Decadal trends in photosynthetic capacity and leaf area index inferred from satellite remote sensing for global vegetation types

In the face of a mounting diversity of experimental, satellite and ground-based observations, it is becoming necessary to simulate all changes associated with vegetation (phenological, structural, physiological and biochemical) and to understand the links between them. In this respect, global land-surface models are an indispensible tool. These models require, above all, a temporally and spatially explicit parameterisation of light- and Rubisco-limited photosynthetic capacity in order to simulate photosynthesis accurately. The current study carries out a novel retrieval of these quantities by combining the standard satellite products of Leaf Area Index (LAI), from the Moderate Resolution Imaging Spectroradiometer (MODIS), with a hyperspectral index of total canopy chlorophyll concentration from the MEdium Resolution Imaging Spectrometer (MERIS). Monthly values of photosynthetic capacity are retrieved for the period 2002-2012 for global 0.5° landpoints and made available to the community. We examine the decadal trends in both photosynthetic capacity and LAI in order to ascertain biochemical and structural responses of vegetation to environmental change. The main conclusion is that these trends, if sustained, are of a sufficient magnitude to vie in importance with other environmental factors which affect vegetation productivity and carbon uptake (e.g. CO2 fertilisation and climate). The decadal trends for Rubisco-limited photosynthetic capacity, which tend to be negative, depend more on plant functional type than latitude, suggesting that biochemical change, like physiological response (e.g. owing to CO2 fertilisation), might best be monitored in terms of vegetation type rather than climate zone. We record an LAI trend which, globally, is flat (−0.2 ± 0.4% per decade) and, for the (mid-)northern latitudes, is much smaller (1.5-2.7% per decade) than that inferred by previous authors for Normalised Difference Vegetation Index (NDVI) during the 1980s (9-13%).