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