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Recent Study Seeks to Explain Global Forest Diversity Patterns

July 5, 2017
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Kyle Gay measuring tree diameter

The well-known trend of global diversity decreasing from the tropics to the poles is often discussed but never adequately explained.  A paper that came out in the June 30, 2017 issue of the journal Science is shedding new light on potential reasons behind this global phenomenon.   The study, headed by Joe LaManna at Washington University in St. Louis, and with 49 other authors including Harvard Forest Ecologist David Orwig, shows that global patterns of plant diversity are strongly affected by conspecific negative density dependence (CNDD), in which an individual is negatively affected by the number of nearby neighbors of the same species.  Using data on 3000 species and nearly 2.4 million trees from 24 large forested plots around the world, in each of which every woody individual is tagged, mapped, and measured, the authors found that sapling recruitment decreased with increasing numbers of conspecific adults but was unaffected by adult densities of other species. This result suggests that conspecific interactions are stronger and likely maintain tree diversity.  The analyses also found a difference in these neighborhood interactions between temperate and tropical forests.  There were stronger negative effects of conspecific neighbors in diverse tropical forests than in temperate forests like Harvard Forest.  Further, rare species experienced stronger CNDD than common species in the tropics, whereas common species in temperate forests experienced similar or stronger CNDD.  Orwig was excited by the findings, “Only by examining data from sites around the globe was such a robust analysis possible, which highlights the importance of the ForestGEO network in contributing to our understanding of worldwide forest diversity and associated dynamics.”

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