Articles | Volume 12, issue 15
Biogeosciences, 12, 4693–4709, 2015
Biogeosciences, 12, 4693–4709, 2015

Research article 06 Aug 2015

Research article | 06 Aug 2015

Probing the past 30-year phenology trend of US deciduous forests

X. Yue1, N. Unger1, T. F. Keenan2, X. Zhang3, and C. S. Vogel4 X. Yue et al.
  • 1School of Forestry and Environmental Studies, Yale University, 195 Prospect Street, New Haven, CT 06511, USA
  • 2Department of Biological Sciences, Macquarie University, Sydney, New South Wales 2109, Australia
  • 3Geospatial Sciences Center of Excellence, South Dakota State University, Brookings, SD 57007, USA
  • 4The University of Michigan Biological Station, Pellston, MI 49769, USA

Abstract. Phenology is experiencing dramatic changes over deciduous forests in the USA. Estimates of trends in phenology on the continental scale are uncertain, however, with studies failing to agree on both the magnitude and spatial distribution of trends in spring and autumn. This is due to the sparsity of in situ records, uncertainties associated with remote sensing data, and the regional focus of many studies. It has been suggested that reported trends are a result of recent temperature changes, though multiple processes are thought to be involved and the nature of the temperature forcing remains unknown. To date, no study has directly attributed long-term phenological trends to individual forcings across the USA through integrating observations with models. Here, we construct an extensive database of ground measurements of phenological events across the USA, and use it to calibrate and evaluate a suite of phenology models. The models use variations of the accumulative temperature summation, with additional chilling requirements for spring phenology and photoperiod limitation for autumn. Including a chilling requirement or photoperiod limitation does not improve model performance, suggesting that temperature change, especially in spring and autumn, is likely the dominant driver of the observed trend during the past 3 decades. Our results show that phenological trends are not uniform over the contiguous USA, with a significant advance of 0.34 day yr−1 for the spring budburst in the east, a delay of 0.15 day yr−1 for the autumn dormancy onset in the northeast and west, but no evidence of change elsewhere. Relative to the 1980s, the growing season in the 2000s is extended by about 1 week (3–4 %) in the east, New England, and the upper Rocky Mountains forests. Additional sensitivity tests show that intraspecific variations may not influence the predicted phenological trends. These results help reconcile conflicting reports of phenological trends in the literature, and directly attribute observed trends to long-term changes in temperature.

Short summary
We performed model inter-comparison and selected the best model capturing the spatial and temporal variations of observations to predict trends of forest phenology over the past 3 decades. Our results show that phenological trends, which are dominantly driven by temperature changes, are not uniform over the contiguous USA, with a significant spring advance in the east, an autumn delay in the northeast and west, but no evidence of change elsewhere.
Final-revised paper