Ted,
I agree with the previous posters but would add that I think it is a
good idea to measure both relative growth rate AND include initial
fresh weight as a covariable. I would measure rgr derived from as
many different measures of size as is reasonable (e.g., ln(final dry
wt) - ln(initial dry wt derived from fresh rhizome wt), ln( final
height) - ln (initial height), etc.,).
The reason why I think it is a good idea to measure both rgr and
include initial size as a covariable is because initial size (e.g.,
fresh wt) can influence relative growth rate independently of your
treatments. At the end of the experiment, you simply examine the
correlation between initial fresh weight and rgr for each treatment.
If it is consistently low and weak, you can exclude the covariable
from the analysis and simply examine treatment effects on rgr. If
there is a consistently positive relationship for all treatments,
then you should keep it in. If the relationship varies with
treatment, then you need to shift your attention to investigating the
interaction between initial size and the treatments and should not
pool the covariable-treatment interactions with the error term, as is
typically done in ANCOVA.
Good luck,
Steve Brewer
At 7:11 PM -0400 8/4/09, Ted Turluck wrote:
Hello,
I've posted before with questions about other methods and got a lot of
responses. Thank you to all who responded before.
I'm going to be starting an experiment in which I plan to measure growth
rates of two different haplotypes of Phragmites australis under varying
water depths and salinities. I plan to use change in height and number of
shoots over time as a measure of growth. I'm planting rhizome fragments.
I'll measure length, diameter, number of nodes, and number of buds before
planting. I'll try to make the fragments as similar in size as possible.
I would like to measure biomass before and after as well (or maybe
instead). But I don't know if measuring fresh biomass before planting and
after harvest is a legitimate method.
Would there be a chance that there would be a difference in moisture
content between each rhizome fragment that could mess up my data?
Is height change a good way of measuring growth in grasses?
Is fresh biomass a good thing to measure before and after a growth
experiment?
Thanks for any advice.
--
Department of Biology
PO Box 1848
University of Mississippi
University, Mississippi 38677-1848
Brewer web page - http://home.olemiss.edu/~jbrewer/
FAX - 662-915-5144
Phone - 662-915-1077
