Secondary Succession in Tropical Dry Forests

Secondary Succession in Tropical Dry Forests

Drivers and Mechanisms of Forest Regeneration

G?raldine Derroire

Bangor University College of Natural Sciences School of Environment, Natural Resources and Geography

& Swedish University of Agricultural Sciences

Faculty of Forest Sciences Southern Swedish Forest Research Centre, Alnarp

Doctoral Thesis Bangor, United Kingdom & Alnarp, Sweden - 2016

Acta Universitatis agriculturae Sueciae

2016:66

Cover: Tropical dry forest of Santa Rosa, Costa Rica (photo: G?raldine Derroire)

ISSN 1652-6880 ISBN (print version) 978-91-576-8634-3 ISBN (electronic version) 978-91-576-8635-0 ? 2016 G?raldine Derroire, Bangor & Alnarp Print: SLU Repro, Alnarp 2016

Secondary Succession in Tropical Dry Forests. Drivers and Mechanisms of Forest Regeneration

Abstract Secondary succession is a complex process involving numerous factors acting across scales. Understanding secondary succession in tropical dry forests is important for the conservation and restoration of this highly threatened biome. My research aims to improve knowledge of the trajectories and drivers of secondary succession in this biome, and the underlying mechanisms. I used a combination of literature synthesis, observational and experimental approaches to study plant-plant interactions and community changes during succession.

Through review of published studies, I showed that established trees have a mainly positive effect on the seed dispersal, survival and germination of the subsequent generation of woody plants. However, the balance between positive and negative effects is more complex at the seedling establishment stage and can be influenced by the precipitation regime. Meta-analyses of chronosequence studies showed an increase in tree and shrub species richness with succession and a slow convergence of successional forest species composition with that of old-growth forests. Using survey of young woody plants establishing under isolated trees in pastures, I showed that the attributes of the trees influence the functional composition of the regeneration assemblages but are only weakly related to their taxonomic composition. The position of isolated trees in the landscape is also influential, but this is complex and sitespecific. Through extensive sampling of leaf functional traits of sapling communities in secondary forests of different successional age, I found that community functional composition shifts from conservative towards acquisitive strategies of resource economics, through both species turnover and intraspecific variation of trait values. Five of the measured traits also showed directional changes with tree ontogeny. Lastly, an experimental test of seed fate showed that leaf litter reduced seed removal in successional forests. Seed germination rate was higher in successional forests compared with open sites and generally benefited from the presence of litter. Overall, this research suggests a higher predictability of successional trajectories when studied through functional rather than taxonomic composition. It also shows heterogeneity in successional trajectories among tropical dry forests that require further study.

Keywords: biodiversity, community assembly, Costa Rica, environmental gradients, facilitation, plant functional traits, regeneration niche

Author's address: G?raldine Derroire, Bangor University, School of Environment, Natural Resources and Geography, Deiniol Road, Bangor, Gwynedd, LL57 2UW, UK E-mail: g.derroire@bangor.ac.uk

Contents

List of Publications

9

Abbreviations

11

1 Introduction

13

1.1 Importance of tropical secondary forests

13

1.2 Secondary succession in tropical forests

15

1.2.1 Models of forest succession: determinism and stochasticity 15

1.2.2 Factors influencing secondary succession in tropical forests 16

Abiotic environmental factors

16

Surrounding landscape

17

Past and current management

18

Initial vegetation

18

1.3 Importance of plant-plant interactions for succession

19

1.3.1 Types of plant-plant interactions

19

1.3.2 Core processes for deterministic niche-based models of

succession

20

1.4 Species-based and functional trait-based approaches

21

1.4.1 From context-specific accuracy to global comparisons

21

1.4.2 Functional traits and plant strategies

22

1.4.3 Variation of functional trait values within and among species 24

1.5 Tropical dry forests

26

1.5.1 Definitions, distribution and state of conservation

26

1.5.2 Highly seasonal environment

28

1.5.3 Specificities of regeneration in tropical dry forests

30

1.5.4 Changes during succession in tropical dry forests

32

1.6 Ecological restoration in tropical forests

33

2 Objectives

37

3 Material and methods

41

3.1 Study area and study sites

41

3.2 Data acquisition

44

3.2.1 Systematic review of the literature

44

3.2.2 Observational approach: vegetation survey

44

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