Abstract
Although tropical forests are rich in plant diversity and deliver essential ecosystem services, goods and services that benefit our well-being, tropical forests are under increasing pressure of deforestation and degradation. In response and under the assumption that biodiversity and ecosystem services are positively related, conservation measures are increasingly focussing on
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ecosystem services. Yet, for tropical forests, such relationships remain unclear. This thesis identified three important knowledge gaps. First, it is unclear how plant diversity in a broad sense (including taxonomic, functional and structural diversity) is related to the stock and the flow components of ecosystem services. Second, questions have been raised on how using a different plot size or geographical extent can affect relationships. Third and last, it is unknown how the combination of ecological factors and socio-economic factors can influence relationships. To close these knowledge gaps, this thesis explored the relationships between plant diversity, carbon storage, timber provisioning and the provisioning of non-timber forest products (‘NTFPs’; products such as medicines, food and cultural totems) in old-growth tropical forests. As there were many potential relationships to consider, this thesis aimed to “see the forest through the trees” and to come to general implications for tropical forest conservation. A systematic review of pan-tropical studies found mainly positive relationships between plant diversity in a broad sense and carbon storage, regardless of the stock or flow. By contrast, relationships concerning timber and NTFPs had been poorly studied. Although the reported positive relationships were in line with predictions, the discovered mix of positive and negative relationships was surprising. Plot size was found to moderate relationships, where relationships were more often significant when smaller plots were used. Direct analyses of the relationships between woody species richness and the stock component of the three ecosystem services within and across Amazonia showed that woody species richness was consistently positively related to carbon stock. This thesis provides evidence suggesting that previously reported zero to negative outcomes for this relationship were due to moderating effects of sampling a large amount of environmental heterogeneity by including a large geographical extent. The found positive relationship suggests that protecting carbon-rich tropical forests in the Amazon is likely to protect concentrations in woody species diversity as well. By contrast, timber stock and NTFP stock were not consistently related to woody species richness in the Guiana Shield, a region of Amazonia. Instead, these stocks were related to differences in floristic composition across forest types and biogeographical subregions. In Suriname, a country on the Guiana Shield, NTFP stocks were mainly determined by a very small number of woody species called ‘NTFP oligarchs’, which were associated to specific floristic compositions. However, NTFP flows were ultimately determined by socio-economic factors such as the demand, costs and harvest rules. These factors determined which NTFP stocks were harvested, regardless of how much stock or plant diversity was present. These findings suggests that forests with important timber and NTFP stocks will often be located near forest-dwelling communities. Therefore, protection of these stocks could benefit from allowing community management of forests.
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