From picturesque coastal estuaries of Cape Cod to the soaring White Mountains, much of New England's rich native flora is fighting for survival against increasing odds, according to what conservationists call the most comprehensive accounting ever made of the region's plant life. The report, to be released Thursday by the New England Wild Flower Society, studied more than 3,500 known plant species and determined that 22 percent are considered rare, in decline, endangered or possibly extinct. Many plants also range over a much smaller geographical area than they once did.