Hydrochory in the Florida Everglades: Temporal and Spatial Variation in Seed. Dispersal Phenology, Hydrology, and. Restoration of Wetland Structure.
RESEARCH ARTICLE
Hydrochory in the Florida Everglades: Temporal and Spatial Variation in Seed Dispersal Phenology, Hydrology, and Restoration of Wetland Structure Dean Monette and Scott H. Markwith Abstract Hydrochory may move species into areas appropriate for establishment, increasing the probability that species can take advantage of restored habitat. This research examined the influence of Everglades ridge and slough degradation on hydrochory, the interaction of hydrochory, hydrology, and ecosystem structure, and the role of hydrochory in ecosystem restoration. We identified 41 seed species from 2343 and 2849 seeds trapped in the intact and degraded sites, respectively. Month and the interaction of month and site type had significant effects within subjects in repeated measures ANOVA for seed density/trap, species richness, water depth, and water velocity. Percent cover of standing vegetation and periphyton, and Sørenson’s Similarity Index comparing standing vegetation and the dispersing seed pool differed significantly between sites in both the wet and dry seasons. Regression analyses indicated that seed species richness and seed density/trap were significantly positively related to water depth and velocity. The timing of increasing water depths and velocity coincided with the dispersal phenology strategy in intact sloughs in late spring/early summer, which may assist the dispersal of slough species through the system into physically restored deeper water areas once they are restored. Ridge species in degraded sloughs, such as swamp sawgrass (Cladium jamaicense) and common buttonbush (Cephalanthus occidentalis), timed their seed release with peak water depths immediately before drawdown, and during substantially reduced water velocities. This strategy may increase the probability of these species being deposited on elevated peat locations. Keywords: dispersal phenology, hydrochory, ridge and slough, seed dispersal, sheet flow
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verglades National Park and its partners involved in the Comprehensive Everglades Restoration Plan (CERP) have a direct interest in restoring natural flow regimes and biological and ecological processes to the ecosystem. Restoration plans involve addressing anthropogenic barriers to flow with decompartmentalization, i.e. removal/breaching of canals and levees to restore natural flow. Unfortunately, few studies have examined the interaction of flow with biological processes or its role in sustaining or restoring the prevailing ecosystem Ecological Restoration Vol. 30, No. 3, 2012 ISSN 1522-4740 E-ISSN 1543-4079 ©2012 by the Board of Regents of the University of Wisconsin System.
(Aumen 2003), and the scientific community is not certain that function and structure will show resilience once some disturbances are removed. The ridge and slough landscape occupied 55% of the historical Everglades wetland, Florida, USA (McVoy et al. 2011) and was characterized by elongated, swamp sawgrass (Cladium jamaicense) dominated peat ridges elevated above and alternately interspersed among long hydroperiod open water sloughs dominated by water lilies (Nymphaea odorata), bladderworts (Utricularia spp.), and other floating and submerged species (Ogden 2005, Larsen et al. 2011). Dissection of the Everglades by canals and levees has compartmentalized the Everglades system, interrupting flow and altering
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depth, distribution, and timing of water in the landscape. These disturbances, along with nutrient addition, have resulted in the replacement of the dominant ridge and slough ecosystem with a uniform topography and emergent vegetation community in a large proportion of this landscape. Over wide areas, mainly where water depth and duration is decreased over historical conditions, swamp sawgrass and southern cattail (Typha domingensis) density and dominance has increased. Where sloughs were once continuously interconnected and acted as open flow-ways that often held water through the dry season, many are now patchy, unconnected, or completely filled (Larsen et al. 2011).
