Research Collection Doctoral Thesis Impacts of woody invasive species on tropical forests of the Seychelles Author(s): Küffer Schumacher, Christoph Publication Date: 2006 Permanent Link: https://doi.org/10.3929/ethz-a-005212597 Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library Diss. ETH No. 16602 Impacts of woody invasive species on tropical forests of the Seychelles A dissertation submitted to the SWISS FEDERAL INSTITUTE OF TECHNOLOGY ZURICH for the degree of Doctor of Natural Sciences presented by Christoph Küffer Dipl. Umwelt-Natw. ETH born April 13, 1974 citizen of Täuffelen BE Accepted on the recommendation of Prof. Dr. Peter J. Edwards, examiner Dr. Hansjörg Dietz, co-examiner Prof. Dr. Andrew Hector, co-examiner 2006 Contents Summary 1 Zusammenfassung 3 General Introduction 7 Chapter 1 Woody plant invasions on the granitic islands of the Seychelles: Background 17 Chapter 2 Impacts of invasive trees on litter decomposition in phosphorus-poor tropical 33 forests in the Seychelles Chapter 3 Strong belowground root competition shapes tree regeneration in invasive Cinnamomum verum forests 61 Chapter 4 The role of invasive woody plants in plant-frugivore interactions in the Seychelles 81 Chapter 5 Integrative ecological research: Case-specific validation of ecological knowledge for environmental problem solving 105 General Conclusions and Management Implications 123 Annexes 143 Acknowledgements 153 Curriculum Vitae 155 Summary 1 A considerable body of recent research has focussed on why a small proportion of plant species spread rapidly when introduced into a new area. In this context, species invasiveness and habitat invasibility have been extensively studied, mostly in productive habitats, but information on how invasive plants affect ecosystem function remains scarce. In particular, there have been very few studies on the invasibility of and invasion impacts in very nutrient poor habitats, including many tropical forests. In this thesis, heavily invaded tropical rainforests on nutrient poor soils on one of the granitic islands of the Seychelles group were investigated. The aims of the study were: (i) to evaluate the invasiveness and persistence of woody invasive plants in these forests, (ii) to assess the impacts of the invaders on soil nutrient availability and forest development, and (iii) to derive conclusions relevant for forest management and rehabilitation. 2 Two main conclusions emerged from an expert-based evaluation of the state of plant invasion in the Seychelles. First, although many tree species have been introduced to the Seychelles, rather few are invasive in the mid-altitude and montane forests. Second, both local environmental conditions - in particular the very nutrient-poor soils - and land-use history, which included periods of nearly complete deforestation, were important factors determining which species became invasive. 3 Litter properties of the six most abundant native and invasive woody species in mid-altitude forests were compared. Leaf litter of the invasive species had markedly higher N and P contents but lower polyphenol concentrations. It also tended to decompose faster than the litter of native species. Soil nutrient availability was investigated under stands of three species that differed greatly in their litter properties - the N-fixing invasive species Falcataria moluccana, the very widespread invasive Cinnamomum verum, and the endemic Northea hornei. The results revealed only small differences in soil conditions. It appears that invasive species adapted to very nutrient poor soils are particularly effective in nutrient uptake which may have counterbalanced a fast release of nutrients from litter in established stands. 4 In a root trenching experiment, the dominant invasive canopy tree Cinnamomum verum showed strong belowground competitive effects; these were attributed to the dense root mat produced by this species which apparently suppresses tree regeneration. Release from 1 Summary belowground competition through trenching was stronger for seedlings than saplings, and for juveniles of invasive than for juveniles of native tree species. Thus, forest dominance by C. verum appears to shape tree regeneration not only by generally reducing rates of colonization but also by inhibiting the growth of small-seeded or invasive trees at the seedling stage. Undisturbed C. verum forests may, therefore, represent good nurse habitats for the regeneration of native trees. 