 9. Most outlier loci with high mexicana introgression show no signals of sweeps or local sourcing from sympatric populations and so likely represent ancestral introgression sorted by selection, resulting in correlated but distinct outcomes of introgression in different contemporary maize populations.

Low temperatures impact species distributions of jumping spiders across a desert elevational cline

Temperature is known to influence many aspects of organisms and is frequently linked to geographical species distributions. Despite the importance of a broad understanding of an animal’s thermal biology, few studies incorporate more than one metric of thermal biology. Here we examined an elevational assemblage of Habronattus jumping spiders to measure different aspects of their thermal biology including thermal limits (CTmin, CTmax), thermal preference, V̇CO2 as proxy for metabolic rate, locomotor behavior and warming tolerance. We used these data to test whether thermal biology helped explain how species were distributed across elevation. Habronattus had high CTmax values, which did not differ among species across the elevational gradient. The highest-elevation species had a lower CTmin than any other species. All species had a strong thermal preference around 37 °C. With respect to performance, one of the middle elevation species was significantly less temperature-sensitive in metabolic rate. Differences between species with respect to locomotion (jump distance) were likely driven by differences in mass, with no differences in thermal performance across elevation. We suggest that Habronattus distributions follow Brett’s rule, a rule that predicts more geographical variation in cold tolerance than heat. Additionally, we suggest that physiological tolerances interact with biotic factors, particularly those related to courtship and mate choice to influence species distributions. Habronattus also had very high warming tolerance values (> 20 °C, on average). Taken together, these data suggest that Habronattus are resilient in the face of climate-change related shifts in temperature.

Integrative taxonomic analysis reveals a new species of Leptobrachium (Anura: Megophryidae) from north-eastern Cambodia and central Vietnam

ABSTRACT Species boundaries within the red-eyed Leptobrachium of southern Indochina have been uncertain. Leptobrachium pullum and L. mouhoti from upper and lower elevations, respectively, of the Langbian Plateau of southern Vietnam and eastern Cambodia differ in body size but have relatively low interspecific mitochondrial DNA divergence, leading to speculation that these might represent a single species with an elevational cline in body size. The recent discovery of an allopatric high-elevation population of red-eyed Leptobrachium on the Kon Tum Plateau (= Central Highlands) of north-eastern Cambodia and central Vietnam has been referred to both species, and to a putatively undescribed species. We examine variation in morphology of adults and tadpoles, mitochondrial DNA, 11 nuclear genes and advertisement calls, and show corroborating lines of evidence for the existence of three species of red-eyed Leptobrachium in southern Indochina. Although the three species are reciprocally monophyletic in mitochondrial DNA, their shallow mitochondrial DNA divergences are not mirrored by morphology, advertisement calls, or – in part – nuclear DNA, and probably reflect past mitochondrial introgression rather than recent speciation. The Central Highlands taxon is described herein as a new species. http://www.zoobank.org/urn:lsid:zoobank.org:pub:CE6650AF-D9FB-40F4-8A2E-9B4E23AAC205

Elevational cline in herbivore abundance driven by a monotonic increase in trophic-level sensitivity to aridity.

The abiotic environment drives species abundances and distributions both directly and indirectly through effects on multi-trophic species interactions. However, few studies have documented the individual and combined consequences of these direct and indirect effects. We studied an ant-tended aphid along an elevational gradient, where lower elevations were more arid. Hypotheses of stronger species interactions at lower elevations and a greater sensitivity of higher trophic levels to climate led us to predict increased top-down control of aphids by natural enemies (third trophic level) but even stronger protection from mutualist ants (fourth trophic level) with increasing aridity. As a result, we predicted that mutualism strength and aphid abundance would increase with aridity. We documented patterns of aphid abundance and tested for both the direct and multi-trophic indirect effects of aridity on aphid performance. To do so, we used both observational and manipulative methods across two years in replicate high- and low-elevation valleys, where summer temperatures decreased by 3.7°C and precipitation increased by 27mm/mo from low to high elevations. Aphid colonies were 75% larger in the most (vs. least) arid sites, and this was best explained by changes in interactions with predators and ants. Aphids were unaffected by the direct effects of the abiotic environment or its indirect effects via host plant quality. In contrast, natural enemy effects increased with aridity; under ant exclusion, natural enemies had no effect on aphids in the least arid sites but depressed colony growth by 252% in the most arid sites. Ant activity also increased with aridity, with ants discovering more aphid colonies and experimental baits and allocating more foragers per aphid, although there was no effect of aridity on ant abundance or community composition. Correspondingly, the mutualist services provided by ants increased with aridity; ants provided no benefits to aphids in the least arid sites but doubled colony growth in the most arid sites. In summary, an elevational cline in herbivore abundance was driven by a monotonic increase in trophic-level sensitivity to aridity. These findings illustrate that predicting species responses to climate change will require a multi-trophic perspective.

