Spatially-explicit, individual-based plant competition model

Peter Stoll , Jakob Weiner (KVL, Denmark), Helene Muller-Landau (Princeton University, USA), Amal Jasentuliyana (State University, New York), Elke Müller (University of Bern), Toshihiko Hara (Hokkaido University, Japan)

Zone of Influence model

We have developed a spatially-explicit, individual-based plant competition model based on overlapping zones of influence. Plants are modelled as circles growing in two dimensions. The area of the circle represents resources available to the plant, and is allometrically related to its biomass. Plants compete for resources in areas in which they overlap. The size-asymmetry of competition is reflected in the rules for dividing up the overlapping areas: in asymmetric competition the larger individual obtains all the resources, in symmetric competition areas of overlap are divided equally among competing individuals. The computer code for the model is freely available to researchers (ZOIcode.txt). We ask only that the program be acknowledged if results are published.

We have two research projects underway using this model:

• The effects of density, spatial pattern and competitive symmetry on size variation in simulated plant populations.
  Theoretical plant populations were grown in random and in perfectly uniform spatial patterns at four densities under size-asymmetric and size-symmetric competition to investigate the potential contribution of spatial pattern and competitive size asymmetry to size variation in plant populations.
  
• Effects of size-asymmetric competition and morphological plasticity on plant biomass-density relationships.
  
  We use the model to investigate the effect of size-asymmetric competition on density-dependent mortality (self-thinning), and we test the predictions with experiments with normal and "far-red blinded" Arabidopsis thaliana populations. "Far-red blinded" plants show reduced morphological plasticity and more size-asymmetric competition.

Keywords: power law - shade avoidance - spatial effects

Publications

Stoll P & Bergius E (2005) — Pattern and process: competition causes regular spacing of individuals within plant populations — Journal of Ecology 93: 395–403

Stoll P, Weiner J, Muller-Landau H, Müller E & Hara T (2002) — Size symmetry of competition alters biomass-density relationships — Proceedings of the Royal Society of London B - Biological Sciences 269: 2191–2195

Weiner J, Stoll P, Muller-Landau H & Jasentuliyana A (2001) — The effects of density, spatial pattern, and competitive symmetry on size variation in simulated plant populations — American Naturalist 158: 438–450