Where do conifers regenerate after selective harvest? A case study from a New Zealand conifer-angio sperm forest

Ensuring the regeneration of selectively harvested canopy tree species remains challenging in mixed forests where species have different arts of New Zealand's North requirements for successful recruitment. Mature conifer trees (Podocarpaceae) have been selectively harvested from p Island conifer-angiosperm forests. Forest managers require guidance on podocarp restoration, given current dominance by the shade-tolerant angiosperm, Belischmiedia tawa. We surveyed seedling densities of podocarps and B. tawa ca. 40 years after harvesting and found that B. tawa seedlings outnumbered the combined total of podocarp seedlings by approximately 3:1. There were significant, positive associations between seedlings of most species suggesting that safe sites for establishment were similar, in part, for the suite of study species. These sites are the same as those where adult podocarp trees are reported to occur. We developed and tested candidate models predicting the influence of environmental factors on seedling regeneration; these focused on the roles of soil nutrients, landform, canopy openness, tree fern cover, ground cover by ferns and disturbance. We found the most support for models that used a combination of soil nutrients, canopy composition, landform, index and disturbance type to predict seedling occurrence. A positive relationship was found between soil nitrogen (N) and seedling occurrence of all species surveyed, and this relationship alone had most support in explaining the occurrence of Prumnopitys taxifolia and Dacrycarpus dacrydioides seedlings. We found little difference in the current sites of young Prumnopitys ferruginea and B. tawa; both species occur in dense stands of adult B. tawa, although in contrast to B tawa, P ferruginea seedling occurrence declined with increasing soil phosphorus (P). Dacrydium cupressinum also declined with increasing soil P. Given that the studied forest fits the apparent global trend for angiosperm ascendancy we suggest that manipulations will be required to restore podocarps. Possible interventions are planting seedlings and/or removing some of the B. tawa canopy. Given that podocarp seedlings were found on soils with low soil P, but high soil N concentrations, soil nutrient status should be taken into account during management. (c) 2007 Elsevier B.V. All rights reserved.