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Carbon emitted to the atmosphere by human activities is the primary cause of global climate change. On average, the land sequesters over 30% of anthropogenic carbon emitted to the atmosphere each year. The only mechanism for carbon uptake by terrestrial ecosystems is photosynthesis, also called Gross Primary Productivity (GPP) at the ecosystem scale. This flux is estimated to be between 100-200 Petagrams of Carbon annually and shows substantial spatiotemporal variations due to climate. Space-based observations can be used as a proxy for GPP across the globe, providing insights into the effects of climate on regional carbon cycling in ecosystems of critical importance. In this dissertation, I study the spatiotemporal impacts of a changing climate on GPP in the ABZ and Tropics using contiguous solar-induced fluorescence (CSIF) as a proxy for GPP along with several climate datasets to assess the sensitivity of productivity to climate in two critical ecosystems: The Arctic Boreal Zone (ABZ) and the Tropics. These critical ecosystems have the two largest stocks of carbon stored on land and both are projected to experience large climate trends.
Within boreal forests, there is evidence that boreal ecosystems in Eurasia show stronger greening trends compared to North America, suggesting regional differences in the trends in GPP to trends in climate. In this thesis, I confirm that North American and Eurasian boreal forests show different sensitivities to climate variations. Our analysis suggests that temperature is a more important control of Eurasian forest productivity than North American productivity. Eurasian boreal forest productivity on average is more sensitive to both spatial and interannual temperature variations than North American boreal forest productivity. Long-term warming decreases productivity gains in western North American boreal forests, which may be partly due to the fact that I found that North American boreal forests are more strongly controlled by moisture than their Eurasian counterparts. My analysis of North American forests showed that productivity is controlled by moisture, and leaves a discernible imprint on soil moisture reserves, as measured by terrestrial water storage. The contrasting controls of temperature and moisture across Eurasia and North America respectively, suggests that we may see diverging responses to productivity in Eurasia and North America in the future with a warming climate.
Within the tropics, there are also open questions as to whether temperature or moisture controls productivity variations, especially during different phases of the El Niño-Southern Oscillation (ENSO). We learn that Central Pacific El Niño shows a larger spatial decline in terrestrial water storage and productivity when compared to Eastern Pacific El Niño. Productivity saw higher spatial coherence during most Central Pacific ENSO events from 2001-2018. However, productivity saw much lower spatial coherence compared to the spatial coherence for climate, suggesting divergent regional responses in tropical productivity. This thesis contributes to important understanding about how different drivers impact carbon uptake in critical ecosystems.