[R-sig-eco] Help with model selection

[Altaf Hussain]

Hello everyone,

I have some field collected data from *90* random trees on *6* sites and
I am comparing the trees for the concentrations of different chemical
compounds. The chemicals are of two different types, *(Group 1 = A, B, C,
D, E, F and their sum, X)* & *(Group 2 = G, H, I, J and their sum, Y)*. I
also have the sum of X and Y that I call Z. All variables are continuous
and use the same units. The 6 sites are closely located but are unique in
terms of their soil nutrients (N, P K, S and their sum, Tot.Nutrients,
*continuous/same
units*) and soil moisture. I further grouped the sites for soil moisture
and I now have 2 Dry, 2 Medium dry and 2 Wet sites, (6/3=2), character
variables. Each site has 15 trees and an equal number of trees i.e., (5)
were treated using different fungal inoculation densities i.e., (0 for
control, 4 and 16).  The trees were sampled in a pre and post inoculation
manner, where the pre inoculation samples serve to determine the baseline
chemistry of trees and the post inoculation samples will show the chemical
responses of the trees to inoculations *(if any!).*

Alright, first thing first, I admit that I am very new to statistical
modeling, which is why I have such a complicated design, and which is why I
am posting ☺.  The following are the questions that I want to answer using
my dataset introduced above:

1) How do I compare my 6 sites for trees' chemical responses to the
different inoculation densities?
2) How do I check if any of the nutrients, their interaction, or their
interaction with soil moisture is playing a role in the chemical variation?

I am open to any suggestions that will improve my study!

I am using RStudio version 3.4.3 for the analysis.

I appreciate any input that will help me with the analysis.

Regards,

//Simon

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