The contribution of nonnative tree species to the structure and composition of forests in the conterminous United States in comparison with tropical islands in the Pacific and Caribbean
Nonnative tree species have received less scientific attention than nonnative species in general, but when a forest is colonized by a nonnative tree species, the ecological effects can be significant as a change in tree species composition can alter the structural and functional attributes of forest ecosystems. We assess the abundance, geographic distribution, contribution to forest structure (including carbon), and temporal trends of nonnative tree species between the most current inventory and the previous one, ranging from 3 to 15 years earlier, within the conterminous United States (CONUS) and U.S.-affiliated islands in the Caribbean and the Pacific. We used publicly available data from the U.S. Department of Agriculture Forest Service, Forest Inventory and Analysis (FIA) program. Our analysis is by ecological section (ecosection) within ecological provinces of the CONUS and islandwide for Pacific and Caribbean islands. We found that the forest land area with nonnative tree species in the CONUS is 18.8 million acres (7.6 million ha) and is expanding at about 500,000 acres (202 343 ha) per year. The contribution of nonnative tree species in the CONUS to the structural component of forests (basal area and tree density) increased slightly. The mean live aboveground tree carbon of nonnative tree species ranged from 0.39 ton per acre (0.88 Mg/ha) for saplings (small trees with diameter at breast height [dbh] ≥1–<5 inches [≥2.54–<12.7 cm]) to 2.47 tons per acre (5.54 Mg/ha) for all trees (≥1 inch dbh), saplings included. These numbers are equivalent to 19 and 10 percent of the total carbon storage for their respective size classes in the forest plots where they occur, and they slowly increased between previous and current inventories. The contribution of nonnative tree species to the carbon storage of CONUS forests is 92.6 gigapounds (42 Tg) of C or about 0.05 percent of the amount stored in those forests. Nonnative tree species also sequester 1.3 gigapounds (0.6 Tg) of C annually or about 0.5 percent of the carbon sink of CONUS forests. The type and intensity of human activity is generally associated with the presence of nonnative tree species. A similar relationship is at play in Caribbean and Pacific islands and in the mainland forests of the CONUS. Additionally, a greater concentration of human activities in islands makes the nonnative tree species more common there than in the CONUS.