Publications

Li, XL; Carrillo, CM; Ault, T; Richardson, AD; Friedl, MA; Frolking, S (2024). Evaluation of Leaf Phenology of Different Vegetation Types From Local to Hemispheric Scale in CLM. JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES, 129(10), e2024JG008261.

Abstract
Accurate simulation of plant phenology is important in Earth system models as phenology modulates land-atmosphere coupling and the carbon cycle. Evaluations based on grid cell average leaf area index (LAI) can be misleading because multiple plant functional types (PFTs) may be present in one model grid cell and PFTs with different phenology schemes have different LAI seasonal cycles. Here we examined PFT-specific LAI magnitudes and seasonal cycles in the Community Land Model versions 5.0 and 4.5 (CLM5.0 and CLM4.5) and their relationship with the onset of growing season triggers in the Northern Hemisphere. LAI seasonal cycle and spring onset in CLM show the best agreement with Moderate Resolution Imaging Spectroradiometer (MODIS) for temperature-dominated deciduous PFTs. Although the agreement in LAI magnitude between CLM5.0 and MODIS is better than CLM4.5, the agreement in seasonal cycles is worse in CLM5.0. Agreements between CLM and MODIS leaf phenology are primarily determined by the PFT and phenology scheme. While productivity depends on the environmental factors to which the plant is exposed during any given growing season, differences in phenology sensitivity to its environment necessitate a decoupling between the seasonality of LAI and GPP, which in turn could lead to biases in the carbon cycle as well as surface energy balance and hence land-atmosphere interactions. Because the discrepancy not only depends on parameterizing phenology but phenology-environment relationship, future improvements to other model components (e.g., soil moisture) could better align the seasonal cycle of LAI and GPP. The timing of leaf growth and senescence modifies the exchange of water, carbon, and energy between the land and the atmosphere. However, discrepancies exist between how land surface models simulate leaf area and what remote sensing products show. In this study, we examined how different types of plants (like evergreen and deciduous trees) vary in leaf area and seasonality in two versions of a land surface model-the Community Land Model versions 5.0 and 4.5 (CLM5.0 and CLM4.5). In the Northern Hemisphere, the timing of spring leaf growth and growing season length match satellite data best for temperature-sensitive deciduous plants. The newer model version (CLM5.0) is more accurate in representing the magnitude of leaf area but is less accurate in seasonal timing. We also observed that the timing of leaf changes is mainly determined by plant type, while plant productivity is more affected by environmental factors. This misalignment between seasonal leaf area and productivity can lead to errors in understanding the carbon cycle and interactions between the land and the atmosphere. Improving other parts of the model, like soil moisture, could help better align leaf area with productivity in future models. CLM LAI exhibits the best agreement with Moderate Resolution Imaging Spectroradiometer (MODIS) in seasonal deciduous PFTs and deciduous broadleaf trees Agreements in LAI magnitudes and seasonal cycles between CLM and MODIS are primarily determined by the PFT and phenology scheme Discrepancies in LAI result in biases in GPP, but improvements in one variable may not lead to better results in the other

DOI:
10.1029/2024JG008261

ISSN:
2169-8961