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    THẠC SĨ Modelling Growth and Yield of Dipterocarp Forests in Central Highlands of Vietnam

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  6. Modelling Growth and Yield of Dipterocarp Forests in Central Highlands of Vietnam

    iii
    Abstract

    Dipterocarp forests in Vietnam are distinct ecosystems with specific characteristics
    which are different from other forest types such as evergreen forests, semi-deciduous
    forests and conifer forests. According to inventory results of the Forest Inventory and
    Planning Institute of Vietnam in 2005, the area of the Dipterocarp forests is approximately
    680.000 ha, accounting about 5.4% the total forest area of the country and concentrates
    mainly in the Central Highlands of Vietnam.
    The main objective of this study is to develop a size class model based on systems
    of differential equations for supporting sustainable management of the Dipterocarp forests
    in Vietnam. Two data sets collected in the Dipterocarp forest in YokDon National Park
    were used in this study to construct the growth model and calculate the main stand level
    characteristics. They include plot group A consisting of twelve one-hectare permanent plots
    with two measurements of a 5-year growth interval, and plot group B of 21 one fourth
    hectare plots with a single measurement. For calibrating the growth model, only data set of
    group A plots was used. In addition to be used to calculate the main stand level parameters,
    the group B plots will supply reliable data sources to recalibrate the model in the future.
    The study area was classified into three site quality levels based on mean height of the 20
    largest trees in each plot. The measurements on these permanent plots recorded a total of
    4,975 trees belonging to 64 species with diameter at breast height (dbh) from 6 cm and
    above. Based on biological characteristics, trees on these plots were grouped into three
    species groups: Dipterocarp species, evergreen tall species, and small-sized, lower species.
    The diameter distribution of the average stands follows the form of negative exponential
    distribution for all three species groups in accordance with the distribution rule of natural
    uneven-aged forests. The number of trees per hectare has a tendency to decrease when
    diameter increases. Stand basal area ranges from 10.15 to 26.9 m 2 ha -1 and the range of
    basal area increment is between 0.27 and 0.48 m 2 ha -1 yr -1 . Standing volume ranges from
    53.8 to 208.8 m 3 ha -1 and the range of standing volume increment between 1.5 and 3.86
    m 3 ha -1 yr -1 . The number of tree per hectare ranges from 223 to 1.156 trees ha -1 .
    The four major components of the growth model are diameter increment, mortality,
    recruitment and harvesting. The first three models were developed separately for each
    species group and site quality level. Multiple linear regression, non-linear regression and
    logistic regression were used to estimate the parameters of diameter increment, recruitment
    and mortality functions. Significant stand level variables included stand basal area, basal
    area in larger trees, tree number, site quality, and significant individual-tree level variables
    were diameter, diameter squared and reciprocal of diameter. Selecting the model equations iv
    was based on the following criteria: suitability of biological interpretation and goodness-of-
    fit statistics. The results indicated that diameter growth level of three species groups on
    different site quality levels was significantly different with the exception of species group 3
    on good and medium site quality. Trees grow more quickly on good sites than on poor
    ones. However, the effect of site quality on mortality rate was not obvious in this study.
    These major components were then embedded to the final growth model which is a size
    class management-oriented model. The model was implemented in the framework of the
    modelling software Vensim DSS 5.7a. It consists of 76 one-cm diameter classes ranging
    from 6 to 81cm dbh for three species groups, the last class gathering all trees with diameter
    above 80.5cm. Time interval for each simulation step of the model was set one year and
    diameter class width was one cm.
    A thorough evaluation of the growth model showed that the models were fitted very
    well with the empirical data. Simulation results with the models showed that the difference
    between observed and predicted values of basal areas and tree number distribution by
    diameter class for a growth period of five years was small. The long-term performances of
    the simulation proved plausible states of the stand evolution which is consistent with
    general knowledge of stand growth over long time. This indicates that the model can be
    applied in practice.
    The example applications of the growth model in determining appropriate
    silvicultural regimes based on the method of scenario analysis. Given the initial condition
    of the stand, the model estimated the state of the stand after given years with the alternative
    assumed prescriptions. The simulation results indicated that, with a selection harvesting
    cycle of 10 years, different initial stand distributions will produce different sustainable
    yields. The q-factor method was applied to determine the target diameter distributions that
    produce maximum sustainable yields on three site qualities. The maximum diameters for
    each species group were selected based on management purpose and diameter growth level
    as follows: for species group 1 and 2, maximum diameters are 70, 60 and 50 cm for good,
    medium and poor site quality, respectively. For species group 3, maximum diameter is 35
    cm for all site qualities. From the simulation results of the model, the following target
    distributions have been defined: on good site quality with following parameters: basal area
    equal to 20 m 2 ha -1 , q-quotient (slope of the stem number-diameter distribution of 5 cm
    classes) equal to 1.4, with the sustainable yield of 3.91 m 3 ha -1 yr -1 . For medium site quality:
    basal area equal to 18 m 2 ha -1 , q-quotient equal to 1.5, sustainable yield of 3.22 m 3 ha -1 yr -1 .
    And on poor site quality: basal area equal to 16 m 2 ha -1 , q-quotient equal to 1.6, sustainable
    yield of 2.75 m 3 ha -1 yr -1 . In addition, the model was also used to estimate the return time
    that regulates a given stand towards the target distribution stand for the twelve plots of
    group A and to assess effect of wildfires on long-term yields of the Dipterocarp forests. v
    The example applications presented in this study provide valuable information to
    the forest managers for supporting decision making in sustainable management of
    Dipterocarp forests. Other applications of the model need to be further explored in specific
    contexts of the production practice.
    Although there were several studies on growth and yield of natural uneven-aged
    forests in Vietnam before, those studies modeled only important species in the forests and
    produced yield tables dependent on the age of trees that provide less information for forest
    management. In comparison to those studies, this growth model was constructed
    incorporating competition effects as well as mortality and recruitment so that it has the
    advantage of being able to estimate the growth of forests dynamically and independent on
    the tree age for long time spans with reliable results.
    However, due to the comparably small amount of data available in this study, all
    data was used to calibrate the model, there was no data set aside for validating the model.
    So, it is necessary to obtain more data from permanent plots and when it is available the
    model should be recalibrated in order to expand the geographic research area and achieve
    more accurate results. Although the growth model in this study was developed for
    Dipterocarp forests that are uneven-aged, multi-species deciduous forests, the approach can
    be applied to develop models for other forest types such as evergreen, semi-evergreen
    forests or plantation forests. vi
    Zusammenfassung

