Annals of Botany

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Crowns above the crowd may help to get ahead

Rain forests are critically important ecosystems and contain a very large proportion of the planet’s biodiversity. But, despite this, there is much to learn. One example is our relative ignorance of how so many different species of forest trees can become established and successfully coexist. This problem has been addressed by Masahiro Aiba and Tohru Nakashizuka (Otsu and Kyoto, Japan, pp. 313–321). The authors investigated juvenile specimens of 18 Shorea species that occur within the same forests in Borneo. They focused especially on biomass allocation and plant architecture, where they describe two trade-offs. One is that between a less branched narrow crown on a slender stem, versus a wide crown on a thicker stem. The other is between what the authors call a ‘robust structure’ and a structure that favours photosynthetic carbon gain. Several features relating to these trade-offs were measured at different phases of growth of young trees. The first point the authors make is that even in this group of congeneric species, there is a very great variation in sapling structure, as great as has been observed between much less closely related species. Secondly, when the data are subjected to principal component analysis, crown architecture and biomass allocation to leaves emerge as major components of the variation between species. Not surprisingly, the latter showed some correlation with photosynthetic capacity but the correlation with wood density was stronger. Crown architecture, on the other hand, was correlated with the size of the seed with, perhaps unexpectedly, large-crowned species emerging from smaller seeds. Overall, it is difficult to relate these features to regeneration, although the authors suggest that poorly dispersed, large-seeded species require small crowns on taller slender stems in order to escape shade. They are clearly right in stating that ‘further… studies… are required to… understand how tree species with a variety of structural traits compete and coexist’.