Plastic collective endothermy in a complex animal society (army ant bivouacs: Eciton burchellii parvispinum)

Endothermic animals do not always have a single adaptive internal temperature; some species exhibit plastic homeostasis, adaptively allowing body temperature to drop when thermoregulatory costs are high. Like large‐bodied endotherms, some animal societies exhibit collective thermal homeostasis. We tested for plasticity of thermoregulation in the self‐assembled temporary nests (bivouacs) of army ants. We measured core bivouac temperatures under a range of environmental conditions and at different colony developmental (larval vs pupal brood) stages. Contrary to previous assertions, bivouacs were not perfect thermoregulators in all developmental stages. Instead, bivouacs functioned as superorganismal facultative endotherms, using a combination of site choice and context‐dependent metabolic heating to adjust core temperatures across an elevational cline in ambient temperature. When ambient temperature was low, the magnitude of metabolic heating was dependent on colony developmental stage: pupal bivouacs were warmer than larval bivouacs. At cooler high elevations, bivouacs functioned like some endothermic animals that intermittently lower their body temperatures to conserve energy. Bivouacs potentially conserved energy by investing less metabolic heating in larval brood because the high costs of impaired worker development may require more stringent thermoregulation of pupae. Our data also suggest that site choice played an important role in bivouac cooling under high ambient temperatures at low elevations. Climate warming may expand upper elevational range limits of Eciton burchellii parvispinum, while reducing the availability of cool and moist bivouac sites at lower elevations, potentially leading to future low‐elevation range contraction.

Differential elevational cline in the phenology and demography of two temporally isolated populations of a damselfly: Not two but one taxon?

1. Temporal isolation by cohort splitting is a life‐history mechanism that has been reported in many temperate insects, including those inhabiting freshwater habitats. Although the cohorts seem to maintain separate temporal niches in a specific location, the temporal isolation may be disrupted across a geographic gradient due to constraints imposed by seasonality.2. This prediction was tested on two temporally isolated populations of the obligatory univoltine Lestes virens (Odonata, Lestidae) in north‐east Algeria. Although the two cohorts emerge at the same time in spring, one cohort reproduces in summer, while the second cohort estivates in summer and reproduces in autumn. A survey assessing the phenology and abundance was conducted on eight ponds across an elevational gradient (5–1012 m asl) using capture–mark–recapture and adult density sampling.3. In all sites from low to high elevation, the species showed cohort splitting. The phenology of reproduction of both cohorts showed a delay with elevation, but the cline was 2.2 days for the summer cohort and 0.7 days for the autumn cohort per 100 m of elevation. Moreover, the density of adults in the autumn cohort was higher than that of summer cohort across the entire elevational range, and the difference increased with elevation.4. These findings regarding the differential elevational cline in the phenology show that the temporal isolation of the two cohorts becomes narrower at high elevation, suggesting potential inter‐cohort temporal overlap at higher elevations.5. The claim that the two cohorts of L. virens are true temporally isolated species needs further investigation.

