Does dynamically modelled leaf area improve predictions of land surface water and carbon fluxes? - Insights into dynamic vegetation modules.

Land-surface models represent exchange processes between soil and atmosphere via the surface by coupling water, energy and carbon fluxes. As it strongly mediates the link between these cycles and, vegetation is an important component of land-surface models. In doing so, some of these models include modules for vegetation dynamics which allow adaptation of vegetation biomass, especially leaf area index, to environmental conditions. Here, we conducted a model-data comparison to investigate whether and how vegetation dynamics in the models improves the representation of vegetation processes and related surface fluxes in two specific models ECLand and Noah-MP in contrast to using prescribed values from look-up tables or satellite-based products. We compare model results with stations from the FLUXNET 2015 dataset covering a range in climate and vegetation types, the MODIS leaf area product, and use more detailed information from the TERENO site "Hohes Holz". With the current implementation, switching vegetation dynamics on did not enhance representativeness of e.g. leaf area index and net ecosystem exchange in ECLand, while Noah-MP improved it only for some sites. The representation of energy fluxes and soil moisture was almost unaffected for both models. Interestingly, for both models, the performance regarding vegetation- and hydrology-related variables was unrelated, such that the weak performance regarding e.g. leaf area index did not detoriate the performance regarding e.g. latent heat flux. One reason, we showed here, might be that implemented ecosystem processes diverge from the observations in their seasonal patterns and variability. Noah-MP includes a seasonal hysteresis of the relationship between leaf area index and gross primary production that cannot be found in observations. The same relationship is represented by a strong linear response in ECLand which substantially underestimates the variability seen in observations. For both, water and carbon fluxes, the current implemented modules for vegetation dynamics in these two models yielded no better model performance compared to runs with static vegetation and prescribed leaf-area climatology. [ABSTRACT FROM AUTHOR]