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Conifers

coniferGymnosperms and angiosperms are often depicted as arch rivals in the competition for space in forest canopies. This adversarial image derives from paleo-ecological reconstructions that suggest that angiosperm radiation during the Cretaceous occurred at the expense of gymnosperm diversity and abundance. Yet one gymnosperm group, the conifers, remain as a major contender for canopy dominance across the globe. A stimulating hypothesis formulated by Bond (1989) is that conifer persistence can be explained by vegetative competition in the seedling phase. By portraying conifer seedlings as slow "tortoise" regenerators compared with the fast-growing angiosperm "hare" seedlings, Bond suggested that conifers persist where angiosperms are unable to realize their maximum growth potential because of environmental limitations on photosynthesis and growth. Thus abiotic stress is argued to reduce the growth of angiosperm seedlings to similar rates as those of conifers, which are intrinsically constrained by inferior water transport in wood and leaves.

In a recent review1, we reevaluated Bond's hypothesis in the light of recent discoveries. We explored the evolutionary history of conifers, highlighting the distinct nature of Podocarpaceae and Pinaceae in the Southern and Northern Hemispheres, respectively. The contrasting history and ecology of these key conifer families raises questions about the concept of a predictable rule for conifer–angiosperm competition. In an attempt to reconcile the observed diversity in conifer biogeography with the functional characterization of conifers, we reexamined key aspects of conifer physiological and trait evolution, seeking general principles that may be used to define global conifer ecology. We concluded that, although many conifer species have conservative traits that enable persistence in stressed environments, others are successful pioneers of disturbed habitats. This allows modern conifers to escape head-on competition for light with broad-leafed angiosperms and to occupy a much broader range of habitats than is included in Bond’s hypothesis. We emphasize that different lineages of conifers (specifically the Cupressaceae, Podocarpaceae, and Pinaceae) are distinct in the environmental stressors they tolerate, giving rise to distinct biogeographic distributions of these clades1.

Podocarps attain their greatest diversity in the tropics. A few species of podocarp eke out a living in dense lowland tropical forests, these having developed flat leaves which may function similarly to those of slow-growing shade-tolerant angiosperms. However, the vast majority of temperate and tropical podocarps are restricted to nutrient poor soils, including ombrotrophic bogs2,3. In New Zealand's Fiordland National, podocarp are uncommon on rich alluvial soils but dominant on P-depleted marine terraces. Intriguingly, ferns turn out to determine where podocarps regenerate: ground ferns are so prolific on the richer soils that practically the only opportunity for seedlings to establish arise on the trunks of tree-ferns, and only a few small-seeded angiosperms do this 4. We have used an individual-based simulation model to further explore whether competition keeps podocarps out of alluvial sites, and found support for this hypothesis5. In other parts of New Zealand, podocarps dominate on alluvial surfaces by escaping head-on competition with angiosperms: they turn out to be excellent colonisers of sites disturbed by catastrophic earthquakes.

  1. Brodribb, T. J., Pittermann, J., & Coomes, D.A. (2012) Elegance versus speed: Examining the competition between conifer and angiosperm trees. Special Issue: Major Transitions in Angiosperm Ecology and Functional Biology. International Journal of Plant Sciences, 173, 673-694.
  2. Cernusak, L.A., Adie, H., Bellingham, P.J., Biffin,E., Brodribb, T.J., Coomes, D.A., Dalling, J.W., Dickie, I.A., Enright, N.J., Kitayama, K., Ladd, P.G., Lambers, H., Lawes, M.J., Lusk, C.H., and Morley, R.J., and Turner, B.L. (2011) Podocarpaceae in tropical forests: a synthesis. Smithsonian Contributions to Botany, 95, 189-195.
  3. Coomes, D.A., & Bellingham, P.J. (2011) Temperate and tropical podocarps: how ecologically alike are they? Smithsonian Contributions to Botany, 95, 119-140.
  4. 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
  5. 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. (in review) Century-scale effects of invasive deer and rodents on the dynamics of two forests growing on soils of contrasting fertility. Ecological Monographs.