Hello.
It downs young trees flush to soil, no stumps. Grapple? shears? cutter? jaws ? ... Some hydraulic, some cordless, some AC electric, some are ratcheting, up to 10cm diameter, perhaps larger... Cutting edges are concave.
The one in your picture is listed as hydraulic cutting shears here in Oz, so cutting shears generally but likely to have different names in different countries.