Bank Failure & Cypress Success

 One of the most fascinating things about bald cypress, water tupelo, and swamp tupelo trees (Taxodium distichum, Nyssa aquatica, N. biflora) is that they have very specific conditions for their natural dispersal, germination, and establishment. Because of this, where the trees are established tells a story about conditions at the times the trees got started. Certainly, this is of intrinsic botanical and ecological interest, but it also provides evidence of hydrological and geomorphological conditions, and in some cases, changes.

The seeds are dispersed mainly by water. To germinate, they need to be deposit on wet 

A mass of floating water tupelo seeds on the floodplain of the Pee Dee River, S.C.

or at least moist soil. They cannot germinate underwater, and seedlings must grow tall enough to rise above any subsequent flooding. Once that happens, the trees can survive in perpetually inundated conditions, but the spot where establishment occurs must have been subaerially exposed—wet maybe, but not underwater—for at least one growing season. One of the things that particularly interests me is cases where tupelo and cypress are growing in sites that are always underwater. They can’t have been that way when the trees got started, so something changed. For example, the Google Earth^TM image below shows a 0.5 km (0.3 mile) stretch of the shoreline of the Chowan River, N.C. 

Cypress trees growing in standing water, Chowan River. 

Here cypress and a few tupelo are growing in always-flooded conditions because shoreline erosion and drowning by rising sea-level has overcome the sites where the trees germinated. You can measure and see that >100 m of shoreline retreat has occurred over the lifetime of the oldest, farthest from the modern bank trees. 

But there are cases where trees are growing in constantly inundated conditions where it is not obvious how they ever dried out long enough for trees to establish, or how once exposed sites got drowned. Again, the tree survival and growth is no surprise—cypress and tupelo can do that—but they cannot have gotten started underwater. 

Trees growing in constantly flooded conditions along the Trent River, NC. 

Where this occurs, one of several things must have happened—an extreme drought or diversion of flow away from the site long enough for trees to germinate, perhaps, or channel changes that leave a non-flooded depression that is later filled. Or, the trees germinated on a higher spot, which could have been a log or stump from a predecessor that died. This blog will investigate more of these situations in, as they say, the fullness of time.

The situation I address now is that of trees growing along, but away from, a distinct bank (in these swamps a distinct bank is not always present—many banks are very low, or even absent, with just a gradual transition from open water to deepwater swamp). Like the example above, the trees could have started on a stream bank which later eroded, and some of them probably did. But in other cases, the field evidence makes that unlikely, and therefore a puzzle. 

Trees in perpetually flooded sites along the banks of Cedar Creek, SC (top), and Tar River, NC (bottom).

Then last week, I was paddling along Holly Shelter Creek (tributary to the Northeast Cape Fear River), and I noticed something that I should have noticed a long time before—bank failures. Slumps, slides, and rotational failures along banks can dump sediment along the stream edge where a tupelo or cypress could get established before the failure material gets washed away.

A line of tupelo and cypress of apparently similar age along a section of Holly Shelter Creek that has experienced a series of bank failures. 

Older cypress resprouting from stump—note the slump scar on the bank behind it. 

Slump scars along the Trent River.

Slump sites along Island Creek, NC. 

I feel a bit foolish for not thinking of this before, because I’ve seen more than one example like the one below, which I photographed in 2012. The slump does not have to have trees already growing on it, however—in fact, unless such trees are hydrophytes such as cypress, tupelo, some willows, etc., they won’t last long with their bases underwater. All they must do is provide a substrate that stays mostly above water for a growing season.

Bank slump along the Sabine River, Louisiana.