Here is the probable dataset used : 

https://geodata.environnement.brussels/client/view/eef335e3-fb5b-4b46-983a-0df333f6191b

I don't find this on WMS layer i have from environnement.brussels (some tree need alignement). 

For information there's is the another data set used (exemple : https://www.openstreetmap.org/node/3116851763) : https://data.mobility.brussels/fr/info/trees/ 