Two of the oldest living trees known on earth, the Pando and the Old Tjikko, are clonal plants—they share a root system with other trees. Their longevity has been attributed to the mutual benefit of sharing resources and the accumulation of beneficial somatic mutations. Similar effects are known for mycorrhizal networks and trees connected by grafted roots. Could it be that networking is generally advantageous, increasing forest integrity under resource limitation or harsh conditions? This would revolutionize our understanding of forest dynamics, which are currently believed to be driven by competition and facilitation among single trees, and not among single trees and tree networks. Natural root grafting could play a pivotal role in forest resilience, since grafting can be a facilitative interaction potentially increasing individual fitness. To understand this role, it is necessary to investigate the conditions that make resource sharing (e.g. water under drought stress) beneficial for networked compared to individual trees. Field studies would be very difficult, but individual-based models let us synthesize the few empirical results and develop new understanding of tree network function, group fitness, and its impact on forest dynamics.
Results/Conclusions
We provide an individual-based model serving as a virtual laboratory to investigate how formation of networks in plant populations depends on spatio-temporal patterns of resource availability. We designed simulation experiments mimicking black mangroves exposed to water stress, and compared the results with field data from Mexico. Field data showed that root grafting depends on trees local configuration, tree density and water availability. We explored alternative hypotheses of cooperation among trees in the model, with the best reproducing observed tree networks with 4 - 5 connected trees on average and a predominant linear structure. Our results agree with evolutionary game theory, which predicts that cooperation flourishes most if organisms are strongly tied pairwise, since the costs (for grafting) are quickly payed off by reciprocal benefits (sharing water, nutrients etc.). Our findings thus support the recent hypothesis that root networking is an evolutionary beneficial adaptive behavior that improves resource acquisition and survival under stressful environmental conditions.