The Genetics and Ecology of Post-Fire Eucalyptus globulus Recruitment in an Isolated Stand in Central Portugal

Abstract

Eucalyptus globulus Labill. is a widespread forest tree species, reported as naturalized across the introduced range, often showing abundant natural regeneration after wildfires. This paper studies a post-fire cohort of seedlings derived from a small, isolated E. globulus stand in central Portugal. The aim is to better understand the genetic dynamics and dispersal mechanisms of naturally established E. globulus populations in the introduced range. The seedling density at 55 m from adult trees was 12,000 ha−1, the farthest seedling being registered at 101 m. Post-fire expansion occurred in a southward direction, in accordance with predominant wind. Seedlings had significantly lower levels of genetic diversity (Ae = 5.8.; He = 0.8) than adult trees (Ae = 6.5; He = 0.8). Crossings were strongly unbalanced, with only eight trees contributing to the sampled seedlings, and one single tree contributing to 52% of these seedlings. Moreover, the co-ancestry amongst seedlings more than doubled (from θ = 0.03 to 0.076), and the population status number was around one third of the value registered for the adult population (from Ns = 16.2 to 6.6). Despite its isolation, external pollen was detected in the stand, and appears to have travelled at least 700 m, contributing to 22% of the offspring. Overall, the seedling cohort is much less diverse than the parent trees, with expected lower resilience and persistence to environmental stresses.

Eucalypt Recruitment and Invasion Potential in Protected Areas of the Iberian Peninsula under Current and Future Climate Conditions

“…Eucalyptus globulus is considered to be naturalised in many world regions, including North, Central and South America, Indonesia or the Pacific Islands [7], i.e., it can originate thriving offspring, normally nearby parent plants. In some circumstances, E. globulus can show invasive behaviour, producing offspring into surrounding habitats [8][9][10][11], as reported in Iberia [12][13][14][15][16].…”

“…The current climatic suitability for E. globulus recruitment (Section 2.1) was combined with a set of additional local scale variables known to influence E. globulus recruitment (Table 1), in order to define levels of recruitment potential for eucalypt stands established inside and around Natura 2000 sites (Table 2). Eucalypt stands outside the sites, distancing up to 100 m (maximum dispersal distance recorded in [12,13]), were also classified to estimate the potential of invasion coming from outside stands (Section 2.3). Recruitment potential was defined as the likelihood of eucalypt stands to originate recruitment, meaning that stands with higher recruitment potential had more chances of originating recruitment.…”

“…Eucalypts lack long-distance seed dispersal adaptations [20], meaning that the presence of mother plants is required for recruitment in a particular area. Wildfires are associated with recruitment and invasion events [12,13,53,54] by triggering a massive, synchronised seed shed from canopies in the weeks following fire [55] and by disrupting the resistance of plant communities to eucalypt establishment [12,17,19]. Higher residence time (a proxy for tree age and stand rotations in coppicing systems) may translate into greater propagule pressure and enhance recruitment [12,13,56].…”

“…Wildfires are associated with recruitment and invasion events [12,13,53,54] by triggering a massive, synchronised seed shed from canopies in the weeks following fire [55] and by disrupting the resistance of plant communities to eucalypt establishment [12,17,19]. Higher residence time (a proxy for tree age and stand rotations in coppicing systems) may translate into greater propagule pressure and enhance recruitment [12,13,56]. A minimum residence time of 15 years allows younger stands to be distinguished from older, mature stands submitted to at least one rotation, which is normally 10-12 years [57].…”

“…However, seeds from E. globulus lack long dispersal adaptations [20]. Dispersal distances up to 80-100 m have been recorded [12][13][14], and a probable maximum distance of 175 m was reported in a study from Australia in a drainage line [21]. The species is not legally identified as invasive in Portugal and Spain even though different stakeholders recognise its invasive potential [22].…”