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Plant diversity increases with the strength of negative density dependence at the global scale

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Science  30 Jun 2017:
Vol. 356, Issue 6345, pp. 1389-1392
DOI: 10.1126/science.aam5678

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  • RE: Giving no quarter: Habitat, biotic and abiotic factors smash plant diversity and density
    • Jeshima Khan Yasin, Scientist, Division of Genomic Resources, ICAR-NBPGR
    • Other Contributors:
      • Sakshi Chaudhary, Student, ICAR-NBPGR

    Jeshima Khan Yasin† and Sakshi Chaudhary
    Division of Genomic Resources, ICAR- National Bureau of Plant Genetic Resources,
    PUSA campus, New Delhi, India- 110012
    †corresponding author: yasinlab.icar@gmail.com; Yasin.Jeshima@icar.gov.in

    Apart from host specific enemies explained in Janzen – Connell hypothesis 1,2, plants have to pact with varied environmental interactions involving biotic and abiotic factors. Exposure of plants to stress induces disruption in plant metabolism implying physiological costs at cell level leading to reduction in fitness and ultimately productivity, population density and diversity3,4,5,6. As a consequence of abiotic stresses, negative impacts on plant growth lead severe losses to an extent more than 50 per cent in most plant species. Moreover, biotic stress is an added challenge inducing a strong pressure on plants and adding to the damage through pathogen or herbivore attack. Stress is a stimulus that influences exceptional range of homeostatic control of any organism. If stress exceeds tolerance limit, altered mechanisms are activated in re-establishing homeostasis. When the stress heads off, plants may revive to its original state or to a new physiological situation as in quiescent state3.
    Density and diversity are the result of many other factors too. Seed dispersal, seed dormancy, competition for nutrients, allelopathy...

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    Competing Interests: None declared.
  • Over-interpreting forest tree size distributions
    • Christian Damgaard, Professor, Bioscience, Aarhus Universitet
    • Other Contributors:
      • Jacob Weiner, Professor, Department of Plant and Environmental Sciences, University of Copenhagen

    Scientific inferences are limited by the nature of, not only the amount of, data available. Understanding dynamic processes almost always requires dynamic data, especially in forest ecology, where many factors come into play and variation is ubiquitous. LaManna et al. (1) claim to present evidence for higher "conspecific negative density dependence (CNDD)" in tropical than in temperature forests. They regress the density of trees below a threshold size ("saplings") on the density above this size ("adults") and call the slope CNDD. They then show that this slope is steeper in tropical than in temperate forests, and conclude that intraspecific competition is stronger in the tropics.

    As we recently argued (2), competition (density dependence) is a process, which, like all processes or rates, can only be measured with dynamic data, in this case by measuring the change in plant size or abundance from one point in time period to another. LaManna et al. state that they have analyzed the sapling data using a Ricker model, but this is a difference equation model, which requires time-series data (3). Treating the number of saplings as a proxy for the next generation of adults entails unjustifiable assumptions. At best, the number of sapling sets an upper limit on the next generation of adults, as most saplings will die due to density-dependent mortality before reaching adulthood. Their interpretation of the data is like considering an age distri...

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    Competing Interests: None declared.

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