The liana assemblage of a Congolian rainforest: Diversity, structure and dynamics

This study analyzes the diversity, composition, and dynamics of the liana assemblage of the Ituri rain forest in northeastern DR Congo. In two 10-ha plots of the Ituri Forest Dynamics Plots 15008 liana individuals (≥2 cm diameter at breast height) were found, representing 195 species, 83 genera and 34 plant families. Liana density decreased from 1994 through 2001 to 2007, with concomitant declines in basal area and above-ground biomass. Despite lower stem densities the species richness remained constant over time. Total liana recruitment rates decreased only slightly and mortality rates decreased significantly over the census intervals. Diameter growth and survival increased with liana stem diameter. In contrast to the generally found increase in liana abundance in the neotropics, the Ituri forest showed recent declines. Interestingly, changes in overall liana community structure and composition were mostly driven by one species only: the superabundant Manniophyton fulvum collapsed between the first and the second census. The vital rates recruitment, growth and mortality shared a wide interspecific variation and most species had low to moderate rates. Species that grew fast tended also to recruit and die fast, but recruitment and mortality rates were not directly related, suggesting that species shift in absolute abundance over the 13 year period.

Lianas are thought to globally increase in overall density, but information on species level changes in abundance and on the underlying vital rates that explain changes in liana density is scarce. The Ituri forest showed a pervasive change in liana population densities in the last decade. Many decreasing species are associated with disturbed habitats and are short-lived. Many increasing species are late successional and longer-lived. Increasing species have a slightly higher recruitment, decreasing species a higher mortality. This study suggests that changes in the liana community result from forest recovery from past disturbances. Rising atmospheric CO2 level, the most widely suggested factor driving liana increase, was not a likely explanation for liana change: more species declined than increased, and increasing species did not have higher growth rates. This is the largest study of an African liana community and its dynamics, and I conclude that in the Ituri Forest local stand dynamics override more global drivers of liana change.

Key-words: Liana assemblage, species composition, community, dynamics, canopy openness, Manniophyton fulvum, functional traits, population density, pervasive change