Tree Physiology Advance Access originally published online on January 19, 2009
Tree Physiology 2009 29(3):321-331; doi:10.1093/treephys/tpn040
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Influence of spring phenology on seasonal and annual carbon balance in two contrasting New England forests
1 Complex Systems Research Center, University of New Hampshire, Morse Hall, 8 College Road, Durham, NH 03824, USA
2 Corresponding author (andrew.richardson{at}unh.edu)
3 USDA Forest Service, Northern Research Station, 271 Mast Road, Durham, NH 03824, USA
4 Department of Plant, Soil, and Environmental Sciences, University of Maine, Orono, ME 04469, USA
5 Division of Engineering and Applied Science, Department of Earth and Planetary Science, Harvard University, Cambridge, MA 02138, USA
6 Harvard Forest, Harvard University, Petersham, MA 01366, USA
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Abstract |
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Spring phenology is thought to exert a major influence on the carbon (C) balance of temperate and boreal ecosystems. We investigated this hypothesis using four spring onset phenological indicators in conjunction with surface–atmosphere CO2 exchange data from the conifer-dominated Howland Forest and deciduous-dominated Harvard Forest AmeriFlux sites. All phenological measures, including CO2 source–sink transition dates, could be well predicted on the basis of a simple two-parameter spring warming model, indicating good potential for improving the representation of phenological transitions and their dynamic responsiveness to climate variability in land surface models. The date at which canopy-scale photosynthetic capacity reached a threshold value of 12 µmol m–2 s–1 was better correlated with spring and annual flux integrals than were either deciduous or coniferous bud burst dates. For all phenological indicators, earlier spring onset consistently, but not always significantly, resulted in higher gross primary productivity (GPP) and ecosystem respiration (RE) for both seasonal (spring months, April–June) and annual flux integrals. The increase in RE was less than that in GPP; depending on the phenological indicator used, a one-day advance in spring onset increased springtime net ecosystem productivity (NEP) by 2–4 g C m–2 day–1. In general, we could not detect significant differences between the two forest types in response to earlier spring, although the response to earlier spring was generally more pronounced for Harvard Forest than for Howland Forest, suggesting that future climate warming may favor deciduous species over coniferous species, at least in this region. The effect of earlier spring tended to be about twice as large when annual rather than springtime flux integrals were considered. This result is suggestive of both immediate and lagged effects of earlier spring onset on ecosystem C cycling, perhaps as a result of accelerated N cycling rates and cascading effects on N uptake, foliar N concentrations and photosynthetic capacity.
Keywords: AmeriFlux, bud burst, carbon dioxide, eddy covariance, growing season length, phenology, spring onset, start of spring
Received July 31, 2008; Accepted November 13, 2008