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Keywords:

  • Fynbos Proteaceae;
  • long-distance seed dispersal;
  • model aggregation;
  • phase II dispersal;
  • process-based model;
  • relationship between dispersal distance and seed size;
  • secondary wind dispersal;
  • seed lift-off velocity;
  • tumble dispersal;
  • validation experiment

Summary

  • 1
    Secondary seed dispersal by wind, the wind-driven movement of seeds along the ground surface, is an important dispersal mechanism for plant species in a range of environments.
  • 2
    We formulate a mechanistic model that describes how secondary dispersal by wind is affected by seed traits, wind conditions and obstacles to seed movement. The model simulates the movement paths of individual seeds and can be fully specified using independently measured parameters.
  • 3
    We develop an explicit version of the model that uses a spatially explicit representation of obstacle patterns, and also an aggregated version that uses probability distributions to model seed retention at obstacles and seed movement between obstacles. The aggregated version is computationally efficient and therefore suited to large-scale simulations. It provides a very good approximation of the explicit version (R2 > 0.99) if initial seed positions vary randomly relative to the obstacle pattern.
  • 4
    To validate the model, we conducted a field experiment in which we released seeds of seven South African Proteaceae species that differ in seed size and morphology into an arena in which we systematically varied obstacle patterns. When parameterized with maximum likelihood estimates obtained from independent measurements, the explicit model version explained 70–77% of the observed variation in the proportion of seeds dispersed over 25 m and 67–69% of the observed variation in the direction of seed dispersal.
  • 5
    The model tended to underestimate dispersal rates, possibly due to the omission of turbulence from the model, although this could also be explained by imprecise estimation of one model parameter (the aerodynamic roughness length).
  • 6
    Our analysis of the aggregated model predicts a unimodal relationship between the distance of secondary dispersal by wind and seed size. The model can also be used to identify species with the potential for long-distance seed transport by secondary wind dispersal.
  • 7
    The validated model expands the domain of mechanistic dispersal models, contributes to a functional understanding of seed dispersal, and provides a tool for predicting the distances that seeds move.