Better derivations and an improvement for the specific increment equations of tree growth

Improved derivations are provided for the specific increments of internodal growth. These derivations strengthen the theoretical basis of the concept of specific growth. Despite being based on a more rigorous approach, however, these derivations produce a specific wood volume increment equation that is identical with the standard version. In contrast, a new equation emerges for the specific (annual) increment of cambial area: SAI = ln(r(n)/r(n-1)), where In denotes the natural logarithm and r(n) and r(n-1) are the cambial radii corresponding to the annual rings produced in years n and n-1, respectively. The standard approximation underestimates the specific area increment, especially on young, fast-growing internodes, but even then the error involved may be small. Any standard approximation to SAI can be corrected by multiplication with the provided correction factor (which requires values for r(n) and r(n-1) only).