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Phreatophytic vegetation responses to groundwater depth in a drying mediterranean-type landscape

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... Groundwater levels have dropped 1.8 m between 1998 and 2016 within the Perth metropolitan region (Department of Water and Environmental Regulation 2019), causing declines in groundwater dependent vegetation (Froend and Sommer 2010). Over the last 30 years, there has been improved understanding of the importance of groundwater, including responses of vegetation to reduced access, and management of groundwater resources to minimise impacts on vegetation (Barron et al. 2014;Sommer and Froend 2014;Rohde et al. 2017). Bounded by the Swan River (south), Moore River and Gingin Brook (north), Darling Scarp (east) and the south-west Australian coast (west) (Fig. 4), the Gnangara Groundwater Mound (GGM) is a large sand mound underlying seasonal and permanent wetlands, pine plantations and extensive areas of BWs that contains groundwater which supplies the city with fresh scheme water. ...
... Several studies (Zencich et al. 2002;Veneklaas and Poot 2003;Groom 2004;Froend and Sommer 2010;Sommer and Froend 2014;Muler et al. 2018b) have highlighted species differences in water use patterns relating to rooting depth relative to groundwater depth. Understorey shrubs with relatively deep roots (e.g. ...
... Management interventions to prevent vegetation change or restore original floristics of BWs are considered extremely difficult after ecohydrological habitats have passed a critical threshold. Assessments of alternative vegetation states for BWs were identified over a multi-decadal drying of the Mediterranean-type BWs landscape (Sommer and Froend 2014). Plant community composition varied across a range of groundwater depths suggesting that as groundwater depth increases, the degree of habitat specificity decreases. ...
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The rapid expansion of urban areas worldwide is leading to native habitat loss and ecosystem fragmentation and degradation. Although the study of urbanisation’s impact on biodiversity is gaining increasing interest globally, there is still a disconnect between research recommendations and urbanisation strategies. Expansion of the Perth metropolitan area on the Swan Coastal Plain in south-western Australia, one of the world’s thirty-six biodiversity hotspots, continues to affect the Banksia Woodlands (BWs) ecosystem, a federally listed Threatened Ecological Community (TEC). Here, we utilise the framework of a 1989 review of the state of knowledge of BWs ecology and conservation to examine scientific advances made in understanding the composition, processes and functions of BWs and BWs’ species over the last 30 years. We highlight key advances in our understanding of the ecological function and role of mechanisms in BWs that are critical to the management of this ecosystem. The most encouraging change since 1989 is the integration of research between historically disparate ecological disciplines. We outline remaining ecological knowledge gaps and identify key research priorities to improve conservation efforts for this TEC. We promote a holistic consideration of BWs with our review providing a comprehensive document that researchers, planners and managers may reference. To effectively conserve ecosystems threatened by urban expansion, a range of stakeholders must be involved in the development and implementation of best practices to conserve and maintain both biodiversity and human wellbeing.
... Ecosystem function is generally defined as an ecosystem's ability to maintain multiple functions, such as carbon storage, nutrient cycling, and the transfer of energy via growth and decomposition. Functional responses to stress induced by changes in groundwater have been reported to cause changes in community composition such that native species are outcompeted by non-native species (Keddy and Reznicek, 1986;Moore and Keddy, 1988;Sommer and Froend, 2014). Numerous studies have also shown the ability of non-native invasive species to alter the flow of energy and cycling of materials within an ecosystem, and thus its functioning (Vitousek et al., 1987;D'Antonio, 1992;Vitousek, 1997;Gordon, 1998;Mack and D'Antonio, 1998;Cicchetti and Diaz, 2002;Meyerson et al., 2002;Ehrenfeld, 2003;Kourtev et al., 2003;Levine et al., 2003;Dukes and Mooney, 2004;USEPA, 2016). ...
... Greater species diversity in Tall and Shaw Forests in comparison to Castello Forest was also accompanied by a greater proportion of native plant cover. These observations are consistent with previous research studies, which found changes in groundwater availability to cause a functional shift in the ecosystem away from native habitat toward more favorable conditions for invasive species (Keddy and Reznicek, 1986;Moore and Keddy, 1988;Sommer and Froend, 2014). ...
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Sustainable groundwater management provides an opportunity for environmental water needs to be considered and secured by establishing appropriate groundwater thresholds. Ecosystems that require access to groundwater for some or all their water requirements are referred to as groundwater dependent ecosystems (GDEs). However, large data gaps often exist around the cause-and-effect relationships between groundwater conditions and the impacts they have on GDEs. These data gaps are largely attributed to a lack of shallow monitoring wells near GDEs, and a lack of practical biological metrics to characterize ecosystem health. This transdisciplinary study explores the use of geophysics (electrical resistivity tomography) to fill in our understanding of shallow subsurface soil-hydrological conditions within GDEs. In addition, we develop an approach to characterize ecosystem health within GDEs, using groundwater-dependent vegetation (phreatophytes) as indicators. Ten vegetation variables were used to characterize six biological indicators—growth, diversity, recruitment, structure, native plant dominance, and survivorship—which were used to infer ecosystem health conditions. Health indicators for groundwater-dependent vegetation were found to directly correlate with subsurface conditions, where greater groundwater availability (higher soil moisture content and shallower groundwater levels) was associated with “healthier” vegetation. This study provides a new approach to integrate hydrological, geophysical, and biological data to strengthen monitoring programs and inform water resource management decisions.
... The change of groundwater flow system under strong human intervention has become the main driving force affecting the ecological function and evolution of local and even regional surface vegetation (Dong et al., 2013;Wang et al., 2019). The depth to groundwater table (DWT) determines the structure and function of groundwater-dependent ecosystems (GDEs) Kopeć, Dominik et al., 2013;Sommer & Froend, 2014;Jin et al., 2011). The effects of groundwater on vegetation ecology are as follows: In the range of suitable vegetation groundwater level, groundwater affects the growth and development indexes of vegetation, such as biomass and vegetation coverage; Vegetation community succession will occur when the suitable vegetation groundwater level is exceeded (Huang et al., 2008;Zhang et al., 2019). ...
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Coal mine in arid and semi-arid area is one of the most severely degraded ecosystems on the earth. The continuous decrease in groundwater level caused by coal mining will inevitably affect biogeochemical environment of the vadose zone, and then lead to the replacement of surface vegetation. Yimin open-pit coal mine was taken as an example to reveal the relationship between the groundwater depth and soil water content (SWC), soil salt content, soil electrical conductivity (SEC), soil organic matter (SOM), soil available potassium (SAK), soil available nitrogen (SAN), vegetation coverage, aboveground biomass and species richness. The results show that, the change of groundwater depth can affect soil properties and then change the characteristics of surface vegetation, and the change of surface vegetation can also react on soil properties. Vegetation coverage and aboveground biomass are negatively correlated with groundwater depth, and positively correlated with SWC, SEC, SOM and SAK. The shallow groundwater table is conducive to the accumulation of SOM, so that the surface biomass and vegetation coverage are high. The higher the surface biomass, the more the SAN is absorbed. Under natural conditions, the relative strength of biological nitrogen fixation and plant absorption determine the content of SAN. In the research area, when the depth of groundwater is less than 0.4 m will cause soil salinization, then lead to low species richness; Species richness is exponentially correlated with groundwater depth and decreases with the increase in groundwater depth.
