Standard phenology model
(Jolly, W.M., Nemani, R. and Running, S.W. (2005), A generalized, bioclimatic index to predict foliar phenology in response to climate. Global Change Biology, 11: 619-632. https://doi.org/10.1111/j.1365-2486.2005.00930.x)

With static phenology, the growing season parameters need to be set in the vegetation definition/default file for the start of the growing season and the beginning of leaf drop:

• epc.day_leafon = start of the growing season, the day of the year that 'leaf out' occurs for the vegetation type (day 1-365)
  (epc.day_leafon is the first day - then epc.ndays_expand indicates the number of days that the leaf out period lasts)
• epc.day_leafoff = beginning of leaf drop, the day that signifies the end of the growing season, when leaves begin to fall (day 1 - 365)
  (epc.day_leafoff starts the end of the growing season - then epc.ndays_litfall indicates the number of days that litter fall lasts)

This phenology model works in all ecosystems - except for the Mediterranean type ecosystem.

RHESSys with phenology model implemented
When concerned about the timing of leaf out due to inter-annual climate variability or climate change - so that vegetation expresses growth in the spring in response to environmental cues (change in temperature) rather than a fixed day of the year - then dynamic phenology may be used. This may be more important for deciduous vegetation than evergreen vegetation.

To evoke dynamic phenology, several parameters in the vegetation default file must be set:
At minimum:

• set epc.phenology_flag to "dynamic"
• set epc.day_leafon and epc.day_leafoff to the maximum day of year for these events - so the phenology routine will trigger leaf on or leaf off before these dates based on Jolly's index, but if they haven't happened by this time, they will get triggered anyway - this prevents situations where leaf out/off never happens.

Optional parameters:
6 optional stratum default files that correspond with Jolly et al. parameters for temperature, vpd and photoperiod (daylength) controls on phenology timing.
In RHESSys these are internally set to the "global" values that Jolly used - note that Jolly used the same parameters everywhere, for all vegetation types - and did a comparison with remote sensing derived phenology timing - so in theory parameters should not be vegetation type specific. However, these may need to be calibrated for grasses.

• epc.gs_tmin
• epc.gs_tmax
• epc.gs_vpd_min
• epc.gs_vpg_max
• epc.gs_dayl_min
• epc.gs.dayl_max

ℹ️ The parameters epc.ndays_expand and epc.ndays_litfall in the vegetation default file are still used - the phenology model only determines the timing of leaf out and leaf drop, not the number of days it takes to happen

Primarily used in a Mediterranean type ecosystem
Leaf out/green up responds to precipitation events rather than seasonality/time of year
More Info on The Mediterranean Grassland Phenology Model