Overwatering, underwatering, wind and heat cause brown edges to leaves – caused by dehydration (or lack of oxygen to roots in the case of overwatered plants). If the soil's continuously wet, leaf edges will turn brown and the leaves will droop and appear dull. If water can’t drain away, or is continuously reapplied, the soil pores remain filled with water, the roots can’t absorb the oxygen they require and they begin to decay. Having a healthy root system helps overcome these problems – keep maples well-draining, watered regularly and consistently to eliminate further harm. Shade maples in containers throughout really hot weather and mist foliage with a fine spray.
Acer leaves will naturally brown and fall once autumn is over