... For the GDEs in the semi-arid to arid ecosystem, the change in GD can alter the ecosystem water balance like soil water (Yao et al., 2018), and consequently leading to shifts in WUE for semi-arid to arid vegetation (Sommer and Froend, 2014). Previous studies on relations between WUE and GD mainly focused on field scale for typical species. ...
Article
Groundwater influences the water and carbon cycle by supplying moisture to plants in the semi-arid and arid zones. However, little is known about the response of ecosystem water use efficiency (WUE) to climate change in different groundwater depth (GD) sections. Recent research has shown that plant photosynthesis and growth are closely related to GD via field experiments but the wider recognition of GD effect on regional-scale ecosystems has not been yet established. In this study, we test whether the GD has an impact on ecosystem WUE and its variability to climate change at the regional scale. Based on the observed data of nearly 3000 wells, meteorological data (precipitation and pan evaporation), and the 0.01°-resolution remote sensing datasets including gross primary production (GPP), evapotranspiration (ET), and normalized difference vegetation index (NDVI), we explored the spatio-temporal variations of WUE and its composites (i.e., GPP and ET), and their characteristics depending on GD under different aridity conditions and biomes across the Ordos Plateau, a semi-arid to arid area in northern China. Results show that WUE increases with decreasing GD due to water availability in the semi-arid lands where WUE variability is mainly regulated by biological processes (i.e., GPP), while WUE is insensitive to the changes in GD across the arid zone where the physical processes (i.e., ET) control WUE change. However, when drought happens the groundwater-independent vegetation in the arid zone can also utilize groundwater, characterized by lower reductions of GPP with decrease in GD. A dense vegetation condition (i.e., large NDVI) is more vulnerable to climatic disturbance over the semi-arid zone because it tends to decrease GPP and WUE, especially in the large GD regions. These findings have important implications for reasonable land use and groundwater management over the semi-arid and arid regions.
... In addition, the relationships between groundwater and plant communities are highly complex (Celentano et al. 2017). On the one hand, groundwater may directly affect plant distribution and thus influence nutrient traits (Sommer and Froend 2014). On the other hand, groundwater may indirectly affect nutrient traits by its interaction with edaphic factors (Zhang et al. 2019). ...
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Aims As the determinant of water availability in drylands, groundwater plays a fundamental role in regulating vegetation distribution and ecosystem processes. Although considerable progress has been made over the past years in the relationship between environment stress and plant community-level traits, the potential influence of water stress induced by groundwater changes on plant community-level stoichiometry remains largely unclear. Here, we examined whether belowground and aboveground community-level stoichiometry responded differently to groundwater changes. Methods We measured nitrogen (N) and phosphorus (P) concentrations in plant leaves and fine-roots of 110 plots under a broad range of groundwater depths in a typical arid inland river basin. We examined the spatial patterns and drivers of community-level N:P stoichiometry in leaves and fine-roots. Important Findings Community-level leaf and fine-root N, P and N:P ratios were mainly determined by groundwater, vegetation types and species composition, among which groundwater played a dominant role. Groundwater indirectly regulated community-level N:P stoichiometry through affecting vegetation types and species composition. Vegetation types and species composition had significant direct influences on community-level N:P stoichiometry. Furthermore, groundwater depth had opposite influences on community-level leaf and fine-root N:P stoichiometry. Groundwater depth regulated vegetation types and further decreased leaf N, P but increased leaf N:P ratios and fine-root N. Groundwater depth had a positive indirect impact on fine-root P but a negative indirect impact on fine-root N:P ratios primarily by affecting species composition. Our findings indicate that groundwater rather than climate conditions effectively regulates community-level N:P stoichiometry, and below- and aboveground N:P stoichiometry has opposite responses to groundwater.
... As groundwater table lowers groundwater depending vegetation in the wetland buffer zone (root depth range 5-10 m) are less vulnerable than flora species that depend on shallow groundwater table due to low adaptive capacity of their roots to reach groundwater (Froend et al. 2010, Canham et al. 2013, Sommer and Froend 2014. ...
Thesis
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Wetlands are considered as important ecosystems and provide multiple benefits to humans, such as recreation and enjoyment of biodiversity. Wetlands are mainly threatened due to human caused effects, such as climate change. The drying climate in Perth has resulted in too low water levels to support the ecology of wetlands. Ideally, wetland management need to respond to this change and should aim to protect the ecological integrity. We present an approach that answers why, when, and how management could respond to protect these important ecosystems.
... Understanding how these ecosystems respond to climate change is important to ensure that they continue to provide these services. Groundwater depth is understood to be the major driver of riparian plant communities (Sommer and Froend 2014;Yin et al. 2015), but water tables are dropping world-wide (Margat and Van Der Gun 2013) and increased variation in rainfall will increase the correlated variation in groundwater stores. We demonstrated that GSFs can be useful tools to identify plant species that are most resilient (or most sensitive) to future increases in environmental variability. ...
Article
Aims Determining the ecological consequences of interactions between slow changes in long-term climate means and amplified variability in climate is an important research frontier in plant ecology. We combined the recent approach of climate sensitivity functions with a revised hydrological ‘bucket model’ to improve predictions on how plant species will respond to changes in the mean and variance of groundwater resources. Methods We leveraged spatiotemporal variation in long-term datasets of riparian vegetation cover and groundwater levels to build the first groundwater sensitivity functions for common plant species of dryland riparian corridors. Our results demonstrate the value of this approach to identifying which plant species will thrive (or fail) in an increasingly variable climate layered with declining groundwater stores. Important Findings Riparian plant species differed in sensitivity to both the mean and variance in groundwater levels. Rio Grande cottonwood (Populus deltoides ssp. wislizenii) cover was predicted to decline with greater inter-annual groundwater variance, while coyote willow (Salix exigua) and other native wetland species were predicted to benefit from greater year-to-year variance. No non-native species were sensitive to groundwater variance, but patterns for Russian olive (Elaeagnus angustifolia) predict declines under deeper mean groundwater tables. Warm air temperatures modulated groundwater sensitivity for cottonwood, which was more sensitive to variability in groundwater in years/sites with warmer maximum temperatures than in cool sites/periods. Cottonwood cover declined most with greater intra-annual coefficients of variation (CV) in groundwater, but was not significantly correlated with inter-annual CV, perhaps due to the short time series (16 years) relative to cottonwood lifespan. In contrast, non-native tamarisk (Tamarix chinensis) cover increased with both intra- and inter-annual CV in groundwater. Altogether, our results predict that changes in groundwater variability and mean will affect riparian plant communities through the differential sensitivities of individual plant species to mean versus variance in groundwater stores.
... One example is in southwestern Australia where the Gnangara groundwater mound not only is the primary water source for the metropolitan area of Perth but also largely supports Banksia (Proteaceae) woodlands: the most widespread plant community of the Swan Coastal Plain biological region of Western Australia. The combination of groundwater abstractions (Sommer & Froend, 2011) coupled with regional increases in aridity (Delworth & Zeng, 2014) is resulting in the ongoing drawdown of the regional water tables and vegetation change (Sommer & Froend, 2014). The capacity of these woodlands to persist under future environmental conditions will hinge largely on the resilience of phreatophytic vegetation to groundwater declines and groundwaterdependent vegetation response to improved, sustainable aquifer management. ...