Adaptive differentiation coincides with local bioclimatic conditions along an elevational cline in populations of a lichen-forming fungus

BackgroundMany fungal species occur across a variety of habitats. Particularly lichens, fungi forming symbioses with photosynthetic partners, have evolved remarkable tolerances for environmental extremes. Despite their ecological importance and ubiquity, little is known about the genetic basis of adaption in lichen populations. Here we studied patterns of genome-wide differentiation in the lichen-forming fungus Lasallia pustulata along an altitudinal gradient in the Mediterranean region. We resequenced six populations as pools and identified highly differentiated genomic regions. We then detected gene-environment correlations while controlling for shared population history and pooled sequencing bias, and performed ecophysiological experiments to assess fitness differences of individuals from different environments.ResultsWe detected two strongly differentiated genetic clusters linked to Mediterranean and temperate-oceanic climate, and an admixture zone, which coincided with the transition between the two bioclimates. High altitude individuals showed ecophysiological adaptations to wetter and more shaded conditions. Highly differentiated genome regions contained a number of genes associated with stress response, local environmental adaptation, and sexual reproduction.ConclusionsTaken together our results provide evidence for a complex interplay between demographic history and spatially varying selection acting on a number of key biological processes, suggesting a scenario of ecological speciation.

Distribución altitudinal de la riqueza y composición de ensamblajes de aves en una zona montañosa al sur de Nayarit, México

Elevation gradient studies have strengthened the evaluation of changes in richness and composition of bird assemblages. They also provide information on environmental variables that determine bird distribution, and the variables that define their population structure. Our aim was to describe their variation through an elevational cline in Southern Nayarit, Mexico. To analyze the behavior of richness across the gradient, we gathered information through point counts in nine elevational intervals (300 m from each other) from sea level to 2 700 m of elevation. With a standardized sampling effort, we produced rarefaction curves and analyzed changes in species composition by hierarchical classification using the TWINSPAN technique. In order to identify variables associated with richness changes, we examined the effect of precipitation and habitat structure via regression trees. An analysis of nonmetric multidimensional scaling (NMDS) was implemented with the purpose to determine if the changes in composition correspond to changes in vegetation types. Species richness varied significantly across the gradient: high in the lower parts of the gradient, reached its peak in the middle, and decreased monotonically with elevation. Species responded to changes in the cline and were grouped in three elevational zones. Analyses suggest that changes in richness and species composition are influenced by vegetation, its structure and precipitation regime, as well as various aspects related to habitat features and disturbance. These aspects should be taken into account in order to design appropriate strategies for the conservation of the birds of Nayarit.

Bud flush phenology and nursery carryover effect of paper birch provenances

Abstract: Paper birch (Betula papyrifera Marsh) is an ecologically valuable species with a broad geographic distribution across the North America. Its diversity, versatility and enduring nature make it an ideal candidate for a selective breeding program in this region. However, an understanding of the genecology of this species is fundamental to deploy it successfully. Ten paper birch provenances were collected from British Columbia (BC, Canada) and northern Idaho (USA) along elevational transects to determine whether observed bud flush phenology was due to genetics and /or environmental variation or their interaction. Seedlings were grown at three different nurseries: University of Idaho (46°44’N), Landing (50°17’N) and Little Forestry (54°00’N) and planted in a randomized single tree interlocking block design in three common gardens at Sandpoint, ID (48°13’N), Skimikin, BC (50°45’N) and Red Rock, BC (53°45’N). Results indicate that variation in the timing of bud flush is a complex interaction among local genetic characteristics and environmental conditions of the growing site. Birch bud flush followed a general geographic trend where provenances at the southern common garden (Sandpoint) required less time (Day of Year, DoY) and fewer growing degree days (GDD) compared to central (Skimikin) and northern (Red Rock) common gardens. Although there were significant differences in the timing of bud flush among provenances along an elevational gradient, none of the regions showed the expected linear elevational cline, trends were inconsistent. Further, birch bud flush was significantly influenced by nursery displacement effects in the initial year of establishment but disappeared within three years. These results provide an opportunity to characterize bud flush phenology of paper birch and would be useful for improving operational paper birch seed transfer programs in BC.

