E mettere in parentesi la spiegazione : "portion of trunk less than 4 m in length"
Mia idea. Visto che l'uso sebra essere della zona di Udine secondo questo testo che vedi qui:
http://books.google.it/books?id=e0MKkL7KQTEC&pg=PA806&lpg=PA...

[NOTE see the Wikipedia link below describing the linguistic differences between 'timber and 'lumber' before commenting, TY.]

bole is a word commonly used in the logging industry;
bottolo is a similar word in Italian having similar - if somewhat limited use.

It is not a question of dialect in either language - but possibly a question of industry specific jargon. [just because it's not common, doesn't make it archaic or dialect]

'bolt' is a similar word as 'bole' in English, but is specifically a shorter log. Here, 'bottolo' has the idea to be a longer piece (see definition for 'bottolo' in Italian below).

This is why I added timber - because lumber/timber is technically a length similar to that described below in Italian for 'bottolo'.

Bole and log, definitions

bole n. a tree trunk (origin Old Norse) [source OED and Web reference 1 below]; the main stem of a tree; usually covered with bark; the bole is usually the part that is commercially useful for lumber [source http://www.audioenglish.net/dictionary/bole.htm ]

'logs' are sections of a bole

log any section of the bole, or of the thicker branches, of a felled tree, after trimming and cross cutting. [source Web reference 1 below]

"In botany, bole refers to the main structural member of a tree that supports the branches and is supported by and directly attached to the roots." [source Wikipedia]

Definition of Bottolon in Italian

Bottoli. Sinonimi: legna scelta, bottolame, cortame.
Questo tipo di assortimento è costituito in prevalenza dalla parte basale dei fusti e risponde a precisi criteri di mercato in quanto la separazione di questa parte del tronco, notoriamente soggetta a particolari tensioni, dal materiale idoneo a produrre assortimenti da sega o travatura, consente di togliere da questi assortimenti di pregio, parti potenzialmente difettose che potrebbero causarne il deprezzamento. Questo assortimento viene commercializzato in lunghezze limitate che variano dai m 2,20 ai m 3,50 e sempre più spesso da m 2,40 o 2,50.

Legname da imballaggio. Sinonimi: tasti, trusi, robusti, bottoli, legna da opera, opera. Si tratta di legname destinato alla produzione di imballaggi per la quale sono tollerati anche difetti o qualità inferiore rispetto ai tronchi da sega. Sotto l’aspetto dimensionale del materiale legnoso attribuito a questo tipo di assortimento, si sono sempre più diffuse ed apprezzate le misure che siano multipli di m 1,20 e in particolare i 2,40 i 4,80 o i 6,00 metri.

[source both http://www.legnotrentino.it/interne/classificazione_e_assort...]

The second definition, I would call something like 'lower grade bole timber'.

Comparison by dimension

Based on the dimensions of in the above definitions in Italian - the adjective timber should be added, as lumber/timber is defined by length in the logging industry. I selected 'timber' as I found more references for 'bole timber' than 'bole lumber.'

Lumber/timber definition

Lumber Lumber must be 2.5 m long, free of rot and fractures. Lumber is a manufactured product derived from a log in a sawmill, ... . [source Web reference 1 and The National Lumber Grades Authority,NLGA]

Lumber vs timber

For a discussion on the differences between 'lumber' and 'timber' - see http://en.wikipedia.org/wiki/Timber#Terminology .

'bottolo' and 'bole' are not the same as 'tree trunk' as they don't have to come from the base or 'butt of the tree as 'trunk' must.

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Note added at 16 hrs (2010-06-28 21:14:26 GMT)
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After all that - I forgot to return to the word "bolt"! Ooops.

Anyway, I meant to add this:

Because the piece to be translated talks about short-thick pieces - i tronchi corti e grossi - you could also use bolt timber. The confusion comes from someone thinking 'bottolo' can be short.

You can see above in my 'novella' that - 'bolt' is a similar word as 'bole' in English, but is specifically a shorter log.

HTH (hide the hungry)