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Resilience of coastal freshwater wetland vegetation of subtropical Australia to rising sea levels and altered hydrology

DOI:10.1007/s10113-018-1399-2
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Rebekah Grieger at Griffith University
Rebekah Grieger
Samantha Capon at Griffith University
Samantha Capon
Wade L Hadwen at Griffith University
Wade L Hadwen
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Abstract and Figures

Coastal freshwater wetlands are amongst the world’s most modified but poorly researched ecosystems and some of the most vulnerable to climate change. Here, we examine vegetation resilience in coastal wetlands of subtropical Australia to altered salinity and flooding regimes likely to occur with climate change. We conducted field surveys and glasshouse experiments to examine plant diversity and regeneration responses of understorey and canopy species across four habitats. Vegetation composition, but not richness, varied between seaward and inland habitats while soil seed bank diversity was greatest in more inland sites. Experimental salinity and flooding treatments strongly influenced emergence from seed banks with most species germinating under fresh, waterlogged conditions and very few in saline treatments. Composition of emerging seedling assemblages was similar across habitats and treatments but differed considerably from the extant vegetation, indicating a relatively minor role of soil seed banks in sustaining current vegetation structure in this wetland. An exception to this was Sporobolus virginicus (marine couch) which was common in both the vegetation and seed banks suggesting a high capacity for this species to re-establish following disturbances. Seedlings of dominant canopy species also reacted strongly to increased salinity treatments with decreased survivorship recorded. Overall, our findings suggest a high probability of constrained vegetation regeneration in this wetland in response to key projected climate change disturbances with implications for vegetation diversity at a landscape scale including declines in the extent and diversity of more landward vegetation communities and expansion of salt-tolerant marshes dominated by Sporobolus virginicus.
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ORIGINAL ARTICLE
Resilience of coastal freshwater wetland vegetation of subtropical
Australia to rising sea levels and altered hydrology
Rebekah Grieger
1,2
&Samantha Capon
1,2
&Wade Hadwen
1,2,3
Received: 12 January 2018 /Accepted: 28 July 2018 /Published online: 29 August 2018
#Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract
Coastal freshwater wetlands are amongst the world’s most modified but poorly researched ecosystems and some of the most
vulnerable to climate change. Here, we examine vegetation resilience in coastal wetlands of subtropical Australia to altered
salinity and flooding regimes likely to occur with climate change. We conducted field surveys and glasshouse experiments to
examine plant diversity and regeneration responses of understorey and canopy species across four habitats. Vegetation compo-
sition, but not richness, varied between seaward and inland habitats while soil seed bank diversity was greatest in more inland
sites. Experimental salinity and flooding treatments strongly influenced emergence from seed banks with most species germi-
nating under fresh, waterlogged conditions and very few in saline treatments. Composition of emerging seedling assemblages
was similar across habitats and treatments but differed considerably from the extant vegetation, indicating a relatively minor role
of soil seed banks in sustaining current vegetation structure in this wetland. An exception to this was Sporobolus virginicus
(marine couch) which was common in both the vegetation and seed banks suggesting a high capacity for this species to re-
establish following disturbances. Seedlings of dominant canopy species also reacted strongly to increased salinity treatments with
decreased survivorship recorded. Overall, our findings suggest a high probability of constrained vegetation regeneration in this
wetland in response to key projected climate change disturbances with implications for vegetation diversity at a landscape scale
including declines in the extent and diversity of more landward vegetation communities and expansion of salt-tolerant marshes
dominated by Sporobolus virginicus.
Keywords Flooding .Germination .Salinity .Seed banks .Seedlings .Climate change
Introduction
Coastal freshwater wetlands (CFWs) are highly valued for
their diverse biota and ecological services (Millennium
Ecosystem Assessment 2005). Typically positioned be-
tween estuarine and freshwater systems, CFWs are neither
fully saline nor exclusively freshwater. Rather, the main
water sources are fresh (i.e. rainfall, terrestrial runoff,
streamflow, groundwater) with saline intrusion events also
occurring during the largest tides and storm surge events
(Office of Environment and Heritage (OEH) 2011). The
unique hydrology of these systems, highlighted by the
variability in moisture and salinity, creates distinct vege-
tation communities (David 1996;Olmsteadand
Armentano 1997;Rossetal.2003;Toddetal.2010).
