S. Ellen Macdonald

The reduction of organic-layer depth by wildfire in the North American boreal forest and its effect on tree recruitment by seed

We compared prefire and postfire organic-layer depths in boreal forest types (14 fires) across Canada, and examined tree recruitment as a function of depth. There was extensive within-stand variation in depth, much of it due to clustering of thinner organic layers around boles. There were no significant differences in postfire organic-layer depth among sites with different prefire forest species composition, but sites in the eastern boreal region had thicker postfire organic layers than those in the western boreal region. Mean organic-layer depth was much greater in intact stands than after fires; overall, fire reduced organic-layer depth by 60%, largely because of increases in the area of thin (<3 cm) organic layers (1% in intact stands vs. 40% in postfire stands). There was more variation in organic-layer depth within postfire than within prefire stands; notably, some areas in postfire stands were deeply combusted, while adjacent parts were only lightly combusted. We speculate that the diminished role of energy loss to latent heat around tree boles increased organic-layer consumption around tree boles. Seedlings were clustered around burned tree bases, where organic layers were thinner, and the dependence of a species on thin organic layers was an inverse function of seed size.

Structure and composition of edges next to regenerating clear-cuts in mixed-wood boreal forest

Abstract We investigated the spatial and temporal pattern of edge influence on primary (forest structure) and secondary (understory structure and composition) responses at forest edges next to regenerating clear-cuts in Populus-dominated boreal forest in Alberta, Canada. We used before/after-harvest comparisons to determine whether there were significant effects of edge creation in the first and second year after harvest. We also investigated 5- and 16-yr-old edges to determine whether their structure and composition were significantly different from interior forest. Distance of edge influence (20 - 60 m) was not extensive compared to findings from studies conducted in other forest types. In the first two years after harvest, there were significant primary responses to edge creation including increased tree mortality, snag breakage and amount of downed coarse woody debris; as well as secondary responses including prolific vegetative regeneration of Populus spp. and a decrease in total shrub cover. Significant edge effects were still detectable 5 and 16 yr after edge creation, despite the rapid regeneration of Populus- dominated forest in the adjacent clear-cuts. These older edges showed evidence of past primary and secondary responses, including a more heterogeneous canopy, along with additional changes in understory composition. At both younger and older edges the distance of edge influence was greater for secondary responses (up to 60 m) than for primary responses (10 - 20 m). We hypothesize that secondary responses to edge creation start later than primary responses and that the distance of edge influence is greater for secondary (vs. primary) responses. Nomenclature: Moss (1983). Abbreviations: CWD = Downed coarse woody debris; DEI = Distance of edge influence; DEIc = Distance of edge influence for change from the previous year.

Predictors of moss and liverwort species diversity of microsites in conifer-dominated boreal forest

Abstract Patterns of moss and liverwort species diversity – species richness and species turnover (β-diversity) – in three conifer-dominated boreal forest stands of northern Alberta, Canada are described. We examined the relationship between bryophyte species diversity and micro-environment at two sample grains, the microsite – substrate types for moss colonization: logs, stumps, tree bases, undisturbed patches of forest floor (dominated by feather moss species), and disturbed patches of forest floor – and the mesosite (25 m × 25 m plots). Microsite type and properties (e.g. decay class, hardwood vs softwood, pH) were the principal predictors of bryophyte species diversity and not micro-environment variation among mesosites. Microsite type was the strongest predictor of microsite species richness and β-diversity was higher among microsites and types and within microsites than among mesosites or stands. Microsite properties were significant predictors of species richness for all microsite types. Log and stump decay classes, influenced also by hardwood vs softwood predicted species richness of woody microsite types and soil pH and moisture predicted species richness of forest floor microsites. β-diversity was highest for tree bases and disturbed patches of forest floor and lowest for logs. Mesosite β-diversity was lower than that among microsites, and mesosite species richness was not well explained by measured environmental parameters. Results suggest that in conifer-dominated boreal stands, species richness of microsites is only negligibly influenced by within-stand variation at the mesosite grain and that substrate characteristics are the most important predictors of bryophyte species diversity in this ecosystem. Nomenclature: Anderson et al. (1990) for bryophytes except for Sphagnaceae (Anderson1990) and Hepaticae (Stotler & Crandall-Stotler 1977); Orthotrichum elegans is recognized as a distinct species from Orthotrichum speciosum (Vitt & Darigo 1997).

