Testing mechanistic models of seed dispersal for the invasive Rhododendron ponticum (L.)

Rhododendron ponticum (Ericaceae) is a serious invasive alien plant in the British Isles and is of considerable conservation and economic concern. While optimal control strategies for single individuals and small stands of R. ponticum are well described, effective regional control of the plant demands an improved understanding of its spatial dynamics, in particular its dispersal ecology. Here, we describe the results of two field experiments designed to quantify the dispersal pattern of R. ponticum seeds: (1) controlled release over a few seconds at known windspeeds and (2) natural release over the peak dispersal period. We then use these results to assess the potential use of two different mechanistic wind dispersal models (WINDISPER and WALD) as descriptors of seed dispersal ecology for this species. Results from both the controlled and natural release experiments indicate that in open landscapes the vast majority of R. ponticum seeds travel less than 10 m, but that a very small proportion (0.001% in controlled trials; 0.02% in natural release) travel more than 50 m. The WINDISPER model provided the best description of seed dispersal for the controlled releases that took place over a few seconds under known windspeeds, but neither model performed well when used to predict seed dispersal from a natural stand over the peak period of dispersal. We suggest that this is due to a lack of knowledge of the exact windspeed at the time of seed release and the poor spatial and temporal resolution of the wind data available to us. The development of mechanistic wind dispersal models offers great potential for helping develop efficient control programmes for invasive alien plants, but further work to investigate the conditions under which seeds are released and the appropriate spatial and temporal resolution of wind data to use is required.