Historical comparisons

All seven of the islands used by nesting waterbirds that we visited appear on the 19^th Century topographic sheets of the region. Although the islands were initially protected because of their nesting birds in the early 1900s, we wanted to be sure that they were not the result of dredging operations. In particular, the Bird Islands (both upper and lower) and the Tarpon Bay Keys, had an appearance of being spoil islands. Examination of the topographic sheets, however, showed that this was not the case. The Tarpon Bay Keys are clearly apparent on Sheet #T-693 (Fig. 29). Bird Key is clearly indicated also on sheet #T-738 (Fig. 30). The other rookery islands also appear on these early maps. This does not mean necessarily that the islands have not changed over time, or that they have not been impacted since the advent of dredging.

Figure 29. The Tarpon Bay Keys (red arrows), Florida, as depicted on topographic sheet T-693 from 1859 (left) and in a 1999 false color infra-red aerial photograph. The two images are not to equal scales. [larger image]

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Figure 30. The old and new of northern Pine Island Sound, Florida. Topographic sheet T-738 from 1859 (left) and a false color infra-red aerial photograph from 1999. Bird Key is located near the center of both images (red arrow). The two images are not to equal scale. [larger image]

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Figure 31. Comparison of the Shell Mound Trail area and West Impoundment area of J. N. “Ding” Darling NWR in 1859 (left) and 1999 (right), Florida. Lack of apparent shoreline change (red arrows) and apparent vegetation change (yellow arrows) are indicated. The two images are not to equal scale. [larger image]

Another interesting comparison is that of the Shell Mound Trail - Power Line Road - West Impoundment area of the refuge (Fig. 31). The open water shoreline of the impoundment appears unchanged, yet some other water bodies have clearly changed over time. Additionally, there is evidence of vegetation change with the possible infilling of a coastal marsh by mangrove forest vegetation (Fig. 31). Determining all of the changes from the 1857 charts to present is beyond the scope of this report.

Pre-hurricane island geomorphology, vegetation structure, and hurricane damage

We used the descriptions for mangrove forests first proposed by Lugo and Snedaker (1974) and used by Odum and others (1982) in their south Florida mangrove forest community profile. The waterbird breeding islands fall into three, broad, categories: overwash forests, fringe forests and basin forests. Several islands have more than one type of mangrove forest present. Tarpon Bay Keys are low in elevation and are inundated at high tide. Therefore, these are overwash islands dominated by red mangrove (Rhizophora mangle) trees. Lower Bird Island is relatively small (<1 ha). It has fringe mangrove forests around its perimeter. The interior of this island is a slightly elevated open area, with woody vines (e.g., nicker bean, Caesalpinia spp.) and exotics (e.g., Brazilian pepper, Schinus terebinthifolius).

The remaining four islands are more complex and have higher elevation areas that lie well above mean high tide. Areas with higher elevations support native and exotic plants. Bird Key, Givney Key, Hemp Island, and Lumpkin Island are fringed by red mangroves, just inland of which lies an elevated berm. These berms have both tropical hardwood species (e.g., gumbo limbo, Bursera simaruba) and invasive exotic plants (e.g., Brazilian pepper) growing on them. Interiors of these four islands are typical basin-type mangrove forests (Table 2). The presence of these basin mangroves sets these islands apart from the others. Basin mangroves are poorly drained and thus are often flooded for long periods of time. On these islands, we found fine to course forest soils with shell hash and high organic matter content (T. J. Smith, personal observation). These interior basin forests were dominated by the black mangrove (Avicennia germinans), with red mangroves abundant only on Lumpkin Island, while the white mangrove (Laguncularia racemosa) was uncommon throughout (Table 2). Hemp Island was further differentiated from the others by the presence of a very high ( 20 ft or 6.1 m above msl) mound that formed a partial ring around the western side of the island. This mound was covered in a tropical hardwood hammock dominated by gumbo limbo with an understory of Jamaica dogwood (Piscidia piscipula). A large gumbo limbo had been tipped-up revealing that the mound is in fact a shell midden, probably dating to the Calusa culture (Walker and others, 1998). Two exotic plant species, both indicative of white settler habitation, lily and papaya, where also present at this location (Fig. 32).

