Twenty-four specimens of six boreal tree species were excavated: eight Pinusbanksiana Lamb., five Populustremuloides Michx., four Piceaglauca (Moench) Voss, three Piceamariana (Mill.) B.S.P., two Abiesbalsamea (L.) Mills., and two Lara laricina (Du Roi) K. Koch. Root systems were described, and percent biomass determined for above- and below-ground components for half these specimens. Tree ages ranged from 3 to 120 years. Horizontally spreading lateral roots dominated the root systems of all species and occurred within 3– 15 cm of the ground surface. Nutrient and moisture absorption appear to be the principal functions of lateral roots. Maximum vertical root growth occurred near the tree stump as: tap-, heart, sinker, and oblique lateral roots. Vertical root morphology is influenced by site conditions. In addition to providing mechanical support, sinker and taproots may be important adaptations for deep-water utilization on xeric sites. Our data suggest that roots may be important in boreal succession processes through two related mechanisms: (i) nutrient and water deprivation occurring through preemptive growth of tree roots from climax species over roots of serai species, and through interception by mosses; and (ii) niche partitioning occurring below ground between serai and climax, and among climax tree species, by vertical separation of the root systems.

Aim The purpose of this study is to develop palaeovegetation zonation models for central and north-central North America, based on late-Quaternary and Holocene pollen stratigraphic data (n ¼ 246 sites). A secondary purpose was to evaluate an hypothesis (Strong & Hills, 2003) to explain the disjunct distribution of species in western Alberta.

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Aim Evaluate the hypothesis that nine disjunct vascular plant species along the eastern slopes of the Rocky Mountains and in the Peace River District of west-central Alberta represent remnants of more southerly vegetation that occupied these areas during the Holocene Hypsithermal (9000-6000 YR BP). Alternatively, these plants represent populations that became established because of independent chance dispersal events.Location This study focuses on the area east of the Rocky Mountain Continental Divide in the Province of Alberta and the State of Montana in western Canada and USA, respectively.Methods Disjunct species were identified and their distributions mapped based on a review of occurrence maps and records, botanical floras and checklists, herbaria specimens, ecological and botanical studies, and field surveys of selected species. A disjunct species was defined as a plant population separated from its next nearest occurrence by a distance of > 300 km. Evaluation of the hypothesis was based on a review of published and unpublished pollen stratigraphy and palaeoecological studies. The potential geographical distribution of Hypsithermal vegetation was based on modern regional-based ecosystem mapping and associated monthly temperature summaries as well as future climatic warming models.

The root density and depth patterns of four boreal forest age sequences were analyzed for successional trends. Rooting depths increased with age on sandy substrates which supported aspen (Populustremuloides Michx.) and jack pine (Pinusbanksiana Lamb.) communities. Rooting depth did not change in an aspen series on fine-textured substrates or in a black spruce (Piceamariana (Mill.) BSP) series growing on organic substrates. Plant communities growing on mineral soils showed a decrease in near-surface root densities and understory vascular plant cover with increasing age. Maximum rooting was deepest on sandy substrates and shallowest on organic deposits. Roots in all stands were concentrated near the ground surface. In most cases 50% of all roots were located within 15 cm of the forest floor. Root densities in this zone ranged from 11 000 to 30 000 roots m−2 of vertical surface. Densities were approximately 4300 roots m−2 for the overall rooting zone.

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A simple to calculate and statistically testable method is proposed for assessing species abundance unevenness or dominance concentration within plant communities. Dominance rather than evenness is considered the preferred measure, because perfect evenness can be defined and used as a benchmark for comparing communities regardless of floristic richness. Four dominance (Simpson, Berger-Parker, McIntosh, and a proposed [DW]) and four evenness (Carmargo, Gini, Shannon, and Williams) indices are comparatively analyzed. The Simpson, McIntosh, Gini, and Williams indices were correlated with species richness, the Berger-Parker index was correlated with total species cover, the Shannon index over-estimated evenness, the Simpson index under-estimated dominance concentration, and a nonlinear relationship occurred between the Simpson and all of the evenness indices. Only DW and the Carmargo indices fulfilled the technical requirements established for evenness with appropriate reversals of criteria for assessing a dominance concentration index. Determination of DW was based on the maximum difference between cumulative proportion (also referred to as Lorenz curve or partial order) and perfect evenness values. The conventional assumption that dominance (D) is the complete opposite of evenness (E), as assessed by indices, was found in practice to be lacking, without the inclusion of an error term (i.e., 1 = E + D + error). Therefore, both dominance concentration and evenness should be reported when characterizing plant communities.Nomenclature: Anderson et al. (1990) for mosses, Esslinger (1997) for lichens, and Kartesz (1994) for vascular plants. Abbreviations: D -dominance concentration, E -evenness, DB -Berger-Parker index, DM -McIntosh index, DS -Simpson index, DW -proposed index, E1/D -Williams index, EG -Gini index, EJ -Shannon index, E' -Carmargo index, H' -Shannon-Wiener index, r -Pearson product-moment correlation coefficient, rI -Spearman rank correlation coefficient.