| 231 | | Describes r6908. The impact of within-season leaf turnover (thus the turnover between plant_status = icanopy and isenescent) can be monitored by two variables: LEAF_TURN_AGEING_c and LEAF_M_MAX_c. When expressed as percentage the within season turnover (thus due to ageing) varies between 0 and 30% for trees and reaches 100-150% for grasses. This seems to be acceptable. See the documentation on the wiki for some additional background on this issue. |
| 232 | | |
| 233 | | Because of the way leaf_age_crit is calculated there is an apparent conflict between the prescribed longevity and observed longevity. The prescribed longevity is used to calculate the leaf_age_crit (as a function of climate). Half the value of leaf_age_crit is used as the threshold to start within season leaf turnover. So the simulated longevity is less than the prescribed longevity … |
| 234 | | |
| 235 | | An example longevity_leaf = 400, leaf_age_crit = 300 at 25 C, 400 at 15 C and 600 at 5 C. Note that turnover starts when leaf age is half this value. Thus at 150 days in the tropics … which is still too high but not as bad as it looks like. |
| 236 | | |
| 237 | | For grasses the prescribed longevity was 7 times longer than the observed. Even with the climate and the leaf_age_crit complexities we will never match the observed longevity. |
| 238 | | |
| 239 | | The observed longevity suggests an annual within season turnover of 100 to 200%. This is what we get if we decrease longevity_leaf but with such a turnover it is almost impossible to achieve realistic LAI’s unless we increase NPP/GPP (which seems to improve several aspects of growth). |
| 240 | | |
| 241 | | ! longevity_leaf is a prescribed parameter for the longevity of a |
| 242 | | ! typical leaf/needle at the average temperature for that PFT. For |
| 243 | | ! PFTs where a large range in longevity has been observed, this is |
| 244 | | ! accounted for in the calculation of leaf_age_crit. Leaf_age_crit is |
| 245 | | ! thus the location-specific longevity. It the age of a leaf/needle |
| 246 | | ! exceeds crit_leaf_age, leaf turnover will start (see stomate_turnover). |
| | 231 | Describes r6908. '''Longevity_leaf''' is a prescribed parameter for the longevity of a typical leaf/needle at the average temperature for that PFT. For PFTs where a large range in longevity has been observed, this is accounted for in the calculation of '''leaf_age_crit'''. Leaf_age_crit is thus the location-specific longevity. It the age of a leaf/needle exceeds half the value for crit_leaf_age, leaf, root and sapwood turnover will start. Because of the way leaf_age_crit is calculated the simulated longevity differs from the prescribed longevity. For example, longevity_leaf = 220, leaf_age_crit = 165 at 25C, 220 at 15C and 330 at 5C. Note that turnover starts when leaf age is half this value. Thus at 110 days in the temperate zone which is within the observations. |
| | 232 | |
| | 233 | Turnover will result in an imbalance between the vegetation components and this balance will be restored in the allocation routines. Leaf turnover will thus replace old leaves (with a low VCMAX) by young leaves (with a high VCMAX). Despite the fact that leaves are being killed, the GPP may increase and eventually the LAI may even increase. |
| | 234 | |
| | 235 | For evergreen PFTs turnover from ageing is easy to understand as it is the only source of turnover. A longevity_leaf of 730 days will result in a two year canopy if the PFT is growing correctly. For deciduous PFTs the meaning of longevity_leaf is a bit harder to grasp. It affects the calculation of leaf_age_crit and once half the value of leaf_age_crit is reached it will determine the magnitude of the turnover but the age of the leaves is mostly driven by the phenology (senescence). For deciduous species turnover from ageing can thus be understood as the leaf turnover between plant_status = icanopy and plant_status = isenescent. The contribution leaf turnover from ageing can be monitored by two variables: '''LEAF_TURN_AGEING_c''' and '''LEAF_M_MAX_c'''. When expressed as a percentage, the within season turnover varies between 0 and 30% for trees and reaches 100 to 150% for grasses. This seems to be acceptable. |
| | 236 | |
