With respect to timber situated within two hundred feet abutting landward of the ordinary high water mark within shorelines of statewide significance, the department or local government shall allow only selective commercial timber cutting, so that no more than thirty percent of the merchantable trees may be harvested in any ten year period of time: PROVIDED, That other timber harvesting methods may be permitted in those limited instances where the topography, soil conditions or silviculture practices necessary for regeneration render selective logging ecologically detrimental: PROVIDED FURTHER, That clear cutting of timber which is solely incidental to the preparation of land for other uses authorized by this chapter may be permitted.