Article
Groundwater dependent ecosystems are often defined by the presence of deeply‐rooted phreatophytic plants. When connected to groundwater, phreatophytes in arid regions decouple ecosystem net primary productivity from precipitation, underscoring a disproportionately high biodiversity and exchange of resources relative to surrounding areas. However, groundwater dependent ecosystems are widely threatened due to the effects of water diversions, groundwater abstraction, and higher frequencies of episodic drought and heat waves. The resilience of these ecosystems to shifting ecohydrological‐climatological conditions will depend largely on the capacity of dominant, phreatophytic plants to cope with dramatic reductions in water availability and increases in atmospheric water demand. This paper disentangles the broad range of hydraulic traits expressed by phreatophytic vegetation to better understand their capacity to survive, or even thrive under shifting ecohydrological conditions. We focus on three elements of plant water relations: 1) hydraulic architecture (including root area to leaf area ratios and rooting depth), 2) xylem structure and function, and 3) stomatal regulation. We place the expression of these traits across a continuum of phreatophytic habits from obligate to semi‐obligate to semi‐facultative to facultative. Although many species occupy multiple phreatophytic niches depending on access to groundwater, we anticipate that populations are largely locally adapted to a narrow range of ecohydrological conditions regardless of gene flow across ecohydrological gradients. Consequently, we hypothesize that reductions in available groundwater and increases in atmospheric water demand will result in either 1) stand replacement of obligate phreatophytic species with more facultative species as a function of wide‐spread mortality in highly groundwater dependent populations, or 2) directional selection in semi‐obligate and semi‐facultative phreatophytes towards the expression of traits associated with highly facultative phreatophytes in the absence of species replacement. Anticipated shifts in the expression of hydraulic traits may have profound impacts on water cycling processes, species assemblages and habitat structure of groundwater dependent woodlands and riparian forests.
... Particularly, the narrow range of water table depths within which hygrophytes are in better physiological state suggests that future changes in the depth of water table could lead these species to high water stress levels and, ultimately, to transitions between vegetation states. Interestingly, the range reported here is in agreement withSommer and Froend's (2014) reported thresholds of transition of hydrotypes: at 3 m groundwater depth they observed a transition from hygrophytic dominant community to a more meso-xerophytic one.In conclusion, we found that, during the dry summer, depth to groundwater is highly important in physiological structuring woody community in a semi-arid sandy ecosystem. We have highlighted the impact hydrological drought can have on plant water-related functional processes in this semi-arid ecosystem. ...
Article
Predicted droughts and anthropogenic water use will increase groundwater lowering rates and intensify groundwater limitation, particularly for Mediterranean semi‐arid ecosystems. These hydrological changes may be expected to elicit differential functional responses of vegetation either belowground or aboveground. Yet, our ability to predict the impacts of groundwater changes on these ecosystems is still poor. Thus, we sought to better understand the impact of falling water table on the physiology of woody vegetation. We specifically ask (i) how is woody vegetation ecophysiological performance affected by water‐table depth during the dry season?, and (ii) does the vegetation response to increasing depth to groundwater differ among water‐use functional types?. We examined a suite of physiological parameters and water‐uptake depths of the dominant, functionally distinct woody vegetation along a water‐table depth gradient in a Mediterranean semi‐arid coastal ecosystem that is currently experiencing anthropogenic water extraction pressure. We found that groundwater drawdown did negatively affect the ecophysiological performance of the woody vegetation. Across all studied environmental factors, depth to groundwater was the most important driver of ecophysiological adjustments. Plant functional types, independent of groundwater dependence, showed consistent declines in water content and generally reduced C and N acquisition with increasing depths to groundwater. Functional types showed distinct operating physiological ranges, but common physiological sensitivity to greater water table depth. Thus, although differences in water‐source use exists, a physiological convergence appeared to happen among different functional types. These results strongly suggest that hydrological drought has an important impact on fundamental physiological processes, constraining the performance of woody vegetation under semi‐arid conditions. By disentangling the functional responses and vulnerability of woody vegetation to groundwater limitation, our study establishes the basis for predicting the physiological responses of woody vegetation in semi‐arid coastal ecosystems to groundwater drawdown. This article is protected by copyright. All rights reserved.
... Management of GDEs can also be informed by improved understanding of the potential for ecosystems to adapt to change in groundwater availability. For example, some GDEs of the Swan Coastal Plain in Western Australia may have shifted to an alternative state (defined by biota and ecological processes that correspond to a particular groundwater regime) in accordance with changes in the extant hydrological support conditions (Froend and Sommer, 2010;Sommer and Froend, 2014). However, the potential of GDEs to adapt can be limited under catastrophic (and largely irreversible) changes in the availability of groundwater, such as the exacerbation of drought-induced drawdown by groundwater abstraction, which has resulted in widespread mortality of groundwater dependent (phreatophytic) vegetation (Sommer and Froend, 2011). ...
... Study sites were located on the Bassendean Dune System that consists of highly leached quartz sand, which has a very low water-holding capacity (McArthur 1991), and a wide range in depth (3-30 m) to the groundwater table across the dunal landscape (Thackway and Cresswell 1995). As a result of this gradient of water availability, there are differences in species assemblage and plant tolerance to water deficits across the dunes (Beard 1990;Froend and Sommer 2010;Sommer and Froend 2014). The field site (31°42′30″S and 115°54′30′E) was located in the mid-slope of the gradient of water availability, and is under a restoration program (Maher et al. 2008). ...
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Plant interaction studies provide a good understanding of the roles of key species, which can assist restoration of natural ecosystems. Among the interactions, facilitation and competition are known to affect ecosystem structure and function. We investigated whether a deep-rooted species could positively affect surrounding seedlings through hydraulic redistribution during dry months. We conducted two experiments in which seedlings from two species were growing together or isolated from source plants (field experiment) and where plants were isolated from source plants that were connected to or separated from a water table (glasshouse experiment). Survival, growth, water relations and soil water content were measured. We also applied δ²H enriched water adjacent to, or into, the roots of source plants to track water movement between plants. Soil water content was higher in shallow layers where source plants could interact with seedlings (field) and when accessing water tables (glasshouse). Seedlings from all treatments had an increase in leaf δ²H. Seedlings of Banksia attenuata that were isolated from source plants had the highest survival, growth and stomatal conductance rates. Seedlings of Gompholobium tomentosum presented higher stomatal conductance rates when growing with source plants than when isolated from them during the first months, but this relationship reversed towards the end of summer. These results suggest that source plants and seedlings competed, but the influence of facilitation and competition might change during the year, at least for the shallow-rooted species. Therefore, competition for water and/or other limiting factors must be considered when planning ecological restoration in such areas.
... Thus, locals would dig SDWs in such areas and often locate water (FOs, FGDs). Interestingly, the practice of using the status of the vegetation health as an indicator of groundwater and/or soil moisture is constant with modern science (Mata-González et al. 2012;Nocco et al. 2013;Sommer and Froend 2014). Locals in ancient times also used vegetation and soil moisture as indicators of groundwater (Kent 2001). ...
