They have been removed to stop the spread of ash dieback, a fungal disease that can infect mature trees and saplings.
Scientists have warned that if it is not contained it could change the landscape and affect our biodiversity.
It has been found on around 100 sites in Northern Ireland.
All those sites were planted with imported saplings, some of which had the infection.
Infected ash trees were destroyed along with any others planted alongside.
That led to the removal of 100,000 young trees.
The disease is widespread in Europe where it is killing mature trees.
In Northern Ireland, it is mostly in saplings planted within the past five years.
It spreads on the wind by spores.
Scientists at the Agri-Food and Biosciences Institute (AFBI) in Belfast are trying to establish if that is happening in Northern Ireland yet.
Prof Alistair McCracken, a plant pathologist with AFBI, said: "This is a serious disease. In Denmark they say up to 90% of the ash trees have been infected.
"Now they haven't all died, but they have been affected and are looking quite sickly so potentially this could have a huge impact on the Northern Ireland environment."
Infected ash trees will not recover, although mature trees can survive for a long time.
The fungus appears between June and September on leaf litter. Each pinhead size fungus can produce large numbers of spores.
When they land on a fresh ash leaf they germinate, penetrate the surface and grow into the stem.
The first symptom is wilting leaves. As it gets worse the trees will develop a diamond-shaped dark-coloured lesion on the bark.
In Denmark, Sweden and east European countries, where the disease has been present for 30 years, large numbers of trees have become infected.
Scientists are worried about its impact in Northern Ireland, where ash is a common tree.
They are carrying out a survey of more than 1,000 sites to identify any further problems.
And they have asked for the public's help.
The authorities have developed a smartphone app called Tree Check that the public can use to report potential problems, including a picture and a GPS position.
They will then be assessed and, if necessary, the site visited by experts.
Jim Crummie, plant health expert with the Forest Service urged the public to download the app and to be the "eyes and ears" of the authorities in the fight to combat the disease.
Tree Check can be used to report all tree diseases, not just ash dieback.

The incident happened at the One Stop Shop in Usk at about 06:00 GMT.
A 15-year-old, from Little Mill, is currently in police custody after being arrested on suspicion of robbery.
Gwent Police said a 36-year-old man from Blaina and a 42-year-old from Little Mill had been arrested and released on police bail pending further enquiries.

It comes as tennis star Andy Murray prepares to tie the knot with Kim Sears in Dunblane.
The tourism promotion agency said nearly a quarter of weddings held north of the border were between people from outside Scotland.
It said breathtaking scenery, celebrity weddings and welcoming hospitality were the biggest draws for couples.
Among those who have married in Scotland are Madonna and Guy Ritchie (Skibo Castle), Mark Owen (Cawdor Parish Church), Philip Schofield (Ackergill Tower, Caithness) and Zara Phillips and Mike Tindall (Canongate Kirk, Edinburgh).
VisitScotland has estimated that the wedding tourism industry is worth Â£80m to the Scottish economy.
The agency said market research suggested "tourist weddings" tended to have fewer guests, but people stayed longer at the locations.
It added that Dumfries and Galloway was the most popular area for marriages because of "the popularity and romantic significance" of Gretna Green. In second place was Argyll and Bute.
VisitScotland chairman Mike Cantlay said: "Put simply, Scotland is one of the most beautiful places in the world in which to get married.
"From the splendour and magnificence of our castles and historic homes, to the wild romantic nature of our lochs, mountains, islands and coastlines, there is a uniqueness and magic to having your big day on Scottish soil.
"The impact the wedding industry has on tourism itself is significant, but more than that, it gives us an opportunity to showcase Scotland to guests who may never have made the trip before and turn their time here into a holiday."
One hundred thousand young ash trees have been destroyed in Northern Ireland in an attempt to combat a potentially devastating disease.
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Police investigating an armed robbery at a shop in Monmouthshire on Sunday have arrested a teenager and two men.
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VisitScotland has launched a drive to promote Scotland as the place to get married.