Questions on Stress-Deciduous Onset Triggers for C3 Grass?

[mengqi]

Hi,

I am working on focusing on the C3 grass phenology. In my case, the simulation results show that the occurrence of onset growth period for simulated data is greater later than that for observed data. Thus, I am going to adjust the relevant parameters.

However, CLM5 states that the C3 grass governed by stress-deciduous phenology starts growing only after their chilling requirements are met and in response to GDD thresholds and soil moisture. In this context, the stress-deciduous onset trigger is a very complex process. So I am a little confused about which parameter is the most important or predominant.

I was wondering if anyone could give me some advice on that. Thank you so much in advance!!

 

[samrabin]

From the User's Guide, it looks like stress-deciduous onset will depend on whether you're looking at a place with a cold season or a place that's warm year-round. Dahlin et al. (2015) say that a gridcell has a cold season if, beginning with the onset of the dormant period, it has 15 days worth of timesteps below freezing (p. 5063).

The actual onset trigger code doesn't seem to have too many parameters, so are you maybe asking more about leaf-up during the onset period?

 

[dll@ucar_edu]

I don't know that there is one parameter that is most important. You can try changing individual parameters to get a sense for which might be the most important. You might also be able to glean some initial insights from the Parameter Perturbation Ensemble (PPE) that our group has run, and I can try to connect you with the appropriate resources if you're interested. In short, I would imagine temperature and water availability parameters to be the most important in most locations. These might include (not an exhaustive list): crit_onset_fdd (freezing), crit_onset_gdd (warm), crit_onset_swi (soil water potential), and rain_threshold (rain amount).

 

[wwieder]

I'd agree with the comments above and add that the limitations or errors are likely due to what ever is more limiting in particular systems (e.g. cold ecosystem = temperature thresholds vs. dry systems = moisture). From our own work at NEON sites it seems like the moisture triggers are important in generating multiple (unrealistic) peaks in the annual cycle of LAI. Sorry we don't have specifics, but please circle back with what you find!