Tropical ash (Fraxinus udhei) invading Andean forest remnants in Northern South America

Introduction: Exotic invasive species represent a major driver of the loss of biological diversity and services provided by ecosystems globally. An important cause of species becoming invasive is the development of commercial afforestation projects using fast-growing and adaptable exotic species, which may become invasive impacting natural environments. The tropical ash (Fraxinus uhdei), native to Mexico, has been widely introduced to Colombia and other countries for timber, live fences, and urban greening. It is now common in many areas of the tropical mountains and has been observed expanding into native forests in Colombia. This study examined the invasion pattern and the demographic structure of tropical ash in remnant Andean forests in Colombia, to find out factors associated with the invasion process. The analysis took into account biotic and physical factors: distance to propagule source, vegetation cover types, and understory and canopy cover. Results: Although the reproductive strategy of the tropical ash is of r type with low survival rates, the results show evidence for an active process of invasion characterized by an aggregated distribution pattern dominated by seedlings and juvenile plants. The study shows the existence of an interaction between the distance to propagule source and the vegetation cover, which has an additive effect on the demographic structure of the population. Conclusions: This study suggests an active invasion process by the exotic tree tropical ash into a remnant of sub-humid forests in the Andean highlands. Our results showed an expanding population process with a decreasing average age from propagule sources and mature reproductive individuals within the native forest, which bring new individuals that are colonizing new areas and advancing into the forest. This research points to the need for urgent plant intervention on two aspects: the use of the tropical ash and the management of the invasion process in native forests.