[R] Help to solve modeling problem with gamm

[Abby Spurdle]

> There was clearly a non-linear growth pattern such that an additive mixed effects model was proposed to model the behavior of biomass as a function of time and treatments.

The presence of non-linearity (in x) does not necessarily mean that
you can't use a "linear model".
In general, linear models work well for biological growth curves with
small to medium sized data.

Technically, this is off topic, however, the notion of what is linear
and what is not linear, is a recurring theme on this mailing list.

> When plotting the residuals a clear cone-shaped pattern was observed

Based on classical statistics, the obvious approach is to transform your data.
This is easy to do, however, some expertise is required to
back-transform parameters/estimates and interpret them.

While you're welcome to seek advice on mailing lists, I'd recommend
you consult a biostatistician.