Evaluating the Community Land Model in a pine stand with shadingmanipulations and 13CO2 labeling

[摘要] Carbon allocation and flow through ecosystems regulates landsurface–atmosphere CO₂ exchange and thus is a key, albeit uncertain,component of mechanistic models. The Partitioning in Trees and Soil (PiTS)experiment–model project tracked carbon allocation through a young Pinus taeda standfollowing pulse labeling with ¹³CO₂ and two levels of shading.The field component of this project provided process-oriented data that wereused to evaluate terrestrial biosphere model simulations of rapid shifts incarbon allocation and hydrological dynamics under varying environmentalconditions. Here we tested the performance of the Community Land Modelversion 4 (CLM4) in capturing short-term carbon and water dynamics inrelation to manipulative shading treatments and the timing and magnitude ofcarbon fluxes through various compartments of the ecosystem. When calibratedwith pretreatment observations, CLM4 was capable of closely simulatingstand-level biomass, transpiration, leaf-level photosynthesis, andpre-labeling ¹³C values. Over the 3-week treatment period, CLM4generally reproduced the impacts of shading on soil moisture changes,relative change in stem carbon, and soil CO₂ efflux rate. Transpirationunder moderate shading was also simulated well by the model, but even withoptimization we were not able to simulate the high levels of transpirationobserved in the heavy shading treatment, suggesting that the Ball–Berryconductance model is inadequate for these conditions. The calibrated versionof CLM4 gave reasonable estimates of label concentration in phloem and insoil surface CO₂ after 3 weeks of shade treatment, but it lacks themechanisms needed to track the labeling pulse through plant tissues onshorter timescales. We developed a conceptual model for photosynthatetransport based on the experimental observations, and we discussed conditionsunder which the hypothesized mechanisms could have an important influence onmodel behavior in larger-scale applications. Implications for futureexperimental studies are described, some of which are already beingimplemented in follow-on studies.

[学科分类] 地球化学与岩石