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Phosphorus Key To Forest Decline

Tue, 29 June 2004

Phosphorus Key To Forest Decline

Phosphorus key to why forests decline Forest ecologists have long wondered why forests decline in the absence of catastrophic disturbances. A new study conducted by David Wardle from Landcare Research (New Zealand) and the Swedish University of Agricultural Sciences (Sweden), Lawrence Walker from the University of Nevada, Las Vegas (USA), and Richard Bardgett from the University of Lancaster (UK) has shed new light on this problem.

This study has been published onthe 'Science Express' website, the portal for rapid publication of particularly significant research findings which appear subsequently in print in the prestigious international publication Science (in this case in mid-July). This study investigated natural forested stands across each of six 'chronosequences' or sequences of soils of different ages since the most recent major disturbance.

These sequences were located in a range of climatic zones, including previously burned but now forested islands in northern Sweden, a glacial moraine in Alaska, the entire island chain of Hawaii, dunes in eastern Australia, a glacial moraine in New Zealand and a set of raised marine terraces in New Zealand. All sequences consisted of forested stands, on soils ranging in age from those formed very recently to those aged between several thousand and 4.1 million years.

For all six sequences, forest biomass (mass of trees per unit area) increased initially as soil fertility increased. However, after thousands to tens of thousands of years, forest biomass declined sharply for all sequences, to a level where some sites could no longer support trees.

The researchers found that this decline in all cases was associated with reduced levels of plant-available phosphorus relative to nitrogen in the soil. As soils age, phosphorus becomes increasingly limiting for trees because it is not biologically renewable in the ecosystem. Conversely, nitrogen is biologically renewable (because atmospheric nitrogen can be converted by soil bacteria into forms of nitrogen that trees can use), so nitrogen limitation does not contribute to forest decline in these systems, contrary to popular views.

There was also evidence from this study that phosphorus limitation during the stage of forest decline negatively affected soil organisms, and therefore reduced their potential to release nutrients from the soil for maintaining tree growth. These results have several implications.

First, they show that major disturbances are necessary for rejuvenating forest ecosystems. Disturbances that rejuvenate the system vary for different forests, but can include wildfire, glaciation, or volcanic activity. In the absence of such disturbances, productive forests do not survive indefinitely; eventually phosphorus becomes sufficiently limiting that forests with a high standing biomass can no longer be maintained.

Second, they reveal that forest stands with high biomass represent a transitional phase in the long term (in the order of thousands to tens of thousands of years) and, if left without major disturbances, will then decline.

Finally, they show that very similar patterns of decline, and mechanisms behind this decline, occur for very different types of forest throughout the world, spanning the boreal, temperate and tropical climatic zones.


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