## 2.16.2. Tissue Stoichiometry[¶](#tissue-stoichiometry "Permalink to this headline")
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As of CLM5, vegetation tissues have a flexible stoichiometry, as described in [Ghimire et al. (2016)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#ghimireetal2016). Each tissue has a target C:N ratio, with the target leaf C:N varying by plant functional type (see [Table 2.16.1](#table-plant-functional-type-pft-target-cn-parameters)), and nitrogen is allocated at each timestep in order to allow the plant to best match the target stoichiometry. Nitrogen downregulation of productivity acts by increasing the C:N ratio of leaves when insufficient nitrogen is available to meet stoichiometric demands of leaf growth, thereby reducing the N available for photosynthesis and reducing the \\(V\_{\\text{c,max25}}\\) and \\(J\_{\\text{max25}}\\) terms, as described in Chapter [2.10](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Photosynthetic_Capacity/CLM50_Tech_Note_Photosynthetic_Capacity.html#rst-photosynthetic-capacity). Details of the flexible tissue stoichiometry are described in Chapter [2.19](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/CN_Allocation/CLM50_Tech_Note_CN_Allocation.html#rst-cn-allocation).

Table 2.16.1 Plant functional type (PFT) target C:N parameters.[¶](#id2 "Permalink to this table")  
| PFT
 | target leaf C:N

 |
| --- | --- |
| NET Temperate

 | 58.00

 |
| NET Boreal

 | 58.00

 |
| NDT Boreal

 | 25.81

 |
| BET Tropical

 | 29.60

 |
| BET temperate

 | 29.60

 |
| BDT tropical

 | 23.45

 |
| BDT temperate

 | 23.45

 |
| BDT boreal

 | 23.45

 |
| BES temperate

 | 36.42

 |
| BDS temperate

 | 23.26

 |
| BDS boreal

 | 23.26

 |
| C3 arctic grass

 | 28.03

 |
| C3 grass

 | 28.03

 |
| C4 grass

 | 35.36

 |
| Temperate Corn

 | 25.00

 |
| Spring Wheat

 | 20.00

 |
| Temperate Soybean

 | 20.00

 |
| Cotton

 | 20.00

 |
| Rice

 | 20.00

 |
| Sugarcane

 | 25.00

 |
| Tropical Corn

 | 25.00

 |
| Tropical Soybean

 | 20.00

 |
| Miscanthus

 | 25.00

 |
| Switchgrass

 | 25.00

 |
