| 264 | | The pft-specific parameter '''always_init''' controls whether the phenology depends on the reserves (set to .FALSE.) or is forced (set to .TRUE.). Note that a forced phenology (thus always_init = .TRUE.) has no ecophysiological basis, it is a numerical approach to stabilize the vegetation cover. A stable vegetation cover is particulary welcome in coupled simulations but likley hides real vegetation dynamics (especially under future climate conditions) or problems in other routines or parameter settings. If forced phenology is used, plants will develop an initial canopy in phenology irrespective of whether the plant had sufficient carbon and nitrogen reserves and for evergreen species irrespective of whether the canopy was viable at all. This setting basically overcomes a mortality event at the expense of taking up carbon and nitrogen from the atmosphere. When used in combination with impose_cn = n, an inconsistency is introduced: impose_cn = n reflect the desire to close the nitrogen cycle, always_init = y opens a backdoor in the nitrogen cycle. |
| 265 | | |
| 266 | | |
| 267 | | |
| 268 | | What we experienced with the trunk version (2.0), is that once there is |
| 269 | | ! no more lai, plants do not grow anymore later. To my understanding [NVui], |
| 270 | | ! there is no recruitment process. As it was a behavior we wanted to avoid |
| 271 | | ! within the frame of the CMIP6 exercise, we decided to implement a patch |
| 272 | | ! (see below) within the trunk. It is still a flagable option with the |
| 273 | | ! ALWAYS_INIT parameter which is now PFT-dependent. |
| 274 | | ! I [SL] think this is not needed in CAN as we have two processes that |
| 275 | | ! should avoid this situation: (1) recruitment. If there is no canopy, lots |
| 276 | | ! of light will reach the forest floor and so recruits will establish and |
| 277 | | ! should take over and (2) irrespective of whether we use recruitment we |
| 278 | | ! expect no leaves -> no gpp -> labile and carbres get depleted -> |
| 279 | | ! hit threshold to be killed -> kill -> replant. |
| 280 | | ! Nevertheless, a similar functionality was implemented in the merge. Note |
| 281 | | ! that the order of code was changed to deal with the circumference classes |
| 282 | | ! and to ensure mass balance closure. |
| | 264 | The pft-specific parameter '''always_init''' controls whether the phenology depends on the reserves (set to .FALSE.) or is forced (set to .TRUE.). Note that a forced phenology (thus always_init = .TRUE.) has no ecophysiological basis, it is a numerical approach to stabilize the vegetation cover. A stable vegetation cover is particulary welcome in coupled simulations but likley hides real vegetation dynamics (especially under future climate conditions) or problems in other routines or parameter settings. If a PFT keeps dying in an area where it is currently present, this would hint at a problem with the current model/parameters. If a PFT keeps dying under future conditions, it may be a real response (depending on the PFT). If forced phenology is used, plants will develop an initial canopy in phenology irrespective of whether the plant had sufficient carbon and nitrogen reserves and for evergreen species irrespective of whether the canopy was viable at all. This setting basically overcomes a mortality event at the expense of taking up carbon and nitrogen from the atmosphere. When used in combination with impose_cn = n, an inconsistency is introduced: impose_cn = n reflect the desire to close the nitrogen cycle, always_init = y opens a backdoor in the nitrogen cycle. |
| | 265 | |
| | 266 | From a conceptual point of view, CN-CAN is all about vegetation dynamics and thus instabilities in the vegetation cover. In CN-CAN there are two processes that can deal with dying PFts including evergreens PFTs. First, ok_recruitment could used. If ok_recruitment = .TRUE. a decreas in the canopy cover will result in more light reaching the forest floor which in turn should trigger recruitment of -for the moment- the same PFT. Generation can take over from each other without loosing the canopy cover entirely. Second, if there are insufficient reserves to grow no leaves, there will be no or insufficient gpp, the crabon reserves will be consumed by respiration processes, the plants will be killed, the biomass transferred to the litter pools and the same or another PFT (see section on species change) will be replanted. CN-CAN was developed to work with always_init = .FALSE. so this has become the default value, contrary to the trunk where always_init = .TRUE. is the default. |
