Post-fire vegetation recovery in northern New Zealand: potential contributions of soil seedbanks, effects of heat-shock, and soil conditions

Abstract

The rarity of fire in New Zealand prior to human arrival meant much of the native flora lacks fire-adaptive traits. As a result, increased fire frequency associated with human settlement has driven widespread forest loss in New Zealand. Remnant forests are continuously threatened by recurrent fires and the invasion of exotic fire-adapted plants. Little, however, is known of either the effects of post-fire soil conditions on influencing post-fire community dynamics, or the potential contributions of soil seedbanks to the early stages of post-fire succession. This study seeks to evaluate the (1) effects of heat-shock (a fire-related germination trigger) on the germination on New Zealand soil seedbank, (2) the contribution (in terms of composition) of soil seedbanks to post-fire vegetation regeneration, and (3) to assess post-fire soils. Vegetation data and soil seedbank samples were collected across four study sites in northern New Zealand, and subsequent heat-shock experiments, shadehouse germination trials, soil bulk density, soil moisture, pH, and total carbon (C) and nitrogen (N) analyses were conducted. Compositional data from the germination trials and field vegetation surveys were analysed using classification and ordination. Comparisons showed a decoupling between the composition of the soil seedbank and post-fire regenerating vegetation. Soil seedbanks were dominated by herbaceous taxa, while the regenerating vegetation comprised predominately of native early-successor species such as Leptospermum scoparium and invasive, exotic fireadapted species such as Hakea sericea and Pinus radiata. Heat-shock experiments show that fire suppressed the overall germination of New Zealand species in the soil seedbank, while promoting the germination of species with persistent and fire-tolerant seeds (e.g. Paraserianthes lophantha and Ulex europaeus). Post-fire soils across the four sites were characterised by high bulk density, low moisture, and low total C and N concentrations. This study identified persistent and heat-tolerant seeds and post-fire soil conditions as contributors to positive fire-vegetation feedbacks in post-fire communities, which would likely increase their flammability and sensitivity to recurrent fires, and slow return-to-forest succession.