Tree rings in the tropics:

Transcripción

1 Tree rings in the tropics: a study on growth and ages of Bolivian rain forest trees Roel J.W. Brienen PROMAB Scientific Series 10

2 The Programa Manejo de Bosques de la Amazonía Boliviana (PROMAB) is an international research, education and extension program advancing the sustainable exploitation and management of timber and non-timber forest resources in northern Bolivia. PROMAB is a joint effort of the Instituto para el Hombre, Agricultura y Ecología, Bolivia, the Universidad Téchnica del Beni, Bolivia, and Utrecht University, the Netherlands. PROMAB, Casilla 107, Riberalta, Beni, Bolivia, ( promab@cotas.net, Internet: or c/o/ Plant Ecology Group, Department of Biology, Utrecht University, PO Box 80084, 3508 TB Utrecht, the Netherlands ( promab@bio.uu.nl). R.J.W.Brienen Tree rings in the tropics: a study on growth and ages of Bolivian rain forest trees PROMAB Scientific Series 10 PROMAB-Riberalta, Bolivia ISBN: Keywords: autocorrelated tree growth, climate-growth relations, growth simulation, release, suppression, temporal growth patterns, tree ring analysis, timber yield, tropical dendrochronology, tropical rain forest PROMAB, Roel J.W. Brienen All rights reserved. No part of this publication, apart from bibliographic data and brief quotations in critical reviews, may be reproduced, re-recorded or published in any form including photocopy, microfilm, electronic or electromagnetic record, without written permission. Printed by: Feboruk BV (Enschede, Utrecht) Cover: Emy Frank Photos: Adhemar Cassanova Arias, Afdeling Vormgeving en Beeldverwerking UU, Roel Brienen

3 Tree rings in the tropics: a study on growth and ages of Bolivian rain forest trees Jaarringen in de tropen: een studie naar groei en leeftijden van Boliviaanse regenwoudbomen (met een samenvatting in het Nederlands) Anillos de crecimiento en los trópicos: un estudio de crecimiento y edades de árboles de bosques tropicales en Bolivia (con resumen en español) Proefschrift Ter verkrijging van de graad van doctor aan de Universiteit Utrecht op gezag van de Rector Magnificus, Prof. Dr. W.H.Gispen, ingevolge het besluit van het College voor Promoties in het openbaar te verdedigen op donderdag 20 october 2005 des ochtends te 10:30 uur door Roel Jacobus Wilhelmus Brienen Geboren op 21 augustus 1974 te Heerlen, Nederland

4 Promotor: Co-promotor: Prof. Dr. M.J.A.Werger Plant Ecology Group, Department of Biology, Utrecht University Dr. P.A. Zuidema Plant Ecology Group, Department of Biology, Utrecht University The studies in this thesis were carried out within the Programa de Manejo de Bosques de la Amazonía Boliviana (PROMAB), Riberalt a, Bolivia, and the department of Plant Ecology, Utrecht University, the Netherlands. Financial support was obtained through grants BO from the Biodiversity Program of the Netherlands Development Assistence (DGIS), Ministry of Foreign Affairs of the Netherlands.

5 To Masako and Stef

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7 Contents Chapter 1 General Introduction 9 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Relating tree growth to rainfall in Bolivian rain forests: a test for six species using tree ring analysis 19 with P.A. Zuidema; Oecologia 2005 Lifetime growth patterns and ages of Bolivian rainforest trees obtained by tree ring analysis 35 with P.A. Zuidema; Journal of Ecology, accepted Autocorrelated growth of tropical forest trees: unraveling patterns and quantifying consequences 53 with P.A. Zuidema & H.J. During; submitted The use of tree rings in tropical forest management: projecting timber yields of four Bolivian tree species 75 with P.A. Zuidema; submitted Chapter 6 General discussion and summary 93 Resumen ejecutivo 105 Samenvatting 123 References 128 Dankwoord/ Agradecimientos 138 Curriculum vitae 141 Promab publication 142

