From Field to Forest

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This project simulates the process of natural evolution of a new stable ecosystem on an abandoned field.

  • Project name: From Field to Forest
  • Class: 4IT495 Simulation of Systems (WS 2013/2014)
  • Author: Alice Peková
  • Model type: Agent-based simulation
  • Software used: NetLogo 5.0.4

Problem definition

When a field is abandoned, it takes aproximately 150 years before it turns into a natural forest. This process called secondary succession was firstly described by F. A. Clements in 1916. In this paper a Tolearance Model of Succession is used as presented in the article Mechanisms of Succession in Natural Communities and Their Role in Community Stability and Organization written by Joseph H. Connell and Ralph O. Slatyer, which is typical for abandoned field behavior. This model of succession suggests that the change in plant species dominance over time is caused by competition for resources. Later species are able to tolerate lower resource levels due to competition and can grow to maturity in the presence of early species, eventually out competing them.

The goal of the project is to simulate the process of secondary succession on an abandoned field. The simulation should answer the question how the initial fertility of the field influences the whole process. The sources used are soil fertility and sunshine available. The model is set in central-european climate, which implies that the ecosystem reached with the succession should be a mainly deciduous forest with oaks prevailing.

Method

The succession is a complex process with number of mutual dependancies. Two essential viewpoints can be taken. The focus can be put either on the feedbacks between various plants, soil and the envirnment, or on the individual behavior of each plant in the changing environment.

The results of feedback loop analysis of the process of secondary succession was presented for example in the article Feedback dynamics analysis of secondary successional transients in ecosystems written by Luis T. Gutierrey and Willard R. Fey.

I have chosen the latter approach, as there are many possibilities of individual plant's reactions. For this approach an agent-based simulation is appropriate where each plant is percieved as an agent with specific requirements and behavior. The NetLogo software was used for its simplicity and easy data analysis.

Model

The species listed below are gathered from a diploma thesis Sekundární sukcese na opuštěných polích v pahorkatině jižní Moravy (Secondary Succession on Abandoned Fields in a Hilly Landscape of South Moravia) by M. Sojneková.

Agents and environment

The model uses 4 breeds af agents (turtles) - grasses, herbs, bushes, and trees. The characteristics of the environment are saved as variables of patches. The initial average fertility of the field is stored as a global variable and can be adjusted by the user before running the simmulation. The patches also store number of seeds in the soil for each type of turtles.

The agent characteristics are described in variables as follows:

  • rfertility: sets the minimum soil fertility requirements of the plant. If the actual fertility of the patch is lower, the plant dies.
  • rsunshine: sets the minimum requirement for exposure to the sun. If the actual exposition on the patch is lower, the plant dies. The exposition is influenced by bushes on the same patch and trees on the same patch and in the neighborhood.
  • death-ratio: sets the percentage of plants dying every winter.
  • spread-ratio: sets the number of seeds which the plant produces and which can sprout and grow. The plant can spread on the same patch or on the neighboring patches.
  • age: stores the current age (in years) of the plant. The age change every step of the simulation.
  • max-age: sets the maximum age the plant can reach. When the plant gets older, it dies.

The agent characteristics are typical for each type of turtles, however it is randomized in order to refer more accurately to the reality. The characteristics are set as a value of normal distribution with mean value typical for the type of turtle and standard deviation set as a fraction of the typical value (in case of rfertility, rsunshine and max-age) or as a constant (in case of death-ratio and spread-ratio). The age is naturally set as 0.

The characteristics of environment are stored for each patch seperately. One patch represents 1m2 of the soil. The characteristics are as follows:

  • local-fertility: percentage of the humus in the soil. The local fertility changes over time according to plants living on the soil and rotting dead plants. The initial value is set as randomized user-selected fertility.
  • sunshine: percentage of sunshine that reaches the ground. The initial values is 100 (no shade is produced), but changes over time according to number of trees on the patch and on the neighboring patches and number of bushes on the patch.
  • seeds-grass: number of grass seeds on the patch. Every spring one half of the seeds manage to sprout.
  • seeds-herbs: number of herbs seeds on the patch. Every spring one half of the seeds manage to sprout.
  • seeds-bushes: number of bushes seeds on the patch. Every spring one quarter of the seeds manage to sprout.
  • seeds-trees: number of trees seeds on the patch. Every spring one fifth of the seeds manage to sprout.

