2.4 SAVANNA © Model description
2.4.1 Vegetation sub-model
In this section the main vegetation sub-models gathered from Coughenour (1993) are described: net primary production (NPP), biomass allocation and plant population. The primary production sub model belongs to the vegetation and soil sub model group. The NPP model primarily simulates light penetration through tree, shrub and herbaceous plant canopies using Beer’s law. Hence leaf areas and extinction coefficients are summed across species and tissue types. Beer’s law is used to calculate the radiation that passes through the overstory, which is then used as incident radiation in the next lower canopy layer.
Potential Transpiration rates
SAVANNA© computes potential NPP as a product of a computed rate of water use through transpiration (trans) and water use efficiency (WUE). Stomatal conductance (Cs) is calculated as
Awpet
Eflp
Rad
Eftp
T min
Efnp
Pnb
,Efsr
Sre)
Efwp Csmx
Cs day (Eq. 2)
The concept of Awpet (Eq. 2) is used rather than actual soil water content for two reasons:
First, because of weekly simulation time steps, substantial amounts of water may be added to the soil which would not otherwise be usable until the next week. Second, the amount of water that is available during a week may exceed the soil water holding capacity due to within-week cycles of rain-days followed by plant water uptake. Most importantly, scaling up to SysPet accounts for drought stress on daily stomatal closure.
Eflp is used to compute the effects of light on photosynthesis and trans as
2
The constant 0.0036 is used to convert units6. Transpirational water demand from soil water layers is distributed similarly to available soil water.
Net primary production (NPP)
The potential plant water demand of all plants rooted in a soil layer may exceed the actual quantity of available water due to the weekly simulation resolution. On a daily time step, stomata would gradually close as soil water is depleted. An approximation is derived by assuming that each plant´s competitive abilities for water are proportional to its potential rate of water use. Thus when total plant water demands in a layer exceed available water the availabe water is partitioned among competitors in proportion to their demands:
nspmx l potential water uptake. Finally, the actual NPP rate is
Nitrogen uptake
The weekly time step creates situations where the total plant demand for N can exceed the available soil N supply. An approximate solution is to apportion N uptake among species according to their demand.
Biomass allocation - herbaceous plants and shrubs
NPP of herbs and shrubs is allocated between shoots and roots. Then shoot growth is allocated among leaves, stems and reproductive tissues. Belowground net primary production (BNPP) of herbaceous plants and shrubs is calculated as
)
Prrt Efsr sre Ewal Awpet Enal Pnb Epal Phen Npp
Rtgr (Eq. 8)
The remainder of NPP is used for aboveground shoot growth (Shgr) Rtgr
Npp
Shgr (Eq. 9)
Herbaceous shoot growth is allocated to leaves in the proportion Prlf and to reproduction in the proportion Prfl, while the remainder is allocated to the stem. Hence leaf growth is
lf Shgr
Grlf Pr (Eq. 10)
while stem growth (Wdgr) constitutes the balance
lf fl
Shgr
Wdgr 1Pr Pr (Eq. 11)
Allocation of Shgr by shrubs depends on the current fraction of shoots that are leaves. As growth begins, the model assumes highest priority to allocation to leaves until the morphometric leaf:stem ratio is attained. Hereafter allocation to leaves is in the morphometric proportion
Wdgr Gbiom
Gbiom lfleaf Pr (Eq. 12)
is the current deficit of leaf mass relative to the morphometric leaf mass, and
Is the allocation of shoot growth to leaves to meet the leaf mass deficit, if any, or it is the allocation to leaves in the proportion Prlf if there is not.
Plant Population sub-models
Herbaceous plant population sizes are calculated on the basis of the number of plants per m2. There is a single plant size class; however, plant size varies in response to tissue growth and mortality. Shrub populations are modelled on the number of plants per hectare. Shrubs that do not grow beneath trees are referred to as overstory shrubs, a distinction used in this study. Shrub population dynamics are computed using a weekly time step and a single size class. However plant size is variable. Tree population dynamics are computed as the number of plants per km2. Tree cover defines the woody zone. The tree population in each grid cell subarea is divided into six classes, simulated on an annual time step. There exist three main points of connection between NPP and the plant population sub-model: seed germination, plant size determination and plant mortality. The latter two are straightforward; when plant biomass increases relative to plant number, plants must increase in size, and when plants die, live tissue biomass is converted to dead tissue biomass.
Seed germination and recruitment of shrubs is modelled in demographic terms in this approach. However, a more complex approach is needed to compute the dynamics of short-lived or annual herbaceous plants. The time lag for short-short-lived herbaceous plants associated with seed development and transfer to soil can lead to significant errors. Here, the biomass of seeds that germinate and become established is used.
Biomass-based seed germination and establishment
This sub-model tracks seeds from allocation of NPP to seeds on the plant to seeds in the soil, to establishment. Seed biomass on the plant is distinguished from seed biomass in soil (Seeds). Seeds are transferred from the shoots to the soil at a daily rate. Soil seeds also originate from an external seed source, representing seed rain from outside the study area.
Seeds that become inviable are removed from the soil seed bank. Seed germination
nsp Seeds Germrb Etgrmb T Ewgrmb Avwpet
Grinis (Eq. 14)
Newly established seed biomass is converted to leaf and root tissue in equal proportions.
The number of plants established (Estnumnsp) due to germination is )
The function Ehrbeb computes shading or other competitive effects from established plants.
Demographic seed dynamic sand establishment
The demographic model calculates establishment, which depends on the number of plants already established, abiotic factors and competition from herbaceous plants. Shrub establishment (Shestb) in a given month, from seed or vegetative spread, is
cov
Ecov of the overstory is then Shcvr Wdcvr
Ecov1 (Eq. 17)
Plant Mortality
Plant mortality is differentiated between herbs and shrubs due to differences in their time scales, absence or presence of size classes. Herbaceous plant mortality (Dthnumnsp) rate is
))
lnum Max Drwatp Awpet DrtmppT Drphnp Phen P
Mean root biomass (Rsize) per plant is a measure of herbaceous plant size,
nsp
Overstory shrub cover (Shcvr) is calculated at