The paper by Veihmeyer and Hendrickson seems definitive in its affirmation about both the field capacity and the wilting point. That they exist and depends upon the soil characteristics. Because I certainly know that field capacity is just an interpretation of phenomena and not some characteristics written in Mose’s tables, I have the doubt that also the second about the wilting point is at most an approximation.
Gardner in his interesting 1965 paper, in fact says the following: “Whatever the exact value of the pressure potential at wilting, the wilting phenomenon is a consequence of a change in the mechanical or elastic properties of the plant leaf and appears to be associated with a rather definite value of the relative water content and water potential for leaves of a given species and given age for a given cell solute content.”
This statement is much less assertive and leave large space to variation among plants types.
Actually I was interested in the wilting point because $theta_w$ is used in traditional standard models for determining the water stress induced by droughts on plants. Actually then, what I am interested in is not the “wilting” point, but the point at which stomata close with respect to this, the CLM model, in fact, acts in a different way:
- It uses suction instead than water content (i.e. formula 2.34 of the manual, Oleson et al, 2013)
- It gives a table according to which different vegetation types close stomata at different values of suction (Table 8.1)
The second fact implies that the water content at which a plant type close its stomata varies with soil types (this is obtained by inverting the suction using the soil water retention functions).
Clearly the two views cannot be both true when applied to transpiration. Either the one or the other must be the correct one (well, this is optimistic: probably they are both wrong).
The reading of the Garner paper also suggests that anyway the wilting point, i.e. when plants wilts, depends upon the properties of plants’ cells, which actually could have a certain homogeneity among the various specie and the one of the wilting point is a topic that needs further browsing of literature (but the issue related to stomata closure, maybe not). Anyway Gardner 1965 set it between -12 and -15 Bars, which is, actually a quite wide range.
Since the arguing went to the topic of stresses it is also to remark that isohydric and anisohydric behave differently. Therefore this has to be reflect in the mathematical form of the stress which actually is not.
…. Still mumbling …….
Keith W. Oleson, David M. Lawrence,Gordon B. Bonan, Beth Drewniak, Maoyi Huang, Charles D. Koven, Samuel Levis, Fang Li, William J. Riley, Zachary M. Subin, Sean C. Swenson, Peter E. Thornton, Anil Bozbiyik, Rosie Fisher, Colette L. Heald, Erik Kluzek, JeanFrancois Lamarque, Peter J. Lawrence, L. Ruby Leung, William Lipscomb, Stefan Muszala, Daniel M. Ricciuto, William Sacks, Ying Sun, Jinyun Tang, Zong-Liang Yang. 2013. “Technical Description of Version 4.5 of the Community Land Model (CLM).” NCAR.
Gardner, W. R. 1965. “DYNAMIC ASPECTS OF SOIL-WATER AVAILABILITY TO PLANTS.” Annu. Rev. Plant. Physiol.
Veihmeyer, F. J., and A. H. Hendrickson. 1950. “Soil Moisture in Relation to Plant Growth.” Annual Review of Plant Physiology 1 (1): 285–304.