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9 General introduction Chapter 1 General Introduction Tropical rain forests harbour a very high diversity of tree species: in a single hectare of forest as many as 300 tree species can coexist (Gentry 1988). Explaining this high diversity has been a major goal for tropical ecology over the last decades and many new theories and hypotheses dealt with this issue (Wright 2002). These theories and hypotheses in general explain species coexistence by interspecific differences in growth conditions for which species specialize (Grubb 1977; Ricklefs 1977). Dynamics play a central role in many theories, and disturbances that cause these dynamics are considered crucial for species maintenance in tropical rain forests (Sheil & Burslem 2003; Baker et al. 2005). As they create spatial and temporal variation in environmental conditions (mainly light), disturbances provide different niches for which species may specialize ('niche differentiation', Denslow 1980). To test whether species do specialise for niches along a disturbance or light gradient, information is required on abundance, spatial distribution and performance of tree species in relation to light conditions and other site conditions (e.g. Denslow 1980; Harms et al. 2001). In particular, it is important to understand how trees respond to canopy openings (gaps) in the forest caused by tree falls, and to what extent they depend on gaps to reach the canopy. Over the last decades a large number of permanent sample plots has been established and monitored in tropical forests, providing necessary data on tree abundance, distribution and performance (Condit 1995). These studies have been central to our current understanding of tropical forest ecology and allowed to test some of the hypotheses on species coexistence (Sheil & Burslem 2003). In spite of their important role in providing the data basis of our current understanding of tree ecology, permanent plot data have the disadvantage of spanning a limited amount of time (generally <20 y) relative to the lifespan of large-statured trees in tropical forests (Lieberman et al. 1985a; Fichtler et al. 2003; Laurance et al. 2004). Long-term data on tree growth and survival are required to better understand the ecology and coexistence of long-lived tree species. For instance, we lack information on the degree to which juvenile trees depend on gaps to reach the canopy, on the amount of gaps they would require, and on their age when reaching the canopy (Clark & Clark 2001). In this thesis I apply a method, which provides long-term data on tree growth; tree ring analysis (dendroecology). This allows me to answer ecological questions that cannot be addressed using permanent sample plots. Ecology of tropical forest trees Light is the main limiting factors of tree growth in tropical rain forests. Under closed canopy generally only 1-2% of the light penetrates to the forest floor (Chazdon et al. 1996). These low light levels may be temporarily enhanced by branch or tree falls (Canham et al. 1990; vandermeer & Bongers 1996b). Juvenile trees growing at the forest floor may be released by such canopy openings and expose higher growth rates (Canham 1985). Generally, it is thought that trees differ in their responses to high light levels and in their capacity to withstand periods of low light availability (Canham 1989). These differences between species in light requirements have often been put forward to explain the high 9

