Connecting mathematical ecosystems, real-world ecosystems, and climate science
Global models of Earth’s climate have become models of the physics, chemistry, and biology of the Earth system. Prominent ecological processes in the models that affect climate include: change in stomatal conductance and water-use efficiency with higher atmospheric CO2; vegetation greening in response to climate change; land use and land cover change; and the carbon cycle. Future use of land to grow food, fiber, and biofuels, and the resulting feedback on climate, is a key aspect of climate policy. This study of biosphere influences on climate by geophysical scientists contrasts with an earlier period in the 1700s and 1800s when the arguments of conservationists and foresters for forest influences on climate was dismissed by climatologists of the day. Today geoscientists embrace the study of biosphere-atmosphere interactions using Earth system models (ESMs). Despite being richly complex in ecological theory and processes, ecologists have not been involved with model development, evaluation, and application. That is changing as ecologists see the commonality of processes represented in ESMs, those in traditional ecosystem models, and those studied in the field. Here, I illustrate some of these common processes and the role of ecological theory and observations to test and improve ESMs. The specific examples pertain to the Community Land Model (CLM), the land component of the Community Earth System Model, and illustrate the importance of deconstructing models into component processes.
(1) Soil carbon is a key feedback with climate change, but is poorly simulated by many ESMs. The CLM simulates low soil carbon compared with observational datasets. Comparison with litterbag decomposition studies in multiple biomes and climates (LIDET) shows that the model decomposition rates are too high and guides model improvement. (2) Most land components of ESMs use a coupled photosynthesis-stomatal conductance parameterization to simulate stomatal conductance (biogeophysics) and gross primary production (carbon cycle). However, the photosynthetic parameter Vcmax is poorly constrained in global models. Constraints on Vcmax imposed from leaf trait databases expose deficiencies in the big-leaf canopy parameterization used in the CLM. (3) Stomatal conductance parameterizations require specification of stomatal closure with soil moisture stress. Most land surface models in ESMs use an empirical scaling factor. Alternatively, stomatal conductance can be modeled directly from plant water relations. These examples highlight the need for better use of ecological theory and observations to guide the development and evaluation of land surface models and provide examples of how ecologists can contribute to ESMs.