How will flooding impact South Carolina’s forests and trees?

There is no doubt that the forestry industry has been and will continue to be impacted by the October 2015 floods. But what about our forests? It will be at least spring before we can begin to estimate the impact the floods will have on South Carolina’s forests and trees. Tree deaths in the next two to five years may be attributed to flood damage, but it will be difficult to determine if that is in fact true.

It seems intuitive that a 1,000 year rain event and flooding that exceeds 500 year flood levels would be damaging to trees in forests. But trees are very resilient. Floods are a natural phenomenon and most trees growing in flood plains are adapted to flood conditions. If flood conditions are not prolonged, they may even boost the growth of the tree.

The northwestern part of Lake Marion, where the Congaree and the Wateree Rivers meet. Most of this is perennially flooded, but we can see the mud that has been flushed down the Congaree River from Columbia. 

 

Forest health workers in states that have dealt with extensive flooding, including Louisiana, Texas and North Carolina, are all very optimistic about the health of South Carolina forests in the wake of the floods. Our colleague at the North Carolina Forestry Commission, Rob Trickel, monitored trees in areas flooded by Hurricane Floyd in 1999 for two years and compared them with trees in areas that were not flooded. They were unable to detect differences in mortality rate of trees in areas that had been flooded and in areas that had not been flooded.

Flooding caused by Hurricane Floyd and the flooding experienced by South Carolina were both of relatively short duration, i.e., less than a week. Most trees can survive such short term inundation of their roots.  We will be paying close attention to excessive mortality in what we might call sentinel species this spring. These include widespread and commonly found species, such as dogwood (Cornus florida), redbud (Cercis canadensis), cherry (Prunus americana) and loblolly pine (Pinus taeda). These species are ideal sentinel species to monitor for flood damage because they very intolerant of even short term inundation AND they can be found as ornamentals in neighborhoods and as natural components of the landscape. If we see high mortality in these species this spring we will know that we need to monitor other tree species more closely for excessive mortality or more subtle declines in growth or health.

Many factors interact to determine whether a particular tree will succumb to flooding.

Species vary widely in their ability to tolerate flooding. In addition to the sentinel species that we already mentioned, white oak (Quercus alba), butternut and mockernut hickories (Carya cordiformis and C. tomentosa), blackjack oak (Q. marilandica), and willow oak (Q. phellos) are all very intolerant of short term flooding and may suffer significant mortality. Other species, such as pecan (Carya illinoiensis) and sweetgum (Liquidambar styraciflua), can tolerate flooded roots for an entire growing season, even longer. And then you have bald cypress (Taxodium distichum), swamp tupelo (Nyssa aquatica), and black willow (Salix nigra), trees that not only tolerate inundated roots but thrive in swampy conditions where other trees can’t survive!

A wetland forest. You can see the expanded bases of the water tupelo trees, Nyssa aquatica, and cypress, Taxodium distichum. These trees thrive in flooded conditions

 

The age and vigor of a tree will also be important in determining whether it survives a flooding event. Mature, healthy trees are better able to survive flooding events than younger trees, overly mature trees, or unhealthy trees.

The time of year in which a flooding event occurs is also important. Almost all trees can survive even long term root submersion during the dormant season. Flooding events late in the growing season, especially close to leaf fall, will be less detrimental than flooding events at the height of the growing season. Higher temperatures during a flood will negatively impact a tree’s ability to cope with flooding.

Growing roots need access to oxygen to function normally. Even short term (five hours) lack of oxygen at the roots reduces photosynthesis and thus growth. Longer term (days to weeks) inundation can destroy mycorrhizal associations with the roots. Mycorrhizal fungi, which increase nutrient uptake from the soil, are intolerant of anaerobic conditions. The buildup of toxic byproducts of anaerobic respiration, especially ethanol, along with the anoxic conditions, damage roots, reducing their already compromised capacity to absorb nutrients from the soil. Tolerant trees can overcome many of these problems by producing adventitious roots above the waterline that can keep tissue aerated.

A beaver has raised the water level in this hardwood stand. The prolonged flooding will kill most of these trees. In their weakened state they will be attacked by insects and diseases, hastening their deaths. Most of the flooding we saw this October receded rapidly enough that we can expect to see most trees remain healthy.

 

Trees that survive a flood may have reduced growth or used energy stores to get survive the flood. This could leave the tree weaker and prone to attacks by insects and disease one or two years later. Physical damage to trees by debris can also weaken them. The prolonged moisture at the roots and crown can support invasion by root rots and crown rots.

One thing is certain… South Carolina’s forests dramatically buffered the state from worse damage! More than 60% of South Carolina is covered by forests, including important wetlands and floodplains. These forests reduce erosion, absorb much of the excess water and keep our waterways healthy by filtering out pollutants.

Let’s take care of the trees… they take care of us!