Abstract
Abstract. Correct representation of seasonal leaf dynamics is crucial for terrestrial biosphere models (TBMs), but many such models cannot accurately reproduce observations of leaf onset and senescence. Here we optimised the phenology-related parameters of the ORCHIDEE TBM using satellite-derived Normalized Difference Vegetation Index data (MODIS NDVI v5) that are linearly related to the model fAPAR. We found the misfit between the observations and the model decreased after optimisation for all boreal and temperate deciduous plant functional types, primarily due to an earlier onset of leaf senescence. The model bias was only partially reduced for tropical deciduous trees and no improvement was seen for natural C4 grasses. Spatial validation demonstrated the generality of the posterior parameters for use in global simulations, with an increase in global median correlation of 0.56 to 0.67. The simulated global mean annual gross primary productivity (GPP) decreased by ~ 10 PgC yr−1 over the 1990–2010 period due to the substantially shortened growing season length (GSL – by up to 30 days in the Northern Hemisphere), thus reducing the positive bias and improving the seasonal dynamics of ORCHIDEE compared to independent data-based estimates. Finally, the optimisations led to changes in the strength and location of the trends in the simulated vegetation productivity as represented by the GSL and mean annual fraction of absorbed photosynthetically active radiation (fAPAR), suggesting care should be taken when using un-calibrated models in attribution studies. We suggest that the framework presented here can be applied for improving the phenology of all global TBMs.
Highlights
Leaf phenology, the timing of leaf onset, growth and senescence, is a critical component of the coupled soil–vegetation– atmosphere system as it directly controls the seasonal exchanges of carbon, C, as well as affecting the surface energy balance and hydrology through changing albedo, surface roughness, soil moisture and evapotranspiration
44.1–44.6 28–44 54–63 77–82 29–44 4–11 the value of J (x) for the default parameters of ORCHIDEE) across all 20 random tests was seen for the boreal needleleaved (BoND) plant functional types (PFTs)
There was a higher spread in the % reduction for natural C3 grasses and the temperate broadleaved deciduous (TeBD) PFTs, suggesting the cost function is not as smooth as for the BoND PFT
Summary
The timing of leaf onset, growth and senescence, is a critical component of the coupled soil–vegetation– atmosphere system as it directly controls the seasonal exchanges of carbon, C, as well as affecting the surface energy balance and hydrology through changing albedo, surface roughness, soil moisture and evapotranspiration. In turn leaf phenology is largely governed by the climate, as leaf onset and senescence are triggered by seasonal changes in temperature, moisture and radiation. It is expected that climate warming will advance leaf onset in temperature limited northern biomes. Such trends have already been observed in the Northern Hemisphere (NH) using either satellite or in situ observations (Badeck et al, 2004; Delbart et al, 2008; Jeong et al, 2011; Myneni et al, 1997; Parmesan, 2007). Future changes of precipitation in a warming climate will likely affect tropical and semi-arid ecosystems that are more controlled by moisture availability (e.g. Anyamba and Tucker, 2005; Dardel et al, 2014; Fensholt et al, 2012)
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