Sitka Spruce Populations Research Articles

Delivering Sitka spruce with resistance against white pine weevil in British Columbia, Canada

The Sitka spruce (Picea sitchensis [Bong.] Carr) breeding program for resistance against the white pine weevil Pissodes strobi Peck (Coleoptera: Curculionidae) is arguably one of the most successful pest resistance breeding programs for plantation forest species in North America, with a substantial proportion of the planting stock in BC and Washington State currently coming from this breeding program. Using conventional selection and breeding, and by screening Sitka spruce populations using artificial weevil infestations, we identified sources of heritable and stable weevil resistance. We also used this program to investigate potential causes behind this resistance and identified several heritable resistance mechanisms, including anatomical characteristics, such as constitutive resin canals and sclereid cells in the bark, terpene defenses and variation in tree phenology. We concluded that resistance is conferred by a suite of traits whose composition varies among resistant sources. In addition, we evaluated the efficiency of screening for resistance using weevil population enhancement as a screening method. Our results culminated in the establishment of seed orchards, and the availability of resistant seed that is contributing to the return of Sitka spruce as a species of choice in coastal British Columbia.

Metabolic dynamics during autumn cold acclimation within and among populations of Sitka spruce (Picea sitchensis)

• Autumnal cold acclimation in conifers is a complex process, the timing and extent of which vary widely along latitudinal gradients for many tree species and reflect local adaptation to climate. Although previous studies have detailed some aspects of the metabolic remodelling that accompanies cold acclimation in conifers, little is known about global metabolic dynamics, or how these changes vary among phenotypically divergent populations. • Using untargeted GC-MS metabolite profiling, we monitored metabolic dynamics during autumnal cold acclimation in three populations of Sitka spruce from the southern, central, and northern portions of the species range, which differ in both the timing and extent of cold acclimation. • Latitudinal variation was evident in the nature, intensity, and timing of metabolic events. Early development of strong freezing tolerance in the northern population was associated with a transient accumulation of amino acids. By late autumn, metabolic profiles were highly similar between the northern and central populations, whereas profiles for the southern population were relatively distinct. • Our results provide insight into the metabolic architecture of latitudinal adaptive variation in autumn acclimation and show that different mechanisms are the basis of early October cold hardiness and autumn-acclimated cold hardiness.

The potential to improve growth rate and quality traits of stem straightness and branching habit when breeding Picea sitchensis (Bong.) Carr

Tree breeders continue to use visual assessments of stem shape and branching habit as part of their selection criteria because these are key features used by timber merchants in assessing stand value. However, it is not known how effective these assessments are in introducing real improvements in stem and branching traits to breeding and production populations of Sitka spruce. This study determined the genetic characteristics of several traits associated with stem straightness and branching, and examined whether these traits are suitable as future selection criteria. Trees were sampled from 33 families selected from a 20-year-old half-sibling progeny trial growing in northern Scotland and a control of directly imported material from Queen Charlotte Islands, Canada. Stem straightness and branching habit were ‘scored’ for quality in addition to direct measurements made on growth and branching characteristics. Results indicate that heritability values for most external stem and branching properties evaluated visually are sufficiently strong to enable effective selection in tree improvement programmes. Estimated genetic correlations suggest that substantial improvements in branch and stem quality could be achieved without a reduction in growth rate. Visual scoring systems appear to successfully target families of good growth and small branch size relative to the control population.

Global monitoring of autumn gene expression within and among phenotypically divergent populations of Sitka spruce (Picea sitchensis)

Cold acclimation in conifers is a complex process, the timing and extent of which reflects local adaptation and varies widely along latitudinal gradients for many temperate and boreal tree species. Despite their ecological and economic importance, little is known about the global changes in gene expression that accompany autumn cold acclimation in conifers. Using three populations of Sitka spruce (Picea sitchensis) spanning the species range, and a Picea cDNA microarray with 21,840 unique elements, within- and among-population gene expression was monitored during the autumn. Microarray data were validated for selected genes using real-time PCR. Similar numbers of genes were significantly twofold upregulated (1257) and downregulated (967) between late summer and early winter. Among those upregulated were dehydrins, pathogenesis-related/antifreeze genes, carbohydrate and lipid metabolism genes, and genes involved in signal transduction and transcriptional regulation. Among-population microarray hybridizations at early and late autumn time points revealed substantial variation in the autumn transcriptome, some of which may reflect local adaptation. These results demonstrate the complexity of cold acclimation in conifers, highlight similarities and differences to cold tolerance in annual plants, and provide a solid foundation for functional and genetic studies of this important adaptive process.

