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Dynamic modelling of vegetation along a 291,000 year chronosequence

waitutuChronosequences are a type of ‘natural experiment’ which offer invaluable insights into the temporal dynamics of plant communities and soil development over time (Walker et al. 2010). Transformations of soil P have become a central paradigm of ecosystem development theory in the context of chronosequences. Specifically, the model of Walker and Syers (1976) proposed that P is most available to plants and microbes in the early stages of primary succession, when minerals such as calcium apatite dissolve to produce inorganic phosphates. Models of retrogressive succession have emphasised the role of phosphorus (P) depletion in driving biomass loss on surfaces of increasing geologic age, but the influence of impeded drainage on old surfaces has received much less attention.

We have conducted a series of studies along a 291,000-year chronosequence in southern New Zealand (the Waitutu chronosequence). The field site is characterized by a series of 13 uplifted marine terraces, separated by steep scarps, the youngest of these is a Holocene raised beach close to sea level whilst the oldest lies 12 km inland and is estimated to be at least 900 000 years old. The terrace sequence is intersected by two large rivers, bordered by alluvial terraces, which are regularly refreshed by alluvium, whilst the terrace surfaces are much older and are poorly drained and P depleted1,2. Typical ground-level appearance of the alluvial and 120,000-year-old terrace forest are shown to the left. (a) Alluvial forest dominated by angiosperms with a dense understory dominated by tree ferns. (b) Marine forest co-dominated by conifers and angiosperms with a sparse understory

We conducted a large-scale fertilization experiment to test whether P was indeed limiting on the oldest stages of a 291,000-year chronosequence in southern New Zealand (the Waitutu chronosequence)1. We also tested whether poor drainage contributed to changes in ecosystem properties along this sequence: soil and ecosystem properties were measured at 24 evenly distributed points within each of eight 1.5 ha plots located on young, intermediate and old surfaces and regression analyses used to test whether drainage, in addition to P, affected ecosystem functioning. We found that most phosphorus depletion occurred in the early stages of pedogenesis (within 24,000 years)2 whereas drainage was initially good but became increasingly impeded with surface age. In the fertilizer experiment, species showed positive responses to both nitrogen (N) and P addition on the oldest surfaces, supporting Walker and Syer’s model 1. However, water table depth was also found to be strongly correlated with plant species composition, forest basal area, light transmission, and litter decomposition when comparisons were made across sites, emphasising that it too has strong influences on ecosystem processes. Experimentally growth seedlings in waterlogged soils demonstrated that some species from the oldest sites were extremely well adapted to saturated soils3. We concluded that poor drainage influences the process of retrogressive succession along the Waitutu chronosequence, and suggested that topography is like to have strong influences on retrogressive processes around the world1. Intriguingly, some of the patterns we observed in New Zealand mirrored those observed along a soil drainage gradient in the Amazon4

We have used simulation modelling to explore forest dynamics in these contrasting systems. We collected all data necessary to parameterise the SORTIE-ND individual based model3,4 (Click to read detailed report for terrace forest), and have used the model for three purposes:

  • To evaluate whether timber harvesting can ever be sustainable in lowland podocarp forests5
  • To examine the long-term effects of invasive deer and rodents on forest dynamics 6
  • To explore how the different traits of conifers and angiosperms affect their relative abundance along the chronosequenceunpub
  1. Coomes, D.A., Allen, R.B., Bentley, W.A., Burrows, L.E., Canham, C.D., Fagan, L., Forsyth, D.M., Gaxiola-Alcantar, A., Parfitt, R.L., Ruscoe, W.A., Wardle, D.A., Wilson, D.J., & Wright, E.F. (2005) The hare, the tortoise and the crocodile: the ecology of angiosperm dominance, conifer persistence and fern filtering. Journal of Ecology, 93, 918-935.  click
  2. Coomes D.A., Bentley, W.A, Tanentzap, A.J., Burrows, L. (2013) Soil drainage and phosphorus depletion contribute to retrogressive succession along a New Zealand chronosequence. Plant and Soil (Special Issue on soil chronosequenceS)
  3. Parfitt, R.L., Ross, D.J., & Coomes, D.A., Richardson, S.J., Smale, M.C., & Dahlgren, R.A. (2005) N and P in New Zealand soil chronosequences and relationships with foliar N and P. Biogeochemistry, 75, 305-328. click
  4. Gaxiola A., McNeill S.M., Coomes, D.A. (2010) What drives retrogressive succession? Plant strategies to tolerate infertile and poorly drained soils. Functional Ecology 24:714–722. DOI link
  5. Kunstler, G., Coomes, D.A., Canham, C.D. (2009) Size-dependence of growth and mortality influence the shade tolerance of trees in a lowland temperate rain forest. Journal of Ecology. 97, 685 - 695. DOI link
  6. Coomes, D.A., Kunstler, G., Canham, C.D., & Wright, E. (2009) A greater range of shade-tolerance niches in nutrient-rich forests: an explanation for positive richness–productivity relationships? Journal of Ecology. 97, 705-717. DOI link pdf (password needed)
  7. Kunstler, G., Canham, C.D, Allen, R.B., Coomes, D.A. (in press). Sustainable management, earthquake disturbances and transient dynamics: modelling timber harvesting impacts in mixed-species forests. Annals of Forest Science.
  8. Forsyth, D.M., Wilson,D.J., Easdale,T., Kunstler, G., Canham, C.D., Ruscoe, W.A., Wright, E.F., Murphy, L., Gormley, A.M., Coomes, D.A. (under review) Century-scale effects of invasive deer and rodents on the dynamics of two forests growing on soils of contrasting fertility. Ecological Monographs.
  9. Coomes, D.A. (1997) Nutrient status of Amazonian caatinga forests in a seasonally dry area: nutrient fluxes in litter fall and analyses of soils. Canadian Journal of Forest Research, 27, 831-839. click