Figure 32. Evidence of human occupation of Hemp Island, Florida. A tip-up mound and pit reveal the existence of past Calusa habitation in the form of conch, clam, and whelk shells (left). The presence of lily and papaya plants indicate human occupation much after the Calusa habitation (right). [larger image]

An interesting feature of Hurricane Charley was that it was fast moving and did not have a large storm surge (Figs. 8 and 9). We found evidence of overwash and sediment deposition on two waterbird islands, Upper Bird Island and Givney Key (Fig. 33). It appeared, however, to be very limited in extent. Overall, there appeared to be little evidence of sediment having been carried into the interior of the islands.

Figure 33. Two views of overwash deposits from Hurricane Charley. On the left is a close-up showing approximately 13 cm of sediment on Upper Bird Island. The right hand panel shows an overwash lobe of about 50 cm depth on Givney Island, Florida. [larger image]

Hurricane damage to vegetation

Vegetation damage ranged from simple salt burns on plant leaves; to loss of leaves and some branches through heavy defoliation, crown damage and some windthrow; to almost complete loss of the forest canopy with high tree mortality (Fig. 34). In general, we observed the most severe damage in mangrove forests and least severe damage in live oak, cabbage palm, and tropical hardwood hammocks. Damage also appeared to be least in the southern portions of the Complex and increased northward towards the location where the eye of Hurricane Charley crossed the coastal barrier islands. Our quantitative data support this view (Table 2). We measured the greatest damage on Bird Key where the basin forest had 60-90% tree mortality. In geomorphologically similar forests on Givney Key, mortality approached only 20% (Table 2). Data from the SCCF mangrove plots (4) also showed these trends (E. Milbrandt, unpublished data).

Figure 34. Levels of hurricane damage range from: (a) leaf burn from salt, (b) defoliation and branch loss, (c) some stem blown down, and (d) total canopy loss. [larger image]

The area of the Shell Mound Trail - Power Line Road deserves a special comment (Fig. 31). The mangroves in this area appeared to be what Lugo (1997) termed “old growth” forest. This is an unusual and rare form of mangrove forest. The trees are quite large in diameter and height and stem density is low. The crown is dense and there is little or no understory, which means there is no pool of seedling or sapling-sized individuals. Our observations in this area indicated that all of these characteristics were present prior to Hurricane Charley. Unlike the waterbird islands where black mangrove was dominant, the Shell Mound Trail forest was dominated by very large red mangroves ( 30 to 50 cm dbh), with large black and a few white mangroves, scattered throughout. The forest canopy in this area was catastrophically damaged. Mortality of canopy-sized individuals was in the range of 80-100%.

Regeneration following disturbance

Many plants have the ability to regenerate vegetatively following disturbance. This is especially true for plants of tropical origin; however, species differ in this ability. For example, the red mangrove can't coppice or stump sprout (Smith and others, 1994). Black mangroves, however, can (Fig. 35). Tropical species such as gumbo limbo and sea grape also resprout very quickly following disturbance (Fig. 35).

Figure 35. The ability to re-sprout following leaf and branch loss varies greatly among species. In general, species with a tropical origin have higher coppicing ability than do temperate species. These photographs were taken five weeks following Hurricane Charley. The red mangrove, a dominant species in the mangrove forests of Florida, lacks coppicing ability. [larger image]

Another factor affecting forest recovery is advance regeneration. This refers to the presence of seedling and sapling-sized individuals in the forest at the time of disturbance. These individuals, which are already present, form the pool from which the new forest canopy develops. We observed advance regeneration at many mangrove forests, but not all (Fig. 36). In particular, the Shell Mound Trail - Power Line Road forest was devoid of advance regeneration.

