Grid-Cohort Framework

<to do: introduction>

Features of the Framework

The highest level element in the grid-cohort framework is the grid, containing multiple grid cells. Each grid cell contains one or more plant-soil systems (models) which are associated with unique landscape elements. Each landscape element contains one or more cohorts. No assumptions are made as to the shape of the grid nor the contiguousness of the cells.

A landscape element is defined by its topographic parameters: elevation, slope, and aspect. A landscape element is not restricted to specific geographic coordinates, and may comprise non-contiguous geographic areas within a grid cell. Landscapes do not overlap and are completely contained by their cell. The sum of the areas occupied by the landscape elements equals the area of the grid cell.

A cohort is uniquely identified by its combination of landscape element owner, plant functional types, soil type, age since it was last disturbed, and the fraction of the landscape element area that it occupies. A cohort is not restricted to specific geographic coordinates within the grid cell, and can represent either a contiguous geographic unit or an aggregation of geographic units. Cohorts are contained completely within a landscape.

Each grid cell exchanges fluxes with the atmosphere. Incoming fluxes are distributed to each cohort proportional to the area of the cohort. Outgoing fluxes are the aggregation of individual cohort fluxes.

The following algorithms operate upon the cohorts within a grid cell:

• Disturbances "reset" a cohort's plant and soil properties, and create new cohorts. The disturbed cohort is divided into a new cohort and a remaining area of the original cohort, based upon the area of disturbance.
• Succession rules determine the response of a cohort after disturbance.
• Aggregation - combine similar cohorts base upon time since last disturbance.
• Collection and distribution of fluxes across the cohorts within a cell.

The actual implementation of the disturbance, sucession, aggregation are supplied in the software which uses the framework. The framework only provides the "plug-in" points for these algorithms.

The elements of a grid cell are:

Identification Number

Each grid cell has a unique identification number. The identification number is assigned during the creation of a grid cell when the simulation is initialized.

Geography

Each grid cell requires geographic information consisting of latitude and longitude of the northwest corner of the cell, and the area of the cell in kilometers. The geography data is constant, and cannot be modified during the cell's lifetime.

List of Landscape Elements

Each grid cell has list of landscape elements. The list is provided by the initialization data, and cannot be modified during the cell's lifetime.

List of Cohorts

Each grid cell has a list of cohorts which it contains and manages. A list of the initial cohorts is provided by the initialization data. The list is dynamic, and is modified when cohorts are added, divided, or combined, according to the implementations of the disturbance, aggregation, and succession algorithms.

The major elements of a landscape element are:

Identification Number

Each landscape element has an identification number that is unique within a grid cell. These numbers are not modified during the lifetime of the grid cell. The identification number is by the initialization data.

Area Fraction

Specifies the fraction of the grid cell area that is occupied by this landscape element. The area may represent either a contiguous geographic area, or the total area occupied by non-contiguous geographic elements within the grid cell.

Topography

Specifies values describing the topography for the landscape element. Each landscape element has its own topography which is unique within the grid cell. Topography a range of slope, aspect, and elevation. The topography is constant, and so cannot be modified after the landscape element is created.

The major elements of a cohort are:

Identification Number

Each cohort has an identification number that is unique within a grid cell. Though cohorts may be created and deleted during the coarse of a simulation, the identification numbers are not reused during the lifetime of the grid cell. The identification number is assigned during the creation of a cohort. The numbers for the initial cohorts in a cell are provided by the initialization data.

Landscape Element Number

This is a reference to the landscape element to which the cohort belongs. The value is constant for the lifetime of the cohort.

Time of Creation

Specifies the simulation time at which the cohort was created. The age of the cohort is the time since the cohort was created to the current simulation time. When disturbance creates a cohort, the original cohort's time of creation is not affected.

Area Fraction

Specifies the fraction of the landscape element area that is occupied by this cohort. A cohort's area may represent either a contiguous geographic area, or the total area occupied by non-contiguous geographic elements within the landscape element.

Cohort Algorithms

There are two algorithms fundamental to the management of cohorts within a grid cell, disturbance and aggregation. These are triggered by the disturbance and combine events during the course of a simulation.

Disturbance

The primary effect of a disturbance event is to create a new cohort from a portion of an existing cohort. A disturbance event specifies the fraction of a cohort's area that will be transformed into a new cohort. The remaining area remains as the original cohort.

Additional information provided with a disturbance event is the plant, soil, and canopy types for the new cohort, and the time of disturbance (in simulation time units).

Since each cohort has a unique identification number within a grid cell, the new cohort resulting from the disturbance will be assigned its own number. This number is determined from a grid cell algorithm which tracks assigned numbers, and provides a sequential number that has not yet been previously assigned during the grid cell's lifetime.

The particular actions effected by a disturbance are specific to each disturbance, however, all disturbances perform the following actions:

• A new cohort is created from an existing cohort, whose area is specified in the disturbance object. The units of the area are the fraction of the parent cohort's area which will be reapportioned to the new cohort. The area of the parent cohort is set to the remaining amount.
• The new cohort is given the simulation time of the disturbance. This information is provided with the disturbance.
• The new cohort is given plant, soil, and canopy biophysics properties specified in the disturbance object or determined from the succession rules.
• The new cohort is of the same landscape element as the parent.
• The new cohort is given an identification number which is unique within the grid cell.

<to do: Implementation of the disturbance algorithm in the IRC models.>

Aggregation

A combine event will aggregate multiple cohorts into a single cohort. The minimum criteria for combining is (1) the cohorts must be exist in the same landscape element, (2) both cohorts must have a minimum simulation time since the cohorts were last disturbed, and (3) cohorts must have similar plant and soil types. The implementation of the aggregation algorithm determines what is an allowable variance for a pair cohorts such that they can combine.

When cohorts are combined, the following actions are performed:

• Pools are summed.
• A new simulation time is calculated based upon the mean elapsed time since the cohorts were last disturbed.
• The plant objects are combined (according to the implementation algorithm).
• The soil objects are combined (according to the implementation algorithm).
• The lowest cohort identification number is retained, while the higher numbers are discarded.

<to do: Implementation of the aggregation algorithm in the IRC models.>