Clonal response of a recalcitrant understory shrub, Labrador tea (Rhododendron groenlandicum Oeder.) to forest gaps

After canopy removing disturbance, recalcitrant understory shrubs can rapidly occupy the forest understory to the detriment of tree regeneration and growth. The expansion of recalcitrant species has been documented following stand replacing disturbances, such as fire and timber harvest. However, there is little information on how these species respond to much smaller canopy gaps created by the senescence and/or death of single or groups of canopy trees. In this study, we determined the response of Labrador tea (Rhododendron groenlandicum Oeder.), a recalcitrant ericaceous shrub, to canopy gaps in a late-successional boreal forest in northwestern Ontario, Canada. We evaluated functional traits related to the morphology and regeneration strategy of this plant to elucidate the mechanism of gap filling. We found that R. groenlandicum abundance and vigor were greater at the center of treefall gaps than in gap edges or the forest understory due to aggressive sprouting from buried clonal bud banks. Layering was higher in canopy gaps than in the understory. The composition of ground cover and rooting substrate was more influential on the adventitious rooting of the layered stems than increased light availability in gaps.We found a strong response of R. groenlandicum to small canopy openings, suggesting that the species can form recalcitrant understory layers even in the absence of stand replacing disturbance.