Interannual variability of net ecosystem productivity in forests is explained by carbon flux phenology in autumn. 2013. Wu, C.; Chen, J.M.; Black, T.A.; Price, D.T.; Kurz, W.A.; Desai, A.R.; Gonsamo, A.; Jassal, R.S.; Gough, C.M.; Bohrer, G.; Dragoni, D.; Herbst, M.; Gielen, B.; Berninger, F.; Vesala, T.; Mammarella, I.; Pilegaard, K.; Blanken, P.D. Global Ecology and Biogeography

Thu, 07 Feb 2013

Aim To investigate the importance of autumn phenology in controlling interannual variability of forest net ecosystem productivity (NEP) and to derive new phenological metrics to explain the interannual variability of NEP.

Location North America and Europe.

Method Flux data from nine deciduous broadleaf forests (DBF) and 13 evergreen needleleaf forests (ENF) across North America and Europe (212 site-years) were used to explore the relationships between the yearly anomalies of annual NEP and several carbon flux based phenological indicators, including the onset/end of the growing season, onset/end of the carbon uptake period, the spring lag (time interval between the onset of growing season and carbon uptake period) and the autumn lag (time interval between the end of the carbon uptake period and the growing season). Meteorological variables, including global shortwave radiation, air temperature, soil temperature, soil water content and precipitation, were also used to explain the phenological variations.

Results We found that interannual variability of NEP can be largely explained by autumn phenology, i.e. the autumn lag. While variation in neither annual gross primary productivity (GPP) nor in annual ecosystem respiration (R_e) alone could explain this variability, the negative relationship between annual NEP and autumn lag was due to a larger R_e/GPP ratio in years with a prolonged autumn lag. For DBF sites, a longer autumn lag coincided with a significant decrease in annual GPP but showed no correlation with annual R_e. However, annual GPP was insensitive to a longer autumn lag in ENF sites but annual R_e increased significantly.

Main conclusions These results demonstrate that autumn phenology plays a more direct role than spring phenology in regulating interannual variability of annual NEP. In particular, the importance of respiration may be potentially underestimated in deriving phenological indicators.

Interannual and spatial impacts of phenological transitions, growing season length, and spring and autumn temperatures on carbon sequestration: a North America flux data synthesis. 2012. Wu, C.; Gonsamo, A.; Chen, J.M.; Kurz, W.A.; Price, D.T.; Lafleur, P.M.; Jassal, R.S.; Dragoni, D.; Bohrer, G.; Gough, C.M.; Verma, S.B.; Suyker, A.E.; Munger, J.W. Global and Planetary Change 92-93:179-190.

Tue, 19 Jun 2012

Understanding feedbacks of ecosystem carbon sequestration to climate change is an urgent step in developing future ecosystem models. Using 187 site-years of flux data observed at 24 sites covering three plant functional types (i.e. evergreen forests (EF), deciduous forests (DF) and non-forest ecosystems (NF) (e.g., crop, grassland, wetland)) in North America,we present an analysis of both interannual and spatial relationships between annual net ecosystem production (NEP) and phenological indicators, including the flux-based carbon uptake period (CUP) and its transitions, degree-day-derived growing season length (GSL), and spring and autumn temperatures. Diverse responses were acquired between annul NEP and these indicators across PFTs. Forest ecosystems showed consistent patterns and sensitivities in the responses of annual NEP to CUP and its transitions both interannually and spatially. The NF ecosystems, on the contrary, exhibited different trends between interannual and spatial relationships. The impact of CUP onset on annual NEP in NF ecosystems was interannually negative but spatially positive. Generally, the GSL was observed to be a likely good indicator of annual NEP for all PFTs both interannually and spatially, although with relatively moderate correlations in NF sites. Both spring and autumn temperatures were positively correlated with annual NEP across sites while this potential was greatly reduced temporally with only negative impacts of autumn temperature on annual NEP in DF sites. Our analysis showed that DF ecosystems have the highest efficiency in accumulating NEP from warmer spring temperature and prolonged GSL, suggesting that future climate warming will favor deciduous species over evergreen species, and supporting the earlier observation that ecosystemswith the greatest net carbon uptake have the longest GSL.

Publications by D. Dragoni