Is the invasive Rhododendron ponticum L. an emergent threat to mainland Atlantic forests? A population dynamics approach

Abstract

Understanding the population dynamics of invasive species is essential for identifying the key parameters of their success and guiding management actions. European temperate forests of the Atlantic domain are prone to be invaded by the Pontic Rhododendron (Rhododendron ponticum L. subsp. baeticum (Boiss. & Reut.) Hand.-Mazz.), a shade-tolerate evergreen shrub native to the Iberian peninsula. While the invasion has a long history in Ireland and the British Isles, it is an emerging threat in mainland Europe. It is therefore a timely challenge to better understanding the invasive dynamics of this species in this context. We studied the population dynamics of R. ponticum in 27 invaded forests of north-western France, examining population structure, and individuals’ growth, fertility and fecundity among contrasted light conditions. All invasive populations were found on acidic soils. Irrespective of light conditions, R. ponticum exhibited a regular radial growth, with a clear increase since the 1990s. The majority of stems were younger than 38 years on average, indicating a recent increase in population density. Light availability strongly impacted R. ponticum’s canopy structure, with higher canopy cover and stem numbers under sun conditions. Despite the production of a huge number of viable seeds, the observed number of seedlings was always very low irrespective of light conditions, indicating recruitment limitation and thus a limited ability to establish new, distant populations. In contrast, vegetative propagation via intense layering at the invasion front wave ensured local population spread from initially planted individuals, especially on moist soils. We conclude that where it has been planted, R. ponticum is able to actively layer, thereby forming dense bushes in the forest understories as long as the soil is acidic and relatively moist. The establishment of new, distant populations appears a rare event since recruitment is limited to particular microhabitats related to forest management-associated disturbances. Such disturbance should thus be avoided in sites where the species has already established, unless a post-disturbance eradication of R. ponticum’s seedling can be implemented the following years. The plantation of R. ponticum in forest must be avoided when the soil is acidic and moist. Because R. ponticum’s growth performances are increasing since the 1990s, as a likely consequence of climate change, monitoring established populations is recommended to detect a possible increasing invasiveness.