The Liana assemblage of a Congolian rainforest : diversity, structure and dynamics

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

This study analyzes the diversity, composition, and dynamics of the liana assemblage of the Ituri rain forest in northeastern DR Congo. I used data from two 10-ha plots of the Ituri Forest Dynamics Plots, in which all liana stems ≥2 cm diameter at breast height (dbh) were marked, mapped, measured and identified in 1994, 2001 and 2007. In addition, the plot topography and canopy structure were measured.

Chapter 2 analyzes the liana assemblage (in terms of species richness, abundance and diversity), characterizes liana functional traits and determines effects of forest structure, topography and edaphic variation on liana species composition. In 20 ha, 15008 liana individuals were found, representing 195 species, 83 genera and 34 plant families. Per hectare species number averaged 64, basal area was 0.71 m2 and Fisher alpha, Shannon and Simpson diversity indices were 17.9, 3.1 and 11.4, respectively. There was oligarchic dominance of 10 plant families that represented 69% of total species richness, 92% of liana abundance and 92% of basal area, while ten dominant species accounted for 63% of abundance and 59% of basal area. Forty-one species (21%) were represented by one individual only. Most lianas were light-demanding, climbed their hosts by twining, and had conspicuous flowers, medium-sized leaves and animal-dispersed propagules. Liana abundance increased with abundance of medium-sized and large trees but was, surprisingly, independent of small-tree abundance. Canopy openness, soil moisture, and tree size were the most important environmental factors influencing abundance and distribution of lianas.

In Chapter 3 I investigate changes in structural characteristics, diversity, recruitment, mortality and growth of the liana community over the thirteen years (1994 ¬- 2007). Liana density decreased from 750 (1994) through 547 (2001) to 499 (2007) stems ha-1, 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 slightly from 8.6% per year in the first period to 6.6% in the second, but this decrease was not significant. Liana mortality rates decreased significantly from 7.2% to 4.4% per year over the two census intervals. Diameter growth rates and survival increased with liana stem diameter. Surprisingly, liana abundance in Ituri showed recent declines, rather than recent increases, as has been reported for tropical and temperate forests in the Americas. Interestingly, changes in overall liana community structure and composition were mostly driven by one species only: the dramatic collapse of superabundant Manniophyton fulvum between the first and the second census.
In chapter 4 I investigated species-specific dynamics of the 79 most abundant liana species, representing 13,156 of the stems (97% of total) in two 10-ha plots. I evaluated their demographic performance and the relation if the vital rates (growth, mortality, recruitment) to the species abundance and four functional traits (climbing strategy, dispersal syndrome, leaf size and light requirements) to determine across species variations and major strategies characterizing species. Vital rates shared a wide interspecific variation; species-specific recruitment rates varied from 0.0-10.9%, mortality rates from 0.43-7.89% over 13-year, and growth rates from -0.03-3.51 mm y-1. 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. However, with the exception of the collapsing Manniophyton fulvum population, species maintained their rank-dominance over time. Species growth declined with abundance, but recruitment and mortality rates were not related to abundance. The demographic performance of liana species varied weakly with their climbing strategy and dispersal mode but was, surprisingly, not related to their lifetime light requirements. A principle components analysis of liana strategies in terms of functional traits and vital rates showed that light demand, and dispersal syndrome were the most determining traits. Based on the PCA three functional guilds were distinguished. I conclude that old-growth forest liana species show a large variation in abundance and vital rates, and that density-dependent mechanisms are insufficient to explain the species abundance patterns over time.

Lianas are thought to globally increase in density, but we have limited knowledge about the taxonomic patterns of change in liana abundance, and the underlying vital rates that explain changes in liana density. In chapter 5 the changes in abundance of 79 relatively abundant liana species are evaluated. The Ituri forest showed a pervasive change in liana population density in the last decade. 37 species changed significantly in their abundance over time: 12 (15% of total) species increased, and 25 (32%) species decreased. 42 (53%) species did not change. Of the 48 genera, 40% decreased and 52% stayed the same. Five of the 12 increasing species belonged to the Celastraceae, which also was the only significantly increasing family. Surprisingly, none of the four functional traits (lifetime light requirements, climbing mechanism, dispersal mechanism, and leaf size) was significantly associated with species change in population density. Many decreasing species, however, 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 was not a likely explanation for liana change: more species declined than increased, and increasing species did not have higher growth rates. In the Ituri Forest local stand dynamics override more global drivers of liana change.