Remove phenology controls from Growing Degree Days integration?

[mpaiao]

Currently the calculation of GDD is strongly linked to phenological status: GDD stops accumulating when leaves flush, and GDD is reset to zero when plants abscise leaves. This is somewhat different from the definition described in Botta et al. (2000), although the code explains that resetting GDD to zero is to prevent leaves from flushing in the same year if they abscise. Thinking of the crop model, I think this may be a problem, because I think GDD might potentially be useful for predicting harvesting time (though maybe less so in FATES because we calculate growth).

One alternative is to continue integrating GDD, but add checks in the phenology subroutine similar to what is already being done for the drought deciduous. In the drought deciduous, once plants abscise leaves, they must remain leafless for a minimum period of time before flushing again. The advantage of this approach is that it would allow representing multiple leaf flush events in the same year if plants are damaged by a spring frost (D'Andrea et al. 2019).

[mpaiao]

In a second stage, I also think that temperature thresholds, and consequently both GDD and NCD should become PFT-specific quantities. This is how these variables are typically defined outside FATES, each plant/crop may have different temperature requirements.