GETTING TO THE ROOT OF FOREST GROWTH AND DECLINE

Forests need an occasional major natural disaster such as wildfire or a volcanic eruption to flourish, according to research carried out by Lancaster University’s Professor Richard Bardgett. Without such an upheaval, they can go into serious decline, his study reports.

Professor Bardgett, from the Department of Biological Sciences, along with David Wardle (Landcare, New Zealand) and Lars Walker (University Nevada, US), spent six years testing soils from different countries around the world including Sweden, Alaska, Hawaii, Australia and New Zealand.

The results of the study, part funded by the British Ecological Society, show that forests, which need phosphorus to thrive, grow least well in the oldest soils where the supply of phosphorus is dwindling.

Eventually the phosphorus becomes so limited that some forests cannot survive, but where there is a catastrophic disturbance such as a fire, volcano or glaciation, the phosphorus in the soil is replenished, rejuvenating the forest.

The team looked at forest soils in a variety of climatic zones and among a wide range of ages, from several thousand to millions of years old – the oldest soils studied were 4.1 million years old in Hawaii. For all soils, forest density increased initially as soil fertility increased but after thousands to tens of thousands of years, forests went into sharp decline to a point where some sites could no longer sustain trees.

The scientists found that this decline was due to reduced levels of plant-available phosphorus - as soils age, phosphorus becomes increasingly limiting for trees because it is not biologically renewable in the ecosystem. There is a transitional phase during which the forest will thrive for thousands to tens of thousands of years but if left without major disturbances will then decline. However, a drastic natural event can have the effect of rejuvenating forest eco-systems.

“These results are of high significance for the conservation of natural forests around the world since they show that they are transient and need natural disasters to retain their productivity,” said Professor Bardgett.