    [german] Das Ziel dieser Arbeit ist die Entwicklung eines differentialgleichungsbasierten
    Durchmesserklassen-Wachstumsmodells für nachhaltige Bewirtschaftung von
    Dipterocarpaceenwäldern in Vietnam. Die Daten wurden im YokDon Nationalpark
    erhoben. Das Programm besteht aus vier Modulen zur Abschätzung des
    Durchmesserzuwachses, der Mortalität, der Verjüngung und einem Durchforstungsmodell.
    Als Simulationssoftware wurde Vensim DSS 5.7a verwendet. Das Modell wurde eingesetzt,
    um über Szenarioanalysen geeignete Behandlungsstrategien zu finden. vii
    Table of contents


    Preface and Acknowledgements i
    Abstract iii
    Zusammenfassung vi
    Table of contents .vii
    List of figures . xi
    List of tables xiii
    Chapter 1 Introduction .01
    1.1 General Introduction 01
    1.2 Research Questions and Objective of the Study 05
    1.3 Outline of the Dissertation 06
    Chapter 2 Literature Review .08
    2.1 Studies About Forest Structure and Growth in Vietnam in General .08
    2.1.1 Studies about Forest Growth and Yield 08
    2.1.2 Studies about Diameter Distribution Rules .10
    2.2 Studies about Dipterocarp Forests .11
    2.2.1 Studies about the Dipterocarp Forests in the World .11
    2.2.2 Studies about the Dipterocarp Forests in Vietnam 12
    2.3 Historical Development and Classification of Forest Growth and Yield
    Models 18
    2.3.1 Stand Growth Models Based on Mean Stand Variables .18
    2.3.2 Stem Number Frequency Models .19
    2.3.3 Single-Tree Orientated Management Models .21
    2.3.4 Gap and Hybrid Models 22
    2.3.5 Matter Balance Models .22
    2.3.6 Landscape Models 23
    2.3.7 Selection of the Model Approach to be Used in This Study 24 viii

