Does natural root grafting make trees better competitors?

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 (L.) Mill). 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.天然根嫁接（anastomoses）是不同树木根系之间形成的形态学联结结构。共享根系可实现嫁接树木间水分、养分及光合产物的转运，进而影响其生长与生理状态。由于碳水化合物在嫁接树木间重新分配，共享根系的形成可减轻林分内种内竞争对光照或土壤资源获取带来的负面影响。本研究旨在探究根嫁接对香脂冷杉（Abies balsamea (L.) Mill）种内竞争与生长的调控作用。我们针对加拿大魁北克寒温带森林中的3处天然香脂冷杉林分开展了树木间相互关系的研究，这些林分的平均嫁接树木占比达36%。在每个林分中，我们采用树木年代学方法测定了供试树木的年轮宽度与胸高断面积。我们使用混合线性模型，检验了根嫁接与种内竞争对树木年胸高断面积生长量的影响。嫁接前的树木生长速率高于完成嫁接后的个体，由此可见，根嫁接并未提升树木的生长量。嫁接树木的生长受种内竞争的负面影响程度高于未嫁接树木，因此，嫁接树木并不比未嫁接树木具备更强的竞争能力。在高强度种内竞争条件下，体型较大的嫁接树木的生长受影响程度小于体型较小的嫁接个体，这表明它们可在根系联结内部优先获取资源以获益。本研究证实，嫁接树木的生长会相互影响，同时为“嫁接树木在资源竞争中更倾向于以群体而非个体为单位做出响应”这一假说提供了实证支撑。