Journal Article

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

van Mantgem, Phillip J., Bruner, Howard, Urban, Dean L., Harmon, Mark E., Franklin, Jerry F., Stephenson, Nathan L., Bunn, Andrew G., O'Connell, Kari B.

2011

Overview

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.

• Publisher: Ecological Society of America
• Subject: adults / Animal and plant ecology / Animal, plant and microbial ecology / Biological and medical sciences / biomass / Case studies / climate / competition / CONCEPTS & SYNTHESIS: EMPHASIZING NEW IDEAS TO STIMULATE RESEARCH IN ECOLOGY / consumer controls / Demecology / ecological competition / Ecological life histories / Ecosystem studies / ecosystems / Forest biomass / Forest canopy / forest dynamics / Forest ecology / Forest productivity / Forest trees / Forestry / functional traits / Fundamental and applied biological sciences. Psychology / General forest ecology / Generalities. Production, biomass. Quality of wood and forest products. General forest ecology / Life history / life-history trade-offs / Mortality / net primary productivity / permanent sample plots / Plant ecology / plant enemies / Plant populations / Plants and fungi / population dynamics / probability / reproduction / Soil fertility / Species / temperate forest / Temperate forests / Tree growth / tree mortality / Trees / tropical forest / Tropical forests