Optimal sampling strategies for capture of genetic diversity differ between core and peripheral populations of Picea sitchensis (Bong.) Carr

In previous studies we reported that while core populations of Sitka spruce [Picea sitchensis (Bong.) Carr] have little within-population genetic structure, peripheral populations are strongly spatially structured at distances up to 500 m. Here we explore the implications of this difference in structure on ex situ gene conservation collections and estimates of genetic diversity from research collections. We test the effects of varying the number of individuals sampled and the total area they are sampled across on capture of neutral genetic variation in collections from core, continuous versus peripheral, disjunct populations. Bivariate response surface analysis of genetic marker data for eight sequence tagged site loci from core and peripheral populations suggest that a population sample from 150 trees covering at least 225 ha would be adequate for capturing 95% of the genetic diversity (as measured by allelic richness or expected heterozygosity) in core populations. However, a larger sample of 180 individuals from an area of at least 324 ha is needed in peripheral populations to capture the same proportion of standing variation because of stronger within-population spatial genetic structure. Standard population sampling protocols for estimating among and within-population genetic diversity would significantly underestimate the within-population allelic richness and expected heterozygosity of peripheral but not core populations, potentially leading to poor representation of genetic variation in peripheral populations as well as erroneous conclusions about their genetic impoverishment.

Adaptive gradients and isolation-by-distance with postglacial migration in Picea sitchensis

Fossil pollen records suggest rapid migration of tree species in response to Quaternary climate warming. Long-distance dispersal and high gene flow would facilitate rapid migration, but would initially homogenize variation among populations. However, contemporary clinal variation in adaptive traits along environmental gradients shown in many tree species suggests that local adaptation can occur during rapid migration over just a few generations in interglacial periods. In this study, we compared growth performance and pollen genetic structure among populations to investigate how populations of Sitka spruce (Picea sitchensis) have responded to local selection along the historical migration route. The results suggest strong adaptive divergence among populations (average Q(ST)=0.61), corresponding to climatic gradients. The population genetic structure, determined by microsatellite markers (R(ST)=0.09; F(ST)=0.11), was higher than previous estimates from less polymorphic genetic markers. The significant correlation between geographic and pollen haplotype genetic (R(ST)) distances (r=0.73, P<0.01) indicates that the current genetic structure has been shaped by isolation-by-distance, and has developed in relatively few generations. This suggests relatively limited gene flow among populations on a recent timescale. Gene flow from neighboring populations may have provided genetic diversity to founder populations during rapid migration in the early stages of range expansion. Increased genetic diversity subsequently enhanced the efficiency of local selection, limiting gene flow primarily to among similar environments and facilitating the evolution of adaptive clinal variation along environmental gradients.

Strong spatial genetic structure in peripheral but not core populations of Sitka spruce [Picea sitchensis (Bong.) Carr.

We examined spatial genetic structure within eight populations of Sitka spruce classified as core or peripheral based on ecological niche, and continuous or disjunct based on species distribution. In each population, 200 trees were spatially mapped and genotyped for eight cDNA-based sequence tagged site (STS) codominant markers. Spatial autocorrelation was assessed by estimating p(ij), the average co-ancestry coefficient, between individuals within distance intervals. The distribution of alleles and genotypes within core populations was almost random, with nonsignificant co-ancestry values among trees as close as 50 m in core populations. In contrast, the distribution of alleles and genotypes within peripheral populations revealed an aggregation of similar multilocus genotypes, with co-ancestry values greater than 0.20 among trees up to 50 m apart and significant, positive values between trees up to 500 m. The relatively high density of reproductive adults in core populations may lead to highly overlapping seed shadows that limit development of spatial genetic structure. However, in peripheral populations with a lower density of adults, the distribution of alleles and genotypes was highly structured, likely due to offspring establishment near maternal trees and subsequent biparental inbreeding, as well as more recent population establishment at the leading edge of post-Pleistocene range expansion. Conserving genetic diversity in peripheral populations may require larger reserves for in situ conservation than required in core populations. These data on spatial genetic structure can be used to provide guidance for sampling strategies for both ex situ conservation and research collections.

Relationships between gas exchange and carbon isotope discrimination of Sitka x interior spruce introgressive genotypes and ribosomal DNA markers.

Effects of hybridization on physiological performance were investigated in seven natural introgressive populations and one F(1) population of Sitka (Picea sitchensis (Bong.) Carr.) x interior spruce (P. glauca (Moench) Voss. x P. engelmannii Parry ex Engelm.). Each population was represented by a Sitka-interior spruce ribosomal DNA (Si rDNA) index that was calculated from the relative abundance of species-specific DNA polymorphisms. Gas exchange parameters were measured under well-watered conditions on current-year needle tissues, which were also analyzed for carbon isotope discrimination. Sitka spruce populations had higher gas exchange rates (net photosynthesis, stomatal conductance to water vapor), but lower carbon isotope discrimination values than interior spruce populations. The natural hybrid populations had intermediate values of these parameters consistent with their Si rDNA index. The F(1) population had gas exchange parameters resembling those of Sitka spruce populations, but its carbon isotope discrimination was skewed toward that of interior spruce populations. These results confirmed previous findings that physiological performance of introgressive hybrid spruce populations varied as their DNA constitution changed.

