Influence of Landscape Characteristics on Wind Dispersal Efficiency of Calotropis procera

Abstract

Rubber bush (Calotropis procera), a perennial invasive milkweed, infests large swathes of pastoral land in northern Australia and Queensland, diminishing pasture productivity. The seeds of rubber are small with fluffy pappi that confer buoyancy during wind dispersal. Long-distance seed dispersal (LDD) by wind is dependent in part on seed terminal velocity, the height of release above the ground, the surrounding vegetation, and wind parameters such as speed and vertical turbulence. Using empirical dispersal data, spatial population distribution, and historical knowledge of three experimental sites, we examine how seed traits can interact with environmental features to promote dispersal. We expected naturalised rubber bush populations to have the following: (1) higher spatial autocorrelation on open plains where dispersal distances are maximised compared to hilly habitats or those with tall vegetation; (2) southeast to northwest directional bias aligned to prevailing winds; and (3) patchy satellite populations ahead of an infilled continuous main front. Seed dispersal kernels were estimated by releasing seeds from dehiscent fruit for four periods of ten minutes each at three locations from a fixed height while monitoring wind speed. Five alternative models were fitted to the seed dispersal data, of which the log-logistic (Kolgomorov–Smirnov test p = 0.9998), 3-parameter Weibull model (K-S p = 0.9992), and Weibull model (K-S p = 0.9956) provided the best fit in that order. Stem size distribution was similar at the leading edges of populations at all sites (F10, 395 = 1.54; p = 0.12). The exponential semivariogram model of the level of spatial autocorrelation was the best fit and was adopted for all sites (Tennant Creek (TC), Helen Springs (HS) and Muckaty (MU) sites (R2 = 63.8%, 70.3%, and 93.7%, respectively). Spatial autocorrelation along the predicted southeast-to-northwest bearing was evident at all sites (TC kriging range = 236 m; HS = 738 m and MU = 1779.8 m). Seed dispersal distance was bimodal and dependent on prevailing wind conditions, with short distance dispersal (SDD) up to 55 m, while the furthest propagules were 1.8 km downwind in open environments. Dispersal directions and distances were pronounced on plains with short or no vegetation, compared to hilly locations or areas with tall vegetation. In designing management strategies, it should be noted that invasion risk is greater in frequently disturbed open landscapes, such as pastoral landscapes in Northern Australia. Infestations on open xeric grassland plains with shrubby vegetation should be a priority for rubber bush control to maintain high levels of productivity in beef production systems.