Promotors: Prof. dr. ir. Hans Verbeeck Ghent University Faculty of Bioscience Engineering Applied Ecology and Environmental Biology Computational and Applied Vegetation Ecology (CAVElab) Coupure Links 653, B-9000 Gent Prof. dr. ir. Pascal Boeckx Ghent University Faculty of Bioscience Engineering Applied Analytical and Physical Chemistry Isotope Bioscience Laboratory (ISOFYS) Coupure Links 653, B-9000 Gent Dr. ir. Hans Beeckman Royal Museum for Central Africa Departement of Biology Laboratory for Wood Biology and Xylarium Leuvensesteenweg 13, B-3080 Tervuren Decaan: Prof. dr. ir. Guido Van Huylenbroeck Rector: Prof. dr. Anne De Paepe Carbon storage and functional diversity of tropical rainforest in the Congo Basin by Elizabeth Kearsley Proefschrift voorgedragen tot het bekomen van de graad Doctor in de Bio-Ingenieurswettenschappen. Thesis submitted in fulfilment of the requirements for the degree of Doctor (Ph.D.) in Applied Biological Sciences. Op gezag van de rector, Prof. Dr. Anne De Paepe. Dutch translation of the title: Koolstof opslag en functionele diversiteit in tropisch regenwoud in het Congobekken Kearsley, E. (2015). Carbon storage and functional diversity of tropical rainforest in the Congo Basin. Ph.D. dissertation, Ghent University, Ghent, 183 p. ISBN 978-90-5989-814-1 The author and the promoters give the authorization to consult and to copy parts of this work for personal use only. Every other use is subject to copyright laws. Permission to reproduce any material contained in this work should be obtained from the author. Summary Rainforests in the Congo Basin cover an area of over 300 million ha (FAO 2011), store approximately 57 Pg of carbon (FAO 2011) and represent a significant carbon sink (0.34 Pg C yr-1; Lewis et al. 2009). As such, African tropical forests provide an important feedback to the global carbon cycle (Lewis et al. 2006 & 2009). In order to understand and quantify tropical forest carbon uptake and losses, accurate carbon stock reference data are needed (Huntingford et al. 2013). The accuracy of these reference data is essential for the implementation of climate mitigation policies to reduce emissions resulting from deforestation and forest degradation (UN-REDD, United Nations collaborative programme on reducing emissions from deforestation and forest degradation in developing countries, www.un-redd.org) (Gibbs et al. 2007; Somorin et al. 2012). However, central African tropical forests have been underrepresented in studies regarding both carbon storage and biodiversity. This dissertation aims to improving the accuracy of regional and basin-wide estimation of carbon storage in tropical forest of the Congo basin and increase our functional understanding of the forest community, providing useful information regarding ecosystem dynamics. In particular, we investigate the relationships between biodiversity, landscape composition and configuration and carbon storage in oldgrowth and intervened tropical forest (20 one-hectare inventory plots) in the Yangambi Biosphere reserve, as well as two other locations (Yoko and Yambela) in the Democratic Republic of the Congo. Both above and belowground carbon storage and its local variability are studied to improve local estimates for this region and the understanding of drivers behind this variability. Subsequently, we explore functional similarities between species to identify how or if biodiversity contributes to ecosystem functioning and stability. Our research has shown consistently lower above ground biomass values for old- growth forest in Yangambi and surroundings (162 ± 20 Mg C ha-1) compared to the outer regions of the Congo Basin (243-259 Mg C ha-1, Lewis et al. 2009) due to a difference in tree height-diameter allometry. Consequently, the best available tree height–diameter relationships derived for Central Africa (Feldpausch et al. 2012; Banin et al. 2012) do not render accurate canopy height estimates for our study area. In addition, quantifying soil organic carbon (SOC) in old-growth forest in Yangambi, we found an equivalent of 68% of the above ground carbon stored within the first meter of soil. Yet, considerably lower SOC values (23%) were found at the Yoko site, although similar forest composition is present. We suggest that this large variability is related to differences in nutrient limitation, which drives differences in aboveground versus belowground carbon allocation. These findings have important consequences for the assessment of total carbon stored in tropical forests leading to different conclusions regarding the carbon storage capacity of the entire system. The large and unaccounted variability in SOC stocks introduces additional uncertainty in the response of tropical forest systems to climate change and its contribution