Dendrophenology: Inferring the response of North American eastern deciduous forests to an earlier spring from tree rings

There is wide agreement that anthropogenic climate warming has influenced the phenology of forests during the late twentieth and early twenty-first centuries. A critical question for predicting the magnitude of future warming under different emissions scenarios is the degree to which forest productivity responds to longer growing seasons in the face of concurrent changes in other drivers of productivity. Longer growing seasons can lead to increased photosynthesis and productivity, which would represent a negative feedback to rising CO2 and consequently warming. Alternatively, increased demand for soil resources due to a longer photosynthetically active period in conjunction with other global change factors might exacerbate resource limitation, restricting forest productivity response to a longer growing season. In this case, increased spring-time productivity has the potential to increase plant N limitation by increasing plant demand for N more than N supplies, or increasing early-season ecosystem N losses. Long-term direct measurements are not yet available to specifically address this question, but advances in remote sensing and dendroecological methods present opportunities to acquire information retrospectively to advance understanding of how phenological changes and resource availability to trees have been affecting forest productivity. Here we show that for 222 trees representing three species in eastern North America over the past 30 years earlier spring phenology has caused declines in N availability to trees by increasing demand for N relative to supply. The observed decline in N availability is not associated with reduced wood production, suggesting that other environmental changes such as increased atmospheric CO2 and water availability have likely overwhelmed reduced N availability. Given current trajectories of environmental changes, N limitation will likely continue to increase for these forests, possibly further limiting C sequestration potential.