This report profiles the natural history of the rich hardwood forests of the middle St. John River Valley in New Brunswick and in the eastern section of Aroostook County, Maine. Section 1 defines what is meant by the St. John River Valley Hardwood Forest (SJRHF), describes the forest assemblage type in general, and outlines how that type differs from hardwood forest assemblages found in other regions of the Maritime Provinces. Section 2 describes the physical characteristics of the central St. John valley and the vegetation history of the area, from the beginning of the post-glacial period to the present. Section 3 profiles the life history characteristics of ground flora associated with temperate deciduous forest. Section 5 gives detailed descriptions of the rare vascular plant species associated with the SJRHF. The final section reviews conservation prospects for the SJRHF in New Brunswick based on current land use trends and makes management recommendations for its protection.