Assertion (A): The climax community remains stable as long as environment unchanged.

Reason (R): Succession succeeds from small plants to bigger plants with several intermittent plants. 

Assertion is true. During succession change is orderly and sequential, parallel with the changes in the physical environment. Hence if the physical environment remains unchanged, the climax community remains stable.

Reason is true. Succession succeeds from small plants to bigger plants with several intermittent plants.  Ecological succession often involves a gradual shift from early colonizing species (which are often small and opportunistic) to later successional species (which may be larger and more competitive).

A is true R is true and R is correct explanation of A. Succession succeeds from small plants to bigger plants with several intermittent plants because it involve fluctuations and changes in community structure as different species interact and respond to changing environmental conditions.