Multiple seed bank studies across the ecosystem have shown that many species of the historic Everglades ridge and slough reference conditions do not germinate under proposed restored hydrologic conditions, are not present, or are found in very low densities (van der Valk and Rosburg 1997, Leeds et al. 2002, Smith et al. 2002, Miao and Zou 2009, Mossman 2009). Smith et al. (2002) concluded that contributions from the seed bank to development of a restored Everglades community will be negligible. Dispersal of seeds and vegetative reproduction within the existing communities must play an important role in restoration of ecosystem structure in degraded areas. The lack of understanding about seed pool and dispersal patterns, and interactions with ecosystem structure and hydrology in intact and degraded regions of the Everglades, raises questions about our ability to predict how ridge and slough restoration will proceed. Hydrochory, the movement of seeds by water, has a long scientific pedigree recognizing its population and community level importance (e.g. Ridley 1930, Schneider and Sharitz 1988, Skoglund 1989, Nilsson et al. 1991, Johansson and Nilsson 1993, Danvind and Nilsson 1997, van den Broek et al. 2005, Gurnell et al. 2007, Markwith and Leigh 2011, Nilsson et al. 2010). Although no studies are known from the Everglades itself, existing research in wetlands and streams highlights the interaction of hydrologic patterns and ecosystem structure with macrophyte dispersal and deposition processes, genetic patterns, and seed pool diversity. For example, Kudoh and Whigham (1997 and 2001) showed that populations of swamp rosemallow (Hibiscus moscheutos) are more genetically similar due to higher rates of long-distance hydrochory among populations adjacent to a tidal creek than populations isolated from the creek by dense emergent vegetation in the tidal marsh. Existing research also indicates that the diversity and composition of the seed pool may be influenced
by flow barriers that filter seeds with specific traits ( Jansson et al. 2000, Markwith and Leigh 2008). Finally, Merritt and Wohl (2002) found that the timing of dispersal in relation to ascending and descending flows on the hydrograph influenced the transport and deposition patterns of seeds in a model stream environment. The research presented herein demonstrates a direct interaction of flow with biological processes directly related to the ability of the Everglades ridge and slough system to recover once hydrology and sheet flow is improved. The objectives were accomplished by field sampling seed pool and vegetation diversity and composition and hydrologic parameters. The research addressed the following specific questions: 1. What is the composition, diversity, and density of seeds of species dispersed by sheet flow in the Everglades ridge and slough system? 2. What is the relationship of species dispersal patterns to hydrology and vegetation structure? 3. Do hydrochory dispersal patterns and processes vary spatially and temporally in intact and degraded sloughs due the interaction of vegetation structure, seed phenology, and hydrology?
The U.S. Department of the Interior (DOI) has articulated research needs related to decompartmentalization to address the dearth of information related to the interaction of flow and biological processes. The relevant needs include identifying factors sustaining ridge and slough, including ecosystem functions related to the restoration of flow, and quantifying the effects of different hydrologic regimes and ecological processes on restoring and maintaining ecosystem composition and function (US DOI 2005). The Science Coordination Team (SCT) of the South Florida Ecosystem Restoration Working Group (Aumen 2003) echo a number of DOI priorities for future research, including, flow measurements coupled with
ecological surveys to study the specific processes hypothesized to likely alter the landscape and affect transport, examination of the influence of flow on dispersal, and measurements in both intact and degraded ridge and slough habitat. The current empirical research concerning the influence of flow on seed movement in ridge and slough habitat addresses these DOI and SCT priorities.
Methods Study Sites Much of the heralded diversity of the Everglades results from a landscape mosaic that contains ridges, sloughs, wet prairies, cypress swamps, mangrove swamps, pinelands, and tree islands (Brown et al. 2006). Although a single marsh community type may be relatively low in diversity, there may be substantial species turnover between 2 different community types. There is also spatial variation in the mechanism for degradation across the system (i.e. impoundment, drainage, and nutrient enrichment), but the response of the ridge and slough communities to drainage and impoundment mechanisms are to transition from slough to ridge type and ridge to slough type communities, respectively (Watts et al. 2010). One relatively low diversity community is often replaced by another when degraded. Therefore, we chose to intensively sample one intact slough state and one degraded slough state over time to focus on capturing temporal variability within the two states rather than attempting to analyze the recognized low variability within those states over a large spatial area. We conducted field collection from July 2009 to June 2010 in a freshwater wetland ridge and slough ecosystem in the Florida Everglades, Water Conservation Area 3A (WCA 3A) south of Alligator Alley and west of the Miami Canal (Figure 1). The intact site was located at N26°03'38.8" W80°45'23.4", and the
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at the degraded site was filled-in with swamp sawgrass. Relatively small and disconnected open water areas without any clear shape were also interspersed throughout the degraded site and were dominated by submerged, floating, and emergent slough and wet prairie species.
Figure 1. South Florida Everglades system and study sites, including Everglades Agricultural Area (EAA), Water Conservation Areas (WCAs), Everglades National Park, and the extent of the ridge and slough ecosystem.
