In general, you can estimate the DBH by summing the squared dbh of the stems and calculating the DBH from the following equation:

DBH=(dbh1^2+dbh2^2+...+dbhn^2)^0.5

For example, it a plant there is 3 stems, and the dbh are 5,6,7 cm respectively, than the DBH of the plant is 10.5 (=(25+36+49)^0.5) cm.

Wish it is helpful to you.

 Thank you Sir, Wei-Sheng Zeng

Measure DBH of each stem separately and report the number of stems and diameter of each stem. If it is necessary to consolidate the diameters of various stems into a single index, the DBH for the tree as a whole could be taken as the square root of the sum of all squared stem DBHs.

I completely agree with the answer of Tewari: that one is the correct way to estimate Dbh of multistemmed trees.

I have been looking for a answer for the same question. Is it possible for you to give a reference for the same?

Kindly refer to following:

Mac Dicken, K.K., Wolf, G.V. and Briscoe, C.B. (eds.) 1991. Standard research methods for multipurpose trees and shrubs. International Research Centre for Agroforestry (ICRAF) and Winrock International publication (multipurpose tree species network research series; manual no. 5).

 Dear Oshadhi,

1. If it is possible, consider other variables such as crown width or tree height, to classify the size of your threes. They may batter represent difference in tree functions and physiology due to difference in tree size.

2. if you have to use DBH data, you can use basal area instead of DBH. BA can be simply considered as sum up of all stems of a given tree. Then classify BA values.

3. IF you have to use exact values for DBH (e.g. you have a model which only accept DBH as input), use root quadratic mean DBH=[(dbh1^2+dbh2^2+...+dbhn^2)/3]^0.5. In this way, larger size DBHs can contribute more to the final average value.

Good Luck,

Hormoz

I completely agree with the answer of Tewari:

Thank you all 

Does anybody have an actual reference for the square root of summed squares method for multiple DBH? I need it for a paper

Hi Andy Speak - did you ever get a citation for this calculation? I also need it for a paper....... Cheers

does the sqrt of sum of squared dbh's yield a good estimate of tree age using the correct factor for tree species?