CFWs provide many ecosystem services that are valuable
to the organisms which inhabit these areas as well as the
human populations living around them including; but not
limited to, habitat provision and diversity, nutrient
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s10113-018-1399-2) contains supplementary
material, which is available to authorized users.
Editor: Wolfgang Cramer.
*Rebekah Grieger
rebekah.grieger@griffithuni.edu.au
Samantha Capon
s.capon@griffith.edu.au
Wade Hadwen
w.hadwen@griffith.edu.au
1
Australian Rivers Institute, Griffith University, 140 Kessels Road,
Nathan, QLD 4111, Australia
2
School of Environment, Griffith University, 140 Kessels Road,
Nathan, QLD 4111, Australia
3
Griffith Climate Change Response Program, Griffith University, 140
Kessels Road, Nathan, QLD 4111, Australia
Regional Environmental Change (2019) 19:279–292
https://doi.org/10.1007/s10113-018-1399-2
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... For example, bald cypress Taxodium distichum swamps, common in TFFWs of the south-east US, exhibit greater basal areas and growth rates in areas of lower salinity and shorter flood periods (Krauss et al. 2009). Composition also differed from marsh to eucalypt forest across a salinefresh tidal gradient in south-east Queensland, Australia (Grieger et al. 2019). Significant changes in salinity or hydrology due to disturbance events (e.g. ...
... TFFWs in the southern US are similarly structured along a tidal gradient of differing salinity and flooding (Mitsch and Gosselink 2015c), reflected by variation in vegetation structure, species richness and diversity and tree regeneration (Baldwin 2007;Krauss et al. 2009;Anderson et al. 2013;Liu et al. 2017). Within subtropical Australia, dominant CFW canopy species shift along landward elevational gradients, reflecting changes in salinity and flooding (Grieger et al. 2019). For example, the structure of Melaleuca swamp forests in northern Australia differs with the duration of seasonal flooding, with denser canopies generally occurring in areas that are inundated for up to 8 months (Finlayson et al. 1989). ...
... As a result, successive disturbance events (e.g. storms, further saline intrusion) can trigger the death of the canopy, resulting in a ghost forest of standing dead trees and a salt marsh understorey (Grieger et al. 2019;Kirwan and Gedan 2019). Similar responses have been observed in coastal Melaleuca forests of northern Australia, where Traditional Owners and scientists attribute the forest dieback to increased salinity from rising sea levels and the effects of feral ungulates (Bowman et al. 2010;Sloane et al. 2019). ...
Spatial variation and drivers of vegetation structure and composition in coastal freshwater wetlands of subtropical Australia
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Coastal freshwater wetlands (CFWs) are among the most understudied wetlands globally and are highly vulnerable to projected climate changes. To address CFW knowledge gaps in south-east Queensland, Australia, we surveyed the floristic composition and structure of wooded CFWs and explored variation in vegetation patterns in relation to selected environmental drivers. Understorey and shrub assemblages were surveyed using a cover-class scale and stem counts for tree species abundance. Vegetation structure attributes (stem density, basal area) were calculated from survey data. Redundancy analysis was used to investigate drivers of vegetation structure and the species composition of each stratum. Vegetation structure patterns were associated with gradients of rainfall, soil moisture, salinity and pH. Understorey species composition was associated with wallum wetland species, native perennial grass and herb species, and vegetation patterns of the canopy. Common CFW species, namely Melaleuca quinquenervia and Eucalyptus tereticornis, dominated tree assemblage variation. Overall, CFW vegetation exhibited strong associations with gradients of salinity, rainfall, groundwater dependence and disturbance. Alterations to key drivers of vegetation pattern with future climate changes are likely to markedly influence the composition, structure and function of CFW vegetation communities. Action is therefore required to maintain CFW vegetation communities and ecological function in these diverse and unique wetland systems.