5 In a comparative investigation of the nutritional quality of fleshy fruits for fruit-eating birds, invasive tree species were shown to produce fruits of higher quality (relative yield, energy content) than native species. The most abundant invasive tree, C. verum, produced fruits with a particularly high fat and protein content. As a consequence, the native tree species may suffer from limited dispersal due to strong competition from the invasive trees for frugivorous birds. However, if much higher bird densities are supported by the ample food resources provided by invasive trees, dispersal of native trees could also be promoted. 6 The results of this thesis suggest that invasive tree species have important impacts upon nutrient poor ecosystems, and that these are not only distinct from those in nutrient rich habitats but, as a result of variation in growth strategy amongst invasive tree species (i.e. stress-tolerators vs. competitors), are also more varied. Which strategy dominates in a particular patch is likely to influence both nutrient cycling and the long-term development of the vegetation. 7 This thesis formed part of a collaboration between the Geobotanical Institute and the Seychelles Ministry of Environment and Natural Resources (MENR), and was developed in close association with practitioners in nature conservation. Based on these experiences, a research strategy was identified that seeks to explain the complexity of specific field or real- world cases. This strategy, referred to here as ‘integrative ecological research’, is seen as being of particular relevance for problem solving in environmental management. The focus on the case of C. verum secondary forests, for instance, made it possible to develop a promising concept for habitat rehabilitation in the Seychelles. This concept, based directly on the research results presented here, involves increasing native propagule pressure at a landscape scale while taking advantage of C. verum secondary forests as nurse habitats for native species. 2 Zusammenfassung 1 In den letzten Jahrzehnten befasste sich eine umfangreiche Forschung mit invasiven Pflanzen, d.h. mit gebietsfremden (vom Menschen eingeführten) Arten, welche sich schnell ausbreiten und dabei zu problematischen Auswirkungen führen können. Insbesondere wurde, vor allem in nährstoffreichen Ökosystemen, untersucht, welche Eigenschaften der invasiven Arten (‚invasiveness’) und der betroffenen Habitate (‚invasibility’) die schnelle Ausbreitung dieser Arten erklären. Wenig bekannt ist hingegen, wie invasive Arten das Funktionieren von Ökosystemen beeinflussen. Zudem wurden Pflanzeninvasionen in nährstoffarmen Gebieten, wie zum Beispiel viele tropische Regenwälder, kaum erforscht. In dieser Dissertation wurden stark von Pflanzeninvasionen betroffene tropische Regenwälder auf einer ozeanischen Insel der Seychellen untersucht. Die Böden dieser Wälder zeichnen sich durch Granit als Muttergestein und Nährstoffarmut aus. Die übergeordneten Forschungsfragen waren: (i) wie lassen sich die Ausbreitung und langfristige Etablierung von invasiven Gehölzpflanzen in diesen Wäldern erklären, (ii) zu welchen Auswirkungen auf die Nährstoffverfügbarkeit im Boden und die Waldentwicklung führen die invasiven Gehölze, und (iii) welche Empfehlungen lassen sich aus den Forschungsresultaten für den Naturschutz in den tropischen Regenwäldern der Seychellen ableiten. 2 Eine Synthese von Expertenwissen zu Pflanzeninvasionen in den Seychellen ermöglichte insbesondere zwei wichtige Schlussfolgerungen. Erstens ist die Zahl der problematischen invasiven Pflanzenarten auf den Seychellen, abgesehen von den Küstengebieten, relativ klein, obwohl eine grosse Anzahl gebietsfremder Arten auf die Inseln eingeführt wurde. Zweitens sind sowohl die lokalen abiotischen Umweltfaktoren – insbesondere die nährstoffarmen Böden, als auch die Geschichte der Landnutzung – insbesondere die zeitweilige fast vollständige Abholzung der Wälder – von zentraler Bedeutung für das Verständnis von Pflanzeninvasionen auf den Seychellen. 3 Ein Vergleich der Eigenschaften von abgestorbenem Blattmaterial (Streu) der sechs häufigsten einheimischen und invasiven Gehölze der Wälder auf mittlerer Höhenstufe zeigte, dass invasive Gehölze im Mittel Streu mit höherem Stickstoff- und Phosphorgehalt aber tieferem Gehalt an Sekundärmetaboliten (Tannine, Phenole) produzieren. Die Streu der invasiven Pflanzen wurde im Mittel auch schneller abgebaut als diejenige der einheimischen Pflanzen. 3 Zusammenfassung Zusätzlich wurden zwischen Baumgruppen der Stickstoff fixierenden, invasiven ArtFalcataria moluccana, des sehr häufigen, nicht fixierenden invasiven Baumes Cinnamomum verum, und der endemischen Art Northea hornei, entgegen der Erwartung aufgrund der unterschiedlichen Eigenschaften der Streu, nur geringe Unterschiede in der Nährstoffverfügbarkeit im Boden beobachtet. Es scheint, dass sich invasive Pflanzen, welche