Garden with Insight v1.0 Help: Plant next day functions: calculate damage from frost and short day length

It is important to understand the difference betwen growth constraints and damage. Growth constraints reduce the amount of new plant biomass created during photosynthesis, but don't touch any existing plant biomass. Plant damage actually kills existing plant biomass. Plant damage can happen for any of four reasons: cold (frost), short day length, water stress when the plant is senescent and inflexible, and pulling off of leaves and other parts. We will deal with the first two damages here.

You've seen plant damage from frost: parts of leaves turn brown and shrivel up, or if the frost is very bad whole branches fall down and seem to disappear. We calculate a frost damage factor from zero to one with an S curve based on the minimum (night) temperature. The curve is adjusted with S curve parameters in the plant. Unlike the growth constraints, the frost damage factor is worst at 1.0 because it is used to calculate a subtraction from plant biomass. Notice that the model doesn't differentiate between frost and freeze, so this could best be called simply "cold damage".

The day length damage factor is a little harder to understand. The calculation of radiation refers to day length, since it calculates deviation from the maximum possible radiation for the day which incorporates day length. So shouldn't the effect of reduced day length be apparent in the reduced solar radiation? The response of plants to decreased day length isn't externally caused, but externally triggered. Reducing day length is a sign to the plant that it should start to shut down its processes. Annual plants lose flexibility and efficiency (the same way we do), and perennial plants close down and prepare for storage. So the day length damage factor modeled here is more of a blanket shutdown factor as triggered by decreasing day length than the effect of day length per se. Actually, EPIC only uses these damage factors for perennial plants (since annual plants are always harvested before they get to this point), but we wanted to simulate the fate of annual plants left in the garden.

After the frost and day length damage factors are calculated, the worst of the two (the higher one) is used to calculate plant damage.With this worst plant damage factor we can calculate the reduction in plant biomass. The amount of biomass lost is multiplied by the quantity (1.0 - HUI) to confine most of this damage to smaller plants -- new seedlings are the most vulnerable. Finally, the amount of biomass lost is limited to half the shoot biomass of the plant, which represents some physiological resistance to shoot death.

Also, the cold and day length damage factor reduces the leaf area index (LAI) by the same factor shoot biomass was reduced (but without the limit to half the shoot biomass). This simulates the effect on LAI of the loss of damaged leaves.

calculation of potential biomass increase, actual new plant biomass, damage from water stress, radiation
EPIC Winter Dormancy (includes frost and day length damage)
Model contents