Forest carbon uptake recovers from modest tree losses

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Forest carbon uptake recovers from modest tree losses

By some accounts, forests are currently seeing the highest rates of disturbance since the recession of the Pleistocene glaciers over 10,000 years ago. In recent decades the global extent of forest-disrupting events has increased, yet their intensity has been on the decline. Significant tree losses upset carbon and nitrogen cycling, drastically extending recovery times. Throughout the northern temperate zone, forests that established a century ago on clear-cut or burned lands have been increasingly affected by subtle disturbances, like selective logging, pathogenic insects, or age-related mortality. The higher proportion of trees surviving these modest disturbances likely mitigates the effects on carbon and nitrogen cycling, but evidence is limited.

By manipulating an experimental forest at the University of Michigan Biological Station, Nave et al. determine how forests' carbon and nitrogen cycles respond to subtle disruptions. In 2008, the authors and their team culled 39 percent of the forest's tree population using stem girdling—the process of removing a ring of the tree's bark and starving it of its nutrient and water supplies. The aim was to accelerate the loss of short-lived tree species, driving the forest into a more complex state, commonly found in older forests, rather than the homogeneous conditions that result from clear-cutting or a forest-clearing fire. Initially, the authors find that the forest's carbon uptake declined and soil nitrogen availability and leaching increased, reminiscent of severe disturbances. However, most forest ecosystems are inherently limited by the availability of nitrogen, and the newly liberated stores were drawn up by the remaining longer-lived trees, producing new leaf area and mitigating the decrease in carbon uptake. The authors suggest that, as the forest structure continues to change in the wake of subtle disturbances, the forest's carbon uptake will continue to increase.

Source: Journal of Geophysical Research-Biogeosciences, doi:10.1029/2011JG001758, 2011
http://dx.doi.org/10.1029/2011JG001758