Relationships between gas exchange adaptation of Sitka x interior spruce genotypes and ribosomal DNA markers.

Adaptive physiological changes were investigated in seven populations of Sitka (Picea sitchensis (Bong.) Carr.) x interior spruce (P. glauca (Moench) Voss x P. engelmannii Parry ex Engelm.) spanning the Nass-Skeena transition zone in British Columbia, Canada. Each population was represented by an Si rDNA index that was calculated from the relative optical densities on a gel autoradiogram of five ribosomal DNA bands characteristic of Sitka spruce and interior spruce. This index estimates the proportion of the genome contributed by interior spruce. Physiological adaptations were assessed by gas exchange parameters measured under both well-watered and drought conditions. Under well-watered conditions, Sitka spruce populations had higher maximal photosynthesis at saturating light and ambient CO(2), higher quantum yield at the light compensation point, and higher dark respiration than interior spruce populations. Sitka spruce populations also reached maximal photosynthesis at lower photosynthetically active radiation and higher CO(2) concentrations, and had higher stomatal densities that resulted in lower stomatal limitations to photosynthesis than interior spruce populations. In contrast, interior spruce populations exhibited greater drought tolerance than Sitka spruce populations. Their gas exchange rates declined at a slower rate in response to drought. They maintained higher gas exchange rates in response to moderate to severe drought (predawn plant water potentials = -1.5 MPa), and their photosynthetic rates recovered faster when they were rewatered after exposure to drought. Comparison of the seven populations indicated that physiological parameters were significantly related to the Si rDNA index. An increase in Si rDNA index was associated with proportional changes in physiological measurements, suggesting that genetic interchange among species with contrasting ecological adaptations can enhance the environmental adaptation of natural populations.

Relationship between nuclear DNA markers and physiological parameters in Sitka x interior spruce populations.

Eight populations of Sitka spruce (Picea sitchensis (Bong.) Carr.) and interior spruce (Picea glauca (Moench) Voss x Picea engelmannii Parry ex. Engelm.) seedlings were sampled from a zone of Sitka-interior spruce introgression in British Columbia, Canada. Restriction fragment length polymorphisms of the nuclear ribosomal RNA genes (rDNA) were used to define species-specific hybridization patterns for the Sitka spruce and interior spruce populations. Hybridization was estimated from an index based on the relative abundance of polymorphic rDNA combining bands for each population. Sitka x interior hybrid seedlings had an index value for the relative abundance of interior spruce rDNA (Si-rDNA) ranging from 0.07 (Lower Nass; the most westerly collected source) to 0.95 (Bulkley Valley low-elevation seed orchard). During shoot elongation, osmotic potential at saturation (Psi(sat)) and turgor loss point (Psi(tlp)) increased, whereas total turgor (Psi(PTotal)) decreased. After bud set in the summer and throughout the fall, Psi(sat) and Psi(tlp) decreased, whereas Psi(PTotal) increased. At all times of year, populations with a higher Si-rDNA index had lower Psi(tlp) and Psi(sat) and higher Psi(PTotal) than populations with a lower Si-rDNA index. During the fall, Sitka x interior hybrid seedlings exhibited a seasonal decline in the temperature causing 50% needle electrolyte leakage (LT(50)) and in the critical temperature indicating the initial point of freezing injury. Seedlings with a higher Si-rDNA index had lower LT(50) and critical temperature values indicating greater freezing tolerance in the fall. Throughout most of the year, seedling population Si-rDNA index was related to the degree of drought and freezing tolerance.

Genetic diversity in a seed production population vs. natural populations of Sitka Spruce

Isozyme analysis was applied to estimate the level of variation and the genetic structure of a seed-production population (i.e., seed orchard) and 10 range-wide natural populations of Sitka spruce (Picea sitchensis (Bong.) Carr.). Gene diversity and heterozygosity estimates were comparatively high in both the seed orchard and the natural populations studied. The seed orchard population showed a significantly higher number of alleles per locus and percentage of polymorphic loci. Though not significant, mean heterozygosity of the seed orchard was higher than that observed for all natural populations. Genetic distance analysis indicated that the seed-orchard population was genetically similar to three natural populations from which the parent trees were selected. Parent trees sampling breadth has been identified as the major cause for the observed increased level. The impact of recurrent selection and seed orchard biology and management on maintaining the genetic diversity is discussed.

