Competitive effects of fire-resistant saplings on their fire-sensitive neighbors are greater than the reverse

Abstract

Although repeated fires are generally thought to reduce competition, direct tests of this hypothesis are rare. Furthermore, recent theory predicts that fires can increase competitive effects of fireresistant species on fire-sensitive species and thus create stable assemblages dominated by the former. In this study, I quantified competition between saplings of fire-resistant oaks and their fire-sensitive non-oak neighbors in adjacent, repeatedly-burned and unburned 1-ha plots following damage by a tornado that reduced canopy cover to 60%. Using field experiments with established in situ saplings, I tested the hypothesis that competition was greater in an unburned plot than in a repeatedly burned plot by examining both the effects of sapling neighbor identity and experimental sapling neighbor reduction (repeated clipping) on the growth of fire-resistant oak saplings. To test whether competition was increased by fire, I measured diameter growth responses of fire-sensitive non-oaks to repeated clipping and topkill of neighbors by fire. Oaks, which generally had higher diameter:height ratios, were more resistant to topkill than were non-oaks. Although diameter growth of both oaks and non-oaks was greater in the burned than in the unburned plot, growth of oak saplings was not influenced by neighbors in either plot in a year with or without fire. In contrast, competitive effects of non-topkilled saplings (the vast majority of which were oaks) on topkilled non-oaks were significant in the burned plot in 2014. Growth of topkilled non-oak saplings was significantly greater in pairs in which the neighbor had been clipped or topkilled by fire than in pairs in which the neighbor had neither been clipped nor topkilled. Hence, the lower incidence of topkill in oaks in the burned plot after repeated fires increased their competitive effects on their non-oak neighbors in the burned plot. Growth of non-oaks did not increase with oak neighbor reduction in a year without fire. Because avoidance of topkill by fire was positively related to diameter, results suggest that repeated fires could generate a positive feedback between greater fire tolerance by oak saplings and increased competitive effects on their non-oak sapling neighbors.