Forests around the world rely on major disruption such as fire, eruptions or ice ages to renew the soils on which they grow.
Landcare Research ecologist David Wardle says that even the biggest and best native forests will eventually decline and collapse without such interventions.
“Everything is transient,” he said.
“High bio-mass forests that we have assumed to be stable are not – they are transient and will not stay forever.”
Over thousands of years, some disturbances caused by humans also might be transient, said Dr Wardle.
He has just published, on the website of the academic journal Science, a comparison of soil profiles from forest land in six places worldwide.
They all showed that forests grew least well in the oldest soils because their access to phosphorous, which is absorbed by trees, declined in proportion to the amount of nitrogen in the soil.
Poor growth was not caused by a lack of nitrogen, as had been previously thought, but a lack of phosphorous.
Dr Wardle said his study of Waitutu forest on raised marine terraces in western Southland, and on a glacial moraine at Franz Josef, helped to explain why forests fell into decline, even when there had been no catastrophic disturbances.
The study, which he conducted through the Department of Forest Vegetation Ecology at the Swedish University of Agricultural Sciences, with Lawrence Walker from the University of Nevada, Las Vegas, and Richard Bardgett, from the University of Lancaster in Britain, will be published in the next print version of Science.
Dr Wardle said the study looked at natural forests on six soil types of different ages in a range of climatic zones.
The youngest soils were on previously burned islands at Arjeplog in Sweden’s Lapland – where he is now working – and a glacial moraine in Alaska, only a few thousand years old.
The oldest soils, at 4.1 million years, were in Hawaii – which had not suffered an ice age – and in between were dunes in eastern Australia, and the New Zealand soils.
For all six, forest biomass – the number of trees per hectare – increased initially as soil fertility increased, but then declined sharply to a level where some sites could no longer support trees.
“This decline in all cases was associated with reduced levels of plant-available phosphorus relative to nitrogen in the soil,” Dr Wardle said.
As soils aged the low level of phosphorus increasingly limited trees because it was not biologically renewable in the ecosystem. Nitrogen was renewed because atmospheric nitrogen was converted by soil bacteria into forms of nitrogen that trees could use.
The study found evidence that falling levels of phosphorus affected soil organisms and reduced their release of other nutrients from the soil.
Without “major disturbances” such as bushfire, glaciers, or volcanic eruptions, the soils on which forests depended could not be endlessly renewed.
“In the absence of such disturbances, productive forests do not survive indefinitely,” he said.
“Eventually phosphorus becomes sufficiently limiting that forests with a high standing biomass can no longer be maintained.”
If thriving forests had no major disturbances over thousands of years, they would decline – a pattern evident worldwide and across a range of climates.