The Fall And Rise Of Forest Ecosystems

Forest ecologists have long wondered why forests decline in the absence of catastrophic disturbances. A new study, in part funded by the British Ecological Society, and published in this week’s Science, has shed new light on this problem.

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 northern Sweden (a series of forested islands near Arjeplog), Alaska, Hawaii, eastern Australia and two locations in southern New Zealand. All sequences consisted of forest stands on soils ranging in age from those formed very recently to those at least several thousand years old; the oldest soils studied were 4.1 million years old in Hawaii.

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 due to 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 phosphorous limitation during 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 which rejuvenate the system vary for different forests, but can include for example wildfire, glaciation, or volcanic activity. In the absences of these disturbances productive forests do not perpetuate indefinitely; eventually phosphorous becomes sufficiently limiting that forests with a high standing biomass can no longer be maintained. Second, they reveal that high biomass forest stands 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.

The study was conducted by David Wardle from the Swedish University of Agricultural Sciences (Sweden) and Landcare Research (New Zealand), Richard Bardgett from Lancaster University (U.K.) and Lawrence Walker from the University of Nevada (USA).