... Improved coastal modelling, experiments and in situ studies are reducing uncertainties at a local scale about the impact of future sea level rise (SLR) on coastal freshwater terrestrial wetlands (medium confidence) (Shoo et al., 2014;Bayliss et al., 2018;Grieger et al., 2019). Low-lying coastal wetlands are susceptible to saltwater intrusion from sea level rise (SLR) (Shoo et al., 2014;Kettles and Bell, 2015;Finlayson et al., 2017) with consequences for species dependent on freshwater habitats (Houston et al., 2020). ...
... Landscape planning (Bond et al., 2014;McCormack, 2018) helps reduce habitat loss, facilitates species dispersal and gene flow (McLean et al., 2014;Shoo et al., 2014;Lowe et al., 2015;Harris et al., 2018;McCormack, 2018) and allows for new ecological opportunities (Norman and Christidis, 2016). Coastal squeeze is a threat to freshwater wetlands and requires planning for the potential inland shift (Grieger et al., 2019). Adaptations that maintain critical volumes and periodicity of environmental flows will help protect freshwater biodiversity (Box 11.3) (Yen et al., 2013;Barnett et al., 2015;Wang et al., 2018b). ...
... Human changes applied to agriculture reduce the resilience of wetland ecosystems' structure and functionality. Disturbances in the hydrological regime can exceed the limits of resilience mechanisms and, consequently, transform the structure, functionality, and identity of ecosystems (Grieger et al., 2019). ...
... Bad weather conditions (INMET, 2020) and low soil moisture -both caused by Banhado Grande's severe drought -were the determining factors for the rapid burning that has devastated 702 ha of emergent vegetation in less than 24 h. Grieger et al. (2019) found that soil seed banks are essential for the resilience of freshwater marshes after fire or flooding hazardous episodes. However, climate change and soil degradation can drastically affect seed availability and, consequently, the resilience of marshes. ...
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... These coastal forests are dynamic and are highly responsive to changing conditions, and in extreme conditions (e.g. storm surges and large tides), can experience periods of elevated salinities and waterlogging due to marine and groundwater interactions (Grieger et al., 2019;Krauss et al., 2009). If such conditions prevail, tidal freshwater forested wetlands can undergo structural and physiological changes in their transition to a more saline environment (Duberstein et al., 2020;Krauss et al., 2009). ...
... In palustrine forests, specific-specific hydrological thresholds influence based on flooding and influence growth and establishment of Casuarina and Melaleuca (Grieger et al., 2019), and forested communities in the US (Hough-Snee, 2020). However, we are uncertain as to whether the characteristics of CSOF observed on the Minnamurra floodplain reflects controls on growth (species hydrological thresholds), or simply is representative of an immature forested community. ...
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... A paucity of woody species emerging from soil seed banks have been similarly. Observed in other studies in arid and semi-arid savannas (Capon & Brock 2006, Tessema et al. 2012, tropical forests (Jara 2006) and subtropical coastal freshwater wetlands (Grieger et al. 2019). These relatively low numbers indicate that tree species may store viable seeds in other seed banks, if at all (e.g., aerial seed banks), but are unlikely to rely on seed stores in soil or litter for regeneration (Roberts & Marston 2000). ...
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... Various studies have been carried out to evaluate the response of biomass gain and growth in height or diameter to the increase in salinity of swamp tree species that are distributed in the southeastern United States such as Cephalanthus occidentalis L., Fraxinus pennsylvanica, Nyssa aquatica, Nyssa sylvatica, Quercus lyrate Walter, Quercus michauxii, Quercus nigra, Quercus nuttallii, Sapium sebiferu and Taxodium distichum (Allen et al. 1994;Conner 1994;Conner et al. 1998Conner et al. , 1997McCarron et al. 1998;Pezeshki 1990;Pezeshki et al. 1989), in the Caribbean such as Pterocarpus officinalis Jacq. (Bompy et al. 2015;Rivera-Ocasio et al. 2007), and in southeastern Australian wetlands such as Melaleuca ericifolia (Salter et al. 2007) and Eucalyptus tereticornis Sm. (Grieger et al. 2019). In these studies, it was observed that the seedlings biomass, height, and diameter were greater in the treatments with no or low salinity compared to the high salinity treatments. ...
