Rain Forest on Maraca Island , Roraima , Brazil : Forest structure and Dynamics

A forest type dominated by Peltogyne gracilipes Ducke (Caesalpiniaceae) occurs on Maraca Island on a range of soil types. Maraca is located in Roraima State (Brazil) in the Rio Uraricoera and has an area of about 100,000 ha. This study compares the structure and floristic composition of the Peltogyne forest with the most widespread lowland forest type on Maraca and investigates some factors that could be involved in the persistent monodominance of Peltogyne. Three 0.25 ha plots were set up in each of three forest types: Peltogyne-rich forest (PRF), Peltogyne-poor forest (PPF) and forest without Peltogyne (FWP). Within each plot all trees (~ 10 cm dbh) were recorded. Seedlings and saplings were sampled in sub-plots of 2 m x 1 m (seedlings) and 4 m x 4 m (saplings). In the PPF and FWP, Sapotaceae were the most important family with the highest dominance and relative density values. Caesalpiniaceae showed high values in the PRF and PPF. Licania kunthiana, Pradosia surinamensis and Simarouba amara occurred in the forest types. Peltogyne dominated had 20% of stems and 53% of the trees ~ 10 cm dbh, and 91% of the canopy layer the canopy in total basal stems and 97% in all the the PRF and area of all of the total basal area of individuals > 50 cm dbh. In PPF, Lecythis corrugata and Tetragastris panamensis were the most abundant species, followed by Peltogyne. In the FWP the most abundant trees (~ 10 cm dbh) were L. kunthiana and P. surinamensis. In general, Peltogyne had low rates of seed predation and herbivory, but suffered locally high levels of damage to its seeds by leaf-cutter ants and was once observed to have an infestation of larvae of the moth Eulepidotis phrygionia on its young leaves. Peltogyne had no allelopathic effects on tested species and had VA mycorrhizal associations. Its occurrence remains unexplained but is most clearly correlated with soil magnesium. Abstract Acknowledgements CONTENTSAcknowledgements CONTENTS CHAPTER 1. GENERAL INTRODUCTION Introduction 1 1 Proposed mechanisms for the origin and maintenance of monodominant forests in the tropics 3 5 The aims of this study CHAPTER 2. MARACA ISLAND Location ..... Previous research 7 7 7 8 8 9 Topography, geology and geomorphology Soils Climate . . . . Vegetation Human occupation 10 11 CHAPTER 3. STUDY SITES AND PLOTS 12 The forest types 12 plot selection 13 Human disturbance of the plots 14 CHAPTER 4. THE SOILS, FOREST STRUCTURE AND FLORISTIC COMPOSITION 15 Introduction . . . . 15 Methods . . . . 16 Soil sampling and analysis 16 Floristic inventory 17 Data analysis 19 Soil data 19 Vegetation data 19 Results ..... 21 Soil pits 21 Surface soils 21 Forest structure 22 Canopy height 22 Basal area and density 22 Lianas and forest type 23 Floristics . . . . . . . . . 23 Level of identification 23 Family composition 24 Density contribution 24 Basal area contribution 25 Species contribution 26 Similarity and diversity among forests 26 Structural classification 26 Species composition .... 27 Density contribution 27 Basal area contribution 28 Species-area curve 28 Community similarity 29 Diversity and evenness 29 Community structure 30 Discussion . . . . . 31 Physical environment 31 Forest structure . . 32 The forest formation 32 Vertical structure 33 Tree density . . . . 34 Basal area Lianas Floristics . . . Family composition Species composition Conclusions . . . . . . CHAPTER 5 CLASSIFICATION AND ORDINATION OF THE FOREST TYPES Introduction Material and Methods ..... . The data set. ..... . Floristic classification . . Agglomerative hierarchical method ( "Cluster Analysis" ) Divisive hierarchical method Floristic ordination PCA ........... . DCA ........... . Analysis of the environmental variables Regression . . . . . . . Results .......... . Floristic classification Cluster analyses TWINSPAN analysis Floristic ordination . PCA ..... . DCA ••.••• Vegetation-soil relationships PCA of the soil variables DCCA ...... . Regression analysis Discussion ........ . Floristic classification and Species-soil relationship Conclusion .......... . ordination CHAPTER 6. POPULATION DYNAMICS OF FIVE NON-PIONEER TREES IN THREE FOREST TYPES ON MARACA ISLAND Introduction ..... Methods . . . . . . . . Species selection Plot samples . . . Gap samples Data analysis Plant mortality Growth rates Results ....... . population structure of the species Size distribution of trees . Tree mortality . . . . . . . Seedling population dynamics Seedling mortality Seedling height growth Comparisons between the gap and PRF Sapling population dynamics Sapling mortality Sapling growth .... Causes of mortality and suppression Discussion .............. . Population structure of the species Tree mortality Seedling mortality Sapling mortality 34 35 36 36 37 39