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Tropical Dry Forest Recovery Tropical Dry Forest Recovery processes and causes of change Herstel van Tropisch Droog Bos processen en oorzaken van verandering Promoters: Prof. Dr. F.J.J.M. Bongers Persoonlijk hoogleraar bij de leerstoelgoep Bosecologie en Bosbeheer Wageningen Universiteit Prof. Dr. J.A. Meave del Castillo Departamento de Ecología y Recursos Naturales Facultad de Ciencias Universidad Nacional Autónoma de México Co-Promoteren: Dr. Ir. L. Poorter Universitair docent, Centrum voor Ecosysteem Studies Wageningen Universiteit Dr. E.A. Pérez-García Departamento de Ecología y Recursos Naturales Facultad de Ciencias Universidad Nacional Autónoma de México Promotiecommissie: Prof. Dr. F. Berendse, Wageningen Universiteit Dr. Ir. N.P.R. Anten, Universiteit Utrecht Prof. Dr. R. Boot. Tropenbos International, Wageningen Dr. J. F. Duivenvoorden, Universiteit van Amsterdam Dit onderzoek is uitgevoerd binnen de C.T. de Wit Graduate School Production Ecology & Resource Conservation. Tropical Dry Forest Recovery processes and causes of change Edwin Lebrija-Trejos PROEFSCHRIFT ter verkrijging van de graad van doctor op gezag van de rector magnificus van Wageningen Universiteit, Prof. Dr. M.J. Kropff, in het openbaar te verdedigen op maandag 6 april 2009 des namiddags om half twee in de Aula Lebrija-Trejos, E. (2009) Tropical Dry Forest Recovery: processes and causes of change. PhD thesis, Wageningen University, Wageningen, The Netherlands With summary in Dutch. ISBN: 978-90-8585-322-0 Subject headings: abiotic environment, chronosequence, community assembly, community dynamics, dendrochronology, environmental filters, forest resilience, functional traits, longitudinal studies, secondary succession, tropical dry forest, Mexico. This study was carried out at the Forest Ecology and Forest Management Group, Centre for Ecosystem Studies, Wageningen University, Wageningen, and the Departamento de Ecología y Recursos Naturales, Facultad de Ciencias, Universidad Nacional Autónoma de México, México, D.F., México. Para los que me dan vida: amigos y familia, y para todos los que se interesan en la ciencia. For those who give me life: Friends and family, And for all those who are interested in science. CONTENTS CHAPTER 1 General Introduction 1 CHAPTER 2 The Potential of Tree Rings for the Study of 13 Forest Succession in Southern Mexico CHAPTER 3 Successional Change and Resilience of a 39 Very Dry Tropical Deciduous Forest Following Shifting Agriculture CHAPTER 4 Community Dynamics of a Secondary 61 Tropical Dry Forest: successional pathways and variability CHAPTER 5 Successional and Seasonal Changes in the 87 Abiotic Environment of a Tropical Dry Forest CHAPTER 6 Functional Traits and Environmental 111 Filtering Drive Community Assembly in a Species-Rich Tropical System CHAPTER 7 General Discussion and Summary 133 REFERENCES 145 ALGEMENE DISCUSSIE EN 173 SAMENVATTING ACKNOWLEDGEMENTS 183 SHORT BIOGRAPHY 186 PUBLICATIONS 187 EDUCATION STATEMENT 188 CHAPTER 1 General Introduction Succession is defined as the directional change in species composition and vegetation structure over time. When this occurs in areas where natural or human disturbances have disrupted forest continuity, and hence opened a space for plant colonization and vegetation regrowth, it is called secondary succession (Corlett 1994, Barbour et al. 1998). Despite these relatively simple definitions, succession is a complex process whose three major causes, availability of space, differential species availability, and differential species performance, are influenced by several factors acting at different spatial, temporal and organizational scales (West et al. 1981, Pickett et al. 1987). For these reasons, the development of ecology has been largely influenced by succession, though not without strong discrepancies and heated debate on the explanation of successional causes and mechanisms, and on finding a unified theory (McIntosh 1981, Finegan 1984, Barbour et al. 1998). This latter problem partially derives from attempts to extract generalizations from a limited number of forest types and circumstances that may strongly influence succession (e.g. land-use history; Ewel 1980, Horn 1981, McIntosh 1981, Finegan 1984). In the case of tropical forests, from the beginning of the twentieth century (e.g. McLean 1919, Kenoyer 1929) a large number of studies in the moist and wet forest types (hereafter referred to only as wet tropical forests) have advanced our understanding of their secondary succession significantly. Such studies have allowed a comprehensive assessment of development trends in community composition, structure and, to a lesser extent, ecosystem functioning (Brown and Lugo 1990, Finegan 1996, Guariguata and Ostertag 2001), of species traits at different successional stages (Bazzaz and Pickett 1980, Popma et al. 1992, Chazdon et al. 2003, Poorter et al. 2005), and, relatively more recently, of successional dynamics (Swaine and Hall 1983, Breugel et al. 2007, Chazdon et al. 2007). The accumulated body of evidence has allowed developing models of species replacement based on species life cycles and life-history strategies (dispersal, germination, growth rates, shade tolerance and longevity), and on competition (Gómez-Pompa and Vázquez-Yanes 1981, Finegan 1996). The developed principles have been effectively used for sound forest restoration and management practices (Finegan 1992, Parrotta et al. 1997, Meli 2003). Unfortunately, the picture for tropical dry forests (TDF) lags far behind in detail in comparison to that of wet Chapter 1 – GENERAL INTRODUCTION – 2 tropical forests, and it is doubtful whether the theory developed from tropical wet forests can be applied to TDF. Tropical dry forests have lower rainfall levels (400-1800 mm) and, most importantly, higher rainfall seasonality (3-8 months of dry season) and ratio of precipitation to potential evapo-transpiration (PET/P > 1) than wet forests (Murphy and Lugo 1986, Gerhardt and Hytteborn 1992). This lower water availability of TDF results in a clearly distinct species composition, structure, and system functioning. In general, TDF have lower species richness, stature, basal area, leaf area index, fewer canopy strata, and a larger ground cover and root/shoot biomass ratio than wet forests (Murphy and Lugo 1986, Holbrook et al. 1995). Similarly, processes such as reproduction, growth, primary productivity, litter production, organic matter turnover, and nutrient cycling, are largely related to seasonality and exhibit lower values compared to wetter forests (Murphy and Lugo 1986, Bullock 1995, Holbrook et al. 1995, Martínez-Yrízar 1995). Because of these differences between wet and dry tropical forests, they may also be expected to differ in their rates, pathways, patterns, and mechanisms of succession (Gerhardt and Hytteborn 1992, Guariguata and Ostertag 2001). STUDYING SUCCESSION The time scales at which secondary forest succession takes place (decades to hundreds of years) poses a problem for its study. Most successional knowledge is derived from chronosequences, in which long-term vegetation changes are inferred from surveying sites with different ages since disturbance (Chazdon et al. 2007). This approach can be useful to define basic patterns and formulate hypothesis of succession as long as the selected sites share a (relatively) similar landscape structure, landform, substrate conditions, and disturbance history and regime (Foster and Tilman 2000). Chronosequences, however, cannot directly inform on rates or on causal processes of succession. Surveys over time (longitudinal studies) are therefore needed for these purposes (Bakker et al., 1996). Furthermore, as chronosequence requirements cannot always be met and assumptions are thus Chapter 1 – GENERAL INTRODUCTION - 3 made (e.g. same propagule availability), longitudinal studies are very useful for the validation of chronosequence studies (Bakker et al. 1996, Foster and Tilman 2000). To our knowledge, up to now all the information on secondary succession in TDF comes from chronosequence studies. PATTERNS AND RATES OF SUCCESSION Unless cleared areas have been seriously degraded, secondary succession in the humid tropics involves four phases characterized by shifts in dominance from herbs and shrubs, to short-lived pioneers, longed-lived pioneers and, finally, shade tolerant species typical of mature forests. In general, several decades elapse (>10) before this latter stage is reached (Finegan 1996, Richards et al. 1996). At the same time, the forest recovers in diversity and species richness, and in structural features such as height, basal area, foliage cover, and stem density (Brown and Lugo 1990, Guariguata and Ostertag 2001, Sheil 2001). It has been suggested that secondary succession of TDF is less variable, floristically simpler and that it has fewer seral stages compared to wet forest (Ewel 1980, Murphy and Lugo 1986). These characteristics, in combination with the high occurrence of sprouts and the relative lower floristic and structural complexity of mature dry forests, led Ewel (1980) to hypothesize that TDF have the potential to recover faster, and therefore to be more resilient, than wet forests. This statement was toned down later in a conceptual model of resilience that gave greater weight to the negative effects of environmental harshness on absolute rates of forest recovery (Ewel 1983). Without further support by empirical data, the original hypothesis nonetheless rooted in later secondary succession theory (Murphy and Lugo 1986, Kennard 2002, Vieira and Scariot 2006,
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