The answer to 'fence turning brown' is that there are several common causes, including weathering, age, and pest damage. Over time, exposure to the elements can cause wood fences to lose their original color and take on a weathered, brown appearance. Additionally, pests like termites or wood-boring insects can eat away at the wood, leading to discoloration.
If the browning is due to natural weathering, a simple cleaning and re-staining or sealing of the fence may be all that's needed. However, if the browning is caused by pest damage, it's best to call in a professional landscaping or pest control expert to assess the extent of the problem and recommend the appropriate treatment.