THE INTERACTION BETWEEN MASTING AND FIRE IS KEY TO WHITE SPRUCE REGENERATION

We used the mast-seeding tree Picea glauca (white spruce) to examine wheth- er the timing of mast years relative to fire had a lasting effect on the density and timing of regeneration. We studied 17 fires that occurred in mast years and in years with low cone production between 1941 and 1994. Trees were carefully aged by crossdating procedures. Over the 59-yr period studied, there was significantly more regeneration after fires that occurred in mast years than after fires that occurred in years of low cone production. Spruce density was significantly lower after fires that occurred 1-3 years before a mast year than after fires during mast years. The cohort of trees that regenerated in the first mast year after a fire was critical to white spruce regeneration for fires that occurred 0-1 year before a mast year, but mast years occurring three or more years after a fire contributed few recruits. Our results suggest that masting is a key process that interacts with fire to shape stand composition in boreal mixedwoods. For species like white spruce, for which establishment is linked to disturbance, masting may have a contingent, historical effect on succession and landscape structure.

Factors Influencing Bryophyte Assemblage at Different Scales in the Western Canadian Boreal Forest

Abstract This study examined bryophyte community composition in relation to microsite and microenvironmental variation at different scales in three conifer-dominated stands in the boreal forest of Alberta, Canada. We documented bryophyte assemblage on specific microsite types (physiognomic forms providing substrates for moss colonization: logs, stumps, tree bases, undisturbed patches of forest floor, disturbed patches of forest floor), and at coarser scales: mesosites (625 m2 plots within stands), and stands (10 ha). Patterns of variation in bryophyte composition arising from the microsite sampling were clearly related to microsite type and, for woody substrates, to microsite quality (decay class; hardwood vs. softwood). Microenvironment (moisture, pH, temperature, light) also had some influence on bryophyte composition of woody microsite types. Forest floor moisture, pH, and light were related to bryophyte composition of undisturbed patches of forest floor while forest floor moisture and temperature were significant correlates for disturbed forest floor. At the coarser-scale, surface moisture and forest floor moisture were related to bryophyte assemblage of mesosites; this was partially reflective of differences among stands. We conclude that bryophyte species composition in these forests is related to a hierarchy of factors including fine scale variation in the type and quality of available microsites along with microenvironmental variation at different scales. Management efforts to preserve bryophyte biodiversity will need to incorporate this complexity.

Evolution of phenotypic plasticity in the Stellaria longipes complex : comparisons among cytotypes and habitats

Variation in the amount and pattern of plasticity was studied in three cytotypes (4x, 6x, and 8x) of Stellaria longipes and diploids of its suspected progenitor S. longifolia. All 13 traits considered showed plasticity. There were significant differences among cytotypes and habitats in plasticity for many traits. Overall, the diploids, S. longifolia, were most plastic, and the three cytotypes of S. longipes did not differ in amount of plasticity. Stellaria longifolia showed divergence from S. longipes in the pattern of plasticity as well. In general, cytotypes with more similar chromosome numbers had the same pattern of plasticity for more traits. Individuals from tundra populations differed in their pattern of plasticity from those of montane, boreal, and prairie origin, which were more similar to one another. Differences in plasticity among cytotypes were due primarily to divergence in amount, while differences among habitats were most often accounted for by divergent patterns of plasticity. We conclude that both polyploidy and natural selection have affected the evolution of plastic responses in this species complex. Analysis of the correlation between pairs of traits provided evidence that the pattern and amount of plasticity operate independently of one another and may be evolving separately.