Article
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Dodoma city, central Tanzania, seats in a semi-arid region of East Africa with limited rains and surface water resources. Consequently, the area largely depends on shallow and deep aquifers for its freshwater needs. Owing to harsh climatic conditions, chronic lack of year-round surface water bodies and, limited development of water distribution infrastructures, over year’s local people have nurtured, developed and, passed on important indigenous knowledge (IK) on exploiting and managing shallow aquifers (SAs). However, there is no clear documented administrative plans for the SAs and the roles of IK, which is widely practised in developing SDWs and managing SAs, are not properly documented. This study intended to assess the extent of shallow dug wells (SDWs) utilization and contribution of IK on management of SAs of indigenous people of Dodoma Municipality. The methods followed include critical field observations, measurements and, focus group discussions done during both the dry season (Sep.–Oct. 2013) and wet season (Dec. 2013–Feb. 2014). The results show that SDWs occur widely in the city, particularly in the suburbs, where they often serve as the only sources of freshwater and heavily dependent by the populace. It is clear that there is rich IK on management of SAs including on groundwater exploration, digging, water allocation, pricing, and even on water quality and, water treatment skills. The aforementioned IK clearly contribute to water sufficiency to the populace and general management of groundwater such as enhancing recharge mechanisms where about 1% of local rainfall is recharged through a network of SDWs compared to ~ 5–10% that is naturally being recharged by rainfall through the vadose zone. Thus, as much as the current policy framework and groundwater managers do not recognize the roles of IK and contributions of SDWs as key water sources, it is clear that IK contributes to the groundwater management and SDWs already support large part of the society. While it is globally appreciated that vital skills on SDWs management are vanishing, local people in Dodoma still retain them and should, therefore, be preserved. It is further recommended that IK are strengthened, improved and most importantly, incorporated in the local water resources management plans that already advocate on integrated approaches but which clearly ignores the IK and the local people’s efforts to explore and manage water resource, particularly SAs.
... In sandy soils, where there is little water retention during dry season, groundwater present at shallow depth could potentially be an important water source for vegetation ( Zencich et al., 2002). As such, changes in groundwater can alter the ecosystem water balance and, consequently, lead to shifts in species composition, water-use and physiological performance for terrestrial and riparian vegetation (Sommer & Froend, 2014;Villalobos-Vega et al., 2014). These impacts of groundwater changes will depend not only on local factors, such as groundwater recharge, land-use changes, water holding capacity of the soil layers, precipitation dynamics and human extraction rates, but also on plant species (Asbjornsen et al., 2011). ...
Article
1.Groundwater lowering can produce dramatic changes in the physiological performance and survival of plant species. The impact of decreasing water availability due to climate change and anthropogenic groundwater extraction on coastal dune ecosystems has become of increasing concern, with uncertainties about how vegetation will respond in both the short and long terms. 2.We aimed to evaluate the water‐use responses of different plant functional types to increasing groundwater table depth and how this would affect their physiology in Mediterranean coastal dune systems differing in aridity. 3.We modeled water table depth, quantified the contribution of different soil layers to plant water through Bayesian isotope mixing models, and used a combination of spectral and isotope data to characterize plant ecophysiology. We found that increasing depth to groundwater triggered water uptake adjustments towards deeper soil layers only in the dry season. These adjustments in water sources use were made by conifer trees (Pinus pinea, P. pinaster) and hygrophytic shrubs (Erica scoparia, Salix repens) but not by the xerophytic shrub Corema album. Moreover, we observed a greater use of groundwater under semi‐arid conditions. Accompanying the greater use of water from deep soil layers as a response to increasing groundwater depth, the semi‐arid dimorphic‐rooted conifer tree P. pinea and hygrophytic shrub E. scoparia declined their water content (WI), without implications on photosynthetic parameters, such as chlorophyll content (CHL), photochemical index (PRI) and δ13C. Unexpectedly, under semi‐arid conditions, the shallow‐rooted xerophytic shrub C. album, associated with an absence of water‐sources‐use adjustments, showed a decline in WI, CHL, and PRI with groundwater table lowering. 4.We provide insight into how different species, belonging to different functional types, are acclimating to groundwater changes in a region experiencing climatic drought and a scarcity in groundwater due to anthropogenic exploitation. Greater depth to groundwater combined with limited precipitation can have a significant effect on plants’ water‐sources use and ecophysiology in semi‐arid coastal dune ecosystems. This article is protected by copyright. All rights reserved.
... Gradual changes in the water table provide a greater opportunity for plants to adapt to the effects of water stress, but in the process can cause changes in the ecosystem structure and community composition (Froend and Sommer 2010). Gradual increases in depth to groundwater within a GDE with historically shallow groundwater levels tends to result in an altered species composition due to the migration of more opportunistic invasive species that have deeper rooting systems and are better adapted to deeper groundwater conditions (Keddy and Reznicek 1986;Moore and Keddy 1988;Sommer and Froend 2014). ...
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Groundwater is a vital water supply worldwide for people and nature. However, species and ecosystems that depend on groundwater for some or all of their water needs, known as groundwater dependent ecosystems (GDEs), are increasingly becoming threatened worldwide due to growing human water demands. Over the past two decades, the protection and management of GDEs have been incorporated into several water management policy initiatives worldwide including jurisdictions within Australia, the European Union, South Africa, and the United States. Among these, Australia has implemented the most comprehensive framework to manage and protect GDEs through its water policy initiatives. Using a science-based approach, Australia has made good progress at reducing uncertainty when selecting management thresholds for GDEs in their water management plans. This has been achieved by incorporating appropriate metrics for GDEs into water monitoring programs so that information gathered over time can inform management decisions. This adaptive management approach is also accompanied by the application of the “Precautionary Principle” in cases where insufficient information on GDEs exist. Additionally, the integration of risk assessment into Australia's approach has enabled water managers to prioritize the most valuable and vulnerable ecologic assets necessary to manage GDEs under Australia's national sustainable water management legislation. The purpose of this paper is to: (1) compare existing global policy initiatives for the protection and management of GDEs; (2) synthesize Australia's adaptive management approach of GDEs in their state water plans; and (3) highlight opportunities and challenges of applying Australia's approach for managing GDEs under other water management policies worldwide.
... Groundwater levels on the Swan Coastal Plain have been declining since the 1970s due to changes in climate, land use and groundwater extraction. Declining groundwater c20.indd 506 04-01-2017 20:48:22 levels have affected vegetation in groundwater- dependent wetlands in a number of ways, including: reduced surface water levels and in some cases dry- ing and loss of wetland habitat; peat fires; declining health and death of some groundwater-dependent vegetation; changes in flow from springs/seeps; and acidification of groundwater and wetlands (Sommer and Froend 2014). With changing hydrological conditions, many permanent/semi-permanent wet- lands are developing vegetation characteristics of seasonally waterlogged wetlands, while formerly waterlogged areas are being encroached by dryland species. ...
... Species-specific susceptibility to drought events, as documented here, may result in structural shifts to vegetation composition over time. These vegetation shifts may lead to a change from one or several dominant species to a new species that is better adapted to survive periods of water deficit (e.g., Mueller et al. 2005;Sommer and Froend 2014). In the Kings Park bushland A. fraseriana is increasing in abundance (Beard 1967;Crosti et al. 2007). ...
