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Causes and implications of the correlation between forest productivity and tree mortality rates Public Deposited

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  • At global and regional scales, tree mortality rates are positively correlated with forest net primary productivity (NPP). Yet causes of the correlation are unknown, in spite of potentially profound implications for our understanding of environmental controls of forest structure and dynamics and, more generally, our understanding of broad-scale environmental controls of population dynamics and ecosystem processes. Here we seek to shed light on the causes of geographic patterns in tree mortality rates, and we consider some implications of the positive correlation between mortality rates and NPP. To reach these ends, we present seven hypotheses potentially explaining the correlation, develop an approach to help distinguish among the hypotheses, and apply the approach in a case study comparing a tropical and temperate forest. Based on our case study and literature synthesis, we conclude that no single mechanism controls geographic patterns of tree mortality rates. At least four different mechanisms may be at play, with the dominant mechanisms depending on whether the underlying productivity gradients are caused by climate or soil fertility. Two of the mechanisms are consequences of environmental selection for certain combinations of life-history traits, reflecting trade-offs between growth and defense (along edaphic productivity gradients) and between reproduction and persistence (as manifested in the adult tree stature continuum along climatic and edaphic gradients). The remaining two mechanisms are consequences of environmental influences on the nature and strength of ecological interactions: competition (along edaphic gradients) and pressure from plant enemies (along climatic gradients). For only one of these four mechanisms, competition, can high mortality rates be considered to be a relatively direct consequence of high NPP. The remaining mechanisms force us to adopt a different view of causality, in which tree growth rates and probability of mortality can vary with at least a degree of independence along productivity gradients. In many cases, rather than being a direct cause of high mortality rates, NPP may remain high in spite of high mortality rates. The independent influence of plant enemies and other factors helps explain why forest biomass can show little correlation, or even negative correlation, with forest NPP.
  • Keywords: Functional traits, Plant enemies, Forest dynamics, Temperate forest, Net primary productivity, Tropical forest, Competition, Consumer controls, Tree growth, Life-history trade-offs, Tree mortality, Permanent sample plots
  • This is the publisher’s final pdf. The published article is copyrighted by Ecological Society of America and can be found at: www.esa.org/
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  • Stephenson, N. L., van Mantgem, P. J., Bunn, A. G., Bruner, H., Harmon, M. E., O'Connell, K. B., . . . . (2011). Causes and implications of the correlation between forest productivity and tree mortality rates. Ecological Monographs, 81(4), 527-555. doi: 10.1890/10-1077.1
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  • 81
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  • 4
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  • Data from Oregon and Washington were funded through NSF’s Long-term Studies Program (DEB-0218088), the Wind River Canopy Crane Program through cooperative agreement PNW 08-DG-11261952-488 with the USDA Forest Service Pacific Northwest Research Station, various awards through the USDA Forest Service’s Pacific Northwest Research Station, and the McIntire-Stennis Cooperative Forestry Program. Data from California were funded through a number of awards from the U.S.National Park Service and the U.S. Geological Survey (USGS). The Forest Dynamics Plot at Barro Colorado Island (BCI) has been made possible through 15 U.S. National Science Foundation grants to Stephen P. Hubbell, and generous support from the Center for Tropical Forest Science, the Smithsonian Tropical Research Institute, the John D. and Catherine T. MacArthur Foundation, the Mellon Foundation, the Celera Foundation, and numerous private individuals.
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