HermantheGerman, rain thing is a bit misleading, too much and you can't tap, too dry no good either.
Water table is the big factor, I'm in Issan, get more rain than most out here, mountains and jungle, cool air causes rain, but the run off keeps a high water table, 3 to 5 meters year round.
Rubber tap root goes deep, that's why good rubber land follows the mountains and the Mekong, really big difference, in rain fall and water table over short distances.
Thais plant without thought and many places in Issan are just too dry. Jim