degraded location was N26°04'12.1" W80°39'41.5". Each of these locations was chosen based on the comparison of historic and current aerial imagery. Nungesser (in press) compared ridge and slough patterning in 15 large-scale quadrats spread across WCA 3A and 3B using aerial photos from 1940, 1952, 1972, 1984, and 2004 and determined whether ridge and slough patterning remained largely unchanged, degraded, or improved in these quadrats over the 64-yr period. The intact slough was located in Nungesser’s quad G1, an area that remained largely unchanged over the years, and the degraded slough was located in quad N3, an
area that showed linear degradation from 1940 to 2004. Historic aerial photos from 1940 were also consulted in the degraded area for delineation of the original slough boundary because infilling with swamp sawgrass at that site has obscured the boundaries in the modern landscape. The intact site was composed of a typical slough with a largely symmetrical elongated shape outlined by the elevated sawgrass ridge vegetation community, but also bounded by a tree island to the northeast. The slough vegetation was an open water habitat dominated by spikerush (Eleocharis spp.), water lily, and bladderwort. The majority of the historic slough
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Field Sampling Design Within each site 5 transects perpendicular to the predominant slough (or historical slough) orientation were permanently placed 100 m apart, with the location of the most upstream/ northerly transect randomly placed within the upstream/northern half of the slough. Along each transect 5 seed traps were evenly distributed across the width of the slough. Seed trap design was based upon Middleton (1995) with minor variation. Traps were constructed from standard 2-gal buckets with the bases cut out. We lined each trap with nylon mesh fabric with hole sizes 0.71 mm2 to capture seeds, attached flotation devices, and attached each trap by rope to stakes so they could reorient with changes in water level and flow direction. The approximate area of the trap opening and internal trap area when floating was 215 cm2 and 2000 cm2, respectively. The traps were deployed monthly for 1 wk, and the collected seeds were taken back to the Biogeography Lab on the Boca Raton campus of Florida Atlantic University for identification. To characterize species diversity and seed pool patterns, the collected seeds were visually sorted, counted, and identified using a seed (taxonomic) key. Seeds too small to be identified with the naked eye were examined under a Leica S6 D microscope (Leica Microsystems) and a digital image captured with a digital camera to determine its diagnostic traits and for archiving. Seeds not found in the taxonomic seed key were identified by the Florida Wetland Plants Identification Manual or germinated and identified from mature plants. Remaining unidentified seeds were
grouped by seed characteristics and designated as unknown based on their similar characteristics. Unknown seed species accounted for only 1.5% of the seeds trapped (76 seeds) over the entire sample. Flow velocity was measured 0.5 m upstream from the stake at each seed trap monthly during the time of seed collection. Water velocity was measured in cm/s using a Marsh-McBirney Flow-Mate Model 2000 (Hach Company, Loveland, CO) portable flow meter 2.5 cm below the water surface. Water depth was recorded in centimeters using a measuring rod. Water depth measured daily between July 2000 and June 2010 was downloaded for South Florida Water Management District gage 3AS, and means for each month were calculated across all years to create a historical hydrograph of water depth for comparison. Emergent and floating vegetation adjacent to the seed traps were sampled to characterize density and cover of emergent and floating stems and periphyton that may influence dispersal patterns. Vegetation sampling was conducted in November 2009 and June 2010. A 2-m × 0.5-m sampling frame was positioned with the long side parallel to the dominant flow direction, and the downstream short side placed 2 m in front of the seed trap stake. The sampling frame was temporarily placed in front of each trap during the vegetation sampling period and removed immediately after to avoid obstruction of flow into the trap. Emergent and floating vegetation was identified to the genus or species level and abundance of each recorded by stem count. Plants were recorded if their root base was within the PVC frame perimeter. Statistical Analysis All Analysis of Variance (ANOVA) and multivariate regression statistical tests were performed using Stata Release 9 (StataCorp 2005). Tests for significant differences between intact and degraded slough sites and transects, and within traps for month of
sampling and interactions with site and transect were conducted using repeated measures ANOVA. For hydrologic parameters and the seed trapping analysis each seed trap was the observational unit, and water velocity, water depth, seed density/ trap, and seed species richness were tested. Standing vegetation and periphyton parameters were tested for significant differences among sites during the wet and dry seasons with one-way ANOVA, and each quadrat was the observational unit. Standing vegetation stem density and percent cover and periphyton percent cover were all tested. Sørenson’s Similarity Index between trapped seeds and standing vegetation was calculated by summing seed abundance/species/trap over all months and comparing to the sum of June and November standing vegetation abundance/species/quadrat, and by independently comparing seeds trapped in June and November with standing vegetation sampled in June and November, respectively. Sørenson’s Similarity Index for each calculation method was tested using one-way ANOVA for significant differences between sites. Nonmetric Multidimensional Scaling (NMDS) was used to examine the species composition patterns of the seed pool and standing vegetation. The NMDS analyses were run in PC-Ord v. 5.1 (McCune and Mefford 2006). For each analysis the following conditions were maintained: a) NMDS was first run in autopilot mode to determine the best dimensionality for the analysis, in each case the 2-dimensional solution was best b) maximum number of iterations was 500 c) starting coordinates were random and d) 250 runs with real data and 250 runs with randomized data. For the seed pool, seed density/ trap was summed per species over all 12 months excluding species that occurred in