    Chapter 3 Study Area and Establishment of Research Plots .26
    3.1 General Information about the Study Area 26
    3.1.1 Geographic Position and Boundary of the YokDon National Park 26
    3.1.2 Forest types in the Park 27
    3.1.3 Topography and Hydrography 28
    3.1.4 Climate 30
    3.1.5 Flore and Fauna Resources 31
    3.1.6 Social Economic Conditions 32
    3.2 Establishment of Research Plots as an Empirical Data Base for
    Modelling Growth and Yield in Dipterocarp Forests 33
    Chapter 4 Data and Description of Stand Characteristics 38
    4.1 Ecological Classification of the Research Plots by Species Composition 38
    4.2 Establishment of Stand Height Curves and Site Quality Classification 43
    4.2.1 Selecting Height Curve Functions .43
    4.2.2 Categorizing Species Groups .44
    4.2.3 The Results of Height Curve Fitting 46
    4.2.4 Site Quality Classification .47
    4.3 Data sets 48
    4.3.1 Data for Calculating Stand Characteristics .48
    4.3.2 Data Used to Calibrate the Growth Model 49
    4.4 Stand Variables 53
    4.4.1 The Method of Calculating Stand Variables 53
    4.4.2 Calculation of Stand Variables 53
    4.4.3 Relationships between Stand Variables 57
    Chapter 5 Model Conception and Parameterization 60
    5.1 Model Conception 60
    5.1.1 The Concept of System Dynamics Diagrams 60
    5.1.2 Model Structure and Implementation 62 ix
    5.2 Development of the Major Components of the Growth Model .71
    5.2.1 Diameter Increment Model 71
    5.2.2 Mortality Model 74
    5.2.3 Recruitment Model 76
    5.3 Results of Model Parameterization 77
    5.3.1 Diameter Increment Model 77
    5.3.2 Mortality Model 81
    5.3.3 Recruitment Model 84
    Chapter 6 Model Evaluation .88
    6.1 Evaluation of the Model Approach .89
    6.2 Validation of the Growth Model 90
    6.2.1 Short-Term Prediction of a 5-Year Period .91
    6.2.2 Long-Term Validation of Steady States 94
    6.3 Evaluation of the Growth Simulator 99
    Chapter 7 Applications of the Growth Model DIGROW . 101
    7.1 Estimation of the Growth and Yield of Forest Stands and Determination
    of the Target Diameter Distributions 102
    7.2 Estimation of Time to Regulate a Given Stand to Target Stand . 110
    7.3 Evaluation of Effects of Wildfires on Long-Term Sustainable Forest Yield 114
    Chapter 8 Discussion . 118
    8.1 Growth Model Approach and Parameterization . 118
    8.2 Simulation Results of the Growth Model 121
    8.3 Effects of Wildfire . 123
    Chapter 9 Conclusion and Perspective 124
    9.1 General Conclusion . 124
    9.1.1 The growth Model Approach and Development 124
    9.1.2 Model Applications 125
    9.1.3 Data Assessment 126
    9.2 Perspective of the Study 127 x
    9.2.1 Recalibration of the Growth Model and Extention of its Applications .127
    9.2.2 Development of Growth Models for Other Forest Types in Vietnam 128
    9.2.3 Technical Development 128
    Literatures . 130
    Appendix 149



















