Paleoenvironmental aspects of the macrophytic plant assemblage from page-Ladson

Abstract

In this chapter I describe paleobotanical remains from the Page-Ladson site and discuss the ecological relationships of the taxa identified to provide basic information about the vegetation communities that existed in and around the site during the various periods of site formation. The deposits and remains analyzed generally span the period of roughly 10,000 years, during approximately 18,000-8,000 yr BP (this volume, see Chapters 3 and 4). The paleobotanical data ultimately help to forward the reconstruction and our understanding of the paleoenvironmental aspects of the deposits, including the vegetation dynamics that were associated with climatic change in the transition from the late Pleistocene to the Holocene epoch. Macrophytic remains provide the basic data sets for this analysis, which involves study of the various types of macroscopic plant constituents of the deposits, most of which are visible without magnification. Among the specimens classified and analyzed are degraded (waterlogged) wood, carbonized wood, bark, thorns, seeds, fruits, leaves, and more. Together the information garnered from this relatively rich data set provides complementary information to compare with the pollen analytical and other microbotanical data (Hansen, this volume). When used in tandem, the micro-and macrobotanical data sets provide a more complete proxy record for the reconstruction of the late Quaternary paleoecology and environmental history of the Aucilla River area. In general, pollen and other plant microremains typically provide a regional-scale perspective on paleoenvironments due to the inherent nature of pollen dispersal, which typically occurs on a relatively broad scale due to dispersal by wind and water action. Thus palynology provides a regional and somewhat composite picture of paleoenvironments, although with certain limitations (e.g. some taxa have inherently low pollen profiles [see Hansen, this volume], and in forested settings filtering of pollen through dense woody vegetation may obscure the immediate area of study). The plant macroremain or macrophytic component, on the other hand, typically provides less of a regional view of vegetation but tends to yield finer-grained resolution of site/community level environmental details, that is, they typically describe the local ecology and microclimate. For example, the habitat zones of a pond or lake (e.g. profundal, limnetic, littoral, wetland [Fernald and Patten, 1984]) based on the characteristic vegetation of each zone, or the mosaic of forest taxa according to hydrology or other parameters, may be recognized on the basis of preserved seeds, wood, or other plant tissues. This is possible because macrophytic remains tend to be deposited in or near the immediate area of the parent plant due to their generally larger size and necessarily having a tighter range of dispersal by wind, faunal, and other dispersal agents. This is especially true of quiet, low-energy aquatic environments - as in the Aucilla River - where the seeds, fruits, wood, etc., from the aquatic and adjacent terrestrial vegetation settle in the wet bottom deposits where they become gently entombed and accumulate over time in the waterlogged, essentially anaerobic deposits where they may be preserved for extended periods. Understanding these benefits and differences in dispersal mode, range, and scale of environmental resolution is critical to the interpretation of macro-and microbotanical data sets and their potential to provide a detailed reconstruction of paleoenvironments. The Page-Ladson site formed in an ancient sinkhole in the Aucilla River, which is part of a freshwater spring and stream ecosystem located in northwest Florida, bordering Jefferson, Taylor, and Madison counties (Fernald and Patten, 1984:218-232). Physiographically, the Aucilla transcends the western region of the Tallahassee Hills and traverses through the Gulf Coastal Lowlands to Coastal Swamp at the Gulf Coast; elevations run from sea level or nearly so in the coastal swamps, lowlands, and river valleys to over 200 feet above sea level in the Tallahassee Hills and the area is generally characterized by limestone karstic topography (Fernald and Patten, 1984:221-222). The soils are predominantly classified as Utisols in the hill region and Spodosols in the lowlands (Fernald and Patten, 1983:58). The climate is classified as humid subtropical, with average annual temperatures of around 68-72°F and annual precipitation of around 56 in. (1422 mm). A rainfall maximum generally occurs during the summer months, whereas the spring is typically the period when droughts of varying intensity and severity may occur, which on average happens once every eight to ten years (Fernald and Patten, 1984:222). The modern vegetation in the vicinity of the site is predominantly comprised of hardwood-dominated bluff, bottomland, and swamp forest associations. The forested floodplains of the Aucilla and its tributary Wacissa River are extensive: 10,930 and 14,647 acres, respectively (Fernald and Patten, 1983:94). This provides for a strong presence of the bottomland hardwood ecological community, which is characterized by a high diversity of hardwoods and is best developed and typical of the floodplains of the river systems of northwest Florida (USDA Soil Conservation Service, 1985 and 1989:108; Ewel, 1990). The annual moisture regime is a critical factor in perpetuating this plant community, where the relatively rapid rise of water levels during seasonal flooding events, but with little or no inundation during the growing season, is followed by quick receding of floodwaters. The retention of floodwater in depressional areas within the alluvial floodplain results in the development of associated swamp hardwoods forest. The floristic composition of the Ochlockonee River valley (Clewell, 1980), just west of the Aucilla River system, provides a good proxy for that of the Aucilla River to portray some of the microclimatic aspects and related details of the bottomland and adjacent forest habitats. On the highest elevations, specifically the uplands adjacent to the river bluffs, grows the longleaf pine-turkey oak (Pinus palustrus, Quercus laevis) forest with an open understory of post oak (Q. stellata) and Ericaceae (shrubs in the blueberry family). The bluff rim is primarily vegetated in laurel oak (Q. hemisphaerica) and mockernut hickory (Carya tomentosa). White oak (Q. alba) predominates on the upper slopes, and southern magnolia (Magnolia grandiflora) and beech (Fagus grandiflora) occur prominently on the lower slopes. Other trees found generally on the slopes are spruce pine (P. glabra), southern sugar maple (Acer saccharum), wild olive (Osmanthus sp.), flowering dogwood (Cornus florida), American holly (Ilex opaca), pignut hickory (C. glabra), basswood (Tilia spp.), sweetgum (Liquidambar styraciflua), and white ash (Fraxinus americana). The primary vegetation of the Tallahassee Hills is the magnolia-beech community, which is essentially the same bluff forest described here. The floodplain vegetation on higher, better-drained soils is similar to that of the bluff slopes, but common includes also swamp-chestnut oak (also known as basket oak, Q. michauxii). Lower, wetter sites demonstrate a predominance of water hickory (C. aquatica), overcup oak (also known as water white oak, Q. lyrata), diamond leaf oak (Q. laurifolia), red maple (Acer rubrum), ironwood (Carpinus caroliniana), American elm (Ulmus americana), green ash (F. pennsylvanica), pop ash (also known as Carolina ash, F. caroliniana), and sweetgum. Still wetter, low areas include and may be dominated by water tupelo (Nyssa aquatica) and cypress (bald cypress, Taxodium distichum). The river banks are frequently bordered by Ogeechee-tupelo (N. ogeche) and river birch (Betula nigra), and this woody riparian vegetation typically also includes willow (Salix sp.), wild rose (Rosa palustris), and stiff-cornell (also known as swamp dogwood, Cornus foemina). The Coastal Lowlands mentioned above, away from the river floodplain, bluffs, and higher terrain of the Tallahassee Hills are primarily vegetated in pine flatwoods, including several species of pine, some oaks, and in the low acid swamps very commonly are found cypress, black gum (Nyssa sylvatica), wax myrtle (Myrica spp.), possum haw (also known as swamp haw, Viburnum nudum), and titi (Cyrilla racemiflora).