Evaluation of leaf phenology of different vegetation types from local to
hemispheric scale in CLM
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 (PFT) 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
amplitudes 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 MODIS for
temperature-dominated deciduous PFTs. Although the agreement in LAI
amplitude between CLM5.0 and MODIS is better than CLM4.5, the agreement
in seasonal cycles is worse in CLM5.0. CLM5.0 also simulates higher soil
moisture and shows lower influences of soil moisture on LAI amplitudes
and seasonal cycles. 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.