Carbon accumulation in Colorado ponderosa pine stands

Woody encroachment and thickening have occurred throughout the western United States and have been proposed as important processes in the US carbon (C) budget despite large uncertainty in the magnitude of these effects. In this study we investigated ponderosa pine encroachment near Boulder, Colorado. We reconstructed a time series of forest structure to estimate changes in C storage by the trees. Advantages of this technique include the ability to estimate changes in C stocks over time with a single series of measurements (i.e., no historical measurements), and the ability to calculate accumulation rate changes through time. Substantial variation occurred in the C amounts and accumulation rates among the three plots resulting from differences in slope, aspect, and soil conditions. Accumulation rates increased exponentially as trees increased in size and additional trees established within the plots, and were highly variable among plots (0.09–0.7 Mg C·ha–1·year–1 during 1980–2001). These rates were less than those used in studies of the US carbon budget, and only by assuming no mortality for the densest stand could we generate a projected rate in 2050 that was similar. Thus, time since the initiation of encroachment and rate of encroachment are variables that should be considered for accurately computing the continental C budget.