Drought in the Southern Appalachian Temperate Rainforest? Part 1


With record droughts in the Midwest, unprecedented flooding in Louisiana and the increased frequency of drought over the past few decades in Macon County, water has been on a lot of people’s minds this summer. Because many often talk about droughts and flooding as “natural disasters,” sometimes the ways that human actions shape water flows and availability and the ways that people are impacted are overlooked. In our next two columns, we focus on water issues in southwestern North Carolina. Today we will highlight some of the major impacts of drought, and in two weeks we will look at how society and nature interact to create drought.

Southwestern North Carolina typically receives ample rainfall associated with the mountainous terrain lifting moist air from the Gulf of Mexico-Western Atlantic Ocean. The occasional tropical system and seasonal frontal passages also contribute to rainfall totals that are distributed evenly across the year.

In addition, because of our deep soils and relatively intact forests, large amounts of water are stored in our hillside soils, providing a substantial buffer against dry seasons. The high rainfall and natural storage capacity is why some people refer to the Southern Appalachian Mountains as the “water tower” for the Southeast. But anyone who remembers the droughts of 1985- 1988, 1998-2001, or 2007-2008 can tell you that we are certainly not immune to water shortages.

In fact, we are prone to natural wet and dry cycles as a result of changes in atmospheric pressure over Iceland and off the coast of Africa (known as the North Atlantic Oscillation) that push the jet stream north or south.

The cyclical patterns have changed since the 1970s, and the timing and quantity of rainfall is now more variable. This suggests that we have both drier and wetter years to look forward to.

Researchers from the US Forest Service working with the Coweeta Long Term Ecological Research (LTER) Program have picked up on some of the subtler ecological impacts of low rainfall, often when it has disrupted their research.

During the four-year drought of 1985- 1988, Coweeta researchers Jim Vose and Wayne Swank found a very simple, direct effect: the extended drought caused pine trees in a 33-year-old plantation to grow 22-38 percent less; oaks fared much better, perhaps because their deeper roots could reach water deep in the soil.

Many impacts of drought, however, are less direct. For example, Coweeta LTER research found that drought makes pines more susceptible to the southern pine beetle. In normal years, pines ooze sap and essential oils to defend against the beetles, but drought reduces their ability to do so.

Researchers Barry Clinton, Lindsay Boring, and Wayne Swank found that drought has a similar effect on oaks. To cope with lack of rain, oaks convert starch to easier-to-use simple sugars. However, these simple sugars are also easier for the shoestring root rot fungus to use. The fungus depletes energy stored in the roots and kills the trees, sometimes years later. When the oaks die, they leave gaps in the forest that provide space and sunlight for other species to grow. However, drought-related gaps are particularly small and unsuitable to many of our traditional forest species, like sweet birch, hemlock, and pines. Over the long term, regular droughts could change the mix of trees in our forests.

Streams, rivers and the plants and animals that make these waterways their homes are also impacted by dry years. During dry years water temperature increases, and dissolved oxygen decreases, which makes it difficult for some species, like trout, to survive in their typical locations. Together with decreased shade and increased sediment in the water, this contributes to the process of native invasion. Species which are native to the region begin to make their way into smaller, cooler tributaries that they did not traditionally occupy, displacing the species that already live there.

Although dry years might not have a strong impact within our homes, they do impact our environment in other ways. In the next column we will discuss the ways in which our activities can influence these impacts.

Do you have any memories of a particular drought year or impact on your home or environment? We would like to hear your thoughts. Please send us your remembrances and observations about drought to the email or postal address below.

 

This column is produced by members of the Coweeta Listening Project (CLP), a branch of the Coweeta Long Term Ecological Research Program. Views expressed here are not representative of the USDA Forest Service or the Coweeta Hydrologic Lab. Please share questions, comments, or suggestions for future topics at cwtlistn@uga.edu or Coweeta Listening Project, UGA, 210 Field St., Room 204, Athens, Georgia 30602.

 

Original Citation: The Coweeta Listening Project. Franklin Press. Column on "Science, Public Policy, Community." Page A5. Oct 5, 2012.