Trees can form root grafts and exchange resources when tissues are fully fused. Group characteristics, such as group size, vary across environmental gradients. In black mangroves (Avicennia germinans), the frequency of groups increased with increasing salinity (i.e., drought stress), but they consisted of fewer members. These results suggest that there is a mechanism controlling group characteristics, possibly related to physiological drought. To better understand the physical limits of water exchange by grafting, we used a mechanistic model. We identified factors that determine the direction and potential magnitude of water exchange between individuals. We also measured radial sap flow profiles of black mangroves in vivo using the 'heat field deformation' method. It proved difficult to identify direct signals of water exchange as we observed unexpected flow patterns such as simultaneous bidirectional flow, suggesting a sectoral sapwood structure and that a complex hydraulic compensation mechanism may contribute to water balance.