Journal
TREE PHYSIOLOGY
Volume 29, Issue 3, Pages 321-331Publisher
OXFORD UNIV PRESS
DOI: 10.1093/treephys/tpn040
Keywords
AmeriFlux; bud burst; carbon dioxide; eddy covariance; growing season length; phenology; spring onset; start of spring
Categories
Funding
- Office of Science (BER)
- US Department of Energy
- Terrestrial Carbon Program Under Interagency Agreement [DE-AI02-07ER64355]
- Northeastern Regional Center of the National Institute for Climatic Change Research
- Northeast Regional Center of the National Institute for Global Environmental Change under Cooperative Agreement [DE-FC02-03ER63613]
- Terrestrial Carbon Program [DE-FG02-03ER83751]
- National Science Foundation LTER program [DEB-0080592]
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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 CO, 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 mu mol 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.
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