A geographic cline in leaf salicylic acid with increasing elevation in Arabidopsis thaliana

Salicylic acid (SA) occupies a key role as a hormone central to both plant resistance to bacterial pathogens and tolerance of abiotic stresses. Plants at high elevation experience colder temperatures and elevated UV levels. While it has been predicted that SA concentrations will be higher in plants from high elevation populations, few studies have addressed this question. Here, we asked how concentrations of SA vary in natural populations of Arabidopsis thaliana collected across an elevational gradient on the Iberian Peninsula. In a series of common garden experiments, we found that constitutive SA concentrations were highest in genotypes from the low elevation populations. This result was in the opposite direction from our prediction and is an exception to the general finding that phenolic compounds increase with increasing elevation. These data suggest that high constitutive SA is not associated with resistance to cold temperatures in these plants. Furthermore, we also found that leaf constitutive camalexin concentrations, an important defense against some bacterial and fungal enemies, were highest in the low elevation populations, suggesting that pathogen pressures may be important. Further examination of this elevational cline will likely provide additional insights into the interplay between phenolic compounds and biotic and abiotic stress.

Use of intraspecific variation in thermal responses for estimating an elevational cline in the timing of cold hardening in a sub-boreal conifer.

To avoid winter frost damage, evergreen coniferous species develop cold hardiness with suitable phenology for the local climate regime. Along the elevational gradient, a genetic cline in autumn phenology is often recognised among coniferous populations, but further quantification of evolutionary adaptation related to the local environment and its responsible signals generating the phenological variation are poorly understood. We evaluated the timing of cold hardening among populations of Abies sachalinensis, based on time series freezing tests using trees derived from four seed source populations × three planting sites. Furthermore, we constructed a model to estimate the development of hardening from field temperatures and the intraspecific variations occurring during this process. An elevational cline was detected such that high-elevation populations developed cold hardiness earlier than low-elevation populations, representing significant genetic control. Because development occurred earlier at high-elevation planting sites, the genetic trend across elevation overlapped with the environmental trend. Based on the trade-off between later hardening to lengthen the active growth period and earlier hardening to avoid frost damage, this genetic cline would be adaptive to the local climate. Our modelling approach estimated intraspecific variation in two model components: the threshold temperature, which was the criterion for determining whether the trees accumulated the thermal value, and the chilling requirement for trees to achieve adequate cold hardiness. A higher threshold temperature and a lower chilling requirement could be responsible for the earlier phenology of the high-elevation population. These thermal responses may be one of the important factors driving the elevation-dependent adaptation of A.sachalinensis.

Soil microbial communities from an elevational cline differ in their effect on conifer seedling growth

Sub-alpine environments consist of altitudinal gradients associated with dramatic changes in plant growth and community composition, but the role of soil feedbacks and microbe interactions is largely unknown. Here, we examine the influence of the overall soil microbial community, with a focus on ectomycorrhizal and dark septate endophytic root colonizing fungi, from low, mid, and high elevations on the growth of Pinus contorta and Picea glauca × engelmannii. The influence of the soil microbial community was tested on seedlings from the same three elevations in order to determine ‘home’ versus ‘away’ effects on conspecifics of differing elevations. The low elevation soil was the most fertile and harbored a soil microbial community with an overall negative effect on seedling growth. In contrast, the high elevation soil was the least fertile and had a microbial community that enhanced seedling growth. However, only the soil microbial community in the highest elevation soil resulted in a stronger influence on the native P. contorta seedlings than seedlings originating from lower elevations. Despite the overall influence of the soil microbial community, ectomycorrhizal colonization was significantly correlated with P. glauca × engelmannii growth rates, but colonization by dark septate endophytes showed no relationship with seedling growth. The results provide evidence that plant—soil microbial community relationships are dependent on soil environment. Moreover, our results provide further support for the importance of soil microbes in facilitating seedling growth toward the edge of their elevational range.

