Table Of Content"ASSESSMENT AND PREDICTION OF WOOD YIELD FROM
AGROFORESTRY SYSTEMS IN KENYA"
UNIVERSITY OF NAIROBI
I I P A R v»
P. O. Box 30197
NAIROBI
DAVID MURIUKI KAMWETI
Vjg
oFKn acceded
......
THB D■®>*»»“ m
A C‘;‘
A Thesis submitted in fulfillment of requirements for the Degree of
DOCTOR OF PHILOSOPHY'
in the University of Nairobi.
1996
DECLARATION
This thesis is my original work and
has not been presented for degree
in any other university.
......... Signature
David Muriuki Kamweti
0 .> .......... Date
This thesis has been submitted for examination with our approval as the
university supervisors.
Professor R.W. Michieka
Signature Date
Dr. N. Karanja
(i)
ABSTRACT
The aim of the study was to assess and predict the growth and yield of wood in agroforestry
areas of Embu District. The District was stratified into eight agro-ecological zones (AEZs)
where a total of 14,746 trees were measured in 254 random farms. The species mostly
investigated was Grevillea robusta .
G. robusta was the most common tree species occurring in majority of the AEZs and had an
overall weighted frequency of 37% out of the 104 tree species found in the sampled farms.
Frequencies of the other 103 tree species found in various farms ranged from 0.001 to 11.4%.
Planted trees accounted for 83% of all trees found in the farms and the number of trees planted
per ha were positively related to the population density. High potential areas had more trees on
farms than the low potential areas despite decreasing farm sizes. A spacing of 2.0 m was the
most common for the trees which were not open grown. Boundary planting was the commonest
pattern with a frequency of 42%.
The yield of G. robusta ranged from 0.007 to 0.025 m3/tree/year at ages three and six years
respectively. Weighted mean volume of the species was 0.181 m3/tree. Biological rotation was
8, 9, 10,12,15 and 15 years for AEZs three, four, five, six, seven and eight respectively.
Farmers felled trees at an early age because of increasing demand for tree products and a need
to reduce tree crop competition. Approximately 80% of G. robusta trees were harvested
(ii)
before attaining 7 years which was below biological rotation. Corresponding diameters and
heights were less than 17 cm and 12 metres respectively. Branch volume contributed an
equivalent of about 26% of the stem volume.
G.robusta accounted for about 2.4 million trees with a volume of 440,000 nv in all AEZs.
Stocking of planted trees ranged from zero to 65 trees per hectare in the eight agro-ecological
zones while that of regeneration trees was generally low with ranges of 1 to 12.
The wood yield models for AEZ three to nine indicated that diameter, height, age, spacing, land
slope and tree crown height were the important independent variables which influence wood
yield in farms and they were significant at 0.05% probability level. Diameter was the most
important independent variable which could be used to estimate tree volume with a precision
level of 80%.
On validation, 57% of the sample plots indicated that there was no significant difference between
observed and predicted volumes. Standard error of models ranged from 1.89 to 5.25% with a
95% confidence interval range of + 4.50 to 12.93%.
