Variation in understorey floristic composition regeneration and fire hazard under Acacia invaded forest canopy in New Zealand

Abstract

Acacia species are considered invasive exotic species in many regions globally, and influence the floristic composition and fire hazard of ecosystems they invade. Managing such species requires an understanding of their invasion capability, and persistence through succession in the community. New Zealand is one of the countries that have experienced Acacia invasion in the forest canopy, however, the potential long-term successional trajectories and future fire hazards of these communities have rarely been studied impeding management. We investigated the variation in woody plant species composition that was regenerating at different height tiers, and the comparative flammability of constituent species, under the canopy of Acacia-invaded forest sites in New Zealand, and evaluated the likely long-term changes in vegetation succession and fire hazards. From the floristic survey of 24 10 × 10 m plots in Acacia-invaded sites, we found a significantly higher number of native species (7.33 ± 0.67) than exotic species (4.83 ± 0.38) regenerating under Acacia-invaded canopies. While exotic stems dominated the lowest height tiers (<0.3 m), native stems outnumbered exotics at other understorey height tiers (0.3–2 m, 2–5 m and 5–12 m) and were recruiting successfully. We also found that invaded Acacia species and other exotic species at the sites were comparatively higher in flammability than most of the native species in the area, and can potentially increase the flammability of the invaded forest area if not managed appropriately. The successful regeneration and recruitment of natives in all height tiers under the Acacia canopy indicates a potential native succession in the longer term. Thus, the Acacia invasion can be managed through passive restoration where the dominant Acacia-invaded canopy in the forest area is replaced by native species, while also likely decreasing the fire hazard of the vegetation. This study provides evidence for the regeneration potential of native species in the understorey of Acacia-invaded forest sites and the knowledge helps forest managers make appropriate decisions to manage this woody exotic invasion and fire hazard of the invaded forest sites in New Zealand as well as other Acacia-invaded sites globally.