Improving Understanding of Forest and Carbon Dynamics

Science Center Objects

The community's ability to understand and predict changes in forests and their feedbacks to the global carbon cycle increasingly relies on models spanning several scales of biological organization – from tree leaves to entire forested landscapes. Yet many model assumptions about key processes – such as tree growth and mortality – require long-term data that are sometimes difficult and time-consuming to get. Dr. Nathan Stephenson partners with multiple Federal and state agencies, universities, and non-profit organizations to provide this information to scientists and the public.

Given the great longevity of trees, many key questions can only be answered by research that spans decades. Established in 1982, USGS’s globally unique network of forest research plots in California’s Sierra Nevada is the source of the world’s longest annual-resolution forest dynamics data (>30,000 trees, and >500,000 tree-years of unbroken data), providing unique insights into the inner workings of forests.

WERC Plot measuring
Taking several measurments at a single plot.    

We put a great deal of energy into improving our basic mechanistic understanding of how forests work. We often use comparative forest studies along environmental gradients (variation in space) to inform us about the potential effects of environmental changes through time.

Examples of the kinds of questions we address include:  What are the interactions between internal (e.g. competition between trees) and external (e.g. climatic variability) drivers of forest change? How do short- and long-term fluctuations in water balance affect tree recruitment, growth, and mortality? What is the relative importance of biological (e.g. insects) and non-biological (e.g. wildfire) causes of tree mortality? Do answers to these questions differ between energy-limited and water-limited forests, among different taxonomic groups, and along global environmental gradients?  What are the drivers of broad-scale geographic patterns in forest dynamics?  What are the implications for the terrestrial carbon cycle?

WERC Beetle Damage
Galleries created by Bark Beetles.

Recently, we collaborated with dozens of scientists from around the world to explore the fundamental nature of tree growth.  We found that, contrary to some long-held assumptions, tree mass growth rates – hence rates of carbon gain – of most species increased continuously with tree size.  We additionally resolved the apparent paradoxes of individual tree growth increasing with tree size despite declining leaf- and stand-level productivity.  Our results resolve conflicting assumptions about the nature of tree growth, inform efforts to model forest carbon dynamics, and have additional implications for theories of resource allocation and plant senescence. Visit the published article in Nature for more information.

We have additionally explored causes and implications of the correlation between forest productivity and tree mortality rates, reaching several key conclusions.  (1) Contrary to a pervasive modeling assumption, environmental changes that increase site potential for productivity also increase the probability of tree mortality.  (2) Along climatic productivity gradients, rather than high forest productivity causing high tree mortality rates, forest productivity remains high in spite of high mortality rates.  (3) The influence of plant enemies (top-down control) and other factors helps explain why forest biomass and carbon sequestration can show little correlation – or even negative correlation – with forest productivity.  (4) The growing body of global ecosystem models based on plant functional types must also incorporate environmental effects on the nature and strength of ecological interactions, especially plant enemies.

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