Add crown damage documentation

docs/source/CLM50_Tech_Note_References.rst

@@ -1859,6 +1859,16 @@ fraction absorbed PAR from year one of MODIS data. Remote Sens. Environ.
 
 .. _Neffetal2005:
 
+|br:|
+
+Needham, J.F., Arellano, G., Davies, S.J., Fisher, R.A., Hammer, V.,
+Knox, R., Mitre, D., Muller-Landau, H.C., Zuleta, D., Koven, C.D. 2022.
+Tree crown damage and its effects on forest carbon cycling in a tropical forest.
+Global Change Biology 28:5560-5574. DOI:10.1111/gcb.16318
+
+
+.. _Needhametal2022:
+
 |br|
 
 Neff, J.C., Harden, J.W. and Gleixner, G. 2005. Fire effects on soil

docs/source/fates_tech_note.rst

@@ -3860,7 +3860,90 @@ Zeng, Xubin. 2001. “Global Vegetation Root Distribution for Land
 Modeling.” *Journal of Hydrometeorology* 2(5): 525–30.
 
 
+Crown Damage Module
+^^^^^^^^^^^^^^^^^^^^^^
+The crown damage module represents crown damage as a reduction in crown area and the biomass of tissues in
+the crown (leaves, sapwood, storage, structural and reproductive tissues), 
+implemented via changes to allometric relationships. Damage currently does not change the height of cohorts or the biomass of the stem. 
+
+We treat damage as a categorical variable with each cohort associated with a ‘damage class’ that describes its degree of crown loss. 
+Damage classes can be set in the parameter file via damage_bin_edges, which sets the lower bin edges for the percentage of crown loss 
+in each damage class. Damage classes do not need to be evenly spaced. Damage class is an argument to allometric equations and 
+is used to reduce the biomass of crown tissues. For example: 
+
+.. math:: bl = bl * (1 - crownreduction)
+
+where :math:`bl` is leaf biomass.
+
+We reduce sapwood and structural tissues in proportion to their branch fraction.
+
+.. math::  bsap = bsap - (bsap * agbfrac *  branchfrac * crownreduction)
+where :math:`bsap` is sapwood biomass, :math:`agbfrac` is aboveground biomass fraction and :math:`branchfrac` is the branch fraction. 
+Branch fraction is calculated as the sum of the first three coarse woody debris pools (i.e. excluding the main stem).
+
+Damage is not currently linked to explicit drivers. The timing of damage events is set by the damage_event_code parameter - described in table  
+:ref:`crown_damage_event_table`.
+
+.. _crown_damage_event_table:
+
+.. list-table:: Crown damage event codes
+   :widths: 25 25 
+   :header-rows: 1
+   :stub-columns: 0
+
+   * - Event code
+     - Description
+   * - 1
+     - Damage is off
+   * - 2
+     - Damage occurs on the first time step
+   * - 3
+     - Damage occurs every day (not recommended)
+   * - 4
+     - Damage occurs once a month (on the first day)
+   * - negative number
+     - Damage occurs annually on the specified day of the year
+   * - YYYYMMDD
+     - Damage occurs on a given date.
+
+The damage_frac parameter determines the proportion of a cohort that is damaged with each damage event. 
+Part of the cohort keeps its current damage class, while the damaged portion of the cohort is equally divided into 
+cohorts with higher damage classes. In the figure below there are five damage classes including undamaged and 
+damage_frac is set to 0.1. Of the intial cohort of 1000 individuals 
+25 individuals are therefore moved into each of the four higher damage classes.    
+
+.. figure:: images/Damage_1.png 
+
+
+Recovery from crown damage is set via the damage_recovery_scalar parameter. A value of one means that during daily
+allocation of NPP, no recovery occurs and damaged cohorts will allocate all available carbon to growth along their 
+altered allometric trajectories. A value of 1 means that cohorts will use all available carbon to regrow damaged tissues, 
+at the expense of dbh growth. The maximum number of individuals of a cohort that can recover in each timestep (:math:`nmax`) is
+a function of the available allocatable carbon to grow with (:math:`C_b`) and the change in carbon between 
+the damage class i and i-1 (:math:`C_r`):
+
+.. math:: nmax = n_i * C_b / C_r
+
+Where n_i is the initial number density of the cohort. The number of plants that recover is 
+then :math:`nmax * fr` where :math:`fr` is the damage_recovery_scalar parameter.
+
+
+
+.. figure:: images/Damage_2.png 
+
+Crown damage in FATES can lead to mortality via carbon starvation. However, to capture mortality associated with crown loss 
+from mechanisms not currently in FATES (e.g. pathogen entry) an additional mortality term describes an increase
+in mortality with crown loss :math:`M_{d,coh}`.  
+
+
+.. math::
+
+	 M_{d,coh} = \frac{1}{1 + e^{(-r_d * (damage - p_d))}}
 
+where :math:`damage` is the fraction of crown loss, :math:`r_d` is the rate that mortality increases with crown loss,
+and :math:`p_d` is the inflection point of the curve, i.e. the crown loss at which annual mortality rate has increased to 50%.
+
+For an application of the FATES crown damage module see :ref:`Needham et al. (2022)<Needhametal2022>`.  
 
 FATES Reduced Complexity Configurations
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^