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RESOURCE AVAILABILTY AND USE IN AGROFORESTRY: RADIATION AND WATER
Radiation distribution and use
Radiation not utilised for photosynthesis immediately cannot be stored and is, therefore, lost (Kropff, 1993a). The quantity and quality of solar radiation that plants intercept is dictated by a number of factors, such as altitude, canopy structure, clouds, latitude, time of year, time of day and topography (Ong et. al., 1996; Berlyn and Cho, 2000). Plant utilise radiation based on their internal and external structures and water and chlorophyll contents of their leaves (Kropff, 1993b; Berlyn and Cho, 2000). Radiation, then, takes part in photosynthesis, stomatal aperture, crown and leaf characteristics, photoperiodism, enzyme activation, radiation driven reactions and cold hardiness (Berlyn and Cho, 2000). Indirectly, radiation affects plant use of other growth resources (Huxley, 1999).
In agroforestry, interspecies competition for radiation (Nair, 1993; Ong et al., 1996), heterogeneity and constant variation in crown architecture makes radiation distribution complex. Shading of field crops is a function of closeness to trees (Ong et al., 1996), tree height (Reifsnyder 1989), nature and structure of tree canopy, tree foliage density, position of the sun, latitude and altitude (Ong et al., 1996; Berlyn and Cho, 2000). Management also plays and important role in radiation distribution of agroforestry. The spatial distribution of transmitted radiation to an agroforestry floor is dependent on tree canopy management (Ong et. al., 2000), planting geometry of component species, tree spacing and row orientation, (Ong et al., 1996; Berlyn and Cho, 2000). Radiation distribution can be optimisation using appropriate design of tree spacing and row orientation and management of tree canopies especially during establishment phases of understory crops (Ong et al., 1996; Garrett and McGraw, 2000).
Water balance
Interspecies competition for water takes place directly or indirectly. When water is limited, the component with the between developed root system dominates. When water is not limited, potential growth of component species dictates potential competition for water at later stages (Kropff, 1993b). In agroforestry, trees affect water availability to crops by enhancing soil physical properties, decreasing (runoff and evaporation) losses and intercepting rain and competing for water in the crop-root zone (Wallace, 1996).
Even when water is not limited, trees affect crop transpiration by affecting other growth limiting resources (Kho, 2000). According to van Noordwijk et al. (1996), hydraulic lift, which is the redistribution of soil water from deeper soil horizons to drier upper horizons via plant root systems (Richards and Caldwell, 1987), plays a role in soil water recycling in agroforestry.
Agroforestry can have higher rainwater use efficiency than mono-cropping (Ong et al., 1996; Wallace, 1996). Reduction in evaporation is due to mulching and perennial tree presence shielding the soil from radiation and wind (Wallace, 1996). Trees can utilise post-harvest residual water in the crop root zone (Leyton, 1983) and rainfall between crop growing seasons. According to Lott et al. (2003), tree water use between crop growing seasons could be as much as 25% of annual tree water use and 16% of annual rainfall. While a fraction of canopy-intercepted rain is evaporated, the rest falls through plant canopies depending on rainfall intensity and atmospheric demand (Ong et al., 1996; Wallace, 1996). Runoff is reduced and infiltration is higher due to modification of kinetic energy of raindrops, reduced soil crusting, improved soil hydraulic conductivity, mulching and increases in soil faunal and floral communities that modify soil structure and permeability (Wallace, 1996).
CHAPTER 1 GENERAL INTRODUCTION
1.1 RATIONALE
1.2 INTERACTIONS IN AGROFORESTRY
1.2.1 Tree – crop interactions
1.2.2 Plant – environment interactions
1.2.3 Economic implications of interactions
1.3 RADIATION DISTRIBUTION AND WATER BALANCE IN AGROFORESTRY .
1.3.1 Radiation distribution and use
1.3.2 Water balance
1.4 MODELLING IN AGROFORESTRY
1.4.1 Overview
1.4.2 Potential source models for developing a mechanistic hedgerow intercropping mode
1.5 OBJECTIVES AND HYPOTHESES
1.6 THESIS OUTLINE
CHAPTER 2 SHOOT ALLOMETRY OF JATROPHA CURCAS L.
2.1 INTRODUCTION
2.2 MATERIALS AND METHODS
2.2.1 Definition
2.2.2 Treatments
2.2.3 Sampling
2.2.4 Data analyses
2.3 RESULTS
2.3.1 Allometry between basal diameter and AG variables
2.3.2. Allometry between crown depth and AG variables
2.3.3 Effects of BG interspecies competition and tree spacing on allometry
2.3.4. Validation of the allometric relationships using independent data
2.4 DISCUSSION
2.5 CONCLUSIONS
CHAPTER 3 EFFECTS OF TREE-PERENNIAL GRASS INTERACTIONS ON TREE PRODUCTIVITY
3.1 INTRODUCTION
3.2 MATERIALS AND METHODS
3.2.1 Treatments
3.2.2 Sampling
3.2.3 Calculations and data analyses
3.3 RESULTS AND DISCUSSION
3.3.1 Effects of treatments on stem growth rate
3.3.2 Treatment effects on post-pruning growth rate of trees
3.3.3 Effects of treatments on seed yield
3.3.4 Effects of treatments on harvest index
3.4 CONCLUSIONS
CHAPTER 4 RADIATION, WATER DISTRIBUTION AND PLANT GROWTH IN HEDGEROW INTERCROPPING SYSTEMS
4.1 INTRODUCTION
4.2 MATERIALS AND METHODS
4.2.1 Sampling
4.2.2 Data Analysis
4.3 RESULTS AND DISCUSSION
4.3.1 Radiation distribution
4.3.2 Water balance and dynamics
4.3.3 Plant growth
4.4 CONCLUSIONS
CHAPTER 5 DEVELOPMENT OF A HEDGEROW INTERCROPPING MODEL
CHAPTER 6 EVALUATION OF THE HEDGEROW INTERCROPPING MODEL .
CHAPTER 7 MODELLING DESIGN SCENARIOS IN ORDER TO MAXIMIZE INCOME FROM HEDGEROW INTERCROPPING SYSTEMS
CHAPTER 8 GENERAL CONCLUSIONS AND RECOMMENDATIONS
REFERENCES
