Studies of competition in mangroves are mostly limited to seedlings and artificial settings like forestry projects. We conducted the first experimental study of intraspecific competition among adult mangroves in a natural mangrove forest to examine how important competition is in determining tree size compared with abiotic conditions. We conducted a study near Port Aransas, TX, USA, which is near the geographical limit of mangroves and dominated by monospecific stands of ‘scrub’ form black mangroves, Avicennia germinans. We thinned 10 plots to create a gradient of mangrove cover, and quantified the effects of mangrove cover on the growth of tagged mangroves from 2013 to 2019, and the mangrove canopy height in 2019. The relative growth rate of tagged mangroves declined as mangrove cover increased, and plants in the plot with 100% mangrove cover did not grow, indicating that they had attained their maximum size. In plots with reduced mangrove cover, plant height increased sharply, with plants in the plot with 11% mangrove cover growing ~52% taller over 6 years. Canopy height was ~30% taller in the plot fringe than in the interior, and canopy height in both fringe and interior declined as mangrove cover increased. Measures of leaf chlorophyll concentration and light interception suggested that plants were primarily limited by nitrogen. Our results showed that scrub mangroves compete strongly despite being limited by abiotic conditions, and that the importance of competition was greater in magnitude than that of abiotic differences between the fringe and interior.