10 Chapter 1 species diversity of tropical rain forests. Many studies use light requirement for successful regeneration as a basis to classify species into functional groups (e.g, Clark & Clark 1992; Poorter 1998; Peña-Claros 2001). Two groups can be clearly distinguished: pioneer tree species ( short-lived pioneers, sensu Swaine & Whitmore 1988) and non-pioneer tree species (classified in various ways, cf. shade-tolerants, primary species, climax species ). Pioneer species have high light requirements for germination and growth, and are generally short-lived. Non-pioneer species do not need high light for germination and are able to survive prolonged periods in the dark forest understorey, growing at low rates. The differences between those two major groups are well known, but the majority of tropical rainforest tree species belong to the non-pioneer group (Swaine & Whitmore 1988). There is an ongoing debate to what extent the coexistence of the many non-pioneers can be explained by differences in light requirements. Many studies have tackled this problem, often proposing different sub-divisions of the non-pioneer group, based on degree of light requirement or shade tolerance. So far, these classifications have mainly been based on seedling performance (survival: Augspurger 1984, growth: Popma & Bongers 1988; Poorter 1998) or sapling performance (Poorter & Werger 1999), on the occurrence of saplings or larger trees under different natural light conditions (Clark & Clark 1992; Poorter & Arets 2003), or on maximum diameter growth of trees (Clark & Clark 1999). Classifications resulting from such studies are not consistent, and differences between species are less clear for large trees (Zagt & Werger 1997), which seem to share the same characteristics (Hubbel & Foster 1990; Clark & Clark 1992; Lieberman et al. 1995). One of the most important criteria that has been put forward in these classifications is whether a species requires a gap for successful regeneration (Clark & Clark 1992). Commonly, it is suggested that all canopy species depend on gaps for successful regeneration (Hartshorn 1978; Denslow 1980; Brokaw 1985), but that they may vary in the size and frequency of gaps needed for successful regeneration. Canopy accession of most non-pioneer trees is a process that may take more than 50 years (Clark & Clark 2001). This is long compared to the periods covered by most studies on tree growth in permanent sample plots, which typically lasted for <20 y (Condit 1995). Clearly, these time periods are to short to follow individual trees all the way from seedling until they reach the canopy and empirical data describing the growth trajectory towards the canopy do no exist for tropical forest trees (Clark & Clark 2001). Hence, a direct assessment of the degree of gapdependence of individual trees over their entire life has not been possible so far. Tree ring analysis may assist to fill this knowledge gap as it allows for the reconstruction of historical growth patterns over the entire life of trees. Such long-term growth data may reveal differences between species in temporal patterns of canopy accession, which are an indication of differences in life history. For instance, the length of periods of slow growth observed in tree ring series may indicate tolerance to shade (Orwig & Abrams 1994; Landis 1999) and the number of instances of sudden growth increases may indicate the number of disturbances that species require to reach the canopy (Lorimer et al. 1988). Tree ring analysis has greatly improved the understanding of tree ecology and forest dynamics in temperate trees, (cf. Canham 1985; Orwig & Abrams 1994; Lusk & Smith 1998; Cao & Ohkubo 1999; Wright et al. 2000), and a similar contribution can also be expected for tropical forests. 10

11 General introduction Tropical forest tree ages Comparisons of tree ages among tree species in tropical forests tell us something about the variation in life history and may be useful to classify species. Predictions of tree ages on large numbers of species, indicate that tree longevities vary highly: pioneer species may complete their whole life cycle within 8 years, whereas non-pioneer trees may become as old as 1000 years (Laurance et al. 2004). This wide range reveals remarkably large differences in life history. Tree ages also hold key information for the development of sustainable forest management schemes, as they give indications on the time required to replace harvested trees. Such data are very useful when assessing species potential for sustainable timber exploitation. In recent years there has been a major debate about the ages of tropical trees. This discussion was strongly hindered by the inability to accurately determine ages of tropical trees. Since the findings of millenarian trees by radiocarbon dating (Chambers et al. 1998) this debate increased in intensity (cf. Worbes & Junk 1999; Chambers & Trumore 1999; Williamson et al. 1999; Martinez-Ramos & Alvarez-Buylla 1999, 1998), but crucial questions regarding the ages of tropical trees are still open. Tree ages can be determined by direct or by indirect methods. Direct methods estimate or determine tree ages by radiocarbon dating (Stuiver & Becker 1986) or by counting of annual tree rings (Worbes et al. 2003). Indirect methods use projections of short-term growth data to estimate the ages of trees at particular size (for an overview of methods see: Martinez-Ramos & Alvarez-Buylla 1998; Baker 2003). Some methods are more suitable than others for application to tropical trees. Radiocarbon dating (a direct method) is very costly and can therefore not be used on a large scale. Another disadvantage is that the method can only be used to determine ages of trees older than 350 years. For younger trees the method is too inaccurate and may give errors of more than 100 years (Stuiver & Becker 1986). Tree ring analysis - another direct method - is the most powerful tool to accurately determine tree ages, and can be applied at large scale. This technique can only be applied however to species that form annual rings. Yet, for most tropical tree species we lack information on the occurrence of annual rings. The abovementioned drawbacks of direct ageing methods and the availability of short-term data from permanent plots, caused most tree age estimates to be based on indirect methods using projections of short-term growth data (Martinez-Ramos & Alvarez-Buylla 1998; Baker 2003). Different approaches have been developed to estimate ages of tropical trees (Lieberman & Lieberman 1985; Condit et al. 1993; Laurance et al. 2004; Bullock et al. 2004). The main problems with these methods are their underlying assumptions. Ages are calculated by inferences of short-term growth data over long time intervals, assuming that short-term growth is representative for long-term realized growth. Of the many seedlings and saplings that are present in the forest understory only a few individuals make it into the canopy. It has often been proposed that these successful individuals probably belong to the faster growing proportion of trees in the population. Trees that can maintain a fast growth over long time periods, for example those in a canopy gap, will reach the canopy much faster compared to slow growing individuals. As a result these trees have a higher chance to survive until maturity, as they experience the high mortality risks in the forest understorey over a relatively short period. In contrast, many of the slow growing saplings 11