Forest restoration following surface mining disturbance: challenges and solutions

Many forested landscapes around the world are severely altered during mining for their rich mineral and energy reserves. Herein we provide an overview of the challenges inherent in efforts to restore mined landscapes to functioning forest ecosystems and present a synthesis of recent progress using examples from North America, Europe and Australia. We end with recommendations for further elaboration of the Forestry Reclamation Approach emphasizing: (1) Landform reconstruction modelled on natural systems and creation of topographic heterogeneity at a variety of scales; (2) Use and placement of overburden, capping materials and organic amendments to facilitate soil development processes and create a suitable rooting medium for trees; (3) Alignment of landform, topography, overburden, soil and tree species to create a diversity of target ecosystem types; (4) Combining optimization of stock type and planting techniques with early planting of a diversity of tree species; (5) Encouraging natural regeneration as much as possible; (6) Utilizing direct placement of forest floor material combined with seeding of native species to rapidly re-establish native forest understory vegetation; (7) Selective on-going management to encourage development along the desired successional trajectory. Successful restoration of forest ecosystems after severe mining disturbance will be facilitated by a regulatory framework that acknowledges and accepts variation in objectives and outcomes.

Sex- and habitat-specific responses of a high arctic willow, Salix arctica, to experimental climate change

Dioecious plant species and those occupying diverse habitats may present special analytical problems to researchers examining effects of climate change. Here we report the results from two complementary studies designed to determine the importance of sex and habitat on gas exchange and growth of male and female individuals of a dioecious. circumpolar willow. Salix arctica, in the Canadian High Arctic. In field studies, male and female willows from dry and wet habitats were subjected to passively enhanced summer temperature (∼ 1.3°C) using small open-top chambers over three years. Peak season gas exchange varied significantly by willow sex and habitat. Overall net assimilation was higher in the dry habitat than in the wet, and higher in females than in males. In the dry habitat, net assimilation of females was enhanced by experimental warming, but decreased in males. In the wet habitat, net assimilation of females was substantially depressed by experimental warming, while males showed an inconsistent response. Development and growth of male and female catkins were enhanced by elevated temperature more than leaf fascicles, but leaf fascicle development and growth varied more between the two habitats, particularly in males. In a controlled environment study, male and female willows from these same wet and dry habitats were grown in a 2 × 2 factorial experiment including 1 × or 2 × ambient [CO 2 ] and 5 or 12°C. The sexes responded very differently to the experimental treatments, but we found no effect of original very differently to the experimental treatments, but we found no effect of original habitat. Net assimilation in males was affected by the interaction of temperature and CO 2 , but in females by CO 2 only. Our results demonstrate (a) significant intraspecific and intersexual differences in arctic willow physiology and growth. (b) that these differences are affected by environmental conditions expected to accompany global climate change, and (c) that sex- and habitat-specific responses should be explicitly accounted for in studies of dioecious species.

Rhizome plasticity and clonal foraging of Calamagrostis canadensis in response to habitat heterogeneity

1) A glasshouse experiment was conducted in which individuals of the rhizomatous grass Calamagrostis canadensis were grown in soil-filled wooden boxes. Directly adjacent to each plant on one side was a control area (same conditions as those in which the plant was growing), while immediately adjacent on the other side was an area with one of the experimental conditions («shade», «cool soil», «cool soil + shade», or «cool soil + intraspecific competition»). Rhizomes of each plant were allowed the opportunity to grow simultaneously into both the control and the treated side

Edge influence on vegetation at natural and anthropogenic edges of boreal forests in Canada and Fennoscandia

Although anthropogenic edges are an important consequence of timber harvesting, edges due to natural disturbances or landscape heterogeneity are also common. Forest edges have been well studied in ...