    xi
    List of Figures

    Fig. 1.1 Geographic position of the Central Highlands in Vietnam .03
    Fig. 3.1 Geographic position of the YokDon National Park in the Dak Lak
    province 27
    Fig. 3.2 Hydrography system in the YokDon National Park 29
    Fig. 3.3 Average air temperature in the period 2001-2006 in the study area 30
    Fig. 3.4 Average atmosphere humidity in the period 2001-2006 in the study
    Area 30
    Fig. 3.5 Average rainfall in the period 2001-2006 in the study area .31
    Fig. 3.6 Forest state map of YokDon National Park .35
    Fig. 4.1 Association type 1: Dipterocarpus tuberculatus as dominating species 39
    Fig. 4.2 Association type 2: Dipterocarpus tuberculatus forest with
    Shorea obtusa .40
    Fig. 4.3 Diameter-height curves of plot A1 and plot A4 for three species groups .46
    Fig. 4.4 Diameter-height curves for three species groups of the twelve group A
    plots 47
    Fig. 4.5 Average tree number by diameter class distribution per hectare of
    the two plot groups 49
    Fig. 4.6 Average number of trees per hectare for three species groups over
    twelve plots of group plot A .50
    Fig. 4.7 Relationships between important stand variables .58
    Fig. 5.1 System Dynamic Diagram notation .61
    Fig. 5.2 Stock- and Flow-structure of a diameter class .63
    Fig. 5.3 Principle of tree transition from one class to the successive higher class .64
    Fig. 5.4 Diagram of recruitment to the smallest diameter class of 6cm 65
    Fig. 5.5 Structure of the mortality model for each diameter class .66
    Fig. 5.6 Structure of harvesting method of diameter limit cut and proportion xii
    harvesting rule 67
    Fig. 5.7 Structure of the harvesting model for q-factor guide 68
    Fig. 5.8 Complete SD Diagram of the simulation model DIPGROW 71
    Fig. 5.9 Partial effect of variables on diameter increment .80
    Fig. 5.10 Plots of residuals against the fitted values of individual-tree diameter
    increment model for three species groups 81
    Fig. 5.11 Partial effect of variables on mortality rate of three species groups 84
    Fig. 5.12 Partial effect of variables on recruitment .86
    Fig. 5.13 Annual predicted vs. observed recruitment of 12 plots for the three
    species groups .87
    Fig. 6.1 Observed vs. predicted values after a simulation period of five years for all
    group A plots 94
    Fig. 6.2 Simulated basal area evolutions over one thousand years in total and
    seprated species roup of an undisturbed stand on three site qualities .96
    Fig. 6.3 Predited long-term diameter distribution evolutions of an undisburbed
    forest stand for three site qualities 98
    Fig. 7.1 Results of a scenario simulation of a stand .104
    Fig. 7.2 Simulation results of mean annual volume increment obtained by the
    stands with different basal areas and q-values .107
    Fig. 7.3 The target diameter distributions of three species groups for three site
    qualities 109
    Fig. 7.4 Simulation results of the growth model for plot A8 following the harvesting
    method of q-factor guide .111
    Fig. 7.5 Simulation results of the growth model for plot A8 following the harvesting
    method of q-factor guide with slight modification .112
    Fig. 7.6 Diameter distribution of example plots .113
    Fig. 7.7. Simulated effects of wildfires with different frequencies and intensities on
    long-term yields 115
    Fig. 7.8 Diameter distribution of the stands with different wildfire frequencies at the
    time of 200 years 116 xiii
    List of tables


    Table 3.1 Areas of different forest types in the YokDon National Park .28
    Table 4.1 Species association on the research plots .39
    Table 4.2 Diversity of species composition for plot group A .41
    Table 4.3 Diversity of species composition of group B 42
    Table 4.4 Summary statistics for individual trees data on 12 plots .50
    Table 4.5 Data for developing the recruitment function 51
    Table 4.6 Summary of the data of mortality status in the plots used to develop
    mortality functions .52
    Table 4.7 Summary statistics of the mortality data used for the model
    development .52
    Table 4.8 Growth and yield characteristics of plot group A 54
    Table 4.9 Growth and yield characteristics of plot group B 56
    Table 4.10 Range of mean diameter and mean height in the stands of group A 57
    Table 4.11 Range of mean diameter and mean height in the stands of group B 57
    Table 5.1 The estimated parameters and fit statistics of individual tree diameter
    increment models by species group .78
    Table 5.2 The estimated parameters and fit statistics of mortality rate models by
    species group .83
    Table 5.3 The estimated parameters and fit statistics of recruitment models .85
    Table 6.1 Predicted vs. observed basal areas for each plot of group A for the
    three species groups .91
    Table 6.2 Predicted vs. observed average number of trees per hectare for each
    site quality by 5-cm diameter classes 93
    Table 7.1 Mean annual volume increments produced by various initial stands 105
    Table 7.2 The target diameter distribution for three site quality levels 108
    Table 7.3 Return time of the group A plots .113
    Table 7.4 Average annual long-term yields with different intensities and
    frequencies of wildfire .116

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