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Background and Aims Drought-induced mortality of tree species is increasing globally. We aimed to investigate spatial patterns and size dependence of mortality of two dominant tree species (Banksia menziesii R. Br and B. attenuata R. Br) capable of accessing shallow watertables in a Banksia-Allocasuarina-Eucalyptus woodland. Methods Living and dead trees were mapped within two plots: a high site (55 m to watertable) and a low site (9–20 m to watertable). Diameter at breast height (DBH) (cm) was measured and year of death estimated for deceased trees. Results Tree mortality was higher for most species in the high site. Across sites mortality was greatest during 2011 for most species including Banksia trees following the 2010 drought. Species differences in mortality were observed between B. attenuata and B. menziesii in the high site only. A greater number of large dead Banksias was observed in the high site. Spatial analysis indicated that local scale competition did not contribute to the death of these Banksias, however stand-level competition may have occurred. Conclusions We conclude that drought-induced mortality of Banksia trees is more prevalent in landscape regions where trees cannot access the watertable and due to greater water demands of larger trees, mortality is more frequent in these individuals.
... Management of GDEs can also be informed by understanding the potential for ecosystems to adapt to changes in groundwater availability. For example, some GDEs of the Swan Coastal Plain in Western Australia may have shifted to an alternative state (defined by biota and ecological processes) in accordance with changes in the groundwater regime (Froend and Sommer 2010;Sommer and Froend 2014). The potential of GDEs to adapt, however, can be limited under catastrophic (and largely irreversible) changes in the availability of groundwater, such as the widespread mortality of groundwater-dependent (phreatophytic) vegetation by groundwater abstraction in times of drought (Sommer and Froend 2011). ...
... Long-term monitoring (over decades) indicate that higher rate of groundwater drawdown will result in the mortality of forests or the transition to an alternative ecohydrological state. Besides the small scale studies mentioned above, regional monitoring also indicates that groundwater decline negatively affects groundwater-dependent plants (Stromberg et al., 1996; Cooper et al., 2006; Elmore et al., 2006; Cunningham et al., 2011; Barron et al., 2014; Kath et al., 2014 and Sommer and Froend, 2014). For example, lowering water table reduced groundwater contribution to plant water use by 62% in the San Luis Valley, Colorado, USA (Copper et al., 2006). ...
... Management of GDEs can also be informed by understanding the potential for ecosystems to adapt to changes in groundwater availability. For example, some GDEs of the Swan Coastal Plain in Western Australia may have shifted to an alternative state (defined by biota and ecological processes) in accordance with changes in the groundwater regime (Froend and Sommer 2010;Sommer and Froend 2014). The potential of GDEs to adapt, however, can be limited under catastrophic (and largely irreversible) changes in the availability of groundwater, such as the widespread mortality of groundwater-dependent (phreatophytic) vegetation by groundwater abstraction in times of drought (Sommer and Froend 2011). ...
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Article
Coastal groundwater‐dependent ecosystems (GDEs), such as wetlands, estuaries, and mangrove forests, are globally important habitats that promote biodiversity, provide climate regulation and serve as refugia for plant and animal communities. However, global warming, coastal development, and over‐abstraction threaten the availability and quality of groundwater in coastal aquifers and, by extension, the ecohydrological function of dependent ecosystems. Because ecohydrological knowledge of coastal groundwater is disparate across disciplines and habitat types, we begin by summarising the physiochemical, biological and hydrological processes supported by groundwater across coastal watersheds. Groundwater makes a significant, but poorly recognised contribution to the function and resilience of coastal ecosystems and will play an essential role in climate change mitigation and adaptation. This review then explores how critical ecosystem processes supported by groundwater will be affected in areas of the humid subtropics that are expected to be impacted by climatic drying. Where rainfall is predicted to decrease, reduced groundwater recharge will interrupt the hydrology of coastal GDEs, while anthropogenic pressures, such as land‐use intensification and pollution, will diminish the quality of remaining groundwater. The challenges of managing groundwater for multiple purposes under climate change predictions are highlighted. To improve the management of coastal GDEs, research should be aimed at developing robust conceptual models of coastal groundwater systems that quantify biophysical linkages with ecological communities across relevant spatiotemporal scales.
Article
Questions Predicting the influence of climate change on riparian plant communities improves management strategies. The sensitivity of riparian vegetation to climate and other abiotic factors depends on interactions between properties of the ecosystem, like flood regime, and plant characteristics. To explore these interactions, we addressed three questions: Does the composition and diversity of riparian vegetation vary with the flood regime? Do abiotic correlates of vegetation, including climate and groundwater, differ between sites that flood compared to locations that did not experience floods? Which plant functional groups account for differential plant community sensitivity to abiotic factors between flood regimes? Location Middle Rio Grande Valley, New Mexico Methods We used long‐term observations of plant community composition, groundwater depth, precipitation and interpolated temperature from 24 sites spanning 210 km of the Rio Grande riparian cottonwood‐willow forest to explore the relative importance of climate and hydrologic correlates of riparian vegetation diversity and composition. Results Riparian plant diversity was higher at sites flooding compared to non‐flooding sites. Plant diversity positively tracked shallower groundwater depth at flooding sites, but was best predicted by intra‐annual groundwater variability at non‐flooding sites. Plant community composition correlated with groundwater depth and air temperature at all sites, but at non‐flooding sites, also with intra‐annual groundwater variability and precipitation. Relationships between native plant cover and potential environmental drivers diverged strongly between the two flood regimes; non‐native plant cover had only weak relationships with most environmental predictors. Conclusions The current flood regime of a site determined the climate and hydrologic factors that best predicted riparian plant community composition and diversity. Relationships between plant diversity or total cover and groundwater, temperature, precipitation, or groundwater variability can change in strength or direction depending on a site’s flood history, highlighting the importance of flood regime to predicting the sensitivity of riparian woodlands to future environmental change.
Article
Environmental heterogeneity significantly affects the structure of ecological communities. Exploring vegetation distribution and its relationship with environmental factors is essential to understanding the abiotic mechanism(s) driving vegetation succession, especially in the ecologically fragile areas. In this study, based on the quantitative analysis of plant community and environmental factors in 68 plots at 10 different transects in the Minqin oasis-desert ecotone (ODE) of northwestern China, we investigated desert vegetation distribution and species-environment relationships using multivariate analysis. Two-way indicator species analysis (TWINSPAN), detrended correspondence analysis (DCA), and canonical correspondence analysis (CCA) methods were used. A total of 28 species, belonging to 27 genera in 8 families, were identified. Chenopodiaceae, Zygophyllaceae, Gramineae, and Leguminosae were the largest families. Annual and perennial herbs accounted for 28.60% of the total number of plants, while shrubs (42.90%) were the most dominant. Nitraria tangutorum was the constructive species of the desert plant community. We divided the 68 plots surveyed in this study into 7 community types, according to the results of TWINSPAN. The distribution of these 7 communities in the DCA ordination graph showed that species with a similar ecotype were clustered together. Results of CCA indicated that groundwater was the dominant factor influencing vegetation distribution, while distance between plot and oasis (Dis) and soil electrical conductivity (EC) were the local second-order factors. Our study suggests that optimizing the utilization of groundwater in oases is key to controlling the degradation of desert vegetation. The favorable topographic conditions of sand dunes should be fully utilized for vegetal dune stabilization, and the influence of soil salinity on the selection of afforestation tree species should be considered.
