Large gains in leaf scale photosynthetic rates of sparsely vegetated arid and semi-arid lands

Photosynthesis drives carbon into terrestrial ecosystems via gross primary production (GPP), rising ~3% decade−1 globally, paralleled by green leaf area expansion. Both trends stem from CO2 fertilization (CFE), climate change, and landcover change, yet spatial mismatches exist between GPP and leaf area changes. Canopy scale GPP changes are separated into those resulting from leaf area changes and those from changing photosynthetic rates per unit leaf area. The latter, by normalizing for leaf area, isolate production dependency on CO2, light and water availability, and the kinetics of the process itself. Arid/semi-arid shrublands and grasslands show substantial CO2 assimilation gains, dominated by CFE and beneficial climate effects. Conversely, croplands and forests exhibit declining leaf assimilation rate due to increased shading from excessive greening and adverse climate shifts (e.g., reduced precipitation, higher VPD). The Sudano-Sahelian zone exemplifies assimilation increases in sparse vegetation, while conspicuously greening regions like China and India face assimilation declines. This pattern of canopy GPP increases from gains in both leaf scale assimilation rates and leaf area in sparse vegetation of inhospitable climes, and only from leaf area gains in more productive vegetation of favorable environments, whilst counter-intuitive, suggests an unforeseen potential for carbon sequestration in sparsely vegetated lands. Vegetation in arid regions benefits from higher leaf-scale assimilation rates and expanded leaf area, boosting carbon uptake, while productive forests and croplands experience declines due to increased shading and climate stress, based on remote sensing data and multiple process-based models.