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Tree ferns

treeferntop 1The first time you encounter tree ferns, you cannot fail to be impressed by their magnificently prehistoric appearance. These arborescent ferns are common in tropical mountains and southern hemisphere rain forests, yet remarkably little is known of their ecology. For instance, how long do they live, and under what conditions do their sporophytes establish? Working in a New Zealand forest, We calculated the growth, death and recruitment rates of five tree fern species using a 38-year record of stem heights, collected within a mapped block of lowland forest, and measured electron transport rates of photosystem II of their fronds1. We discovered that the largest tree ferns in the forest dated back to a massive earthquake which struck the area in 1855, destroying the established forest and providing a window of opportunity for dense groves of tree ferns to establish. The five species had distinct niches along a shade tolerance spectrum and differed in their responses to disturbances.

In two other studies, we found that tree ferns play a peculiar yet pivotal role in the dynamics of forests in the south of New Zealand, by filtering which trees can regenerate. Seedlings are often observed to grow epiphytically on the fibrous trunks of tree ferns, and these seedlings can eventually over-top and kill their hosts. We found that most trees of Weinmannia racemosa - the most common tree in the alluvial rain forests we were investigating2 - had started life as epiphytes. The tiny seeds of Weinmannia were better able to lodge on tree-fern trunks than those of other species. Even though growing conditions on the trunks were tough for these seedlings3, they were marginally better than those encountered by other species on the forest floor, giving Weinmannia enough of an edge to become the forest's dominant species.

We have also investigated the evolution of tree ferns. In general, the current distribution of tree ferns indicates highly conserved climatic niche preferences for warm temperature combined with high humidity and limited seasonal variation in both temperature and rainfall. However, some species are also found at high elevations in tropical mountains (e.g. up to 3500 m in New Guinea and the Andes), and others are found in temperate Australasia, including New Zealand and Tasmania. We reconstructed a climatic niche of scaly tree ferns (Cyatheaceae) using a rigorous analytical procedure which combines climatic niche modelling with reconstruction of continuous characters given a phylogenetic hypothesis4. Our results confirmed a general pattern of niche conservatism with occasional events resulting in niche transformations. Neotropical, Afro-Madagascan and the sampled New Guinea species generally showed similarity in their climatic niches while climatic niches of South-Asian and New Zealand species varied considerably, indicating that climatic niche transformation had taken place. These results are consistent with those of Janssen et al. (2008)5, who did not find evidence for climatic niche differences among species of Madagascan scaly tree ferns belonging to the Cyatheaceae clade. Thus, both studies suggest a strong influence of both geography and phylogenetic (historic) factors on the evolution of the climatic niche of the scaly tree ferns.

  1. Bystriakova, N., Bader, M. and Coomes, D.A. (2011) Long-term tree fern dynamics linked to disturbance and shade tolerance. Journal of Vegetation Science 22, 72–84. DOI link
  2. 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
  3. Gaxiola, A., Burrows, L.E. & Coomes D.A. (2008) Tree-fern trunks facilitate seedling regeneration in a productive lowland temperate rain forest. Oecologia, 155, 325–335. DOI link
  4. Bystriakova, N., Schneider, H., & Coomes, D. (2010). Evolution of the climatic niche in scaly tree ferns (Cyatheaceae, Polypodiopsida). Botanical Journal of the Linnean Society, 165(1), 1-19. DOI link
  5. Janssen, T., Bystriakova, N., Rakotondrainibe, F., Coomes, D.A.,, Labat, N.L., Schneider, H. (2008) Neoendemism in Madagascan scaly tree ferns results from recent, coincident diversification bursts. Evolution 62: 1876-89. DOI link

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