Article
Globally, the provision of groundwater‐supported ecosystem services is threatened by climate change, water extraction and other activities that alter groundwater regimes (defined as temporal dynamics in groundwater pressures, storage and levels). Research on how altered groundwater regimes affect the ecology and ecosystem services of diverse groundwater‐dependent ecosystems (GDEs) is currently fragmented with little integration across different GDEs, hampering our ability to understand and manage ecological responses to anthropogenic changes to groundwater regimes. To address this, we present a framework for assessing ecological responses to groundwater regime alteration (FERGRA). FERGRA is a logical approach to investigating how alterations to groundwater regimes change the timing, variability, duration, frequency and magnitude of groundwater connections to different GDEs, in turn affecting their ecological processes and ecosystem service provision. Using FERGRA, multiple GDEs can be assessed concurrently, optimizing their integrated management. Unifying the concepts of ecological responses to altered groundwater regimes and groundwater connections of different GDEs across the landscape, FERGRA provides a framework for (i) organizing the currently fragmented research on GDEs to better identify commonalities and knowledge gaps, (ii) formulating and testing hypotheses for quantifying ecological responses to groundwater regime alteration in GDEs to derive general principles to guide research and management, and (iii) facilitating assessments of the trade‐offs between the benefits of groundwater extraction (e.g. to support mining, agriculture, etc.) versus conservation of GDEs to protect other ecosystem services.
Article
Groundwater-level fluctuations at a large scale have a significant effect on the preservation and restoration of vegetation. This study determined suitable groundwater depth within which natural vegetation grows well, and analysed the effect of groundwater regulation on vegetation restoration in Tianjin City, northern China. Normal and lognormal distributions were used to fit the curve of the relation between vegetation and groundwater depth. The groundwater depth range corresponding to the higher frequency of vegetation distribution was regarded as the ‘suitable water depth’ range for vegetation growth. The suitable groundwater depth for shrub growth was 3–5 m and for grass growth 1–3 m. A groundwater flow model predicted the future changes of groundwater depths in the vegetation distribution area under the condition that the current levels of groundwater extraction are maintained. The results showed that there is potential for the extraction of groundwater in shrubland areas, but for grassland areas the water-table elevation showed a downward trend, meaning that water shortages in some areas may be more severe in the future. Finally, based on the current groundwater extraction regime, two regulation schemes were developed: (1) for shrubland, groundwater extraction was reduced by 10% in the ecological water deficit areas, and extraction was increased by 10% in the ecological water surplus and suitable areas, and (2) for grassland, groundwater recharge was increased by the restoration of the wetland areas. In conclusion, the groundwater depths in most of the area would be more suitable for vegetation growth under the regulation schemes.
Article
Balancing the water demand between human usageand ecosystems remains a challenge in water-limited regions where one of the main groundwater uses is short-term pumping. A 23-day pumping test was performed in a semi-arid site of northwest Chinato test the hypothesis that native phreatophytes can tolerate short-term pumping. The monitoring indicated that sap flow velocityof groundwater-dependent willow trees began to decrease on day 4 after pumping and almost fully recovered on day 9 after the cessation of pumping. Numerical simulations using HYDRUS-1D were conducted for the period of 1954-2013 to assess the response of phreatophytesto groundwater withdrawal under various climatic conditions. The modelingresults reveal that phreatophytes can recover from the stress induced by groundwater pumping because they can adapt to short periods of water stressusing physiological and morphological traits and the degree of water stress depends on the amount and/orfrequency of rainfall duringandafter pumping,. Therefore, pulsed pumping, under particular climatic conditions, could be used toreduce negative impacts of groundwater extraction on phreatophytes, while still providing groundwater for socio-economic activities in semi-arid zones.
Article
Understanding hydrological processes in water-limited systems requires consideration of temporal and spatial vegetation water use patterns at the landscape scale. We used data derived from the MODerate Resolution Imaging Spectroradiometer (MODIS) satellite instrument and interpolated climate data covering a ten-year period to contrast the spatio-temporal patterns of actual evapotranspiration (AET) from known phreatophytic and non-phreatophytic vegetation overlying a large superficial aquifer. We assessed shallow to deeper groundwater habitats and compared AET responses to seasonal and inter-annual variation in precipitation. Overall, vegetation in shallow groundwater habitats had higher AET rates during the growth season (spring and summer) than vegetation growing in deeper groundwater habitats, suggesting that the former was not physiologically constrained by water deficit. Vegetation in areas of consistently high (ground-)water availability maintained higher AET, reaching a peak of 95 mm in mid-summer. In contrast, plantation maritime pines had the highest AET rates at deep groundwater habitats. Inter-annual variability in AET correlated with rainfall and AET rates peaked two months after the majority of effective rainfall had fallen. During low rainfall years, maximum AET peaked one month earlier relative to higher rainfall years. The results of this study suggest that remote sensing of AET can give a conditional indication of where groundwater is important in supporting vegetation and can be a valuable tool in identifying management focus areas where vegetation is variably sensitive to water deficit. Copyright © 2016 John Wiley & Sons, Ltd.
Chapter
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The southwestern Australian flora is unique in the world, not only for its biodiversity and endemism, but also for its functional biodiversity. It also contains the world's most nutrient-impoverished soils, has a prolonged-summer period and the vegetation is extremely fire-prone. These conditions have engendered an array of survival adaptations that have evolved in these harsh conditions across a diverse range of species. It is well recognised that the southwest flora has the toughest and most spiny vegetation of the world, the greatest number of species that store their seeds in woody fruits, and the most specialised means of obtaining limited soil nutrients and water. This book focuses on the survival mechanisms, adaptations and ecology of the unique Southwest Australian flora (restricted here to flowering plants). The book begins with an examination of how the flora has evolved into the present forms. It describes further in detail the adaptive responses of the flora to the main environmental pressures influencing survival-fire, summer drought, nutrient-impoverished soils, pollination and seed dispersal agents. Specialised responses to obtain essential nutrients are presented in three chapters – carnivorous plants, parasitic plants and specialised roots. An entire chapter is devoted to leaves, with an insight into how leaves may assist in protecting flowers and fruits from herbivores and seed-eaters. The book provides an ecological perspective on how the flora has evolved complex strategies to ensure species survival in the relatively harsh seasonal climate of a Mediterranean-type ecosystem.
Article
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This paper addresses the problem of uniqueness of catchment areas in relation to model representations of flow processes. The uniqueness of field measurements as a limitation on model representations is discussed. The treatment of uniqueness as a residual from a modelled relationship may conceal information about the uniqueness of catchments, while the treatment of uniqueness as a set of parameter values within a particular model structure is problematic due to the equifinality of model structures and parameter sets. The analysis suggests that a fully reductionist approach to describe the uniqueness of individual catchment areas by the aggregation of descriptions of small scale behaviour will be impossible given current measurement technologies. A suggested strategy for the representation of uniqueness of place as a fuzzy mapping of the landscape into a model space is suggested. This will lead to a quantification of the uncertainty in predictions of any particular location in a way that allows a conditioning of the mapping on the basis of the available data. This process can incorporate a hypothesis testing approach to model evaluation but the problem of multiple behavioural models may provide an ultimate limitation on the realism of process representations: not on the principle of realism but on the possibility of unambiguous process representations.
