Early life history transitions and recruitment of Picea mariana in thawed boreal permafrost peatlands

Black spruce (Picea mariana) is the most abundant tree species in the boreal biome, but little is known about how climate warming may change recruitment in peatlands, especially those affected by permafrost thaw. We used results from a seven-year study in northern Manitoba, Canada, to address the following questions: (1) What is the relative importance of early life history transitions on P. mariana recruitment? (2) How are these transitions mediated by biological and environmental factors, including competition, facilitation, disease, herbivory, water table depth, and soil nutrients? (3) Do interactions among these factors create additional recruitment limitations beyond those imposed by environmental factors changing with climate warming, such as hydrology? Seed rain was measured over six years on forested permafrost plateaus and in neighboring collapse scar bogs. Seed germination and seedling survival and growth were measured over 4-5 years in collapse scars and assessed across a three-level water table treatment. Survival and growth experiments examined additional combinations of above- and belowground vascular plant competition and fertilizer addition. Results showed that failure of germination and survival on growing moss surfaces and reduced survival of seedlings in wetter microsites were primary constraints. Seed influx was significantly lower in collapse scars but likely did not limit recruitment. Biological and environmental factors mediating these life history transitions also differed in relative importance, and interactions among them tended to amplify recruitment limitation. Seedling survival was most strongly controlled by fast-growing mosses in wet microsites but also was influenced by apparent drowning in wet plots, herbivory, and loss of foliage caused by a fungal pathogen. Seedling growth was strongly controlled by water table depth, nutrient and competition levels, and fungal pathogens. Multiple, interacting factors will affect P. mariana establishment in boreal peatlands during climate warming. Generalizations about recruitment relying on few environmental gradients sensitive to climate change, such as water table, may therefore not fully capture the complexities of establishment.

Plant diversity, composition, and rarity in the southern boreal peatlands of Manitoba, Canada

Plant diversity and rarity have been relatively well studied for bryophytes in Canadian western boreal peatlands, but little information exists for vascular plants. Diversity, community composition, and rarity of bryophytes and vascular plants were determined and relationships examined among these and environmental variables in five peatland types at Duck Mountain, Manitoba: wooded bogs, black spruce swamps, wooded moderate-rich fens, open moderate-rich fens, and open extreme-rich fens. Total diversity was 298 species comprising 86 bryophytes and 212 vascular plants. Mean diversity followed a unimodal distribution over a bog – rich fen gradient. Wooded moderate-rich fens (59.0) and black spruce swamps (53.4) had the highest mean diversity, whereas wooded bogs (32.3) and open extreme-rich fens (34.7) had the lowest mean diversity. Occurrences of locally rare species followed the same general pattern, and provincially rare vascular plants were found primarily in wooded moderate-rich fens and black spruce swamps and were mostly orchids. Reasons for these patterns are complex, but high diversity appears to be related to high habitat heterogeneity and moderate environmental variables, e.g., pH and alkalinity, and low diversity appears to be related to environmental extremes, e.g., pH and alkalinity. Boreal wooded moderate-rich fens and black spruce swamps have comparatively high plant diversity and rarity and require consideration if the focus is biodiversity conservation. This will become increasingly important in landscapes where development pressures are high.