Answer:
Climatic variation
Step-by-step explanation:
Climatic variation lead to formation of annual rings in the activity of cambium because In climates with alternations of either cold or dry seasons, the growth cells or cambium of wood stems produced when water is available and growth is rapid are always large and have thinner walls and where the climates is dry, the growth is slower and the cambium produced short and thicker walls but where there is uniform climate, there won't be much different as the growth will be uniform.
The cambium is responsible for secondary growth of root and stem.