Natural root grafting in hybrid poplar clones

Abstract

Root grafting occurs in 15-year-old hybrid poplar clones; and tree proximity, tree and root sizes are key factors enhancing its frequency. Root grafting in trees is a well-recognized phenomenon allowing nutrient and photosynthate redistribution between connected trees. This study assessed the influence of tree spacing on root graft formation in a 15-year-old hybrid poplar plantation with two clones (747215: Populus balsamifera × Populus trichocarpa and 915319: Populus maximowiczii × Populus balsamifera) planted and two spacings (1 × 1 m, 3 × 3 m) in Amos, Quebec, Canada. Root graft occurrence was characterized and tree growth metrics including tree diameter and height, number of roots, root cross-sectional area and root age were measured after hydraulic excavation of root systems. We showed that root grafts were present between trees distant of 1 m but not between trees planted at the 3 × 3 m spacing. An average of 38% of excavated trees was grafted in the 1 × 1 m plots with a mean of 1.14 grafts per tree, and 86% of the grafted trees were located in the exterior part of plots (the border rows), where the trees had grown larger. Root grafts were relatively young (1–6 years) and most had not yet completed, preventing us from determining the effect of root grafting on tree growth. Root grafts were more prevalent in clone 915319, and in trees that had greater above and belowground growth. These results strengthen the postulate that tree proximity enhances the frequency of root grafting.