Natural root grafting is a common phenomenon in many woody species. Through functional root grafts, i.e., fully fused root tissue, trees can exchange resources such as water. Mapping the frequency and size of connected trees in a black mangrove forest showed that the grafting frequency increased with increasing salinity, but group size decreased. These results suggest that there is a mechanism controlling group characteristics, possibly related to physiological drought.
Here, we hypothesize that root grafting may be an additional vector of horizontal and evaporative hydraulic redistribution that mitigates water stress. We developed a mechanistic, individual-based model to quantify the amount of water that can be exchanged (relative to whole-tree water uptake) and to identify the driving mechanisms. In the model, water exchange is driven by the water potential gradient between connected trees. The exchange can be bidirectional, i.e., it can flow from tree A to tree B or vice versa, depending on, among other things, temporal and spatial differences in water availability. We apply the model to a forest site, where changes in hydroperiod can increase soil salinity, which has similar physiological effects in mangroves as drought stress has on terrestrial trees. Simulation experiments allow us to assess the role of water exchange in maintaining the water balance under different environmental scenarios.
Mangroves are particularly suitable for investigating the ecological effects of root grafting because the extent of their roots is visible above ground and grafting is relatively easy to detect. Since our individual-based model can be transferred to terrestrial forests, the study provides a good opportunity to investigate the consequences of root grafting for terrestrial trees. Therefore, we would like to find collaborators to further investigate this topic in terrestrial forests and to verify the simulated exchange patterns, e.g., with sapflow measurements.