Why xerarch and hydrarch both leads …

Mesic is one such condition which represents a stable ecosystem ... anything that happens in nature spontaneously....happens so because the end product will bring stability. Hydrarch amd xerarch are not so stable ecosystems as is the mesic one

Its not that always xerarch and hydrarch will lead to mesic condition.....see wht an ecological succession is what we predict that a certain series of events if occur will lead to a certain result. On the land the ultimate Condition will be a mesic one or else what cn u think of it to be... land will ultimately have a forest on it unless the land is barren..and on barren land like a desert which has meagre amount of water...u cnt expect xerarch or hydrarch succesions to take place. Supposee the these successions during its developing period from one to another seral community faces a natural or human induced disaster say drought or no rains or say we found some mineral there n we start mining ....in those cases ultimate result wont be mesic condition. If a piece of land is having water on it with no other interference of human or natural activity ... naturally over a period of time mesic conditions will cm. U cn imagine how first forests would have evolved on this earth .

The climatic changes lead to mesic conditions

No one !