Geographic pattern of local optimality in natural populations of lodgepole pine

Adaptive optimality of local populations is central to evolutionary biology. It also provides a genetic framework for planning gene conservation. We examined local optimality using 20-year height growth as an indicator of population fitness. The height growth data were obtained from a network of 57 long-term field trials in interior British Columbia involving a range-wide samples of 142 lodgepole pine ( Pinus contorta Doug) populations. Local optimality was defined statistically and tested, and its geographic patterns were examined in both multi-dimensional mode and component dimensions along latitude, longitude and elevation. Fitness values of local and optimal populations were derived by fitting population variation to response functions for individual sites. Local optimality was an interval estimate, i.e. within the 95% confidence limit of the projected fitness value of the optimal populations. Both biological and physical distances between local and optimal population were used to examine geographic pattern of local optimality. Study leads to following key results: (1) local optimality prevails at majority of sites; (2) populations at northeast bordering the northern Rocky Mountain Trench are most local optimal, whereas non-local optimality most evident along the eastern foothills of the Coastal Mountains in the coast-interior transition climate zone and the high mountains in the southern interior; (3) there is a geographic cline oriented from southwest to northeast, but with a steep west-east incline; and (4) along component dimensions, a steep longitudinal cline, i.e. the farther the west, the lesser the local optimality, and an elevational cline, i.e. the higher the elevation, the lesser the local optimality. Based on the ecological and life-history characteristics of the species and the climate in interior British Columbia, we propose that a selection gradient coupled with directional gene flow was the process influencing the formation of the observed geographic pattern. The selection gradient parallels the general climate pattern from continental northeast to sub-continental southwest. The gene flow brought the migration of undesirable genes from the coastal subspecies contorta to interior populations of the subspecies latifolia, which further diminished the effectiveness of natural selection. Practical applications through seed transfer in reforestation and genetic resources management are discussed.

Spatial pattern of allozyme variation in a contact zone of Pinus Ponderosa and P. arizonica (Pinaceae)

The spatial distribution of genotypes for nine polymorphic allozyme loci was examined in a contact zone between Pinus ponderosa var. scopulorum and another tree regarded as either a separate species, Pinus arizonica, or variety, Pinus ponderosa var. arizonica, in southern Arizona. Previous work had identified a steep elevational cline for a key taxonomic trait, number of leaf-needles per fascicle, on the south slope of Mt. Lemmon. The present results indicate that the taxa are not fully interbreeding in this contact zone, because allozyme genotypes are considerably more spatially structured than expected for the dispersal characteristics of pines. The amount of spatial differentiation is also much less than that observed for needle number. It appears that this is due to the lack of differentiation for allozyme gene frequencies for the two types of trees, which is further evidenced by analysis of samples from two other populations away from the contact zone. It is likely that if the two taxa were isolated in the past, it was not for long enough nor complete enough to allow mutation-drift to create substantial differentiation between them. Another possible explanation is that introgression after recontact is so advanced that any differences have been erased throughout the Santa Catalina mountain range.

Evaluación de los límites específicos entre <i>Parthenium hysterophorus y P. glomeratum (Asteraceae-Ambrosiinae)</i>: evidencias morfológicas, anatómicas, cromosomáticas y fitoquímicas.

Species boundaries between Parthenium hysterophorus L./P. glomeratum Rollins were examined. Our working hypothesis states that the delimitation of these taxa is based on the dissection of a continuous variation pattern from an elevational cline. We chose a 70 km gradient from 300-3500 m in the Province of Salta (Argentina) where both taxa are sympatric. The study was centred in phenetic analyses of morphological data. In addition, foliar and caulinar anatomy, chromosome numbers and karyotype, and sesquiterpene lactone composition were investigated. Transplant experiments were performed, individuais that occur naturally at high elevations were cultivated at lower ones to analyse the phenetic plasticity of some key characters. Our results indicate that based on morphological and anatomical data no group of specimens can be delimited. The main characters used to recognise both species are significantly correlated with the elevation at which they grow, suggesting a clinal variation. Cytological and phytochemical data show that there is variation among the populations and that no discontinuities can be detected to separate them. Accordingly, it is convenient to consider the studied population as belonging to P. hysterophorus , and P. glomeratum as a synonym of that species.