Natural root grafts (anastomoses) are morphological unions formed between roots of different trees. Common root systems allow translocation of water, nutrients and photosynthesis products between grafted trees, affecting their growth and their physiology. As carbohydrates are redistributed among grafted trees, the formation of a common root system could reduce the negative effect of intraspecific competition for light or soil resources within stands. The aim of this study was to investigate the role of root grafting on intraspecific competition and growth of balsam fir Abies balsamea. We studied inter-tree relationships in three natural balsam fir stands of the boreal forest of Quebec (Canada) that contained an average 36% of grafted trees. At each stand, ring width and basal area of trees were measured using dendrochronology techniques. We used mixed linear models to test the effect of root grafting and intraspecific competition on annual basal area increment of trees. Trees before grafting had higher growth rates than trees once grafted. Thus, root grafting did not improve tree growth. Growth of grafted trees was more negatively affected by intraspecific competition than growth of non-grafted trees. Thus, grafted trees cannot be considered as better competitors than non-grafted trees. Under high intraspecific competition, growth of larger grafted trees was less affected than that of smaller trees suggesting that they were able to divert resources at their advantage within a union. Our study demonstrated that grafted trees acted on each other's growth and provides support for the idea that grafted trees respond to competition for resources more as a community rather than as individual trees.

Les greffes racinaires naturelles résultent de la fusion des tissus vasculaires de deux racines provenant d’arbres distincts. Elles permettent la redistribution d’eau, de nutriments et de produits de la photosynthèse entre les arbres affectant leur croissance et leur physiologie. Bien que des greffes aient été observées chez plus de 200 espèces d’arbres, leur processus de formation et leur rôle écologique dans les interactions entre arbres restent mal connus. Dans ce contexte, trois peuplements de sapins baumiers (Abies balsamea (L.) Mill.) en forêt boréale au Québec ont été excavés hydrauliquement afin de déterminer la fréquence des greffes et de caractériser leurs facteurs de formation. Dans un second temps, l’effet d’interaction entre la compétition intraspécifique et les greffes racinaires sur la croissance des sapins a été évalué en utilisant des modèles linéaires de croissance. Enfin, l’hypothèse selon laquelle les arbres pourraient libérer des composés allélopathiques dans le sol pouvant stimuler ou inhiber la formation des greffes a été testée. Pour cela, les profils métabolomiques d’exsudats et de racines fines d’arbres greffés et non-greffés ont été comparés. Nous avons démontré que : i) en moyenne 36 % des arbres étaient greffés au sein des peuplements et que la proximité entre les arbres était le meilleur prédicteur de la formation des greffes; ii) une fois les arbres greffés, leur croissance était fortement réduite lors de l’augmentation de la compétition intraspécifique; iii) les stilbènes et flavonoïdes libérés dans le sol par les racines de sapins seraient de potentiels inhibiteurs de la formation des greffes. Les greffes pourraient donc jouer un rôle clé dans la dynamique des peuplements en rendant les arbres plus sensibles à la compétition intraspécifique et potentiellement en accélérant la phase d’auto-éclaircie. Les profils métabolomiques différents entre les racines et exsudats d’arbres greffés ou non constituent une première évidence d'un déterminisme chimique des greffes.

Natural root grafts (anastomoses) result from the fusion of the vascular systems of two roots and allow trees to share water, nutrients and photosynthesis products, affecting tree growth and physiology. The aim of this study was to investigate the frequency of root grafting in balsam fir (Abies balsamea (L.) Mill) of the boreal forest of Quebec (Canada), and to identify ecological drivers such as tree proximity or size of stems and roots. One 50 m2 area was hydraulically excavated in each of three natural balsam fir stands of various ages, tree diameters and densities. For each area, we measured the number of roots and grafts per tree, and the diameter and age of all stems, roots and grafts using dendrochronology techniques. Percentages of grafted trees and number of grafts per tree were similar between stands, corresponding to 36% (± 2.86 SE) and 1.30 (± 0.03 SE), respectively. Root grafting occurred at a wide range of tree ages from 12 to 106 years old. Mean distance between grafted trees was 47.91 cm (± 5.10 SE) and we did not observe any graft beyond a 2 m distance between trees. The number of grafts per tree increased with number of roots per tree and decreased with distance between trees. Root grafting also occurred at a wide range of root ages, from 5 to 64 years old. However, roots were relatively small at graft initiation, with an average root diameter of 3.94 cm (± 0.33 SE). These results demonstrate that balsam fir stands are highly connected through root grafting, occurring early in stand development and continuing throughout the life of the stands. The number of roots per tree and distance between trees were the best predictors for root grafting.