Our Invaded Forests

Nathan Klaus
Nongame-Endangered Wildlife Program

Download the PDF version of The Citizen Scientist Newsletter - Spring 2006 (Requires Adobe Acrobrat Reader).

What is an invasive exotic species?  In the case of Chinese Privet, Nepal Grass, or Cogon grass the answer seems obvious.  These are all species from far away countries that have invaded Georgia ecosystems, replacing native species and interfering with ecosystem functions.  What about a species from the west coast, such as the house finch?  Most scientists agree that this too is an invasive exotic species, even though it is native to this continent.  House finches have expanded their range as a result of human activity, taking advantage of suburban and agricultural landscapes.  The distance a species travels has less to do with whether a species is considered an invasive exotic than how it got there and what it does once it arrives.  Perhaps a better way to define an invasive species is that it (1) has recently expanded its range, often as a result of human meddling (2) it often replaces species native to that site and (3) it often disrupts ecosystem functions.

By this definition water oak (Quercus nigra) and sweetgum (Liquidambar styraciflua) are invasive species, though they didnt cross any oceans or even any continents to get here.  These trees are the second and third most common trees in Georgia.  Both were once much less common, found only in major river floodplains for thousands of years prior to European settlement.  At that time Georgias uplands were largely comprised of longleaf pine savanna in the coastal plain, and shortleaf pine/post oak woodlands in the Piedmont (Figure 1).  Regular fires kept these forests open, and water oak and sweetgum out.  Birds used these ecosystems, taking advantage of the open canopy and rich herbaceous ground cover.  Logging, farming, and farm abandonment changed the landscape.  In the early 20th century many farms grew back into forests of loblolly pine, only this time fire was not a part of the system.  In the absence of fire these fast growing, highly mobile species invaded the uplands.  Once established in the uplands, water oak leaves suppress fire, as they are very slow to burn.  Instead of grassy open woods, Georgias forests became dark and shady, with little plant diversity, and recent surveys have revealed this translates to greatly reduced bird diversity.

Research conducted by Georgias citizen scientists have revealed just how big an impact the water oak/sweetgum invasion has had on our birds.  Ninety point counts were conducted between paired upland pine forests throughout Georgia.  Each pair had an equal size and density of loblolly pines.  Each had been burned at least once in the last ten years.  One of each pair had no water oak or sweetgum while the other had at least 25% of its canopy trees as water oak or sweetgum.  Pine stands with water oak or sweetgum had HALF the bird species and HALF the total number of birds as pine stands without water oak or sweetgum (see Figure 1).  Many species of high conservation priority declined or totally disappeared from stands once water oak or sweetgum appeared, including: brown-headed nuthatch, blue grosbeak, indigo bunting, red-headed woodpecker, Bachmans sparrow, red-cockaded woodpecker and bobwhite quail.  Even more interesting, no bird species were positively associated with water oak/sweetgum invasion of upland pine stands.  This strongly suggests that a forest type of upland pine/water oak/sweetgum is unnatural, present on the landscape for only a few decades.

Unlike open grassy forests, no birds have had time to specialize on the wateroak/sweetgum/ loblolly pine forest, and it is of reduced conservation value.  On the other hand, open grassy pine forests are extremely valuable, home to many of our declining birds.  This underlies just how important a regular fire regime, burning every 2-3 years, is to keep the invaders out.  Even though they are native to Georgia, some native species may not always be good.

Figure 1.  Landlottery surveys from the 1820s reveal a strikingly different Georgia Piedmont.  Uplands are dominated by a mixture of pine and fire hardy oaks.  Water oak and sweetgum are found only along major rivers such as the Ocmulgee.  On this image PO = Post Oak, Wh O = white oak

Figure 2. Bird species richness and abundance were half as high in stands invaded by water oak or sweetgum and open pine stands.