Article
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The Gnangara Groundwater System meets about 50% of all water needs for the Perth Peel region of Western Australia (population 1.7 million). Much of the water is contained in an unconfined aquifer which occurs in coastal sand dunes and supports ecologically-important throughflow wetlands. The system has been subject to significant climate change since about 1975, although the persistent and unidirectional nature of the change was not recognised for some time. As well as climate, groundwater levels are affected by land use (e.g. plantation forestry, urbanisation) and land management (e.g. how plantations and stormwater are managed) as well as by the amount of groundwater abstraction from each of several inter-connected aquifers. Land, water and forests are managed by different government agencies with their own policy objectives. Maintaining groundwater levels within an agreed range of values to protect the wetlands requires informed and early adaptation by these agencies as well as a supportive community. Adaptation was hampered because there was little or no experience of managing groundwater for climate change and the causes of declining levels were neither clear nor agreed. Even when target water level decisions were agreed, their achievement required the cooperation of parties with different priorities. This paper examines some of the lessons learned from this experience and the current approach to manage the land, water and forest resources to meet multiple objectives in a system that is undergoing transitional change rather than reaching a new equilibrium. Climate change impacts have been progressive and the concept of a system that can respond in a resilient manner after a temporary perturbation is not an appropriate concept in this example. Climate adaptation involves significant social and institutional change as well as biophysical changes to make the most of a changing system.
Conference Paper
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The edges of wetlands flood and dry on a variety of time scales: days, seasons, years or decades. Such variable water regimes provide unpredictable conditions for the germination, establishment and reproduction of plants in the wet/dry ecotone. The communities of this ecotone are often species-rich and have a seed bank of opportunistic species which can germinate, establish and set seed when conditions allow. On the basis of six years of data collection on the germination, growth and reproduction requirements, and preferences for a wide range of wetland plants, this study explored whether the plants which occupy the edges of wetlands fall into functional groups defined by wetting and drying patterns. Clear groupings of 'terrestrial' and 'submerged' plants occupy the upper and lower sections of a wetland with a large group of 'amphibious' plants that tolerate or respond to fluctuations in flooding and drying in the wet/dry ecotone. These groupings are further divided by the life-cycle patterns plants use to cope with the variable water regime in each habitat. For example, 'amphibious fluctuation-responding' plants change their growth form, whereas 'amphibious fluctuation-tolerating' plants tolerate the variations in flooding pattern. This chapter considers how imposed changes to water regimes might change the balance of functional groups of species and the species richness of future communities occupying wetland edges.
Article
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This article synthesizes the ecological concepts and perspectives underpinning the development and application of state- and-transition models, thresholds, and rangeland health. Introduction of the multiple stable state concept paved the way for the development of these alternative evaluation procedures by hypothesizing that multiple stable plant communities can potentially occupy individual ecological sites. Vegetation evaluation procedures must be able to assess continuous and reversible as well as discontinuous and nonreversible vegetation dynamics because both patterns occur and neither pattern alone provides a complete assessment of vegetation dynamics on all rangelands. Continuous and reversible vegetation dynamics prevail within stable vegetation states, whereas discontinuous and nonreversible dynamics occur when thresholds are surpassed and one stable state replaces another. State-and-transition models can accommodate both categories of vegetation dynamics because they represent vegetation change along several axes, including fire regimes, weather variability, and management prescriptions, in addition to the succession-grazing axis associated with the traditional range model. Ecological thresholds have become a focal point of state-and-transition models because threshold identification is necessary for recognition of the various stable plant communities than can potentially occupy an ecological site. Thresholds are difficult to define and quantify because they represent a complex series of interacting components, rather than discrete boundaries in time and space. Threshold components can be categorized broadly as structural and functional based on compositional and spatial vegetation attributes, and on modification of ecosystem processes, respectively. State-and-transition models and rangeland health procedures have developed in parallel, rather than as components of an integrated framework, because the two procedures primarily rely on structural and functional thresholds, respectively. It may be prudent for rangeland professionals to consider the introduction of these alternative evaluation procedures as the beginning of a long-term developmental process, rather than as an end point marked by the adoption of an alternative set of standardized evaluation procedures. Resumen
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The Global Change and Terrestrial Ecosystems project was set up by the IGBP with the objectives of predicting the effects of changes in climate, atmospheric composition and land use on terrestrial ecosystems, as well as of feedbacks to the atmosphere and the physical climate system. The present document takes the previously laid-out strategic research plan and develops it into an operational plan, structured into a hierarchy of "foci', "activities' and "tasks'. The four foci are: ecosystem physiology; change in ecosystem structure; global change impact on agriculture and forestry; and a proposed focus on global change and ecological complexity. Each of these comprises sub-sections, which are research activities targeted at defined units of the terrestrial ecosystem and which are drawn together at the end of each section. There are concluding summaries on monitoring and detecting global change, the adopted research strategy, etc. -J.W.Cooper
Article
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We review the evidence of regime shifts in terrestrial and aquatic environments in relation to resilience of complex adaptive ecosystems and the functional roles of biological diversity in this context. The evidence reveals that the likelihood of regime shifts may increase when humans reduce resilience by such actions as removing response diversity, removing whole functional groups of species, or removing whole trophic levels; impacting on ecosystems via emissions of waste and pollutants and climate change; and altering the magnitude, frequency, and duration of disturbance regimes. The combined and often synergistic effects of those pressures can make ecosystems more vulnerable to changes that previously could be absorbed. As a consequence, ecosystems may suddenly shift from desired to less desired states in their capacity to generate ecosystem services. Active adaptive management and governance of resilience will be required to sustain desired ecosystem states and transform degraded ecosystems into fundamentally new and more desirable configurations.
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▪ Abstract Feedback between plants and the soil is frequently invoked on the basis of evidence of mutual effects. Feedback can operate through pathways involving soil physical properties, chemical and biogeochemical properties and processes, and biological properties, including the community composition of the microbiota and soil fauna. For each pathway, we review the mechanistic basis and assess the evidence that feedback occurs. We suggest that several properties of feedback systems (for example, their complexity, specificity, and strength relative to other ecological factors, as well as the temporal and spatial scales over which they operate) be considered. We find that the evidence of feedback is strongest for plants growing in extreme environments and for plant-mutualist or plant-enemy interactions. We conclude with recommendations for a more critical appraisal of feedback and for new directions of research. Let us not make arbitrary conjectures about the greatest matters. Heraclitus (1)
Article
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Banksia (Proteaceae) woodlands are one of a number of groundwater-dependent ecosystems in southwestern Western Australia that are threatened by groundwater abstraction. In addition to this threat is an ongoing decline in regional water tables due to a drying climate. We used ecological resilience theory to analyse and interpret a long-term vegetation monitoring dataset from a site that has experienced an abstraction-induced acute groundwater drawdown in the late 1980s and early 1990s. Despite reduced plant abundance, all dominant over- and understorey species were still found on all transect plots in which they were recorded pre-drawdown. This suggests a notional resilience and a strong likelihood of recovery, in the event that pre-drawdown ecohydrological habitat conditions were to return. However, since the drawdown event, the regional water table continued to decline, with the vegetation responding through progressive and uni-directional change in abundance and composition. The change in composition was primarily manifested as a shift towards non-woody, shallow-rooted species not dependent on specific hydrological conditions. This slow, progressive change in hydrology associated with reduced rainfall and land use changes has continued to force a transition in the floristics towards an alternative ecohydrological state. Despite the absence of an acute drawdown event, the same progressive floristic response was also observed at two reference sites that were not under the immediate influence of production bores. The challenge for adaptive water resource management will be to enhance the capacity for resilience in these groundwater-dependent ecosystems in a drying environment through appropriate regulation of groundwater abstraction. Copyright © 2010 John Wiley & Sons, Ltd.
