The relative contribution of wind, seed traits and demography to the spread of an invasive conifer: Results from high-resolution modelling

Background/Question/Methods

Amongst the world’s invasive plant species, those of the genus Pinus are widespread and conspicuous. In many parts of the world pine trees were planted for production purposes or erosion control, and are now spreading well outside the plantations, threatening biodiversity, ecosystem function, and landscape values. Yet, little is known about the relative importance of mechanistic drivers of spread, particularly when long distance dispersal (LDD) events occur, as they affect spread significantly while being difficult to measure. A wide array of methods exist that assess LDD, or spread, but no analytical method exists to date that combines mechanistic dispersal in a spread model. Here we integrate two models of mechanistic wind dispersal and plant spread to determine the relative effects of dispersal and demographic traits and wind statistics on the spread of an invasive pine. We focus on a New-Zealand mountain where the wind-dispersed Corsican pine (Pinus nigra ssp. laricio), planted around 1910 has since spread to neighbouring grasslands and scrublands. Our models are parameterised using data collected in situ and tested using independent data on historical spread. We carry out a full sensitivity and elasticity analysis of spread rate to demographic, seed trait and wind variables. This method allows us to test (i) the effect of location and wind direction on the spread of Corsican pine, and (ii) the relative importance of demographic and dispersal variables for spread rate.

Results/Conclusions

Predicted spread rates and direction match strikingly with historical spread. Seed terminal velocity has the greatest potential influence on spread rate and three wind properties -turbulence coefficient, mean horizontal wind speed and standard deviation of vertical wind speed- are also important. Fecundity has marginal importance for spread rate, but juvenile survival and establishment are consistently important. This coupled empirical/theoretical framework enables prediction of plant spread rate and direction using fundamental dispersal and demographic parameters. It also identifies the traits and environmental conditions which facilitate spread – a promising method when, around the world, tree species distributions are changing dramatically.