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... These results should be interpreted with caution, however, since, to achieve optimal discriminating accuracy, we had to group some disparate wetland classes, which may mask some of the salt-freshwater wetland transitions. Although there are significant differences in the floral composition and vegetation structure [7,88], determinant factors, such as freshwater inputs and groundwater availability [2,26], might not closely associate with the hydro-geomorphological variables considered in this study. Again, further studies with long-term imagery are required to investigate the interchange of freshwater wetlands due to SLR. ...
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During the last century the mean sea level has been increasing at a rate of 0.2 to 0.4 mm·year -1 , and that rate is expected to accelerate during this century. Coastal wetland ecosystems are sensitive to the potential changes and impacts of resulting from a rise in sea level. In the coastal region of the Gulf of Mexico, freshwater swamps are wetlands located further inland than mangroves, and while influenced by the tides maintain freshwater conditions. Due to their location, the rise in sea level could increase the levels of flooding and salinity in these ecosystems. The objective of this study was to evaluate, under greenhouse conditions, the effect of nine flood and salinity treatments on the survival, growth and increase in the biomass of Annona glabra (pond apple) seedlings. The treatments combined two factors: water level (saturation, flood, flood-drought) and salinity (0, 5 and 15 ‰). Germinated seedlings were used (average height: 18.6 ± 1.61 cm). Seedling survival was greater under freshwater conditions. Increase in height and diameter, as well as leaf and biomass gain, were greater under saturation and freshwater conditions. Based on our results, we conclude that under a scenario of rising sea level, increased flood levels and salinity will negatively affect the natural establishment of Annona glabra seedlings in freshwater swamps.
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We measured an array of biophysical and spectral variables to evaluate the response and recovery of Spartina alterniflora to a sudden dieback event in spring and summer 2004 within a low marsh in coastal Virginia, USA. S. alterniflora is a foundation species, whose loss decreases ecosystem services and potentiates ecosystem state change. Long-term records of the potential environmental drivers of dieback such as precipitation and tidal inundation did not evidence any particular anomalies, although Hurricane Isabel in fall 2003 may have been related to dieback. Transects were established across the interface between the dieback area and apparently healthy areas of marsh. Plant condition was classified based on ground cover within transects as dieback, intermediate and healthy. Numerous characteristics of S. alterniflora culms within each condition class were assessed including biomass, morphology and spectral attributes associated with photosynthetic pigments. Plants demonstrated evidence of stress in 2004 and 2005 beyond areas of obvious dieback and resilience at a multi-year scale. Resilience of the plants was evident in recovery of ground cover and biomass largely within 3 y, although a small remnant of dieback persisted for 8 y. Culms surviving within the dieback and areas of intermediate impact had modified morphological traits and spectral response that reflected stress. These morphometric and spectral differences among plant cover condition classes serve as guidelines for monitoring of dieback initiation, effects and subsequent recovery. Although a number of environmental and biotic parameters were assessed relative to causation, the reason for this particular dieback remains largely unknown, however.
