Elm often displays a characteristic speckling of small twig scars which, if intermittent and not too profuse across the timber, are known as pips. These pips are caused in an interesting way.
The elm was brought to Britain by the Romans and introduced as a convenient field boundary fodder crop. These 'crops' readily and speedily reproduced via root suckers, sprouting twiglets from its trunk. These would be grazed by sheep, cattle and horses and with each twig munched, leave a small scar. The next year another twig would sprout and often endure the same fate and this over time would cause small and sometimes very large ‘knobbly’ protrusions to appear on the outside of tree trunks. When cut through, planed and polished, mild knobbles show a small amount of knots/pips while the more severe and sometimes completely knobbly tree trunks show a mad cacophony of knots and convolutions and these are called burrs in Britain and burls in the USA.