12 Chapter 1 are destined to die because of mechanical damage (Clark & Clark 1991; Paciorek et al. 2000), long-term shade-suppression (Kobe et al. 1995; Wyckoff & Clark 2002; Landis & Peart 2005) or other causes. Including these individuals in estimations of time it takes to reach the canopy may lead to serious over-estimates (Ashton 1981). Indirect ageing methods have hardly been validated (Martinez-Ramos & Alvarez-Buylla 1998) although age-estimates for the majority of tropical tree species rely on such indirect methods (Lieberman & Lieberman 1985; Korning & Balsev 1994; Laurance et al. 2004),. Tree ring analysis provides a solution to this problem, for those species that form annual rings (Bormann & Berlyn 1981; Baas & Vetter 1989), as it yields reliable age values and can assist in validating various ageing methods (cf. Baker 2003). Tree rings in the tropics Annual tree rings in tropical trees have been reported already in 1927 in Indonesia by Coster and since that time in more than 20 different countries and in many different tree species throughout the tropics (Worbes 2002). More than 35 studies reported the occurrence of tree rings and many of them have proven that rings were formed on an annual basis. Notwithstanding, many authors have claimed that tropical trees do not produce annual tree rings (Lieberman et al. 1985a; Whitmore 1998) or emphasize the problems regarding the use of tropical tree rings (Bormann & Berlyn 1981; Chambers et al. 1998; Martinez-Ramos & Alvarez-Buylla 1998; Laurance et al. 2004). Formation of annual tree rings in tropical tree species can be caused by annual floodings (Schongart et al. 2002; Dezzeo et al. 2003), by seasonal variation in day length (Borchert & Rivera 2001) or seasonal variation in rainfall (Worbes 1999; Borchert 1999), and possibly by unidentified internal rhythms (Alvim et al. 1978). In most non-flooded forest areas the formation of tree rings is probably induced by the occurrence of an annual dry season of several months (Coster 1927; Worbes 1999; Dünisch et al. 2002). Due to a water deficit, tree species often show a reduced diameter growth or cambial dormancy during the dry season, resulting in the formation of a distinct growth boundary (Worbes 1999). Many ringforming species are deciduous or change all their leaves during the dry period (Worbes 1999; Borchert 1999). The connection between the formation of annual tree rings and seasonal precipitation was well illustrated by Coster (1927). He found that trees of the same species might form clear and annual rings under (seasonal) monsoon climates, while the same species form less distinct or irregular rings under almost everwet conditions. Worbes Number of studies Tree ages and forest dynamics Climate-growth relations Wood anatomy & growth rhythm now Time period Figure 1 Developments of tree ring studies in the tropics. Studies are separated according to the main aim of the study, distinguishing between those that were merely to prove the annual nature of rings (wood anatomy and growth rhythms), those that treated climate-growth relations, and those that present fundamental data on tree ages and forest dynamics. References: Amobi (1973); Ash (1983); Baker et al. (2005); Bhattacharyya & Yadav (1999); Bhattacharyya et al. (1992); Bullock (1997); Chowdhury (1964) Coster (1927); Detienne (1989); Devall et al. (1995) Dezzeo et al. (2003); Dunisch et al. (2002); Enquist & Leffler (2001); Eshete & Stahl (1999); Fichtler et al. (2003,2004); Hasenrath (1963); Jacoby & D'Arrigo (1990); Mariaux (1967a,b, 1969,1970,1995); Ogden (1981); Pumijumnong et al. (1995); Roig (2000) Schongart et al. (2004) ; Stahle et al. (1999a,b) ; Trouet et al. (2001); Tschinkel (1966); van Groenendael et al. (1996); Vetter & Botosso (1989); Worbes (1999); Worbes et al. (2003).

