Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

Long-term development of soil chronosequences

Long-term soil chronosequences are excellent model systems for testing ideas about ecosystem development in the order of thousands to millions of years.

Recent studies and those currently underway have generated new insights into the long-term shifts that occur both above and below ground during ecosystem development. These include investigations of long-term chronosequences both in New Zealand (e.g. along the Franz Josef chronosequence and the Waitutu marine terrace sequence) and elsewhere (e.g. in Hawai'i, Alaska, eastern Australia and northern Sweden). Recent reviews by Wardle et al. (2004, Science 305: 509-513) and Vitousek (2004, Nutrient Cycling and Limitation; Princeton University Press) demonstrate that ecosystem properties and processes follow somewhat predictable long-term trajectories across contrasting systems, in that they are driven initially by nitrogen (N) limitation but ultimately by phosphorus (P) limitation. In a series of previous research projects, we have investigated shifts in both above- and below-ground communities, including vegetation composition, shifts in foliar nutrient contents, plant plasticity, the photosynthetic capacity of canopy trees, soil microbial composition and activity and soil invertebrates, largely along the Franz Josef soil chronosequence and Waitutu marine terrace sequence.

The general issue that we explore using soil chronosequences is what drives ecosystem change over the long term? A closely related question is what causes ecosystem retrogression (i.e. the long-term decline in nutrient availability and primary productivity)? Although we have documented strong shifts in both above- and below-ground communities and ecosystem properties along chronosequences, it is generally unknown how these components interact, whether the observed shifts are generalisable across other ecosystems, and which factors drive long-term ecosystem retrogression.

We are at a stage where the results of various projects are published or nearing publication, but synthesis of ideas is necessary. Some specific questions to be resolved include:

  • Are there general trends in plant communities and attributes that occur during retrogression? For example are there key traits that are characteristic of species that dominate in old versus young soils across chronosequences?
  • Are there predictable changes that occur below ground and do these changes match those that occur above ground? For example, do rates of soil processes (decomposition, nutrient fluxes) decline in tandem with NPP during retrogression? Similarly, do changes in communities of soil organisms (saprophytic microbes, soil fauna, mycorrhizae) match what we see above ground?
  • Can studies on retrogressive chronosequences be scaled up to the landscape scale?

A general review of ecosystem retrogression and its drivers will be used to catalyse future projects and grant proposals. We anticipate that an improved understanding of the patterns and drivers of long-term ecosystem development and retrogression can be used to help resolve such issues as: What are the implications of ecosystem retrogression for the restoration of old landscapes? How much disturbance is needed to reverse retrogression? Can understanding ecosystem development along soil chronosequences help us to understand why P becomes increasingly limiting relative to N as systems age, or in the tropics?


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