The seeds that don't manage to sprout in the spring stay in the soil and may sprout next year or any year after. The initial number of seeds on a patch is generated randomly on given percentage of patches. This simulates the fact that when a field is abandoned, there is significant number of seeds on the soil.

Grasses

The grasses represent annual plants, such as Melica uniflora (wood melica), Poa nemoralis (wood bluegrass) and Brachypodium pinnatum (tor-grass).

The initial values of variables:

  • rfertility: 10
  • rsunshine: 50
  • death-ratio: 43.5
  • spread-ratio: 36
  • max-age: 1

Herbs

The herbs represent mostly perrenial herbs, such as Bellis perennis (daisy), Taraxacum officinale (dommon dandelion) and Convallaria majalis (lilly of the valley).

The initial values of variables:

  • rfertility: 10
  • rsunshine: 50
  • death-ratio: 44
  • spread-ratio: 27
  • max-age: 8

Bushes

The bushes represent mainly pioneering woods, such as Crataegus monogyna (single-seeded hawthorn), Betula pendula (silver birch) and Picea abies (spruce).

The initial values of variables:

  • rfertility: 10
  • rsunshine: 58
  • death-ratio: 20
  • spread-ratio: 27
  • max-age: 50

The bushes produce shade when older than 5 years. This shade consumes 10% of sunshine reaching the ground on the same patch.

Trees

The trees represent mainly deciduos trees typical for central europe oak forests, such as Quercus petraea (sessil oak) or Acer campestre (field maple).

The initial values of variables:

  • rfertility: 20
  • rsunshine: 40
  • death-ratio: 10
  • spread-ratio: 27
  • max-age: 150

The bushes produce shade when older than 10 years. This shade consumes 15% of sunshine reaching the ground on the same patch. Trees older than 20 years produce shade which consumes 25% of sunshine reaching the ground on the same patch and 15% of the sunshine reaching the ground on the neighboring patches.

Model dynamics

Before the model run is started the initial values are set: global fertility set by user is randomized for each patch. Seeds are generated for some patches (80% of patches hold grass and herbs seeds and 40% of pathces hold bush and tree seeds). When the model run starts, the first spring come and then the seasons follow for user-set number of years.

  • Spring: The seeds sprout (i. e. the turtles are initiated). However a lot of sprouts may die. When a sprout manage tu survive, it decreases its likelihood to die a little.
  • Summer: All the plants grow (some of them may die, survivales decrease their death-ratio), grasses and herbs produce new seeds according to their spread-ratio.
  • Autumn: All the plants grow (as in spring and summer), bushes older than 3 years and trees older than 5 years produce seeds.
  • Winter: All the grasses die, 2/3 of herbs die, other plants die according to their death-ratio. The survivals decrease their death-ratio slightly.

When growing, the plants decrease the local fertility (more when older). When dying, the plants increase the loceal fertility (also more when older). Anytime during the year a plant dies when the fertility and sunshine requiremnts are not met.

User interface

Before running the simulation the user can adjust the initial fertility of the soil and set the number of yoers for which the simulation will be run. When hitting the SETUP button, the values are initiated. When hitting the GO button the simulation starts. The data is recorded and displayed every autumn. However the monitors in the left side of the window (counting each type of turtles) may update continuously. The graphs in the right side of the window show the evolution of numbers of grasses, herbs, bushes and trees, average fertility and sunshine exposure and evolution of nubers of bushes older than 5 years and trees older than 15 years. The latest indicates the maturity of the ecosystem.
Picture of the user interface displaying the end of the simulation run. The run lasted 28 years and the initial fertility was set at 20%.

Model limitations

The process of succession is simplified

Results

Citations

  • CONNELL, Joseph H.; SLATYER, Ralph O. Mechanisms of succession in natural communities and their role in community stability and organization. American naturalist, 1977, 1119-1144. [1]
  • GUTIERREZ, Luis T.; FEY, Willard R. Feedback dynamics analysis of secondary successional transients in ecosystems. Proceedings of the National Academy of Sciences, 1975, 72.7: 2733-2737. [2]
  • SOJNEKOVA, M. Sekundární sukcese na opuštěných polích pod Děvínem v Pavlovských vrších, 2011, Ms., dip. pr., PřF MU, Brno. [3]

Conclusion

Code and results files

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