13 General introduction (1999) suggested that a dry period of at least 2 months with less than 50 mm of rain would be required to expect annual rings in tropical tree species, but recently annual rings have been proven for tree species in everwet climate (all months > 100 mm; Fichtler et al. 2003). Even small annual variation in rainfall occurring under everwet conditions may trigger ring formation. Many developments have taken place in the field of tropical tree ring research (Fig 1). Early studies before 1990 were merely performed to test whether tree rings are formed annually focusing on describing and classifying different growth zones (e.g., Coster 1927; Worbes 1995) and on methods to prove the annual nature of tree rings (e.g., Mariaux 1967, & Vetter 1989b). Later, more researchers have begun to use tropical tree rings for studying climate-growth relations and to develop climate sensitive tree chronologies (e.g., Jacoby & D'Arrigo 1990; Pumijumnong et al. 1995; Buckley et al. 1995; Stahle et al. 1999; Enquist & Leffler 2001). Only in the last five years tree rings have been used more intensively to obtain information on tree ages, forest dynamics and forest history (e.g., Worbes et al. 2003; Dezzeo et al. 2003; Fichtler et al. 2003; Baker et al. 2005). This dissertation focuses on the application of tree ring analysis to tree ecology and forest management. This study In this dissertation I try to contribute to the understanding of the ecology of tropical rain forest trees using tree ring analysis. The occurrence of tree rings in six tropical trees species in northern Bolivia allows to reconstruct historical growth rates of trees over their entire lifetime and provides data on tree ages. Using these data, I studied differences between species in ages and growth patterns. I also use these data to determine how individual trees differ in their performance and how this may influence population dynamics of species. Eventually, the long-term growth data are applied to evaluate the potential of study species for sustainable timber exploitation. For tropical trees long-term information is very scarce and part of the analyses presented in this thesis are new to tropical forest research. The results presented in this dissertation are relevant to tropical forest ecology, but also to forest management. Tree ring analysis has great potential as a tool to evaluate forest management practices by providing direct information on tree ages of harvestable trees (Stahle et al. 1999; Worbes et al. 2003), and providing reliable growth data to project future timber yield. This study is one of the first to use tree ring data with this aim and explores the potentials of this new method for tropical forest managers. Objectives of this study The objectives of the present study were: 1. to determine whether tree species in the Bolivian Amazon region form annual tree rings, 2. to study the relation between tree growth and rainfall; 3. to analyze how trees grow into the canopy; 4. to quantify the effect of temporal growth variation on ages of canopy trees;, 5. to study the degree of autocorrelation in tree growth and its impact on long-term variation in growth rates, 6. to evaluate the use of tree rings for forest management evaluations and estimate future timber yields. 13

