Occurrence of native and exotic invasive trees in burned pine and eucalypt plantations: Implications for post-fire forest conversion

Post-fire management objectives for monospecific forest plantations may include conversion to native broadleaved or mixed forests for purposes of diversification of forestry production, enhancing biodiversity and cultural value, or reducing biotic and abiotic hazards. Thus, the potential for post-fire conversion needs to be assessed as a first step in planning such conversion. On the other hand, fire occurrence may foster invasion by exotic species and this equally needs to be assessed when planning post-fire management. We have surveyed 284 plots of the more common plantation types in Portugal (maritime pine and eucalypt) five years after they were burned. We describe the occurrence patterns of native and exotic tree species, and relate these to plantation type, ecological region, previous fire disturbance frequency, and type of post-fire management actions. We have identified 19 native tree species in ca. 50% of the burned plantations, showing that there is potential for post-fire conversion. Ecological region and post-fire management actions involving soil or understory disturbance were the more important drivers of native species occurrence. Four alien woody species occurred in 10% of the plots, indicating that they now constitute a factual risk in post-fire land management in the country. Unlike native species, where post-fire management negatively impacted on their occurrence, alien species were more prevalent in burned plantations where post-fire operations disturbing vegetation or soil had occurred. The fact that the typical post-fire management practices in these planted forests (salvage logging, understory removal and soil tilling) hindered native species recovery but enhanced invasive exotics creates a conundrum that needs careful consideration when planning post-fire interventions.