Peterson’s Challenge, Part 2

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Five years after conducting my last breeding bird census at Congaree, one of the most powerful hurricanes to hit the South Carolina coast in more than a century created catastrophic changes in much of South Carolina’s forestland. Hugo’s powerful winds passed through central South Carolina, and Congaree National Park, early on the morning of September 22, 1989, with sustained wind gusts of over 90 miles per hour.

It was later in October when I finally got down to the park and investigated the damage to my bird census plot. My initial reaction was shock and disbelief at all the daylight streaming through a formerly closed canopy. Downed trees, limbs, and woody debris were everywhere and at times I wasn’t even sure I had the right location. Based on my previous vegetation survey of the plot, I determined that in one night Hurricane Hugo knocked down sixty-eight trees a foot or more in diameter on my twenty-acre plot; most significantly, twelve of the sixty-eight were three feet in diameter or more.

A large Hugo-downed sweetgum on my bird census plot.

In May, 1991, only a year-and-a-half after Hugo, I was again studying the bird population of my Congaree research plot, but using different methods for different purposes. And I would notice an immediate change in bird numbers and species composition from my earlier census work of just seven years ago.

I began a MAPS (Monitoring Avian Productivity and Survival, a nation-wide program created by the Institute for Bird Populations at Point Reyes Bird Observatory in California) bird-banding station at my Congaree bird plot in the late spring of 1991 and continued it until 2004 (2003 was scrubbed due to excessive flooding). At the end of the first 1991 MAPS field season, in August, we had banded a total of 112 breeding-season birds representing 22 species. It was noteworthy that Carolina wrens were nearly twice as abundant after Hugo as before; white-eyed vireos were nearly three times as abundant; and the hooded warbler, five times as abundant. Just as significantly, three species never recorded before Hugo – indigo bunting, Kentucky warbler, and Swainson’s warbler – were banded on the plot in 1991.

The indigo bunting was found on the census plot only after Hurricane Hugo.

All six of the above species were birds of thickets and dense cover, the so-called edge species mentioned by Roger Tory Peterson. This was no surprise as the bird plot had an entirely different look after Hugo as compared to before: the former dark, dense canopy had been replaced by a much more open canopy and the vegetation-free understory with thickets and cover scattered throughout the twenty acres (and it was amazing to see how quickly the dense cover grew up in only one growing season after Hugo, from 1990 to 1991).

It was noteworthy to follow the trends of three edge species over the next thirteen years: hooded warbler numbers peaked out the summer of 1992, when nineteen were banded; their numbers started declining noticeably after 1995, so that by 2004 there were only two banded on the plot. Trends for white-eyed vireos were similar: their numbers peaked at eleven banded in 1995 and only two banded in 2004. The much-less numerous Swainson’s warbler was represented with a maximum of four birds banded in 1994, while only one was banded from 2000 to 2004.

The uncommon Swainson’s warbler was another species only found after Hurricane Hugo.

What all of this was telling me was that vegetation growth at Congaree was so fast that within eight to ten years after the hurricane, the canopy had closed to pre-Hugo conditions. This resulted in so little sunlight hitting the forest floor that the sun-loving thickets and other dense cover thinned out to the point that the edge birds could no longer use them.

I came to the conclusion that bottomland hardwood forests of the South, nearly all of which are within striking distance of hurricanes, were “managed” under pre-settlement conditions by recurrent strong winds, primarily from hurricanes, but on a smaller scale, by tornadoes and strong wind storms. Winds of this magnitude were needed to open the canopy sufficiently to allow the development of large enough thickets favored by bird edge species.

It appears to me that Congaree, and likely many other bottomland hardwood forests of the South, go through decades of hurricane “dormancy,” where a closed canopy limits the development of thickets, followed by a shorter period where an open canopy created from a passing storm allows the growth of thickets and edges. The birds that use these early-successional habitats must, by nature, be nomadic enough to take advantage of a shifting, transitory habitat.

Hooded warblers were five times more abundant after Hugo.

A caveat here is that hurricane damage on our bird study plot was characterized as moderate (although it certainly didn’t seem moderate at the time). There were some areas in the park, however, that received very heavy hurricane damage – where nearly every overstory tree was felled by strong winds – in effect creating small, natural clearcuts of a few acres or less. Recovery of these stands to pre-Hugo conditions will obviously take many, many decades.