Bud-burst modelling in Siberia and its impact on quantifying the carbon budget

Vegetation phenology is affected by climate change and in turn feeds back on climate by affecting the annual carbon uptake by vegetation. To quantify the impact of phenology on terrestrial carbon fluxes, we calibrate a bud-burst model and embed it in the Sheffield Dynamic Global Vegetation Model (SDGVM) in order to perform carbon budget calculations. Bud-burst dates derived from the VEGETATION sensor onboard the SPOT-4 satellite are used to calibrate a range of bud-burst models. This dataset has been recently developed using a new methodology based on the Normalised Difference Water Index (NDWI), which is able to distinguish snowmelt from the onset of vegetation activity after winter. After calibration, a simple spring warming model was found to perform as well as more complex models accounting for a chilling requirement, hence was used for the carbon flux calculations. The root mean square difference between the calibrated model and the VEGETATION dataset was 6.5 days, and was 6.9 days between the calibrated model and independent ground observations of bud-burst available at 9 locations over Siberia. The effects of bud-burst model uncertainties on the carbon budget were evaluated using the SDGVM. The 6.5 days RMS difference in the bud-burst date (a 6% variation in the growing season length), treated as a random noise, translates into about 41 gCm year in Net Primary Production (NPP), which corresponds to 8% of the mean NPP. This is a moderate impact and suggests the calibrated model is accurate enough for carbon budget calculations. In addition to random differences between the calibrated model and VEGETATION data, systematic errors between the calibrated bud-burst model and true ground behaviour may occur, because of bias in the temperature dataset or because the bud-burst detected by VEGETATION is due to some other phenological indicator. A systematic error of one day in bud-burst translates into a 10 gCm year error in NPP (about 2%). Based on the limited available ground data, any systematic error due to using the VEGETATION data should not lead to significant errors in the calculated carbon flux. In contrast, widely-used methods based on the Normalised Difference Vegetation Index (NDVI) from the AVHRR satellite are likely to confuse snowmelt and vegetation greening, leading to errors of up to 15 days in bud-burst date, with consequent large errors in carbon flux calculations. ird -0 04 26 21 7, v er si on 1 26 O ct 2 00 9