## Forest Submodel

The forest plant production model (Figure 3-17) divides the tree into leaves, fine roots, fine branches, large wood, and coarse roots with carbon and nutrients allocated to the different plant parts using a fixed allocation scheme. Maximum monthly gross production is calculated as the product of maximum gross production rate (PRDX(2)), moisture, soil temperature and live leaf-area-index terms. The effect of moisture and temperature on potential productions are the same functions used for the monthly grassland model (Figures 3-8a and 3-9), while the effect of live leaf-area-index on production is shown in Figure 3-18. Plant respiration is calculated as a function of wood N content and temperature using an equation developed by Ryan (1991) and subtracted from the gross production rate in order to get the net potential production rate. The net potential production rate is not allowed to exceed the tree specific maximum net production rate (PRDX(3) times the other limiting factors). The model assumes that only the sapwood part of the tree respires C and the sapwood fraction of aboveground large wood biomass is calculated using the relationship shown in Figure 3-19 . The same sapwood fraction is used for coarse woody roots (Ryan, 1991). The leaf biomass is not allowed to exceed a maximum value that is a function of the live wood biomass (Figure 3-20). This function specifies the effect of tree allometry and structure on maximum leaf area and is potentially different for different species. Some of the important forest specific parameters include the maximum gross and net production rates (PRDX(2), PRDX(3)), the leaf area index to wood biomass relationship parameters (MAXLAI, KLAI), the sapwood to large wood C ratio parameter (SAPK), and the allocation of C into different plant parts (FCFRAC(1-5,1-2)).

### Nutrient Availability Effect upon Production

The actual production is limited to that achievable with the currently available nutrient supply with plant nutrient concentrations constrained between upper and lower limits set separately for different tree parts. Invoking Liebig's Law of the Minimum, the most limiting nutrient (ELIMIT) constrains production. The limits of nutrient content for shoot growth are a function of plant biomass in order to reflect the changing nutrient content with plant age (Figure 3-13). The user specifies the effect of live shoot biomass on maximum and minimum nutrient content (BIOMAX, PRAMN(*,*), PRAMX(*,*)).

C:E minimum, maximum =
PRAMN(E,0) + (range of PRAMN) * conversion_factor * AGLIVC / BIOMAX

The conversion factor is equal to 2.5 for leaves and fine roots, and equals 2.0 for the remaining woody biomass. The upper limit on nutrient content is based on roots and shoots only.

### Carbon Allocation

The model has two carbon allocation patterns for young and mature forests and can represent either deciduous forests or forests that grow continuously. With a continuous growth or evergreen forest the death of the live leaves is specified as a function of month (LEAFDR(1-12), `tree.100`), while with a deciduous forest the leaf death rate is very high at the senescence month. For deciduous forest the leaf growth rate is also much higher during the first month of leaf growth. Dead leaves and fine roots are transferred to the surface and root residue pools and are then allocated into structural and metabolic pools. Dead fine branch, large wood, and coarse root pools receive dead wood material from the live fine branch, large wood, and coarse root pools respectively. Each dead wood pool has a specific decay rate. The dead wood pools decay in the same way that the structural residue pool decomposes with lignin going to the slow SOM pool and the non-lignin fraction going to surface microbes or active SOM pool (above- or belowground material). The decay rates of the dead wood pools are also reduced by the temperature and moisture decomposition functions, and include CO2 losses.

### Tree Removal Events

A tree removal event, which is defined by Tree Removal Parameters in the `trem.100` file, can simulate the impact of different forest harvest practices, fires, and the effect of large scale disturbances such as hurricanes. For each disturbance or harvest event, the fraction of each live plant part lost and the fraction of material that is returned to the soil system is specified. Death of fine and coarse roots are also considered in the removal event along with the removal of dead wood. Another feature is that the nutrient concentration of live leaves that go into surface residue can be elevated above the dead leaf nutrient concentration (e.g. simulating the effect of adding live leaves to surface residue as a result of hurricane disturbance) by specifying the return nutrient fraction of the leaves to be greater than one (RETF(1,*), trem.100).

Currently tree/shrub grazing is not implemented directly. A tree-removal event (TREM) event can be used to remove aboveground biomass, but no C or N/P/S returns to the soil are made for urine or feces.

### Leaf Area Index

Calculation of the true leaf area index uses leaf biomass and a biomass-to-LAI conversion parameter which is the slope of a regression line derived from LAI vs foliar mass for slash pine. The theoretical LAI is calculated as a function of large wood mass (Waring et al., 1981). There is no strong consensus on the true nature of the relationship between LAI and stemwood mass. CENTURY version 3.0 used a negative exponential relationship between leaf mass and large wood mass, which tended to break down in very large forests. Many studies have cited a "general" increase of LAI up to a maximum, then a decrease to a plateau value (e.g. Switzer et al. 1968, Gholz and Fisher 1982). However, this response is not general, and seems to mostly be a feature of young pine plantations. Northern hardwoods have shown a monotonic increase to a plateau (e.g. Switzer et al. 1968). Pacific Northwest conifers have shown a steady increase in LAI with no plateau evident (e.g. Gholz 1982).

In Century5 and DayCent5, we use a simple saturation function in which LAI increases linearly against large wood mass initially, then approaches a plateau value. The plateau value can be set very large to give a response of steadily increasing LAI with stemwood. The effect of live leaf-area-index on production is shown in Figure 3-18. LAI parameters are BTOLAI, KLAI, LAITOP, and MAXLAI.