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. <br/

Une forte saisonnalité du climat et de la phénologie reproductive dans la forêt du Mayombe : l’apport des données historiques de la Réserve de Luki en République démocratique du Congo

In Africa, the reproductive phenology of tropical trees is mostly annual and regular. This study documents the intra- and inter-annual reproductive phenology of trees in the Mayombe forest, from historical data on the Luki Reserve in the Democratic Republic of the Congo. Reproductive diameter, which is a key parameter for forest management but is largely unknown for many timber species, was also documented for the most abundant species in the dataset. Phenological monitoring of 3,642 trees belonging to 158 species and 39 families was conducted in the Luki Reserve every 10 days from 1948 to 1957. Circular statistics were used to test the synchronicity of phenological events among trees, both at the community level, i.e. for the forest as a whole, and individually for 87 species, which included 35 well-represented species (n >= 20 trees), 16 commercial species and 36 other species. Logistic regressions were used to determine the diameter (minimum and regular) of these species on fruiting. Reproductive phenology for the majority of the trees and the species is largely seasonal, annual and regular (81.6%, 71 species). The peaks for flowering are more abrupt than the fruiting peaks and more spread out over time, although the timing of flowering and fruiting is significantly aggregated. Most of the trees and species bloom from December to February, during the short dry season, but flowers and fruits can be observed throughout the year within the community. Only 13 species showed a significant relationship between diameter and reproduction, including seven canopy species, five understory species and one light-demanding species. For these 13 species, the average minimum reproduction diameter was 17.3 cm

An assessment of recent peat forest disturbances and their drivers in the Cuvette Centrale, Africa

The largest tropical peatland complex in the Cuvette Centrale is marked by persistent knowledge gaps. We assessed recent peat forest disturbances and their direct drivers from 2019 to 2021 in Cuvette Centrale, spanning the Republic of Congo (ROC) and the Democratic Republic of Congo (DRC). Utilizing peatland maps and Radar for Detecting Deforestation alert data, we analyzed spatial and temporal patterns of disturbances. Further, we examined 2267 randomly sampled peat forest disturbance events through visual interpretation of monthly Planet and Sentinel 2A data to identify direct drivers. Our findings revealed that between 2019 and 2021, about 91% of disturbances occurred in DRC, with hotspots concentrated in the northwest Sud-Ubangi district. Disturbances predominantly followed a sharp seasonal pattern, recurring during the first half of each year with temporal hotspots emerging between February and May, closely associated with smallholder agriculture activities. Smallholder agriculture accounted for over 88% of disturbances in Cuvette Centrale, representing a leading role both in ROC (∼77%) and DRC (∼89%). While small-scale logging contributed 7% to the disturbances in the region, it constituted an important driver (18%) in the ROC. Other drivers included floods, roads, and settlements. Approximately 77% of disturbances occurred outside managed forest concessions in Cuvette Centrale, with 40% extending into protected areas. About 90% of disturbances were concentrated within 1 km of peat forest edges and ∼76% of the disturbances occurred within 5 km of road or river networks. The insights underscore the crucial need for effective peat forest conservation strategies in Cuvette Centrale and can inform national policies targeting peatland protection, aligning with commitments in the Brazzaville Declaration and the Paris Agreement. Further, our findings on direct driver assessment could serve as a reference dataset for machine learning models to automate the visual interpretation and upscale the assessment across the entire region

Disentangling national carbon fluxes of African rainforest countries.

African tropical ecosystems possess great potential for nature-based solutions in mitigating fossil fuel emissions through absorbing and storing carbon in soil and vegetation. However, past studies mostly focused on pan-continental carbon balance quantification, often ignoring regional differences. Remarkably, few science-informed attempts have been made to refine carbon flux estimates at the national level within African rainforest countries. Yet, such refined estimates are essential to improve the quantification of Nationally Determined Contributions for the United Nations Framework Convention on Climate Change. In this contribution, we present preliminary results on quantifying national carbon budgets for African rainforest countries by disentangling three major carbon fluxes for the period 2001-2015: (1) net carbon uptake in tropical savannas, woodlands, and forests, (2) carbon losses from land use change, and (3) fossil fuel emissions. Carbon fluxes in intact forests are quantified using ground-based data1 , while the carbon uptake by intact savannas and woodlands is based on Net Primary Productivity assessments estimated from remote sensing products2,3. Furthermore, carbon emissions from land-use change are estimated by analyzing various satellite images and related products providing data on land-use change4 6, soil and tree carbon stocks7 12, fire emissions3,13,14, and carbon recovery in regrowing forests15 18 in tropical Africa. Country-level fossil fuel emissions are taken from the Global Carbon Project database19 to complete the national carbon balances. We reveal that most Central and East African rainforest countries acted as net carbon sinks between 2001 and 2015, while West African rainforest countries exhibited minimal net carbon loss. Overall, tropical ecosystems have played an important role in mitigating carbon emissions due to land-use change and fossil fuels in African rainforest countries, particularly in Congo Basin countries. Our insights into nation-level carbon fluxes will be crucial for informing African rainforest countries, guiding climate policies to stay on track to keep global warming well below 2°C

Synthesis of national carbon fluxes of African rainforest countries

African tropical ecosystems possess great potential for nature-based solutions in mitigating anthropogenic green house gas emissions and biodiversity loss. However, past studies mostly focused on pan-continental carbon bal ance quantification, often ignoring regional differences. Remarkably, few science-informed attempts have been made to refine national-level carbon flux estimates within African rainforest countries. Yet, such refined estimates are essential to improving the quantification of Nationally Determined Contributions for the United Nations Frame work Convention on Climate Change. Here, we present preliminary results on quantifying national carbon budgets for African rainforest countries, dis entangling four major carbon fluxes for 2003-2019: (1) the net carbon uptake in intact tropical terrestrial eco systems, (2) land-use change fluxes, 3) CO2 outgassing in inland waters, and (4) fossil fuel emissions. The net carbon uptake in intact terrestrial ecosystems is based on Dynamic Global Vegetation Models TRENDY v111,2 (DG VMs), ground-based data (AfriTRON3 ), CARDAMOM4 , and remote sensing data products of Net Primary Productivity5 and soil heterotrophic respiration6-7. Land-use change emissions are calculated using bookkeeping models (BLUE8 , H&N20179 , OSCAR10), DVGMs1,2, and CARDAMOM4 . Additionally, we estimate carbon emissions from land-use change by analyzing various satellite images and related products providing data on land-use change11 12, soil and tree carbon stocks13 18, fire emissions19 20, and carbon recovery in regrowing forests21 22 in tropical Africa. We also quantify carbon emissions from CO2 outgassing in estuaries23 and inland waters24-25. National carbon balances are complet ed by using data on fossil fuel emissions from the Global Carbon Project2 . Besides calculating national-level net carbon fluxes using a bottom-up approach by summing individual carbon fluxes, we quantify the net carbon flux using a top-down approach based on atmospheric inversion models (GCP-GridFED26, CAMS27, Jena CarboScope28 , MIROC4-ACTM29, NISMON-CO2 30). We reveal that carbon balances of African rainforest countries remain highly uncertain. Our bottom-up estimates show that Congo Basin countries are net carbon sinks, while most West-African countries are net carbon sources. In contrast, our top-down estimates of net carbon fluxes indicate that African rainforest countries are net carbon sources. Overall, tropical terrestrial ecosystems have played an important role in mitigating anthropogenic carbon emissions in African rainforest countries. Our insights into nation-level carbon fluxes will be crucial for informing African rainforest countries, guiding climate policies to help stay on track to keep global warming well below 2°C