Forests provide important ecosystem services - regulating climate via their role in the earth's carbon cycle, providing clean drinking water, and as habitat for wildlife. However, there is great scientific uncertainty surrounding their future behavior. Tree-ring data and national forest inventories are complementary data streams that can help resolve this uncertainty. Forest inventory data are spatially comprehensive and representative, but censuses in the western United States occur on a 10-year interval cycle. Tree-ring data have annual resolution, but are typically more limited in spatial scope and sampling has historically been biased towards climate-sensitive individuals at the edge of the forest biome. Here we present an analysis of tree-ring data from an emerging data network collected by the U. S. Forest Service in Forest Inventory and Analysis plots. We use a (hierarchical Bayesian) hidden process model to draw upon both radial increment (from tree-rings) and diameter data (from inventories) to infer the influences of tree size, climate, stand density, and biophysical variables on individual tree growth. Our analysis focused on Pinus ponderosain northern Arizona, using a set of >800 trees with both tree-ring and diameter data, along with another ~1,000 trees with repeat diameter data.
The most important influence on growth was individual tree size: diameter increments decline with tree diameter, a decline that slows with increasing diameter. Stand density negatively affected tree growth, whereas site index (an indicator of site productivity) positively affected growth. Growth of Pinus ponderosa is moisture-limited, with positive effects of precipitation throughout the year, negative effects of warm-season temperatures, and positive effects of cool-season temperatures. Using allometric equations, we scaled from diameter growth to tree- and stand-level aboveground biomass increments. Positive sensitivity of growth to precipitation and negative sensitivity to warm-season temperature translates into reduced carbon sequestration capacity of Pinus ponderosa forests in northern Arizona with either warming temperature, increased vapor pressure deficit, or increased climatic moisture deficit. The implication is that this ecosystem's ability to mitigate anthropogenic greenhouse gas emissions and climate change will be compromised by climate change anticipated over the course of the 21st century.