14 Chapter 1 The Bolivian Amazon This study was conducted in the northern Bolivian Amazon in the departments of Pando and Beni (Fig 2). The climate in the region is characterized by a mean temperature of 27 o C with little variation throughout the year or between years. Mean total rainfall is 1690 mm for Riberalta and 1760 mm for Cobija. Inter-annual variation in rainfall is high and varied between 913 and 2646 mm over the period for Riberalta (Vose et al. 1995). There is a pronounced dry season from May till September with less than 100 mm rain per month. During this dry season the soil moisture content is clearly lower than during the wet season (Poorter 1998) and a considerable number of tree species is deciduous or change all their leaves during the dry season. The landscape in Pando and Beni is undulating with elevations ranging from 80 to 280 m. Soils are xanthic ferrasols in the west and haplic ferrasoils in the east (DHV/CUMAT 1993). The vegetation in the area has been classified as tropical lowland moist forest or as semievergreen tropical forest (DHV/CUMAT 1993). Forest cover is very high (c. 95%), as agriculture has not been practiced very intensively and timber logging concerns mainly selective logging of a few valuable species (DHV 1993). Canopy height of the forest is between 25 and 35 m high and the average basal area (DBH>20 cm) is 15 m 2 ha -1 with 103 trees per hectare (Superintendencia Forestal 1999). Species diversity of the region is relatively low with about 80 species per ha (>10 cm DBH; Poorter et al. 2001). Figure 2 Map of Bolivia (left panel) indicating the department boundaries, and of the Bolivian Amazon region (right panel) indicating the two areas where this study was carried out (triangles) and the main towns Riberalta (in Beni) and Cobija (in Pando). 14

15 General introduction The Bolivian timber industry The Bolivian timber industry is of great importance to the national economy: it is the strongest growing sector of Bolivia, and together with non-timber forest products represent an export value of ~117 million US$ in 2003, which is 11% of the national export (BOLFOR 2005). Especially in northern Bolivia - the focal region of this study - the regional economy depends mainly on forest products, constituting about 60% of the monetary influx for the region (Bojanic 2001). Most important exported forest products are timber (export of sawn wood in 2003: 24 mln US$; FAOSTAT data 2005) and Brazil nuts (Bertholletia excelsa: export value 2003: 54 mln US$: FAOSTAT data 2005). Timber export was traditionally limited to the high-valued species Mahogany (Swietenia macrophyla), Spanish Cedar (Cedrela odorata), and Spanish Oak (Amburana cearensis), but overexploitation of these species led to a shift to lesser valued species (Bojanic 2001). Currently over 100 tree species are exploited in Bolivia (Fredericksen 2000). In this study, six species have been included, of which four are important timber species and one (Bertholletia excelsa) is exploited for Brazil nuts (Table 1). Amburana cearensis, Cedrela odorata and Cedrelinga catenaeformis are among the most exploited species in the region, and accounted for about 80% of the regional income from the timber industry in the north of Bolivia in 1998 (Bojanic 2001). These three species also rank high on the national list of timber species, belonging to the ten most exploited species in Bolivia in 2004 (Superintendencia Forestal 2005). Together they accounted for more than m 3 of harvested sawn wood in 2004 and represented 15% of the total extracted sawn volume in Bolivia (Superintendencia Forestal 2005). Bolivia adopted a new forest law in 1996 (MDSMA 1997) aiming at a wise forest management that should guarantee sustained timber yield in the long run (Bojanic 2001). The new law requires forest inventories and forest management plans for most timber exploitation. Currently, around 8.5 million ha (~25% of Bolivia's forest area) are under such a controlled exploitation regime (Superintendencia Forestal 2005; BOLFOR 2005). The main prescriptions of this law are a minimum cutting diameter for each species (between cm diameter at breast height), a harvest intensity of 80%, leaving 20% of the harvestable trees behind for seed production (van Rheenen 2005) and a cutting cycle of at least 20 years. These technical guidelines will probably lead to an improvement of the management of the Bolivian forests. This is illustrated by the fact that nearly 2 million ha of forests are certified by FSC standards at this moment (FSC 2005). Yet, whether these prescriptions of the forest law are sufficient to guarantee regeneration of timber species (van Rheenen 2005) and to sustain timber yields over future harvests remains to be determined. Table 1 List of species used in this dissertation, the chapter in which they are included and the products obtained. Used in Scientific name English name Local name chapters Product Amburana cearensis (Allemao) A.C.Smith Spanish Oak Tumi 2,3,4,5 Timber Bertholletia excelsa H.B.K Brazil nut Castaña 2,3 Nut Cedrela odorata L. Spanish Cedar Cedro 2,3,4,5 Timber Cedrelinga catenaeformis (Ducke) Ducke - Mara Macho 2,3,4,5 Timber Peltogyne cf. heterophylla M.F.Silva Purpleheart Morado 2,3,4,5 Timber Tachigali vasquezii J.J.Pipeloy - Palo Santo Colorado 2,

