The Ecology of Hydric Hammocks - USGS National Wetlands ...

I ~ U mapleswamp laurel oak 3:1100bald cypress a- swamp tupelo -,100wgreen ash m jo $sweetgum m rn 3:6IO~IOIIY pine 3: !i100live oak- dowater oakI0 40 80 120 160 200DISTANCE (m)Figure 30. Distribution of tree species along apresumed flooding gradient in Sanchez Prairie,San Felasco Hammock State Preserve. The transectbegins in swamp conditions (left) and endsin drier hydric hammock (right).the gradient, and live oak, water oak,and loblolly pine to the drier(hammock) part. Red maple and swamp1 aurel oak ranged almost throughoutthe swamp/hammock transect. Swamplaurel oak was replaced by 1 ive andwater oaks at the dry end of the gradient,their relative positions in accordancewith flooding to1 erances inferredfrom the studies cited above.Another study transected 200 m of aSemi no1 e County hydri c hammock (Table5), descending 80 cm to river swampdominated by bald cypress, red maple,and sweetgum (G. R. Best and P. Wallace,pers. comm.). The first half ofthe transect probably was slightlyabove average high water, whereas thesecond half was slightly below. Sugarberry'and hornbeam were found onlyin the first 50 m, while swamp laureloak extended to 150 m. Sweetgum andFlorida elm ranged throughout the twohammock sections and into the swamp.Cabbage palm, the dominant tree inthis hydric hammock (Table 5), alsowas ubiquitous, but its abundance declined greatly in the swamp. Redmaple occurred in swamp and hydrichammock, though only as saplings inthe latter. NO 1 ive oaks were sampled,although this species was ob-served in other, generally drier,parts of the hydric hammock.Some of the differences in compositionof plant species among hydrichammocks also result from variablepatterns of flooding and drydown. TheSeminole County hammock probablyfloods more often and for a longer periodthan many others, which was reflectedin the vegetation: Floridaelm was an important species, redmaple and swamp laurel oak sap1 ingswere abundant, no 1 ive oaks were sampled,and bald cypresses were present.Live oak was absent also from theWekiva Springs and Tiger Creek hydrichammocks, both of which receive seepageand are almost always moist. Inthe Wekiva Springs hammock, a high watertable is maintained by dischargefrom a deep aquifer (see Figure 9).The hydrology of the Tiger Creek hammockwas discussed in Chapter 2.4;thi s wet1 and receives up1 and seepagedepending on the season. The profusionof shrubs and ground vegetationin these hammocks probably resultsfrom the nearly constant saturation ofthe soils. The influence of seepageon plant species composition is especial1 y pronounced at Wekiva Springs.Numerous bay trees, needle palm, andcinnamon fern make this forest intermediatein composition between"typical" hydric hammock and bayhead,a community characterized by constantseepage. The re1 atively constantmoisture regime of the Wekiva Springsand Tiger Creek hammocks contrastswith the extremes of drought andflooding experienced by other hydrichammocks. We suggest that hydric hammockslow in species diversity anddominated by cabbage palm and live oakexi st where long, dry periods are interruptedby occasional episodes offlooding. Examples are found in theupper St. Johns and Myakka Riverbasins, where river overflow only sporadicallyfloods the hydric hammocks.In Myakka River State Park, hydrichammock has expanded into freshwatermarsh during the past 30 years (Figure31), at the same time as river levelshave declined due to upstream diversions(Robert Dye, Florida Departmentof Natural Resources; pers. comm.).Whereas the old part of the hydric