Plant and soil biodiversity reveals past and potential future states of naturally regenerating and planted native forests

Abstract Introduction Forest restoration can be achieved by promoting natural regeneration or planting tree seedlings, but the relative benefits of these widely used approaches are questioned. Soil communities may influence restoration outcomes but are usually ignored by monitoring schemes. Objectives We investigated whether analyses of multitrophic soil communities and ecosystem functions can usefully complement conventional plant community measurements for assessing current and potential future states of naturally regenerating and planted native forests. Methods We used DNA metabarcoding to analyze multitrophic soil biodiversity alongside conventional plant community analyses in greater than 20‐year‐old forests of naturally regenerating and planted Leptospermum scoparium (mānuka, Myrtaceae), an important early‐successional tree native to Aotearoa—New Zealand, within two similar sites. Results Naturally regenerating forests had higher understory native plant cover and sapling wood density, and more saplings of species that dominate old‐growth forests. Planted forests had higher aboveground biomass, richnesses of soil bacteria and protists, and soil pathogen loads. Prokaryote metabolic pathways indicated greater importance of aboveground litter versus belowground plant exudates as soil inputs in planted and naturally regenerating forests, respectively. Soil analyses detected legacy effects of pre‐1860 Māori land use in several plots. Conclusions Soil biodiversity assessment provided information about past, current, and potential future ecological states that was not apparent from the analysis of aboveground plants alone. Our combined results show that planted forests initially grow faster and sequester more carbon, but naturally regenerating forests might be healthier and able to shift to an old‐growth forest state sooner.