(iii)
TABLE OF CONTENTS
ABSTRACT....................................................................................................... (i)
TABLE OF CONTENTS..............................................................................................................(iii)
ACKNOWLEDGEMENTS .........................................................................................................(ix)
DEDICATION .......................................................................................................................... (*)
CHAPTER 1: INTRODUCTION.............................................................................................. 1
1.1 Background............................................................................................................. 1
1.2 Problem statement ................................................................................................ 2
1.3 Objectives of the study ......................................................................................... 3
1.4 Study justification................................................................................................... 3
1.5 Organization of the thesis .................................................................................... 4
CHAPTER 2: LITERATURE REVIEW................................................................................. 5
2.1 Wood Yield ........... .................................................................................................. 5
2.2 Grevillea robusta............................................................................................................ 6
2.3 Existing wood yield models in forestry.................................................................. 9
2.4 Wood yield studies in agroforestry areas............................................................... 19
2.5 Theory of tree growth and yield............................................................................ 20
2.6 Factors affecting wood yield ................................................................................. 24
2.7 Yield models .......................................................................................................... 25
(iv)
CHAPTER 3: MATERIALS AND METHODS.................................................. 28
3.1 Background of the study area......................................................... 28
3.2 Stratification of Embu District................................................................................. 29
3.3 Sampling procedures ................................................................................................. 34
3.4 Design and pre-testing data form ............................................................................ 38
3.5 Field procedure...................................................................... .................................... 39
3.6 Frequency distribution of tree species.................................................................... 42
3.7 Tree species spacing and plantation patterns.......................................................... 42
3.8 Estimation of yield and growth ............................................................................... 43
3.9 Yield models development and validation ............................................................ 44
CHAPTER 4: RESULTS............................................................................................................... 46
4.0. 1 Tree measurements and elevation of AEZs....................................................... 46
4.0. 2 Chemical characteristics of soils in Embu District......................................... 48
4.1 Tree species composition and spacing.................................................................... 50
4.1.1 Frequency distribution of trees in the study area.................................... 50
4.1.2 Proportion of planted and regeneration trees....................................... 52
4.1.3 Tree spacing in farms................................ 53
4.1.4 Planting pattern of G. robusta in farms.................................................. 55
4.2 Tree growth and yield............................................................................................... 56
4.2.1 Mean annual yields .................................................................................... 56
4.2.2 Volume of G. robusta for years one to 3 0 .......................................... 57
4.2.3 Age and size structure of G. robusta trees............................................. 64
(V)
4.2.4 Branch volume of G. robusta .................................................................. 67
4.2.5 Tree stocking and volume in Embu District.............•:........................ 70
4.3 Yield models development and validation ............................................................ 74
4.3.1 Development of models ............................................................................ 74
4.3.2 Validation of models................................................................................. 76
CHAPTER 5: DISCUSSIONS ................................................................................................ 79
5.0 Chemical characteristics of soils in Embu District ............................................... 79
5.1 Tree species composition and spacing .................................................................... 81
5.1.1 Frequency distribution of trees in the study area.................................. 81
5.1.2 Proportion of planted and regeneration trees ....................................... 84
5.1.3 Tree spacing in farms................................................................................. 85
5.1.4 Planting patterns of G. robusta in farm s............................................... 87
5.2 Tree growth and yield .............................................................................................. 89
5.2.1 Mean annual yields .................................................................................... 89
5.2.2 Volume of G. robusta for years one to 3 0 .......................................... 90
5.2.3 Age and size structure of G. robusta trees............................................. 94
5.2.4 Branch volume of G. robusta .................................................................. 95
5.2.5 Tree stocking and volume in Embu District.......................................... 99
5.2.6 Management practices in farm s............................................................ 101
5.3 Yield models development and validation .......................................................... 103
5.3.1 Development of models ......................................................................... 103
