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The stress gradient hypothesis:Plant Facilitation at the forest·Tundra transition (Mealy mountains, Labrador, Canada)

Cranston, Brittany. 2009. Memorial University of Newfoundland

Abstract

Facilitative interactions(Stress Gradient Hypothesis) at the Forest-Tundra (F-T) Transition zone around the globe have yet to be evaluated . As climate warms, the balance between facilitation and competition shifts, allowing species to expand their ranges. This study investigates the role of nurse shrubs (Betula glandulosa [dwarf birch] and Vaccinium uliginosum [bilberry]) in facilitating recruitment of Picea mariana and P. glauca (black and white spruce) in the boreal F-T Transition zone. Treatments were designed to disentangle the roles of nurse height, shade, and nurse canopy removal by herbivores in determining recruitment success. Seeds germinated at temperatures 5oC below the minimum required for germination in the open and at significantly greater levels (20% ± 6.12 of P. mariana and12% ± 2.8 of P. Glauca) compared with the non-shrub reference site (0.5%) and previous studies. Treatments generally had negligible effects on growth, but nurse litter increased levels of soil calcium, suggesting a weak net positive association. If viable seed is available, shrubs will facilitate recruitment. The balance between competition and facilitation is highly site dependent and cannot be generalized over larger spatial scales. Understanding how species interactions are affected by regional climate change is critical for predicting range expansion of boreal trees.

Key Words

Stress Gradient Hypothesis,nurse effect,facilitation ,Picea mariana, P. glauca recruitment , Forest-Tundra Transition zone, dwarf shrubs, Labrador