Roots of Cooperation: Can Root Graft Networks Benefit Trees Under Stress?

The occurrence of natural root grafts, the functional union of roots of the same or different trees1–3, is common and shared across tree species2. However, their significance for forest ecology remains little understood. While early research suggested negative effects of root grafting (i.e. increases the risk of pathogen transmission)4,5, recent evidence supports the hypothesis that it is an adaptive strategy that reduces stress6–8 by facilitating resource exchange9,10. Here by analysing mangrove root graft networks, we show evidence of cooperation-associated benefits of root grafting. Grafted trees were found to dominate the upper canopy of the forest, and as the probability of grafting and the frequency of grafted groups increased with a higher environmental stress, the mean group size (number of trees within groups) decreased. While root networks could form randomly (i.e. trees do not actively ‘choose’ neighbours to graft to)11,12, the increased frequency and reduced group sizes in higher-stress environments point to the existence of underlying mechanisms that regulate ‘optimal size’ group selection related to resource use within cooperating groups8,13,14. This work calls for further studies to better understand tree interactions (i.e. network hydraulic redistribution)15 and their consequences for individual tree and forest stand resilience and water-use efficiency.

Natural root grafting in Picea mariana to cope with spruce budworm outbreaks

Spruce budworm (Choristoneura fumiferana Clem.) outbreaks cause extensive mortality and growth reductions throughout boreal forests in eastern North America. As tree vulnerability to defoliation remains partially unexplained by tree and stand attributes, we hypothesized that root grafting might attenuate the negative impact of severe defoliation in tree growth. Two experimental sites in the Abitibi-Témiscamingue region dominated by black spruce (Picea mariana Mill.) were harvested and hydraulically excavated to study tree growth in 36 trees in relation to root grafting and the last spruce budworm outbreak using dendroecological methods. Root grafts reduced the negative effects of defoliation by maintaining stable growth in connected trees during epidemic periods. Among dominant trees, growth releases immediately after the outbreak were uniquely observed in grafted trees. Among suppressed trees, grafted trees tended to grow more than non-grafted trees when defoliation severity was the highest. Carbohydrate transfers through root grafts and enhanced efficiency to acquire resources may explain the better performance of grafted trees under scenarios of limited carbon supply. This study reinforces the growing body of literature that suggests root grafting as a cooperative strategy to withstand severe disturbances and highlights the key role of root grafting in stand dynamics to cope with periodic outbreaks.