Monitoring, modeling, and manipulative experiments for understanding ecological and biogeochemical processes in terrestrial ecosystems

Terrestrial ecosystems cover approximately 30% of Earth's surface and provide indispensable ecosystem goods and services to humanity. Interacting with the atmosphere and hydrosphere, they play a principle role in the global biogeochemical cycles. Over the century, with improved monitoring, modeling, and experiments, we have greatly enriched our knowledge about ecological and biogeochemical processes in terrestrial ecosystems, enabling us to evaluate current and future changes of the terrestrial biosphere in light of changing environments. For instance, we have learned from global forest inventories and remote sensing data that global forests have removed 2.4-2.8 billion tons of carbon each year from the atmosphere, helping to alleviate the buildup of carbon dioxide (CO₂), and offsetting about 30% of global CO₂ emissions from fossil fuel use. However, continuing global deforestation and forest degradation, together with increasing extreme climates and wildfires, are threatening this carbon sink. Ecosystem-level manipulative experiments, such as CO₂ and nitrogen (N) additions, have helped us to understand the impacts of CO₂ fertilization and N restriction, while computer models incorporate new findings to simulate dynamics and shifts of terrestrial ecosystems, probing mechanisms that cannot be observed and projecting the future. However, these pictures are still incomplete because of deficiencies in terrestrial monitoring systems such as forest inventories and eddy flux towers, particularly in the areas we most need to understand for the future such as tundra and tropics. Remote sensing (RS) observations provide invaluable data for timing of biological activities across all continents, yet ground-based measurements in remote areas are still necessary for RS calibration. Manipulative experiments are often limited by relatively simple factorial designs or short durations due to high operating expenses. As we celebrate our achievements in the last century, our future research may need to facilitate these shortages and improve sophisticated synthesis of monitoring, modeling and experimental data to better understand terrestrial biogeochemical dynamics and exchanges between the land, water, and atmosphere.