how to estimate past increases in tree biomass

[Laurence M. Lue]

Greetings,

I'm starting a M.Sc. at the University of Toronto. I want to track past 
changes in tree biomass in a stand of trees (white spruce), but I'm 
unsure of the best (i.e. most accurate and efficient) way to do this. 
Here's what I propose to do:

1. Do a stem analysis on a number of trees (how many should I use?) to 
   get the change in stem volume per year.  I also plan to do stem 
   analyses on sample branches.  I'll be using cookies (sections) or 
   cores.

2. Convert volume to biomass.  I figure that I'll need to estimate the 
   mean density of the wood formed in a given year.  The only way that 
   I think this can be done is by using x-ray densitometry to analyze 
   a number of cores/cookies per tree (again, I'm not sure how many I'll 
   need).  I'm unsure how much density will vary between years and 
   between trees.

Would this be okay for roots as well?  I would appreciate any advice, 
suggestions, criticism, or references that you might have.  Thank you 
very much.

cheers,

Laurence Lue                            

Department of Zoology                lml at zoo.utoronto.ca
University of Toronto                lm.lue at utoronto.ca   
25 Harbord Street
Toronto, Ontario, Canada  M5S 1A1