Acacia invasion triggers cascading effects above- and belowground in fragmented forests

Invasive alien plants like Acacia species are key drivers of ecosystem change, with considerable effects on forest structure, nutrient cycling, and biodiversity. In the Mediterranean region, which is already vulnerable to challenges such as anthropogenic forest fragmentation, Acacia species have become dominant invasive plants at the landscape scale. In this study, we explored the effects of Acacia dealbata Link, and Acacia melanoxylon R. Br. to a lower extent, in a fragmented forest landscape in Central Portugal. We hypothesised that Acacia invasion would alter vegetation structure, litter, soil, and springtail communities, with cascading effects on ecosystem dynamics. We established 25 sampling points within a 25 km 2 grid to collect data on Acacia invasion status, vegetation structure (cover of different plant layers and species richness), litter and soil quality (litter C/N ratio and soil organic carbon), and springtail communities (abundances of epigeic, hemiedaphic, and euedaphic springtails). We considered an Acacia invasion gradient as a continuous variable calculated with the sum of the covers of A. dealbata and A. melanoxylon divided by the total tree cover to study the combined effect of the two species. High levels of Acacia invasion were associated with reduced herb cover and plant species richness. Moreover, as Acacia invasion intensified, there was a significant decrease in the litter C/N ratio, and an increase in soil organic carbon. Subsequently, these Acacia -induced impacts triggered cascading effects on the relationships between shrub cover, litter and soil quality, and springtail functional structure. These findings showed that even low levels of Acacia invasion altered above- and belowground dynamics, thereby highlighting the cascading impacts of these invasive alien plant species on ecosystem functioning. Prioritizing early intervention in areas with smaller infestations (i.e., especially in fragmented landscapes like those in Central Portugal) can help prevent further spread and impacts of both A. dealbata and A. melanoxylon .