purposive sampling of mangrove leaves?

my thesis is about inventory of microscopic fungi associated with the leaves of rhizophora apiculata, a certain species of a mangrove tree, now after surveying the area of mangrove plantation i noticed how dispersed and how large the area is. aside from that, i also noticed that the area is composed of not just 1 species of mangrove tree but there were also other related species and related trees. now how must i perform my data gathering if simple random sampling like stratified method would be to difficult or impossible ? explain in detailed please
 
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