Effects of lateral hydrological processes on photosynthesis and evapotranspiration in a boreal ecosystem

Landscape-scale hydrological processes can greatly alter the local-scale water balance and many ecological processes linked to it. We hypothesized that in humid forest ecosystems, topographically driven lateral subsurface flow (SSF) has significant influence on ecophysiological processes such as gross primary productivity (GPP) and evapotranspiration (ET). To investigate how simplified hydrological conceptualizations influence the simulated ET and GPP in space and time, we conducted a numerical experiment using a spatially explicit hydroecological model, BEPS-TerrainLab V2.0. We constructed three modelling scenarios: (1) Explicit, where a realistic calculation of SSF was employed considering topographic controls, (2) Implict, where the SSF calculations were based on a bucket-modelling approach and (3) NoFlow, where the SSF was turned-off in the model. Statistical analyses of model outputs showed considerable differences among the three scenarios for the simulated GPP and ET. The NoFlow scenario generally underestimated GPP and ET, while the Implicit scenario overestimated them relative to the Explicit scenario, both in time and space. GPP was more sensitive to SSF than ET because of the presence of unique compensatory mechanisms associated with the subcomponents of the total ET. The key mechanisms controlling GPP and ET were manifested through nonlinear changes in stomatal conductance, unique contributions from GPP and ET subcomponents, alterations in rhizosphere wetting patterns and their impacts on upscaling mechanisms and variability in nitrogen dynamics (for GPP). Feedback and interactive relationships between hydrological and ecophysiological processes also exacerbated the biases. Thus, we conclude that ecological models that have simplified hydrological representations could have significant errors in the estimation of GPP and ET. Copyright  2010 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this article.