FOREST trees in the eastern US grow quicker in years after large numbers of cicadas have emerged, and we may have found the reason why.
Cicadas spend most of their lives underground, where they consume tree root juices, depriving leaves of valuable nutrients. Some periodical species then emerge en masse every 13 or 17 years and feast on tender tree branches before laying their eggs and dying. Now Louie Yang at the University of California at Davis has established that, in dying en masse in this way, the insects provide a deluge of compost that fertilises forest soils and helps trees grow faster.
Using insects collected from broods of periodical cicadas that emerged in 2002 and 2003, Yang applied various densities of dead cicadas to 1-metre-square forest plots. After a month, plots laden with a typical 240 cicadas contained more microbes, three times the concentration of available ammonium and 2.5 times the concentration of nitrates compared with untreated plots. And bellflowers growing on soil enriched with cicadas had a higher concentration of nitrogen in their leaves and seeds 鈥� 9 per cent greater than those growing on control plots (Science, vol 306, p 1565).
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