Biophysical Controls of Net Ecosystem Exchange of CO2 across Various Time-scales and Ecosystems in North America

Terrestrial carbon fluxes are controlled by different biophysical processes, which vary across time and space. We investigated the biophysical controls of net ecosystem exchange (NEE) of CO2 at the hourly, daily, weekly, and monthly time-scales during 2000-2015 for 55 AmeriFlux sites representing six diverse ecosystems across North America. Multivariate techniques (correlation matrix, principal component and factor analyses) were employed to identify the emerging linkage patterns of NEE with the climatic and ecohydrological drivers at the different time-scales and ecosystems. The relative climatic and ecohydrological controls of NEE were then estimated with partial least squares regression modeling by resolving multicollinearity among the predictors. NEE had the strongest link to latent heat flux across all time-scales and ecosystem types, indicating a robust pattern of carbon-water-energy interactions in time and space. Three major biophysical components of NEE were identified: 'radiation-energy (RE)', 'temperature-hydrology (TH)', and 'aerodynamic (AD)' components ― exhibiting strong, moderate, and weak controls, respectively, on the hourly time-scale across all ecosystems. However, the control of TH emerged to be stronger at the daily to monthly scales. In contrast, AD persisted to exhibit weak controls at all time-scales and ecosystem types. The findings would guide the development of generalized low-dimensional models to reliably predict terrestrial carbon fluxes.