Evaluating the Community Land Model in a pine stand with shading manipulations and <sup>13</sup>CO<sub>2</sub> labeling

Abstract

Abstract. Carbon allocation and flow through ecosystems regulates land surface-atmosphere CO 2 exchange and thus is a key, albeit uncertain, component of mechanistic models. The Partitioning in Trees and Soil (PiTS) experimentmodel project tracked carbon allocation through a young Pinus taeda stand following pulse labeling with 13 CO 2 and two levels of shading. The field component of this project provided process-oriented data that were used to evaluate terrestrial biosphere model simulations of rapid shifts in carbon allocation and hydrological dynamics under varying environmental conditions. Here we tested the performance of the Community Land Model version 4 (CLM4) in capturing short-term carbon and water dynamics in relation to manipulative shading treatments and the timing and magnitude of carbon fluxes through various compartments of the ecosystem. When calibrated with pretreatment observations, CLM4 was capable of closely simulating stand-level biomass, transpiration, leaf-level photosynthesis, and pre-labeling 13 C values. Over the 3-week treatment period, CLM4 generally reproduced the impacts of shading on soil moisture changes, relative change in stem carbon, and soil CO 2 efflux rate. Transpiration under moderate shading was also simulated well by the model, but even with optimization we were not able to simulate the high levels of transpiration observed in the heavy shading treatment, suggesting that the BallBerry conductance model is inadequate for these conditions. The calibrated version of CLM4 gave reasonable estimates of label concentration in phloem and in soil surface CO 2 after 3 weeks of shade treatment, but it lacks the mechanisms needed to track the labeling pulse through plant tissues on shorter timescales. We developed a conceptual model for photosynthate transport based on the experimental observations, and we discussed conditions under which the hypothesized mechanisms could have an important influence on model behavior in larger-scale applications. Implications for future experimental studies are described, some of which are already being implemented in follow-on studies.

Evaluating the Community Land Model (CLM4.5) at a coniferous forest site in northwestern United States using flux and carbon-isotope measurements

“…Modeling studies have shown that stable carbon isotopes provide a constraint upon stomatal conductance (Aranibar et al, 2006;Raczka et al, 2016;Mao et al, 2016). Aranibar et al (2006) evaluated the performance of ISOLSM at the Metolius Old Pine AmeriFlux site and were able to calibrate the slope of the stomatal conductance equation (m bb in the Ball-Berry stomatal conductance model; see Eq.…”

“…By using an alternative approach in which a nitrogen downscaling factor is directly applied to V cmax25 (maximum rate of carboxylation at 25 • C), they found significant improvement in the simulations, but with results still suggesting that a smaller m bb value (they used the default C 3 value, m bb = 9) would better simulate the site observations. Mao et al (2016) evaluated CLM4.0 at a loblolly pine site in Tennessee, USA, and were able to adequately simulate the observed biomass δ 13 C values with an optimized m bb value of 5.6. Keller et al (2017) used a global tree-ring δ 13 C data set to evaluate the 20th century trend in photosynthetic 13 C discrimination and iWUE as modeled by CLM4.5 and LPX-Bern.…”

“…In addition to energy flux observations that allow for the estimation of canopy conductance, this study leverages the recent inclusion of photosynthetic 13 C discrimination within CLM and also uses δ 13 C observations to better diagnose the simulation of stomatal conductance at the site. We test whether a reduced stomatal conductance at similar needleleaf evergreen temperate forest sites (Mao et al, 2016;Raczka et al, 2016) is appropriate for Wind River. This study also provides further investigation on the nitrogen limitation issue identified by Raczka et al (2016) and the adequacy of the default parameters used in CLM, especially those regulating stomatal conductance.…”

“…We test whether the calibration scheme (optimized parameters, nitrogen limitation) proposed by Raczka et al (2016) for Niwot Ridge is appropriate for Wind River. By comparing the results at Wind River against those at different sites characterized by the same plant functional type (needleleaf evergreen temperate tree) but with different stand composition and age and climatology (Mao et al, 2016;Raczka et al, 2016), this study also seeks to identify general improvements in model parameterization.…”

Temporal and Spatial Variation in Peatland Carbon Cycling and Implications for Interpreting Responses of an Ecosystem‐Scale Warming Experiment

“…Sensitivity analysis can investigate which parameters and related processes are the most significant contributors to these variations. Sensitivity analysis has been applied in a number of global C cycle models and has identified a small number of critical parameters with relatively large impacts on uncertainty, especially the parameters associated with photosynthesis, respiration, and plant allocation processes (e.g., Dietze et al, 2014;Lu et al, 2014;Sargsyan et al, 2014;Mao et al, 2016). When these model parameters represent measurable quantities, the sensitivities can provide useful feedback about which observations are needed to better reduce variability.…”