A: Water source and nutrient utilization in upland ecosystems in the Everglades National Park, Florida, USA

Project 1: Hammocks and pine rocklands of southern Everglades

Project 2: Shark Valley Slough TREE ISLANDS in central Everglades

enp map

 Project 1: Hammocks and Pinelands


Fieldwork  January 2005 - March 2006 at three hammock and pineland sites on the Miami Rock  Ridge


The upland plant communities of tropical hardwood hammocks and pine rocklands in the  predominantly flooded Everglades National Park (ENP) constitute just 4% of the area, yet are  ecologically important as they greatly increase floral diversity and provide most of the habitat for  fauna. Agriculture, flood control and urban development have changed the hydrological and nutrient  regimes thereby jeopardizing these communities, whose ecohydrological functioning is not well  understood. We examine water source utilization and foliar nutrient status in hammocks and  pinelands, to better understand the sensitivity of these communities to water levels and spatial  nutrient distribution.

Pine rocklands suffer from a paucity of soil owing to natural fires that burn off accumulated litter. Thus  groundwater is thought to be the main water and nutrient source for pineland plants. Hammocks have  built up a nutrient-rich organic soil horizon that entraps rainwater and thus could furnish additional pools of water and nutrients to hammock plants. Hammocks are thus viewed as localized areas of  high nutrients and instances of vegetation feedbackupon the oligotrophic everglades landscape  enabling establishment and survival of flood-intolerant tropical hardwood species. We examined the δ18O of stem waters in plants in Everglades National Park and compared those with the δ18O of potential water sources.

In the wet season hammock plants accessed both groundwater and organic soilwater while in the dry season they relied more on groundwater. A similar seasonal shift was observed in pineland plants; however groundwater constituted a much higher proportion of total water uptake throughout the year under observation. Hammock plants on average had a significantly higher annual mean foliar nitrogen and phosphorus concentration. Foliar N was correlated with δ13C while pineland plants showed a higher water use efficiency (higher δ13C) at lower levels of N and P, but correlation was poor. Most hammock species are intolerant of flooded soils and are thus constrained by the high water table in the wet season, yet access the lowered groundwater table in the dry season due to drying up of water in the surface organic layer. This dependence on a relatively narrow annual range of water table fluctuation is to be considered in South Florida water resource management and Everglades restoration.

Poster presented at the 2007 Fall meeting of the American Geophysical Union, San Francisco, CA

Project 2: Tree islands in the everglades

Water source utilization and foliar nutrient status in upland and flooded plant communities in tree islands of the Shark River Slough, Everglades National Park, USA

To be presented as a poster at Ecological Society of America (ESA) Annual Meeting, Milwaukee, WI, August 8, 2008

 

Background/methods

The uplands or highest parts of tree islands in the Shark River Slough constitute the only areas in the central Everglades that are above water in the wet season, thus resulting in tree islands having two sharply distinct plant communities: (i) the upland community consisting of flood intolerant hammock species and (ii) the flooded margins with a hydroperiod of 3-7 months that have flood tolerant species.  The upland areas have built up a largely organic soil layer consisting of decomposing leaf litter that is thought to provide an unsaturated rooting zone for the flood-intolerant hammock species, with potentially greater nutrient availability in comparison with the flooded communities.  Two potential water sources exist: (i) entrapped rainwater in this organic soil (referred to as soilwater) that is nutrient-rich, yet very localized and small in volume and (ii) the regional surfacewater/groundwater that is nutrient-poor.  We investigated the differences in water source utilization in these two communities and then linked these differences with their foliar nutrient levels and photosynthetic performance as measured by δ13C abundance.  Three tree islands were sampled in a gradient from slough to ridge, 50 trees on each (20 upland, 30 lowland ) bimonthly over a year.

Results/Conclusions

 Upland plants used soilwater in the wet season and shifted gradually to greater regional water uptake in the dry season, while lowland plants used regional water throughout the year.  Consistent with the nutrient concentration of the two water sources used in the two communities, uplands had a greater annual mean foliar nitrogen and phosphorus concentration at the community level over lowlands as well as a higher leaf area index, thereby supporting the idea of tree islands being nutrient hotspots in the oligotrophic Everglades.  High foliar N concentration in upland plants was associated with eventual stomatal limitation of photosynthesis.  Upland  species being intolerant of flooded soils are restricted to water uptake in the shallow unsaturated soil layer in the wet season, yet access the lowered groundwater table in the dry season because of drying up of surface soilwater. This dependence on a relatively narrow annual range of water table levels should be considered in South Florida water management and restoration of tree islands, which add ecological complexity to the predominantly flooded ecosystem and are critically important to fauna for being the only terrestrial habitat in the slough.

A talk given at the South Florida Plant Scientists' meeting in March 2008 at the National Tropical Botanical Garden, The Kampong, Coral Gables, FL is available HERE.

Work in progress

Species level analysis