16 Chapter 1 Programa Manejo de Bosques de la Amazonía Boliviana (PROMAB) This dissertation is part of the Programa Manejo de Bosques de la Amazonía Boliviana (PROMAB), a research, education and extension program of the Instituto para el Hombre, Agricultura y Ecología (IPHAE, Riberalta, Bolivia), the Carrera de Ingenieria Forestal of the Universidad Technica del Beni (Riberalta, Bolivia) and Utrecht University (the Netherlands). The main activities of the program, which started in 1995, are ecological and socio-economic research, technical assistance, training of forest users (in particular farmers and rural communities), training of forestry students, and dissemination of information to forest users and governmental bodies. The program s main goal is to improve the living conditions of forest depending people in North Bolivia through the sustainable use and conservation of their forests and forest resources. This dissertation is the tenth publication in the PROMAB Scientific Series. Earlier dissertations in this series dealt with the influences of light and water availability on seedling growth of rainforest tree species (Poorter 1998), the demography and exploitation of non-timber forest products (Zuidema 2000), mechanisms of secondary forest succession (Peña-Claros 2001), an economic analysis of the export of local forest products (Bojanic 2001), and an exploration of the potentials of sustainable livelihoods of forest residents (Henkemans 2002). The most recent dissertation assessed the role of seed trees and seedling regeneration for maintenance of commercial tree species(van Rheenen 2005). A study on the mechanism of co-existence and replacement of tropical tree species at early stages of succession by Galia Selaya is forthcoming in A complete list of all titles in PROMAB Scientific and Technical Series is included in the back of this dissertation. 16

17 General introduction Outline of this thesis In this thesis, I studied growth patterns and ages of six non-pioneer tree species by use of tree ring analysis. All samples were taken from large trees (mostly >50 cm diameter at breast height) that were either felled for timber or had died naturally. Field sites where samples were collected are indicated in figure 2. Names of the species studied and the numbers of chapters in which they are discussed are presented in table 1. Chapter 2 describes the regularity of ring formation of the six study species and tests whether tree rings were formed on an annual basis by correlating ring-widths to patterns in rainfall and by radiocarbon dating and counting of rings on trees of known age. The relations between rainfall patterns and growth are discussed for four species. Chapter 3 presents the growth trajectories of the six study species reconstructed by tree ring measurements. The magnitude and sources of age variation among and within species is studied. In addition, temporal patterns of tree growth towards the canopy are analysed for four species by identification of phases of suppression and release. Chapter 4 analyses the strengths and persistence of autocorrelated growth within individual trees and among different trees for four species. The significance of autocorrelated growth for age estimations is studied using bootstrapped growth simulations. Chapter 5 uses the growth data of four timber species to evaluate their potential for obtaining sustainable yields using the minimum cutting cycle of 20 years and other technical guidelines as prescribed in the Bolivian forestry law. The scope for applying tree ring analysis to assist in designing forest management systems is discussed. Chapter 6 summarizes the main results of the thesis and provides a general discussion. 17

