1. is she going to compare a given volume of wood or will it be based on weight? I would stick to volume as it is easier to measure displacement without worry of water weight throwing off the project.
2. Generally in Prescott those species can be found in relatively close proximity (don't have to worry about different climatic zones) to each other so you could cut green and weigh then oven dry and weigh to determine moisture content. If it were me I'd stick as close to the 5000 ft elevation level as that is the most common regime that all these species are likely to occur. I know that cutting green wood is illegal in AZ but I would stick to pieces 3-4" in diameter. If it concerns you cutting it, go down to the FS office and they will give you a free permit to harvest a few pieces for this.
3. I would oven dry at 150-200 degrees for a set amount of time (maybe 4 hours at 150? just guessing on this one). Weigh the wood again and determine the moisture content removed.
4. All these steps to now are just for her to explain the scientific process and set a hypothesis plus it will make her sound smart and dry the wood out for the experiment.
5. I would then get a controlled situation like a cast iron pot in the garage or somewhere out of the wind. Get an IR thermogun and set them on fire one at a time measuring how long they burn and the temp they burn at. You could record temps at 1 minute intervals. I would do all this with a 3-4 " diameter piece maybe 1-2" long (just a small wafer) to make ignition easier and the project run quicker.
Just a thought