RC10. Podocarpaceae in tropical forests: ecology, ecophysiology, and mineral nutrition

3rd February 2009, at Macquarie University, Sydney.

SPEAKERS INCLUDE Ben Turner - Smithsonian Tropical Research Institute, Republic of Panama - soils and the mineral nutrition of Podocarpaceae Lucas Cernusak - Charles Darwin U, NT - water use efficiency in the Podocarpaceae Peter Bellingham - Landcare Research NZ - ecology of NZ podocarps Edward Biffin - U Adelaide - molecular phylogenetics Tim Brodribb - U Tasmania - ecophysiology of the Podocarpaceae David Coomes - U Cambridge, UK - ecology of angiosperm and gymnosperm co-existence James Dalling - U Illinois - Podocarpaceae in neotropical forests Ian Dickie - Landcare Research, NZ - root nodules and symbiotic organisms in the Podocarpaceae Neal Enright - U Melbourne - ecology of tropical conifers Hans Lambers - U Western Australia - nutrient-mining by Proteaceae Chris Lusk - Macquarie U - woody plant trait-environment relationships Mike Lawes - Charles Darwin U - African podocarps Phil Ladd - Murdoch U, WA - podarps and fire ecology

On Tuesday 3rd February there will be an intensive 1-day research course offered, open to interested researchers and ECR and HDR.

The purpose of the remainder of the week for working group participants will be to synthesize information on the ecology of the Podocarpaceae in tropical ecosystems that is currently dispersed in various disciplines, and to stimulate thought and foster collaborative research on these fascinating trees.

Background
The emergence of angiosperms in tropical forests at the expense of the gymnosperms, their ancestral relatives, was one of the most important events in the evolutionary history of terrestrial plants. Conifers of the Podocarpaceae are one of the few gymnosperm families that still occur in angiosperm-dominated tropical forests. They were an important component of the Gondwanan flora, but were virtually eliminated from the tropics following the evolution of angiosperms in the early Cretaceous 1. Despite this, they persist throughout the Malesian and neotropics and are also common in temperate regions of Australasia and South America, although they remain the least well-known of the three large conifer families. In particular, the ecology of tropical podocarps remains poorly understood, although there are many aspects of their physiology and mineral nutrition that are of considerable interest. What enables podocarps to persist in angiosperm-dominated tropical environments?

Tropical podocarps are often considered to be restricted to montane sites, a feature of their biogeography that is used by palaeoecologists to reconstruct past forest communities. Some of the biogeographical differences between conifers and angiosperms are likely to be explained by differences in their leaf morphology and water conducting tissues 2. In particular, conifer wood is comprised solely of tracheids and does not contain vessels as in the angiosperms, resulting in contrasting water conductance and therefore photosynthetic capacity. However, the assumption that podocarps represent montane forest conditions in the palaeo-record is almost certainly an oversimplification – native podocarps in Panama, for example, occur widely in lowland forests on islands off both the Caribbean (wet) and Pacific (dry) coasts. Why they persist on islands, but not in lowland forests on the mainland, remains unknown.

Podocarps do occur commonly in montane forests throughout the neotropics, although in Panama they seem to be restricted to sites with extremely infertile soils. They are also common on infertile soils in temperate regions, replacing evergreen angiosperms in the late stages of ecosystem development associated with the extreme phosphorus deficiency 3. A variety of adaptations enable podocarps to compete well on infertile soils, including long leaf life-span and slow growth 4, although it also seems likely that they can access refractory forms of nutrients 5. Attempts to elucidate this mechanistically have been puzzling – most podocarps possess distinct nodules on their roots that suggest an ability to fix nitrogen, yet the nodules do not house nitrogen-fixing organisms. Instead, they become infected by arbuscular mycorrhizae, which are known to improve the ability of plants to acquire phosphorus from soil 6. Yet intriguingly the hyphae do not have contact with the external soil environment, being entirely contained within the walls of the nodules. This raises perplexing questions about the function of the fungi and the mechanism by which the plants obtain nutrients. Given that other plants that are adapted to infertile soils (e.g., the Proteaceae) can access refractory soil phosphorus without forming mycorrhizal associations 7, podocarps may possess other adaptations for acquiring refractory soil phosphorus (e.g., the ability to secrete organic anions) that have not so far been detected.

Literature cited:

1. Morley, R.J., 2000. Origin and Evolution of Tropical Rain Forests. Wiley.
2. Brodribb, T.J., Holbrook, N.M., Hill, R.S., 2005. Seedling growth in conifers and angiosperms: impacts of contrasting xylem structure. Aust. J. Bot. 53:749-755.
3. Wardle, D., Walker, L., Bardgett, R.D., 2004. Science 304: 509-513.
4. Richardson, S.J., Peltzer, D.A., Allen, A.R., McGlone, M.S., 2005. Resorption proficiency along a chronosequence: Responses among communities and within species. Ecology 86:20-25.
5. Turner, B.L., Condron, L.M., Richardson, S.J., Peltzer, D., Allison, V.J., 2007. Soil organic phosphorus transformations during pedogenesis. Ecosystems 10: 1166-1181.
6. Russell, A.J., Bidartondo, M.I., Butterfield, B.G., 2002. The root nodules of the Podocarpaceae arbour arbuscular mycorrhizae. New Phytol. 156: 283-295.
7. Lambers, H., Raven, J.A., Shaver, G.R., Smith, S.E., 2008. Plant nutrient acquisition strategies change with soil age. Trends Ecol. Evol. 23:95-103.

 

Last Updated December 2008