We also believe he returned to the UK during WW1 and was in the 102 Canadian forestry corps stationed at Mortimer, Berkshire, England. Any record which shows this would be great. I have searched around the internet for these records but haven't found anything yet.
Did that information come through the family or did you find a Harry Barnes in the Forestry Corps that might be him?
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You can search WWI Canadian Expeditionary Force personnel here:
https://www.bac-lac.gc.ca/eng/discover/military-heritage/first-world-war/personnel-records/Pages/search.aspxI think it includes the Forestry Corps, but I'm not positive. It specially mentions the Newfoundland Forestry Corps but that might be because Newfoundland wasn't part of Canada yet.
I didn't find a matching H* Barnes in the CEF database. I gave it an age range of a few years either way, but nothing.
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There is some information about the 102 Co. Canadian Forestry Corps here:
https://www.bac-lac.gc.ca/eng/discover/military-heritage/first-world-war/Documents/canadian%20forestry%20corps.pdfIt says
Organized as No. 2 Company, Canadian Forestry Corps under the command of Major M. R. McDonald.
Employed at Rapley Lake, Bagshot from May 1916 and at Mortimer, Berkshire from September 1917.
Redesignated as 102nd Company, Canadian Forestry Corps on 7 August 1917.
It was the only Company at Mortimer, Berkshire. - search the page for Mortimer to find the information.
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It mentioned Major M R McDonald, so thought I'd look for him. The most likely was this man, but his record says he went into the 1st Canadian Pioneers. Maybe he was reassigned when they organized the Forestry Corps. His civilian occupation was "miner and axeman." The other M R McDonalds were a farmer, a civil servant, and a policeman.
Malcolm Rod McDonald, born 25 December 1876 Prince Edward Island, trade: miner and axeman, next of kin: Albert G McDonald (brother), at Brittania Mine, Brittania, British Columbia
enlisted, signed September 2, 1915, Victoria BC
unit: 1st Canadian Pioneers, regiment 154085
rank SPR [said to be the Royal Engineers Sapper, equivalent of private "Sapper" at Wikipedia]
location given on medical certificate Work Point[?], Esquinalt, BC
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Thinking that the 102nd might have been organized in British Columbia, I looked in the 1911 Census of British Columbia for H* Barnes, but didn't find your relative. There is a man by that name born in England, with a wife Lucy, but he arrived in Canada in 1882. And a William Henry Barnes that didn't seem like a match either.
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bottom line: nothing found
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P.S. the Canadian censuses are on that site also, and are free. You might have more motivation and patience than I do for looking for H* Barnes in all the provinces.