to the current terrestrial carbon budget. Predicting the future of the overall carbon budget of tropical forests is currently constrained by, amongst others, a proper understanding of how different species are limited by various resources (Prentice et al. 2007). Both light requirement and adult stature of tropical tree species are two traits, which capture a large portion of the variation in other functional traits (Loehle 2000; Turner 2001; Poorter et al. 2006). A classification using these two traits are also expected to form distinct groups within the leaf economics spectrum (Wright et al. i 2004), contrasting inexpensive short-lived leaves vs. costly long-lived leaves. Our work however provides new insights to the current paradigm regarding these classifications. The site-specific classification of trees made in this study, irrespective of species identification, shows that these classifications do not show distinctly different trait values, mainly of leaf traits reflecting resource acquisition and stress tolerance. Investigating functional similarity between species, we show that rare species are predominantly associated with extreme (leaf) traits. Although these results are well established in literature (Belyea & Lancaster 1999; Cornwell & Ackerly 2009; Kraft et al. 2008; Maire et al. 2012), our findings show that rare species show both unique and redundant functional traits, both of high importance for long-term stability of ecosystem functioning. In contrast, more common species mainly support redundant traits. Our study provides accurate descriptions of (rare) species composition and functional community structure related to the local environmental conditions of African tropical forests, which will allow us to better understand current and future species distributions and biodiversity in the African tropical forest. In conclusion, the implications of our research range from basic research assessing the impact on the global carbon cycle over policy making through the REDD+ mechanisms (Somorin et al. 2012) to ecosystem conservation where functional understanding of the forest ecosystem using a trait based approach could allow us to better predict its resilience for land- use change and climate change (Pereira et al. 2010). ii Samenvatting Regenwouden in het Congobekken strekken zich uit over een enorm gebied van meer dan 300 miljoen ha (FAO 2011). Er zit een zeer grote hoeveelheid koolstof (57 Pg; FAO 2011) opgeslagen in die bossen en jaarlijks neemt dit toe met ~0,34 Pg C (Lewis et al. 2009). Om die reden zijn ze belangrijk voor de globale koolstofcyclus (Lewis et al. 2006 & 2009). Voor een beter inzicht in processen van koolstofopname en –afname zijn accurate referentiegegevens nodig van de hoeveelheid koolstof opgeslagen in het systeem (Huntingford et al. 2013). Bovendien dient het beleid betreffende klimaat mitigatie (bv. het UN-REDD programma; ‘United Nations collaborative programme on reducing emissions from deforestation and forest degradation in developing countries’; www.un-redd.org) (Gibbs et al. 2007; Somorin et al. 2012) te steunen op nauwkeurige referentiegegevens van koolstofopslag. Echter, het Centraal Afrikaans tropisch regenwoud is ondervertegenwoordigd in studies met betrekking tot koolstofopslag en functionele diversiteit. Dit proefschrift is daarom gericht op het verbeteren van de nauwkeurigheid van schattingen van koolstofopslag in de tropische bossen in het Congobekken, en op het uitbreiden van onze kennis over het functioneren van de bosgemeenschap, met als doel nuttige informatie bij te dragen met betrekking op ecosysteemdynamiek. In het bijzonder onderzoeken we de relaties tussen biodiversiteit, landschapspatronen en verdeling en opslag van koolstof in pioniersbossen en volwassen tropisch bossen (20 percelen van 1 hectare) in het Yangambi reservaat, evenals twee andere locaties (Yoko en Yambela) in de Democratische Republiek Congo. Zowel boven- en ondergrondse koolstofopslag worden bestudeerd om de lokale schattingen voor deze regio te verbeteren en voor een beter begrip van de drijfveren achter deze variabiliteit. Vervolgens onderzoeken we de functionele gelijkenissen tussen boomsoorten om te bepalen hoe en of de biodiversiteit bijdraagt aan de werking en de stabiliteit van het ecosysteem. In dit onderzoek wordt aangetoond dat de bovengrondse koolstofopslag systematisch lager is in de bossen van Yangambi en omgeving (162 ± 20 Mg C ha-1) in vergelijking met gebieden aan de oost- en westkant van het Congobekken (243-259 Mg C ha-1, Lewis et al. 2009) doordat de bomen doorgaans lager zijn. Hoogte-diameter relaties beschikbaar voor centraal Afrika in de literatuur (Feldpausch et al. 2012; Banin et al. 2012) laten niet toe accurate schattingen te maken van boomhoogte voor ons studiegebied.