getH3: Calculates the slope of asymptotic height (H3) for trees.
In ecology-rocks/MakeMyForests: SORTIE-ND Preparation From Forest Inventory Data

Description Usage Arguments

This function calculates H2/H1 from Pacala et al. (1996). This function calculates the exponent value (B) of the following equation:

h = H1(1-e^{-B*DIA})

Where h is the calculated height of a tree in meters; H1 is the asymptotic height of the species in meters, and DIA is the diameter at breast height, in centimeters, of the tree. B is the exponent of interest. Of note, this does not calculate H2 directly, but H2/H1, as per Pacala et al. (1996). However, since H2/H1 is the parameter required by SORTIE-ND, that's what we're calculating, and tentatively naming "H3" for simplicity.

1	getH3(x, startVal = 1)

`x`	The tree dataframe, it must have H1, HT, and DIA columns. It is generally assumed that the treedb you enter will be trimmed to include only rows for a species or grouping. H1, in particular, will most likely be the same for any individual within a species, but should not be a factor.
`startVal`	A starting number for the coefficient. 1, the default value, is a pretty good guess for most, and you should only need to enter in a different value if the function is not operating as expected.