Globally relevant lessons from a long-term trial series testing universal hypothesis of the impacts of increasing biomass removal on site productivity and nutrient pools

Abstract

It is established that the long-term productivity of planted forests now and into the future requires a consistent supply of nutrients from soil for sustained growth over multiple rotations, and the importance of soil function to other important planted forest ecosystem services such carbon cycling is now increasingly recognised. In addition, advances in analytical techniques have provided capability to investigate the soil biological communities that supports these functions. The New Zealand Long-Term Site Productivity (LTSP) trial series has been a fundamental research infrastructural asset for exploring the impacts of harvest residue and forest floor removal, providing new insights into the sustainability of soil nutrient and organic matter (soil carbon) stocks and the diverse range of soil biological activity that supports many ecosystem level processes essential to continuous forest productivity. This paper provides a globally unique synthesis from a long-term test of a universal hypothesis and discusses how the findings throughout the ~ 30 year lifespan of the trial series have supported the development of knowledge and tools that are leading to changes to forest management practice. Key examples include the development of the Nutrient Balance Model (NuBalM) platform, demonstration of the critical importance of harvest residues and forest floor material at low fertility sites, and new evidence identifying the enduring sensitivity of the soil microbial community to disruption caused by harvesting intensity. Combined, these results have underpinned significant changes in how planted forest soils in New Zealand are managed, in terms of both nutrition and the influence of beneficial soil microbes. Discussion of these outcomes is particularly timely given the increasing global demands on wood and fibre supply, and demonstrates the importance of long-term site productivity trials to the ongoing ability of forest growers and managers to deliver multiple benefits from planted forests. An outlook is provided on what the future might hold in terms of increasing intensification and the implications of multiple rotations over a larger area of forest in the future.