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

Abstract

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.Key words: black spruce (Picea mariana), insect defoliation, dendrochronology, root graft, tree growth.Résumé : Les épidémies de tordeuse des bourgeons de l'épinette (Choristoneura fumiferana Clem.) sont la cause d'importantes réductions de croissance et de mortalité dans la forêt boréale de l'est de l'Amérique du nord. Comme la vulnérabilité des arbres à la défoliation reste en partie inexpliquée par les caractéristiques des arbres et des peuplements, nous avons posé l'hypothèse que le greffage racinaire pouvait atténuer l'impact négatif de défoliations sévères sur la croissance des arbres. Deux sites expérimentaux dominés par l'épinette noire (Picea mariana Mill.) ont été récoltés et excavés hydrauliquement dans la région de l'Abitibi-Témiscamingue afin d'étudier la croissance de 36 arbres en lien avec les greffes racinaires et la dernière épidémie de tordeuse par des méthodes dendroécologiques. Les greffes racinaires ont réduit l'effet négatif de la défoliation en maintenant une croissance plus ou moins stable des arbres greffés durant la période épidémique. Parmi les arbres dominants, la reprise de croissance suite à l'épidémie n'a été observée que pour les arbres greffés. Chez les arbres supprimés, les arbres greffés tendaient à avoir une meilleure croissance que les arbres non greffés quand la sévérité de l'épidémie était à son maximum. Les transferts de sucres par les greffes racinaires et l'amélioration de l'efficacité d'acquisition des ressources pourraient expliquer la meilleure performance des arbres greffés dans des conditions d'approvisionnement en carbone limitées. Cette étude renforce la th...

Unravelling the age of fine roots of temperate and boreal forests

“…Exchange of older C through root grafts and common mycorrhizal networks are alternative pathways which remain to be explored 31 , 32 . A re-translocation of organic substrates in the root–mycorrhiza system could potentially lead to the usage of older C for the construction of fine roots.…”

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

“…Non-functional grafts confer higher mechanical stability through shared anchoring systems 17 , which is particularly relevant for coastal wetland forests with shallow root systems 16,24 due to anoxic sediments, and exposure to strong winds 25 . Functional grafts can additionally facilitate resource exchange 21 and promote growth 7 by mitigating the adverse effects of defoliation and budworm outbreaks 10,26 and increasing the concentration of carbohydrates in shaded trees 21 . By enabling the exchange of water, carbon, and mineral nutrients, severed trees can be kept alive through the support of grafted neighbours 27 .…”

Cooperative root graft networks benefit mangrove trees under stress

“…Root grafts are functional when cambia and vascular tissues are fused 2 . Functional grafts can facilitate resource exchange 9 and promote growth 12 by mitigating the adverse effects of defoliation and budworm outbreaks 13 , 14 and by increasing the concentration of carbohydrates in shaded trees 9 . Still, even if grafts are non-functional, they can increase tree stability as compared to non-grafted trees by sharing anchoring systems 5 .…”