Complex sentence: One consequence of pollarding is that pollarded trees tend to live longer than unpollarded specimens because they are maintained in a partially juvenile state, and they do not have the weight and windage of the top part of the tree.
Split into simpler sentences: One consequence of pollarding is that pollarded trees tend to live longer than unpollarded specimens. Pollard trees may attain a greater age than normal, because they are maintained in a partially juvenile state, and they do not have the weight and windage of the top part of the tree.