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Mangrove trees form intraspecific root grafts, and resources can be exchanged between trees if the tissues are fully fused. Connections can exist between two or more trees, forming groups with diverse structures. In black mangroves (Avicennia germinans), such group structures vary with increasing porewater salinity. Under benign conditions (low salinity and stand density), the groups had more members spread over a larger area. With increasing salinity, the frequency of groups increased, but they consisted of fewer members. These findings suggest there is a mechanism controlling group size, which could be associated with the physiological drought caused by porewater salinity. Here, we hypothesize that root grafting is a cooperative trait that can contribute to mitigating water stress, where the benefits outweigh the costs of cooperation for each individual. Costs arise, for example, during graft formation and resource donation. To better understand the implications of water exchange through grafts, we used a mechanistic individual-based model to investigate the physical limits of water exchange. We examined the factors determining the direction and the potential amount of water exchanged between grafted individuals. Furthermore, we measured black mangroves' in vivo radial sap profiles using the heat field deformation method. Identifying direct signals of water exchange proved challenging, and we observed unexpected flow patterns. For example, some trees had simultaneous bidirectional flow, suggesting a sectorial sapwood structure and that a complex hydraulic compensation mechanism might contribute to water balance. This topic should be further explored to understand sap movement patterns within and between trees, predict forest function in response to environmental change, and better inform climate change adaptation management policies.