When they die and rot back into the soil the nutrients are replaced and the soil becomes less acid. In farming and forestry the plants are harvested, not left to rot back into the soil which gradually becomes more acid. The removal of a whole tree including the branches and roots can be equivalent to the accumulation of 60 years of acid rain. Only taking the trunks reduces this to the equivalent of 20 years of acid rain. When areas are prepared for forestry they are often drained and deep ploughed, This allows more oxygen into the soil, the minerals are oxidised and the soil becomes more acid.
Some environments are able to neutralise the affects of acid rain.
This is referred to as the buffering capacity. Chalk and limestone areas are very alkaline and can neutralise acids very effectively. The underlying rocks over much of Scandinavia, Scotland and northern Canada are granite. They are naturally acid, and have a very low buffering capacity. It is in these areas that there is the worst damage from acid rain.