The answer is depending on hazel variety. Hazels are bushes that usually die far before than oaks. In Catalonia, where hazels are grown for centuries, some orchards with the same trees were kept on the ground for 3-4 generations. Traditionally hazels in Tarragona region were farmed for 60-100 years, with 3-4 regenerations, but nowadays, that hazel farming has sense just if nut yields are pretty high, most hazel varieties when hazel nut production start decreasing, are removed and new hazels planted.
Some varieties, like “peuetet” works until age 18 and “negret” until age 25, later branches dry up and get rotten. Then hazels usually produce new suckers or shoots that become dominant, but some varieties like “peuetet” although it will become a tree will never be so productive, usually producing nuts every two years. On the other hand, “negret” can be renewed from new shoots every 25 years and keep on producing good nut yields. We don´t know if they will work the same for truffles…
The following picture is a 26 years old “peuetet” getting dry:
In the top picture, at the left side you can see a “peuetet” variety of 26 years old, that will probably be removed next year to plant new hazels. At the right side a “negret” variety of 60 years old, that can easily live decades if you “renew” it, just allowing new shoots to become dominant and leave the main trunk to die naturally…
On the last picture you can see a “peuetet” orchard of 15 years old, at 6x4meters, at its highest production. From now on the nut yields will decrease in this variety.
Farmers allow trees to grow up first and when branches from each row start touching each other, mainly for the weight when they are full of nuts and keep that tunnel, that makes harvest easier.
Traditionally when hazels where produced from suckers, so at low cost, most farmers planted 2 trees together to fill faster the row and start producing nuts earlier, but now that hazels come from a more expensive in vitro varieties, just one is planted at this layout.
I know in several countries hazels have been and are still used for truffles with really good results, some of those orchards beyond 20 years old could start decreasing truffle production as more and more hazels could get too old. I would suggest to these growers to allow 2-3 new shoots to grow strong and the main trunk in 3-4 years will dry up naturally. Depending on the hazel variety the strength of the tree and so truffle fruiting will keep the same, but others will decrease or start to get random fruitings…
Marcos S. Morcillo