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A decade-long examination was made of recruitment and establishment in a tidal freshwater high marsh along the Delaware River. Over the 10 yr of the study, seed bank, field seedlings, and vegetation showed variable patterns and significant year-to-year fluctuations. Patterns of each species were unique, perhaps the result of specific germination and/or establishment requirements and seedling morphology. For a given species there was little correlation among seed bank, seedling, and vegetation patterns, and germination success did not guarantee establishment. Species diversity showed significant year-to-year fluctuation, but there was no trend, and perennials did not change in importance during the 10-yr period. Because four annual species (Bidens laevis, Cuscuta gronovii, Impatiens capensis, and Polygonum arifolium) composed over 90% of the seed bank and field seedlings, and 58-89% ( = 70 ± 4) of the cover, community dynamics were dependent on seedling recruitment. For a given species life history stages (seed bank, field seedlings, and vegetation) were not predictable over the temporal scale of a decade. It is predicted, however, that if hydrology remains unchanged, the same suite of species will persist. The importance of the parasite Cuscuta gronovii is noteworthy.
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Article
  • Apr 1996
Sea level rise may alter salinity and inundation regimes and create patches of open water in oligohaline coastal marshes, potentially affecting the composition and germination of seed bank species. We conducted seedling emergence experiments to: (1) examine the effects of standing vegetation on the seed banks of three oligohaline marsh communities in coastal Louisiana (dominated by Paspalum vaginatum Sw., Sagittaria lancifolia L., or Spartina patens (Ait.) Muhl., respectively); and (2) investigate the effects of salinity and inundation regime on germination of seed bank species. We also studied the effect of a temporary increase in salinity (to simulate a salt water intrusion event) on the viability of buried seeds. We found that the presence or absence of vegetation within a community affected the abundance of some species in the seed bank but had little effect on species composition. Also, the seed banks of the three communities exhibited considerable overlap in species composition and had similar species richness (10–11) and diversity (antilog Shannon-Weaver diversity index = 6.5–7.1), despite differences in vegetation type. Higher salinities and flooding reduced seedling emergence for most species; few species emerged at salinities above four parts per thousand (ppt), and only Sagittaria lancifolia and Eleocharis parvula germinated well under flooded conditions. A temporary increase in salinity did not affect species richness or seedling emergence of most species. Our results suggest that differences in vegetation may have little effect on the composition of seed banks of oligohaline marshes. However, higher salinities and greater depth and duration of inundation (anticipated as global sea level continues to rise) may decrease recruitment of seed bank species, reducing their abundance in oligohaline marsh communities.
Forest composition and growth in a freshwater forested wetland community across a salinity gradient in South Carolina, USA
Article
Tidal freshwater forested wetlands (TFFW) of the southeastern United States are experiencing increased saltwater intrusion mainly due to sea-level rise. Inter-annual and intra-annual variability in forest productivity along a salinity gradient was studied on established sites. Aboveground net primary productivity (ANPP) of trees was monitored from 2013 to 2015 on three sites within a baldcypress (Taxodium distichum) swamp forest ecosystem in Strawberry Swamp on Hobcaw Barony, Georgetown County, South Carolina. Paired plots (20 × 25-m) were established along a water salinity gradient (0.8, 2.6, 4.6 PSU). Salinity was continuously monitored, litterfall was measured monthly, and growth of overstory trees ⩾10 cm diameter at breast height (DBH) was monitored on an annual basis. Annual litterfall and stem wood growth were summed to estimate ANPP. The DBH of live and dead individuals of understory shrubs were measured to calculate density, basal area (BA), and important values (IV). Freshwater forest communities clearly differed in composition, structure, tree size, BA, and productivity across the salinity gradient. The higher salinity plots had decreased numbers of tree species, density, and BA. Higher salinity reduced average ANPP. The dominant tree species and their relative densities did not change along the salinity gradient, but the dominance of the primary tree species differed with increasing salinity. Baldcypress was the predominant tree species with highest density, DBH, BA, IV, and contribution to total ANPP on all sites. Mean growth rate of baldcypress trees decreased with increasing salinity, but exhibited the greatest growth among all tree species. While the overall number of shrub species decreased with increasing salinity, wax myrtle (Morella cerifera) density, DBH, BA, and IV increased with salinity. With rising sea level and increasing salinity levels, low regeneration of baldcypress, and the invasion of wax myrtle, typical successional patterns in TFFW and forest health are likely to change in the future.