Modeling Intraspecific Root Grafting in Trees: a Theoretical Perspective on Predicting the Frequency and Effects of Root Grafting

Abstract

Understanding inter-tree interactions is a vital aspect of the study of forest dynamics. One pathway for tree interactions is through intraspecific root grafting, which occurs when the root tissues of two trees of the same species fuse underground, forming a connection that can transfer nutrients, sap, water, poisons and pathogens. However, root grafting is an understudied phenomenon with a small body of empirical work and very little theoretical modeling. To investigate whether root grafts form solely by random root contact between trees and whether some species form more or fewer grafts than explicable by chance alone, a neutral model that assumes grafts form by chance contact was created and its predictions were explored. Comparing this model’s predictions to empirical data from the literature found that the neutral model was able to explain some, but not all, of grafting behavior and found that different tree species were differently well predicted by the model. Further simulation models were used to investigate under what conditions resource sharing via root grafting could have meaningful benefits for trees at the stand level. Grafting appeared to be associated with stand-level increases in survivorship in environments where resource availability was variable in time and space, but did not have positive effects when resources were evenly or statically distributed to trees, and had negative effects when mean levels of resources in the simulation were very low regardless of resource variation. Thus, grafting may be adaptive by providing a resource averaging effect under uncertain environmental conditions.