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19 Rainfall and tree growth Chapter 2 Relating tree growth to rainfall in Bolivian rain forests: a test for six species using tree ring analysis With Pieter A. Zuidema Oecologia 2005 : online first. DOI: /s y Abstract Many tropical regions show one distinct dry season. Often, this seasonality induces cambial dormancy of trees, particularly if these belong to deciduous species. This will often lead to the formation of annual rings. The aim of this study was to determine whether tree species in the Bolivian Amazon region form annual rings and to study the influence of the total amount and seasonal distribution of rainfall on diameter growth. Ring widths were measured on stem discs of a total of 154 trees belonging to six rain forest species. By correlating ring width and monthly rainfall data we proved the annual character of the tree rings for four of our study species. For two other species the annual character was proved by counting rings on trees of known age and by radiocarbon dating. The results of the climate-growth analysis show a positive relationship between tree growth and rainfall in certain periods of the year, indicating that rainfall plays a major role for tree growth. Three species showed a strong relationship with rainfall at the beginning of the rainy season, while one species is most sensitive to the rainfall at the end of the previous growing season. These results clearly demonstrate that tree ring analysis can be successfully applied in the tropics and that it is a promising method for various research disciplines. Keywords: Climate-growth relation, Radiocarbon dating, Tropical rain forest, Tropical dendrochronology, Wood anatomy. 19

20 Chapter 2 Introduction Most tropical rainforests (sensu Whitmore 1998) experience some seasonality in rainfall and have a distinct dry season (Worbes 1995). This leads to annual rhythms in tree physiology (e.g., Borchert 1994c) and often causes leaf-abscission (Worbes 1999). A dry season of at least two months with less than 50 mm of rain, results in reduced diameter growth or complete cambial dormancy for many species (Worbes 1999). In this way, a distinct growth boundary is formed in many tree species. Indeed, various studies have proven the occurrence of growth rings in tropical rainforest species throughout the world (Detienne 1989; Vetter & Botosso 1989; Devall et al. 1995; Pumijumnong et al. 1995; Worbes 1999; Stahle et al. 1999; Dünisch et al. 2003; Fichtler et al. 2003, 2004), in spite of the traditional believe that tropical rainforest trees do not produce rings (Lieberman et al. 1985a; Whitmore 1998). Yet, the analysis of growth rings of tropical trees is less straightforward than that for temperate regions, as rings are not always formed annually. Some tropical ring studies have reported the occurrence of two rings per year (Jacoby 1989; Gourlay 1995) or an irregular formation of rings (Sass et al. 1995). Deviations of annuality in tree rings can be linked to species-specific differences in physiology and wood anatomy, but can also be due to exogenous factors such as rainfall patterns. Two distinct dry seasons per year for example, can lead to the formation of two rings per year (Jacoby 1989; Gourlay 1995). In the same way irregular patterns of rainfall, such as exceptional droughts during the regular rainy season, can induce the formation of intra-annual rings (Borchert 1999; Borchert et al. 2002). The response to rainfall patterns differs between species, often in relation to leaf-fall behaviour (Borchert et al. 2002). Annual tree rings are more often found in deciduous species than in brevi-deciduous or evergreen species (Worbes 1999; Borchert 1999). Furthermore, it is shown that responses vary between sites for the same species (Breitsprecher & Bethel 1990). Given these problems, there is a clear necessity to better understand the relations between rainfall, leaf fall behaviour and ring formation of tropical rainforest trees. The potential of tree ring analysis for obtaining reliable age estimates and long-term growth data is very high (Bormann & Berlyn 1981; Baas & Vetter 1989; Eckstein et al. 1995). Information on tree ages and growth rates is crucial to understand the dynamics of tree populations (Enright & Hartshorn 1981) and to develop sustainable management systems for tropical timber species (Stahle et al. 1999; Worbes et al. 2003). Most existing data about tree ages however, are either indirect estimates based on projections (Lieberman et al. 1985a; Laurance et al. 2004) or are highly disputed in literature (Martinez-Ramos & Alvarez-Buylla 1998; Chambers & Trumore 1999). In this paper we report on a tree ring study on six tree species in the Bolivian Amazon, focussing on the presence of tree rings and their relation with local rainfall. Our study is one of the few on tree rings in the Amazon basin (Worbes 2002) and provides important information on the potential of our study species for ecological and forest management research using tree rings. As rainfall in the study region is clearly seasonal, with a prolonged dry season, we expected tree growth to be periodically limited by shortness of water and year-to-year variation in rainfall to be reflected in the chronological pattern of tree ring widths. 20