The impact of lianas on tree regeneration in tropical forest canopy gaps: evidence for an alternative pathway of gap-phase regeneration

Regeneration in forest canopy gaps is thought to lead invariably to the rapid recruitment and growth of trees and the redevelopment of the canopy. Our observations, however, suggest that an alternate successional pathway is also likely, whereby gap-phase regeneration is dominated by lianas and stalled in a low-canopy state for many years. We investigated gap-phase regeneration in an old-growth tropical forest on Barro Colorado Island (BCI) in Panama to test the following two hypotheses: (i) many gaps follow a pathway in which they remain at a low canopy height and are dominated by lianas and (ii) the paucity of trees in this pathway is a function of liana density. 2. We surveyed a total of 428 gaps of varying ages (c. 5, c. 10, and 13 years old) and identified those which followed the conventional pathway of regeneration and others that remained stalled in a low-canopy state for many years and were dominated by either lianas or palms. Each of these pathways will likely have different successional trajectories that will favour the growth of a distinct suite of mature species and ultimately result in contrasting species composition. 3. The successional pathway of liana-dominated, stalled gaps is common throughout the forest. We estimate conservatively that 7.5% of the gaps that form each year will follow this pathway, probably due to the suppression of tree regeneration by lianas, and that many of these stalled gaps will persist for much longer than 13 years. Consequently, a high proportion of gaps in the forest at any given time will be stalled. Furthermore, liana tangles, which persist in the tropical forest understorey for extended periods of time, almost certainly originate in these gaps. 4. Liana abundance was positively correlated with pioneer tree abundance and diversity while negatively correlated with non-pioneer tree abundance and diversity. Thus, lianas appear to inhibit non-pioneer tree survival while indirectly enhancing that of pioneer trees. 5. Lianas are abundant in many types of tropical and temperate forests and a successional pathway involving liana-dominated, stalled gaps may therefore be frequent and widespread