Disturbances

Overview

A disturbance is any physical event that disturbs the growth of the plant community. "Physical" because disturbances do not include environmental events like droughts which can also kill plants.

There are three types of disturbances in STEPWAT2:

1. Fire
2. Grazing
3. Animal disturbances

Fire

Fire causes a complete killing of all above-ground biomass for every species in the cell. Some species, most notably grasses, can be allowed to recover a proportion of their biomass following a fire by setting the proportion recovery input parameter to a value greater than 0.

There are two mutually-exclusive fire regimes: prescribed and cheatgrass-driven.

Prescribed fire

Prescribed fire is a way of simulating fires started by humans for rangeland management purposes. The input parameters responsible for prescribed fire are located in the resource group input file. Because it is defined in the rgroup file, it is possible to cause a fire for some resource groups but not others. It might seem unintuitive to allow a fire to affect some plants in a plot while leaving others alone, but this can reflect a real-world scenario if you are running a simulation including trees.

There are two mutually exclusive types of prescribed fire.

Stochastic prescribed fire

Stochastic prescribed fire occurs randomly based on a percentage specified in inputs.

Periodic prescribed fire

Periodic prescribed fire occurs on a schedule specified in inputs.

Cheatgrass-driven wildfire

Cheatgrass-driven wildfire assumes that cheatgrass (brte) is the main determinant in the wildfire cycle. In order for this option to work correctly cheatgrass must be one of the species included in the species input file.

The algorithms and constants behind cheatgrass-driven wildfire come from Balch et al and Whisenant et al.

Grazing

Grazing simulates large herbivores eating a portion of the above-ground biomass of the plants in the cell. The amount of biomass consumed is defined at the rgroup level and can be customized in the rgroup input file. The frequency of grazing can be either stochastic or periodic and is also defined in the rgroup input file.

End of Year Grazing

grazing_EndOfYear() is called every year before stat_collect(). This simulates grazing on the individuals of each resource group. There are three functions regarding grazing, and are called according to the following: grazing_EndOfYear() -> Species_Proportion_Grazing() -> indiv_proportion_Grazing().

Functions

grazing_EndOfYear() cycles through each resource group, making sure first that the current year is past the grazing start year, and then cycles through all of the species in that group, accumulating the grazing for each Species using Species_Proportion_Grazing(). Species_Proportion_Grazing takes Species[sp]->extragrowth and multiplies it by proportionGrazing to calculate how much of the extra growth for the year has been grazed. Then, the function cycles through each individual plant in that species and calls indiv_proportion_Grazing() to simulate grazing on each plant. indiv_proportion_Grazing() calculates grazing on each plant by taking each individual's normal growth and multiplying it by the grazing proportion. This grazing loss is subtracted from the individual's normal growth.

Grazing Flag and Output

Grazing is determined by grazingfrq. If grazingfrq > 0, grazing will occur. Grazing is accumulated for each resource group by the _Grazed variable in Stats.c, but the data will not be outputted unless specified by the graz flag in the bmassflags.in file. Grazing output is the amount of biomass removed by livestock each year for each plant functional type. Output is written separately for each plant functional type as a separate column in the bmass output files, denoted with “_graz”.

Associated Variables

• grazingfrq - Perform grazing on this group at this frequency. Set in rgroup.in (nongridded) or grid_disturbances.csv (gridded)
• grazingfreq_startyr - Start year for grazing based on the grazing frequency. Set in rgroup.in (nongridded) or grid_disturbances.csv (gridded)
• proportion_grazing - Proportion of biomass removed during grazing year. Set in rgroup.in (nongridded) or grid_disturbances.csv (gridded)
• res_grazed - Resource group biomass removed by livestock grazing
• graz - If TRUE output grazed biomass. Set in bmassflags.in
• normal_growth - biomass the plant gained this year excluding superfluous biomass.

Animal disturbances

Animal disturbances represent animals disturbing the soil and preventing plants from growing. These disturbances can be turned on and customized in the environment input file

There are three types of animal disturbances:

Ant mounds

Represent an ant colony building a mound on the plot.

Fecal pats

Represents an animal pat on the plot.

Animal Burrows

Represents an animal burrowing on the plot.