(A) Plant Growth (Initial NPP)
SCUAF does not simulate Plant Growth based on the underlying processes of photosynthesis and respiration). The User enters the Net Primary Production (NPP), for Trees and Crops, at the start of the model run, with the Soil in its present condition, called the Initial NPP. The model then simulates changes in Plant Growth as brought about by changes in Soil conditions.
Initial NPP applies only to aboveground biomass. The NPP of roots is calculated from the fraction entered in the Roots section of the worksheet.
At the User’s choice, the Initial NPP entered may represent either:
In both cases, the model will apply nutrient constraints, but the reduction in calculated growth in Case 2 is likely to be greater.
A Plant may be a Tree or a Crop. Whether a particular plant (e.g. a woody perennial plantation crop) is called Tree or Crop is entirely at the User’s discretion, so long as the names are used consistently throughout the Inputs.
The model does not presently allow for two or more Crops per year. Where this is the practice, the biomass of the successive Crops should be added, and the total entered.
All data are in Kilograms per hectare per year of Plant Dry Matter (kgDM/ha/yr). In Rotational Land Use Systems, data refer to the whole of the system, i.e. one hectare. In Spatial Land-Use Systems, the observed data for each Plant type (Tree, Crop) must be converted to values for a hypothetical hectare under the Plant. It is important to understand this Input convention.
It is important to note that, when the system is established in sloped land prone to erosion and the default NPP is modified, it may also be appropriate to change the cover factor in the erosion equation. For example, if the crop NPP is increased from 9000 to 10000 kg/ha, it may be appropriate to reduce the cover factor by, say, 0.05. This will reduce the rate of erosion.
For land-use systems which begin with clearance of natural vegetation, Year 1 can be specified as a Cutyear (in the Land-use worksheet) and the initial standing biomass can be entered in the appropriate cells, to the right of (A) in the figure above. This is allowed for trees only.
Initial Standing Biomass is expressed as kg/ha of each tree component. The default values for these variables are zeros, which assumes that the agroforestry system is being establshed on cropping (or bare) land.
The treatment of roots differs from the treatment of aboveground biomass in several respects. The user enters initial NPP of roots not as kg/ha but as a fraction. This fraction is expressed relative to the values entered for aboveground biomass NPP. So if the user entered 10000 in the NPP table and enters 0.4 for roots as fraction of aboveground biomass, the resulting root biomass is 4000 kg/ha (10000 x 0.4). This value is calculated and presented on the worksheet (with pink background) for the user's convenience, but it can only be changed indirectly by changing the root fraction.
Fine Roots are defined as those which decay within one year. Coarse Roots remain in the soil for more than one year, they are the equivalent of above-ground Standing Biomass.
The default fraction of Roots that are Coarse Roots is 0.75. Coarse Roots that remain as live Roots in a Cutyear must be entered in the Land-use system worksheet.
Some Plants, usually Trees, have deep roots which grow, and derive their nutrients, from below the Soil Depth considered in modeling. This is entered as a fraction.
In spatial agroforestry systems, Tree Roots usually extend into Soil under Crops, competing for water and nutrients. This is also entered as a fraction.
(C) Apportionment of NPP and plant composition
The NPP accumulated each year is apportioned to four plant parts. The Plant Parts are:
Apportionmnent of NPP to each plant part is entered as a fraction of aboveground biomass. Values entered for aboveground biomass parts (LEAF, FRUIT, WOOD) must add up to 1.0. Root is treated differently and is expressed as the root:shoot ratio.
Under the default values, the growth of Roots is assumed to be 40% of above-ground NPP (root:shoot ratio = 0.4), for both Trees and Crops. These values may be changed by the User. Data refer to kg dry matter per hectare per year, where referring to a hectare entirely under the Tree or Crop, following the same convention as for input of Plant Growth in the Land-use system worksheet.
Below-ground parts of plants which are harvested (e.g. tubers) should be treated as Fruit, not Root.
Plant composition is expressed in terms of the three elements C, N and P in each plant part. The C content of a plant is expressed as a fraction of dry matter (default of 0.5) and is common to all plant parts. The nutrient (N and P) contents of plant parts are expressed as percentages (not fractions) in each Plant Part at maturity.
The Nutrient content of Plant Parts when combined with NPP will determine their Nutrient Requirements.
The effects of changes in Soil properties on Plant Growth are modelled in two ways: through Nutrient Deficiencies and through Feedback Factors. Nutrient Deficiencies (Nutrient Requirements > Available Nutrients) are modelled using the Input values for Plant and Soil Nitrogen and Phosphorus, and the Nutrient Gain and Loss processes.
The Feedback Factors represent the effects on Plant Growth of changes in Soil Carbon/Organic Matter and Soil Depth. It is by means of these Factors that SCUAF takes account of Soil Physical Properties, in particular Available Water Capacity.
The default values of Feedback Factors assume that Trees, owing to their deeper and more permanent rooting systems, are affected more than Crops.
Carbon Feedback Factor
The Carbon Feedback Factor
represents the influence of change in Soil Carbon on Soil Physical Properties
and hence on Plant Growth. It is based on change in Carbon percent in the Topsoil
(assumed to be proportional to total Carbon), as compared with initial Soil
conditions. This factor can be interpreted as the answer to the question: "A
rise or fall in Topsoil Carbon of 0.1 percent causes a percent change in Plant
Growth of..."
Depth Feedback Factor
The Depth Feedback Factor represents the influence of change in Soil Depth on Plant Growth. Decrease in Soil Depth, caused by Erosion, leads to a reduction in space for Root development, and reduced Available Water Capacity. In practice, this effect is small unless the Initial Soil is shallow, or Erosion is severe. This factor can be interpreted as the answer to the question: "A change in Soil Depth of 1.0 cm causes a percent change in Plant Growth of..."