5.3.2 Test of non-linear relationship hypothesis.......................................... 104
(Vi)
5.3.3 Test of model violations on assumptions............................................ 105
5.3.4 Validation of models........................................................ 105
6: CONCLUSIONS AND RECOMMENDATIONS............................................................. 107
BIBLIOGRAPHY......................................................................................................................... Ill
LIST OF TABLES
Table 2.1: Coefficient for the tree volume equations for C. lusitanica, P. patula
and P. radiata in K enya................................................................................................... 9
Table 2.2: Diameter and height of planted O. usambarensis at different ages..................... 13
Table 2.3: Regression equations for different tree species .................................................... 14
Table 3.1: Sampling, demographic and rainfall data for each AEZ .................................... 36
Table 4.1: Population of trees and elevation (m) of the eight AEZs .................................. 47
Table 4.2: Chemical characteristics of the soils collected from Embu District .................. 49
Table 4.3: Frequency Distribution of tree species in Embu District .................................... 51
Table 4.4: Frequency occurrence of planted and regeneration trees in all A EZs................ 52
Table 4.5: Frequency distribution of tree spacing and possible stems/ha in AEZs (3-9) . . 54
Table 4.6: Frequency *of planting patterns of G. robusta ....................................................... 55
Table 4.7: Mean annual yield of G. robusta trees in different A EZs.................................. 57
Table 4.8: Volume of G. robusta from age 1 to 30 years (m7tree) .................................. 58
Table 4.9: Volume/age models for G.robusta for each A E Z ............................................... 63
Table 4.10: On-farm age structure of G. robusta .................................................................... 64
Table 4.11: Diameter structure of G. robusta ............................................................................ 65
(vii)
Table 4.12: Height structure of G. robusta................................................................................. 66
Table 4.13: Wood volume contribution by tree branches ............................... 68
Table 4.14: Statistical interrelationship between growth parameters of G. robusta ............ 69
Table 4.15: Mean farm sizes and tree stocking per ha................................................................ 70
Table 4.16: Total farm sizes and trees in Embu District.......................................................... 72
Table 4.17: Total population and volume of G.robusta in Embu District.......................... 73
Table 4.18: Coefficient estimates of independent variables and
model structures for AEZs 3 to 9 ............................................................................... 75
Table 4.19 Mean biases, standard error, and 95% confidence interval for AEZ 3 to 9 . . 77
LIST OF FIGURES
Figure 2.1: Stages of tree growth ................................................................................................. 22
Figure 3.1: Stratified agro-ecological zones of Embu District ............................................... 33
Figure 3.2: Embu District, agro-ecological zones and sampling plots ............................... 37
Figure 4.1: Volume curves in ( mJ) for G. robusta for different AEZs............................. 60
Figure 4.2: Mean annual increment curves for AEZs (3-8) .................................................. 61
Figure 4.3: Planted trees per ha and the population density.................................................... 71
APPENDICES
Appendix 3.1: Data form used in field...................................................................................... 124
Appendix 4.1: Soil analysis results ........................................................................................... 125
Appendix 4.2: Yield of common tree species found in
association with G. robusta in Embu District..................................................... 129
Appendix 4.3: Number of G. robusta trees measured for ages
1 to 30 for AEZ 3 to 9 .......................................................................................... 130
(viii)
Appendix 4.4.1 Coefficient estimates and Beta values of independent
variables for MODEL AEZ 3 .............................................................................. 131
Appendix 4.4.2 Coefficient estimates and Beta values of independent
variables for MODEL AEZ 4 .............................................................................. 132
Appendix 4.4.3 Coefficient estimates and Beta values of independent
variables for MODEL AEZ 5 .............................................................................. 132
Appendix 4.4.4 Coefficient Estimates and Beta values of independent
variables for MODEL AEZ 6 .............................................................................. 133
Appendix 4.4.5 Coefficient Estimates and Beta values of independent
variables for MODEL AEZ 7 .............................................................................. 133
Appendix 4.4.6 Coefficient Estimates and Beta values of independent
variables for MODEL AEZ 8 .............................................................................. 134
Appendix 4.4.7 Coefficient Estimates and Beta values of independent
variables for MODEL AEZ 9 .............................................................................. 134
Appendix 4.5: Scatter plots from regression data of AEZ 5 ............................................... 135
Appendix 4.6.1 Mean biases, standard error, and 95% confidence
interval for AEZ 3 .................................................................................................. 136
Appendix 4.6.2 Mean biases, standard error, and 95% confidence interval
for AEZ 4 ............... 136
Appendix 4.6.3 Mean biases, standard error, and 95% confidence interval
for AEZ 5 ................................................................................................................ 137
Appendix 4.6 4 Mean biases, standard error, and 95% confidence interval for AEZ 6 . . 137
Appendix 4.6.5 Mean biases, standard error, and 95% confidence interval for AEZ 7 . . 138
Appendix 4.6.6 Mean biases, standard error, and 95% confidence interval for AEZ 8 . 138
Appendix 4.6.7 Mean biases, standard error, and 95% confidence interval for AEZ 9 . . 138
Description:The aim of the study was to assess and predict the growth and yield of wood in agroforestry areas of Embu District. The District was stratified into eight
