Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/3/2010
Publication Date: 11/11/2010
Publication URL: http://hdl.handle.net/10113/48764
Citation: Drewry, D.T., Kumar, P., Long, S., Bernacchi, C.J., Liang, X., Sivapalan, M. 2010. Ecohydrological responses of dense canopies to environmental variability Part 1: Interplay between vertical structure and photosynthetic pathway. Journal of Geophysical Research-Biogeosciences. doi:10.1029/2010JG001340. Interpretive Summary: The existing tools for assessing how ecosystems respond to changes in their environment on a large scale (field-scale and larger) are limited to costly equipment or to models. Deploying equipment is ideal, however this equipment is limited to a certain number of fields and only apply to existing conditions. Models are less certain but can represent both large land areas and can be run for a variety of environmental conditions (for example, conditions predicted for future climate change scenarios). Existing models treat plant canopies in a simple manner; they assume a certain set of conditions will prevail and use basic formulations of how light and temperature change with depth in the plant canopy. In this paper, a new canopy physiology model is presented that provides a comprehensive mechanistic understanding of canopy-atmosphere interactions. The new model, MLCan, breaks that canopy down into numerous layers and vertically resolves the complexity of a plant canopy to the extent that the model is able to predict the consequences of even slight physiological adaptations to the environment. In this paper, the model is developed for maize and soybean and compared with directly measured data from a field monitoring station in Central Illinois. This model will improve the understanding of how corn and soybean, two critical agronomic species in the U.S., respond to their environment.
Technical Abstract: Vegetation acclimation to changing climate, in particular elevated atmospheric concentrations of carbon dioxide (CO2), has been observed to include modifications to the biochemical and eco physiological functioning of leaves and the structural components of the canopy. These responses have the potential to significantly modify plant carbon uptake and surface energy partitioning, and have been attributed with large-scale changes in surface hydrology over recent decades. While the aggregated effects of vegetation acclimation can be pronounced, they often result from subtle changes in canopy properties that require the resolution of physical, biochemical and ecophysiological processes through the canopy for accurate estimation. In this paper, the first of two, a multi-layer canopy-soil-root system model developed to capture the emergent vegetation responses to environmental change is presented. The model incorporates both C3 and C4 photosynthetic pathways, and resolves the vertical radiation, thermal and environmental regimes within the canopy. The tight coupling between leaf ecophysiological functioning and energy balance determines vegetation responses to climate states and perturbations, which are modulated by soil moisture states through the depth of the root system. The model is validated for three growing seasons each for soybean (C3) and maize (C4) using eddy-covariance fluxes of CO2, latent and sensible heat collected at the Bondville (Illinois) Ameriflux tower site. The dataset provides an opportunity to examine the role of important environmental drivers and model skill in capturing variability in canopy-atmosphere exchange. Vertical variation in radiative states and scalar fluxes over a mean diurnal cycle are examined to understand the role of canopy structure on the patterns of absorbed radiation and scalar flux magnitudes and the consequent differences in sunlit and shaded source/sink locations through the canopies. An analysis is made of the impact of soil moisture stress on carbon uptake and energy flux partitioning at the canopy-scale and resolved through the canopy, providing insight into the roles of canopy structure and metabolic pathway on the response of each crop to moisture deficits. Model calculations indicate increases in water use efficiency (WUE) with increasing moisture stress, with average maize WUE increases of 45% at the highest levels of plant stress examined here, relative to 20% increases for soybean.