Microgeographic genetic variation of Sitka spruce in southeastern Alaska

Microgeographic genetic variation among populations of Sitka spruce on Mitkof Island in southeastern Alaska is described. In two common-garden environments, we evaluated genotypes of 208 parent trees from 114 locations in a 17 000-ha area. Two principal components accounted for most of the variation among locations in 11 traits measured to evaluate growth vigor and rhythm of 2-year-old seedlings. Regression analyses of factor scores derived from principal components revealed genetic gradients associated with elevation, slope, aspect, and west–east and north–south direction. Large amounts of additive genetic variation in factor scores occurred among trees within locations. When this variation within locations was used as a scale, variation among locations was also large. In an extreme case, locations differed in factor scores of the first principal component by about 3.0 units of the standard deviation of additive genetic variation in factor scores. Of the total differentiation in this case, elevational range (600 m) contributed 0.7 units of standard deviation, aspect contributed 0.9 units, and distance (16 km) from north central to southeastern parts of the island contributed 1.4 units.

Enzyme variation in natural populations of Sitka spruce (Piceasitchensis). 1. Genetic variation patterns among trees from 10 IUFRO provenances

Genetic variation at 24 enzyme loci was studied for 10 IUFRO populations of Sitka spruce (Piceasitchensis (Bong.) Carr.). The average proportion of polymorphic loci per population was 0.51 ± 0.02 and the average proportion of heterozygous loci per individual was 0.15 ± 0.03. The following generalizations were advanced: (1) the amount of genetic polymorphism varied considerably from locus to locus; (2) many populations were similar in the amount and pattern of genetic variation for most loci; (3) but some loci had large differences among populations.Eight percent of the gene diversity was attributed to interpopulation differentiation; 92% of the total gene diversity resided within populations. Pairwise comparisons of genetic distances averaged 0.014, a value which reflected very little differentiation between geographic sources.

Isoenzyme Identification of Picea glauca, P. sitchensis, and P. lutzii Populations

Electrophoretic techniques were used to identify stands of pure Sitka spruce Picea sitchensis (Bong.) Carr. and pure white spruce P. glauca (Moench) Voss and spruce stands in which introgressive hybridization between the white and Sitka spruce had occurred. Thirteen heteromorphic isoenzymes of LAP, GDH, and TO were the criteria for stand identification. Estimates of likeness or similarity between seed-source areas were made from D determinations. Introgressed hybrid stands had isoenzyme frequencies that were intermediate between the two pure species, but the seedlings were somewhat more like white spruce than like Sitka spruce. Much of the west side of the Kenai Peninsula appeared to be a hybrid swarm area, with stands containing both Sitka and white spruce genes. The presence of white spruce genes in Sitka spruce populations was most easily detected by the presence of TO activity at Rm 52. White spruce showed activity at that position in 79% of its germinants; only 1% of the pure sitka spruce germinants had similar activity. Isoenzyme variation between stands of pure Sitka spruce was less variable than that between interior white spruce stands (mean distinction values were 0.11 for Sitka and 0.32 for white spruce). Cluster analysis showed all six pure Sitka spruce populations to be similar at .93, whereas pure white spruce populations were not similar until .69.

A GENECOLOGICAL STUDY OF THE GENUS PICEA IN BRITISH COLUMBIA*

SummaryThe study is divided into two parts. Part A is an investigation of geographic variation in immature spruce populations and refers to the growth behaviour of 150 spruce populations grown in a relatively uniform environment. The seed, which was collected throughout the white, Engelmann and Sitka spruce complex in British Columbia, was sown in a coastal nursery on Vancouver Island. Detailed measurements were made on these populations during two growing seasons. In the laboratory seed samples of the same populations were X‐rayed to determine embryo development, and subsequently germinated at 25° C, Part B is a study of geographic variation in mature spruce populations and refers to a biometrical assessment of variation in cone scale morphology which was carried out on a mass collection of spruce cones collected in 157 areas throughout the range of spruce in British Columbia. This collection, therefore, embraces all four spruce species of the province. These are Sitka spruce (Picea sitchensis (Bong,) Carr.), black spruce (P. mariana (Mill,) B,S,P,), white spruce (P. glauca (Nlotnc) Voss subsp, glauca) and Engelmann spruce (P. glauca (Moench) Voss subsp, engelmannii). All measurements were subjected to statistical analysis in order to determine the relation between the variation observed and factors of the environment at place of origin. The results of both studies testify to the overwhelming influence of environmental pressures associated with altitude with respect to variation in the white‐Engelmann complex. In regard to the white‐Sitka complex both studies indicate the extent and effect of the penetration of Sitka spruce populations into populations of white spruce. Zones of hybridization between spruce species are characterized. The ecological significance of photoperiodic adaptation in spruce, and the relative importance of flushing and dormancy in the microevolution of the species are considered. The bearing of the results on the taxonomic status of the species in British Columbia is discussed, and recommendations are made in regard to the silviculture of the species in this province.