Challenges for evaluating process-based models of gas exchange

Abstract

Physiologically-based (or process-based) models are commonly applied to describe plant responses mechanistically in dependence on environmental conditions. They are increasingly evaluated with eddy-covariance measurements that integrate carbon and water exchange of an area of several hectares (called the fetch). However, almost all models applied to date in such exercises have considered only the dominant tree species and neglected other species that contributed to the measured gas exchange rates-either in separate patches or in mixture. This decreases the transferability of the model from one site to another because the contributions from other species might be different. It is therefore a major challenge in modeling today to separate the measured gas exchanges by sources.In this study, a detailed physiologically-based biosphere model is applied that allows distinguishing between tree species in mixed forests, considering them as «vegetation cohorts» that interact with each other. The sensitivity of the model to different assumptions about how different tree species contribute to an integrated measurement of standscale gas exchange is investigated. The model exercise is carried out for a forest site in Finland with dominant Scots pine but presence of significant amounts of Norway spruce and birch. The results demonstrate that forest structure affects simulated gas exchange rates indicating a possible importance of considering differences in physiological properties at the species level. It is argued that the variation of stand structure within the range of eddy-covariance measurements should be better accounted for in models and that inventory measurements need to consider this variation.Key words: forest structure; understorey; physiologically-oriented model; eddy-flux measurements; sensitivity analysis. ResumenRetos para la evaluación de modelos basados en procesos de intercambio de gases en rodales con diversas especies Los modelos con base fisiológica (o basados en procesos) se aplican habitualmente para describir las respuestas de la planta mecanísticamente dependiendo de las condiciones ambientales. Son evaluados cada vez más frecuentemente con mediciones de eddy-covariance que integran el intercambio de carbono y el agua de una superficie de varias hectáreas. Sin embargo, casi todos los modelos aplicados hasta la fecha han considerado sólo las especies arbóreas dominantes y se han descuidado otras especies que contribuyen a las tasas de intercambio gaseosos medidas-ya sea en zonas separadas o en mezcla. Esto disminuye la posibilidad de transferir el modelo de un sitio a otro debido a que las contribuciones de otras especies pueden ser diferentes. Por tanto, es un reto importante en la modelización separar el intercambio gaseoso medido por fuente emisora.En este estudio, se aplica un modelo detallado con base fisiológica de la biosfera que permite distinguir entre las especies de árboles en bosques mixtos, considerándolas como «cohortes de vegetación» que interaccionan unos con otros. Se estudia l...

Simulation of CO2 Fluxes in European Forest Ecosystems with the Coupled Soil-Vegetation Process Model “LandscapeDNDC”

“…LandscapeDNDC links modules describing microclimate, water cycle, soil-biogeochemistry, plant physiological processes and dimensional changes by daily time step integration. All processes and state variables are considered in a vertically structured one-dimensional column including tree canopy, humus horizons and mineral soil [36]. Detailed process descriptions and evaluations have been reported in earlier studies including water balance [37,38], soil respiration and N trace gas emission [39] and nitrate leaching [40].…”

Biomass production and nitrogen balance of naturally afforested silver birch (<i>Betula pendula</i> Roth.) stand in Estonia

“…However, without analysing the net ecosystem exchange (NEE) of CO 2 , it is difficult to judge the system's response as a net warmer or cooler of the atmosphere. However, the modelled NEE values for birch forests in Finland show that these ecosystems are net sinks of atmospheric carbon (Grote et al 2011), which was also confirmed in the Estonian case study . The average methane flux for the whole study period was almost below zero, meaning that the stand is consuming CH 4 (Fig.…”

Extending a physiological forest growth model by an observation-based tree competition module improves spatial representation of diameter growth

“…carbon exchange and biomass allocation, have been determined from literature sources and were evaluated using beech trials at other investigation sites (Grote et al 2011a). The setup of a cohort simulation and an application for a mixed stand dominated by Scots pine have been presented in Grote et al (2011b).…”

“…It provides a high spatial differentiation of processes and runs in subdaily to daily time steps, and it is very sensitive to uncertainties in individual initialization (Fontes et al 2010), costly in parameterization and comparatively slow. To reduce complexity while preserving a high degree of mechanistic description, Grote et al (2011b) have represented stand structure in MoBiLE-PSIM as an ensemble of spatially interacting single species cohorts, where each cohort consists of trees that are identical in dimension and are ordered on a regular grid. Presuming this approximation, competition may be covered by exclusively simulating one representative tree per cohort, and computational efficiency is increased by an order of magnitude.…”