Figure 36. Advanced regeneration, i.e., the presence of seedlings and saplings (from seed banks) in a forest's understory is important to recovery following disturbance. It is abundant in some locations, such as Bird Key (left), and almost totally absent in other areas, such as Givney Key (right), Florida. [larger image]

Hurricane disturbance creates a variety of micro sites within the vegetation for regeneration that had not been present prior to the storm. New light regimes exist and the topography of the forest floor is altered by tip-up mounds and pits. Tip-ups were present in all forests we visited (Fig. 37), but appeared to be most abundant in the basin forests of waterbird nesting islands. Tip-ups are especially important in mangrove forests. Here they provide regeneration sites for species not normally encountered in these forests. We found many tip-ups with non-mangrove species regenerating on them (Fig. 38). Following Hurricane Andrew, tip-up mounds in mangroves were extensively colonized by invasive exotics such as Brazilian pepper, papaya, and lather leaf (T. J. Smith, personal observation, see also Table 3).

Figure 37. Tip-ups increase sediment micro-topography by creating mounds and pits: (a) live oak on Legion Curve area, (b) black mangrove on Bird Key, and (c) gum limbo on Hemp Island, Florida. [click on the images above to view a larger version]

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Figure 38. Tip-up mounds also provide places for species to regenerate and colonize. Here, non-mangrove plant species (yellow arrows) colonize a tip-up mound on Givney Key, Florida. [larger image]

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Vines, lianas, and disturbance

We encountered vines at every site we visited, including Upper Bird Island, the smallest of the waterbird islands. The most common vine we found was nicker bean, a native species (Fig 39). We also found Virginia creeper, poison ivy, devils claw, grapes, morning glories, and mangrove rubber vine (Table 3). Interestingly, we did not encounter coin vine, which is common in mangrove forests of Everglades National Park. This is most likely a result of limited surveys rather than true absence. Finally, we found a very aggressive, invasive exotic vine, air potato, at two locations, Shell Mound Trail and near Legion Curve (Fig. 39). We learned that another invasive vine, rosary pea, was present on the refuge, although we did not observe it during our visit.

Figure 39. Proliferation of vines is common following disturbance. Nicker bean, a native species, grows on Hemp Island (left), while, air potato, and invasive exotic, spreads rapidly along Shell Mound Trail, Sanibel Island, Florida (right). [click on the images above to view a larger version]

Observations on exotic plants

Figure 40. Nests of White Ibis (Eudocimus albus) in a Brazilian pepper tree on Hemp Island, Florida. [larger image]

Numerous exotic plant species were encountered during our surveys (see above and Table 3). The most abundant exotic plant on the waterbird islands was Brazilian pepper. On several islands we found wading bird nests in Brazilian pepper plants (Fig. 40). The most problematic exotic we encountered is the vine, air potato (Fig. 39). This species is extremely aggressive and can spread rapidly. It has the capability to become established along the edge of a wetland and then spreading over the wetland using the native plants for support, like a trellis. Other invasive vines also can do this, particularly lather leaf (Table 3), which is a problem in coastal regions of Everglades National Park.

Freshwater wetlands

We did not survey freshwater wetlands which are present on JNDDNWR (Fig. 41). There may be, however, evidence of human activities that have altered the surface hydrology of the refuge in the past (besides the waterbird impoundments). The Shell Mound Trail - Power Line Road area mangroves appear particularly to have been impacted (see Fig. 5 and 31 for location). We found several water control structures along Power Line Road (Fig. 42). It appears, from our one observation, that freshwater was being impounded to the west of Power Line Road. This high water may be another factor that contributes to the lack of advance regeneration in the mangrove forests from Power Line Road westward to Shell Mound Trail. Mangrove propagules and seedlings cannot become established in standing water; they require tidal dry-downs (Smith 1992).

- Figure 41. A dune swale freshwater marsh (left) and white-topped sedge (Rhyncospora latifolia) growing in the marsh (right, enlargement), Legion Curve area, Sanibel Island, Florida. [larger image] - Figure 42. Water control structures along Powerline Road, Sanibel, Island, Florida. A structure (left, yellow arrow) on east side of the road that directs surface water from south to north. A partially collapsed culvert (right, yellow arrow) with water flowing from the Shell Mound Trail area to east under Powerline Road. [larger image]