Scrub is the precursor of woodland. Silver birch is the main invasive woody species on the Forest. It can spread rapidly as it produces huge numbers of wind-borne seeds. Successful heathland restoration means removing saplings quickly, before leaf litter enriches the soil and their shade weakens dwarf shrubs such as Heather beneath. An effective tractor mounted flail mowercan clear large areas of scrub on accessible areas. On many steep slopes and near hazards such as slit trenches, manual cutting with chain saws and hand tools is required. Oak can also invade rapidly, particularly from a woodland edge. Although scrub is a high value habitat on the heath and a range of scrub species, ages and structures is maintained, heathland is scarce by comparison and takes priority.