After an off-season fire: the behavior of exotic Eucalyptus globulus and invasive Acacia longifolia in Portugal

Abstract

The introduction of species into exotic areas has increased, although it results in serious impacts on novel ecosystems. In Portugal, Eucalyptus globulus (exotic species) and Acacia longifolia (exotic invasive species) occupy a vast forested area. Eucalyptus globulus was extensively planted due to its role in pulpwood industries and, while the majority of plantations are managed, some are poorly managed along with isolated trees dispersed in the landscape (seed-trees) that are very huge and old, potentially increasing the risk of dispersal. Acacia longifolia was introduced for dune stabilization, but quickly expanded, becoming invasive without human intervention. As a leguminous species, the ability to establish symbiosis with nitrogen-fixing bacteria seems to be crucial to potentiate this invasiveness; however, these mutualistic interactions will interfere with soil microbiota, altering plant communities and affecting local biodiversity. Being two species adapted to post-fire regeneration, their behavior changes after fire occurrence, however after off-season fires, there is a gap in knowledge about their establishment dynamics. Eucalyptus globulus plantations and surrounding areas of seed-trees affected by June and October 2017 fire events were sampled, as well as unburnt areas. Acacia longifolia root-nodules were collected from unburnt and burnt areas affected by 2017 October fire and bacterial community was isolated and identified. Our results showed that the fire date and pre-fire management restrained E. globulus natural regeneration, with greater establishment in unmanaged plantations affected by the October fire. The presence of seed-trees seems to be less influenced by these factors (fire date and management), and can be considered an important seed source. Also, A. longifolia bacteriome has lower diversity after fire, but the main symbionts seem to be nitrogen-fixing bacteria, indicating a more specialized symbiosis that could enhance post-fire invasion. Bradyrhizobium spp. were the main partners in both studied zones, revealing its role as a facilitating microorganism. Off-season fires specific conditions seem to create more favourable conditions for E. globulus establishment, while A. longifolia seems to be able to establish promiscuous symbioses, but simultaneously adapt to a disturbed environment, managing to outcompete effectively with other plant species