Frequency of root grafting in naturally and artificially regenerated stands of Pinus banksiana: influence of site characteristics

We investigated the frequency of root grafting in naturally and artificially regenerated stands of jack pine (Pinus banksiana Lamb.) in the western boreal forest of Quebec, Canada. Twelve 30-60 m2 plots were hydraulically excavated to determine effects of site characteristics on frequency and timing of root grafting. Naturally regenerated stands had grafted tree percentages similar to artificially regenerated stands (21%-71% across plots) but greater numbers of root grafts per tree (naturally regenerated, 0.73 graft tree-1; artificially regenerated, 0.52 graft tree-1). Mean percentages of grafted trees, number of grafts per tree, and the speed of graft formation were greater in sandy soils (61%, 0.71 graft tree-1 and 2.43 years, respectively) compared with clay soils (44%, 0.54 graft tree-1 and 2.97 years, respectively). Proximity of trees was a better predictor of root grafting than stand density, despite many root grafts being found with distant trees (>2 m) in artificially regenerated stands. Our results suggested that root grafts form early in stand development. Even if trees are initially separate entities, this relatively high level of root grafting produces stands where trees are extensively interconnected.