NetLogo User Community Models

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## WHAT IS IT?

This model explores how an individual-based model of forest dynamics can be linked to soil production. Rules for seed production and root growth are based on resource competition between neighbors. Soil is produced wherever tree roots reach the soil-bedrock interface.

## HOW IT WORKS

This is simpler version of the model tree_soil_pro.

This model of forest dynamics is based on the notion that individual trees compete with their neighbors for resources (e.g., light, water, nutrients). Seeds are produced, saplings are probabilistically recruited, roots grow, and individual trees die. If production and recruitment are too slow, then the population will go extinct. Unlike tree_soil_pro, all individuals live to their maximum lifespan. This means that forest density will grow until the recruitment rate is balanced by the death rate. The key attribute driving model dynamics is that each individual tree onl has fractional resources available (from 0 to 1) due to competition among neighbors. This RESOURCES term assumes size-symmetric resource allocation within a 5-patch radius. In general, individual trees grow fastest when they are young and slow down as the age. This model uses a parabolic growth function that is scales by resource availability. Reseource availability is also used to modulate the number of seed produced by individuals of reproductive age (5% of lifespan). To directly increase the probability of seed germination and sapling recruitment, the user can vary two terms that represent the maximum number of seed produced and a probability of survival. Because most seeds to not germinate and survive, and there is a cost to generating large numbers of agents that are destined to die, these numbers should not be directly compared to actual seed production rates.

## HOW TO USE IT

To run the model using default parameters, first press SETUP to initialize the model and press GO to start the simulation. Each model tick is intended to represent 1 year.

There are six parameters the user can explore using the sliders:
INITIAL-NUMBER-TREES randomly places trees of age 1 into the model domain.

TREE-ROOT-MAX sets the maximum rooting depth an individual tree can obtain if it grows its entire life with a resource availability of 1.

TREE-AGE-MAX sets the lifespan of each individual tree in years. Note that, in alternative versions of this model, trees can die before they reach this age.

SEED-DISPERSE sets the radius of seed dispersal in units of patches.

SEED-FECUNDITY sets the probability of seed germination and sapling recruitment and varies from 0 to 1. The actual probability of recruitment is the complement of this value. In other words, lower values lead to higher recruitment rates.

SEED-MAX set the number of 'seeds' produced by a tree of reproductive age when full resources are available. When resource vailability is less than 1, the actual number of seeds will be probabilistically set to an integer value less than this parameter.

Each of these parameters is set using the sliders in the interface tab.

## QUESTIONS TO ASK

Does the forest achieve any sort of equilibrium? If so, how fast does it achieve this state? If not, why not?

Does the soil achieve any sort of equilibrium? If so, how fast does it achieve this state? If not, why not?

Which parameters values favor higher average resource availability? How is this related to forest and soil properties

What dynamics is this model missing?

## CREDITS AND REFERENCES

https://github.com/mwrossi/Netlogo-Landlab-clinic