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Disentangling the relative roles of biotic and abiotic forces influencing forest structure, function, and local community composition continues to be an important goal in ecology. Here, utilizing two forest surveys 20-year apart from a Central American dry tropical forest, we assess the relative role of past disturbance and local climatic change in the form of increased drought in driving forest dynamics. We observe: (i) a net decrease in the number of trees; (ii) a decrease in total forest biomass by 7.7 Mg ha−1 but when calculated on subquadrat basis the biomass per unit area did not change indicating scale sensitivity of forest biomass measures; (iii) that the decrease in the number of stems occurred mainly in the smallest sizes, and in more moist and evergreen habitats; (iv) that there has been an increase in the proportion of trees that are deciduous, compound leaved and are canopy species, and a concomitant reduction in trees that are evergreen, simple-leaved, and understory species. These changes are opposite to predictions based on recovery from disturbance, and have resulted in (v) a uniform multivariate shift from a more mesic to a more xeric forest. Together, our results show that over relatively short time scales, community composition and the functional dominance may be more responsive to climate change than recovery to past disturbances. Our findings point to the importance of assessing proportional changes in forest composition and not just changes in absolute numbers. Our findings are also consistent with the hypothesis that tropical tree species exhibit differential sensitivity to changes in precipitation. Predicted future decreases in rainfall may result in quick differential shifts in forest function, physiognomy, and species composition. Quantifying proportional functional composition offers a basis for a predictive framework for how the structure, and diversity of tropical forests will respond to global change.
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Based on the analysis of 600 vegetation plots using the method of Braun-Blanquet (1964) the composition of the whole vascular forest plant flora with about 1220 species was studied in the forests of Mt. Kilimanjaro. The altitudinal distribution of all strata (trees, shrubs, epiphytes, lianas and herbs) along a transect of 2400m is discussed with respect to altitudinal zonation and ecological factors. With uni-dimensionally constraint clustering significant discontinuities were revealed that occurred simultaneously in the different strata. Thus even in structurally highly complex, multilayered tropical montane forests distinct community units exist that can be surveyed and classified by the Braun-Blanquet approach. This observed zonation was significantly correlated with altitude, temperature and soil acidity (pH); rainfall was of importance in particular for the zonation of epiphytes. Other key factors were humidity (influenced by stable cloud condensation belts) and minimum temperature (in particular the occurrence of frost at 2700m altitude upslope). The contrary results of other transect studies in East Africa in respect to continuity of change in floristic composition appear to be caused by different sampling methods and intensities or mixing of data from areas with different climate conditions, whereas species richness did not influence the clarity of floristic discontinuities on Kilimanjaro and other parts of East Africa.
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An ecological threshold is the point at which there is an abrupt change in an ecosystem quality, property or phenomenon, or where small changes in an environmental driver produce large responses in the ecosystem. Analysis of thresholds is complicated by nonlinear dynamics and by multiple factor controls that operate at diverse spatial and temporal scales. These complexities have challenged the use and utility of threshold concepts in environmental management despite great concern about preventing dramatic state changes in valued ecosystems, the need for determining critical pollutant loads and the ubiquity of other threshold-based environmental problems. In this paper we define the scope of the thresholds concept in ecological science and discuss methods for identifying and investigating thresholds using a variety of examples from terrestrial and aquatic environments, at ecosystem, landscape and regional scales. We end with a discussion of key research needs in this area.
Article
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While seasonal redistribution of fine root biomass in response to fluctuations in groundwater level is often inferred in phreatophytic plants, few studies have observed the in situ growth dynamics of deep roots relative to those near the surface. We investigated the root growth dynamics of two Banksia species accessing a seasonally dynamic water table and hypothesized that root growth phenology varied with depth, i.e. root growth closest to the water table would be influenced by water table dynamics rather than surface micro-climate. Root in-growth bags were used to observe the dynamics of root growth at different soil depths and above-ground growth was also assessed to identify whole-plant growth phenology. Root growth at shallow depths was found to be in synchrony with above-ground growth phenophases, following increases in ambient temperature and soil water content. In contrast, root growth at depth was either constant or suppressed by saturation. Root growth above the water table and within the capillary fringe occurred in all seasons, corresponding with consistent water availability and aerobic conditions. However, at the water table, a seasonal cycle of root elongation with drawdown in summer followed by trimming in response to water table rise and saturation in winter, was observed. The ability to grow roots year-round at the capillary fringe and redistribute fine root biomass in response to groundwater drawdown is considered critical in allowing phreatophytes, in seasonally water-limited environments, to maintain access to groundwater throughout the year.
Article
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In Mediterranean ecosystems vegetation overlying shallow, transient aquifers is often dominated by woody phreatophytes, trees and shrubs that have been shown to be dependent on groundwater for their water requirements. Natural and anthropogenic alterations of groundwater tables (abstraction) are of clear importance to phreatophytic vegetation as reduction of water tables may sever these plants from their natural water sources. Seasonal water sources were determined for species growing on a coastal dune system that overlies a shallow sandy aquifer in south-western Australia. The plants studied grew over groundwater that ranged in depth from 2.5 to 30 m. The naturally occurring stable isotope of hydrogen (deuterium, &#392H) was used to distinguish potential water sources. Isotopic ratios from vascular water of the dominant species of the study area (Banksia ilicifolia R. Br. and Banksia attenuata R. Br. trees) were compared with those of potential sources of precipitation, soil moisture and groundwater. A relatively shallow-rooted perennial shrub, Hibbertia hypericoides Benth., was also included as an isotopic reference. The results suggest that both B. attenuata and B. ilicifolia are phreatophytic as they derived some of their water from groundwater throughout the dry-wet cycle, with the exception of B. attenuata at the site of greatest depth to groundwater (30 m) which did not use groundwater. A high proportion (>50%) of groundwater use was not maintained throughout all seasons. With the onset of the hot Mediterranean summer, progressive drying of the surface soils resulted in increased use of groundwater and deep soil moisture. During the wet winter plants used proportionately more water from the upper layers of the soil profile. The degree to which groundwater was utilised by the study species was dependent on the proximity of groundwater, availability of moisture in shallower horizons of the soil profile, root system distribution and maximum root depth.
Book
Ecohydrology: Vegetation Function, Water and Resource Management describes and provides a synthesis of the different disciplines required to understand the sustainable management of water in the environment in order to tackle issues such as dryland salinity and environmental water allocation. It provides in the one volume the fundamentals of plant ecophysiology, hydrology and ecohydrology as they relate to this topic. Both conceptual foundations and field methods for the study of ecohydrology are provided, including chapters on groundwater dependent ecosystems, salinity and practical case studies of ecohydrology. The importance of ecologically sustainable development and environmental allocations of water are explained in a chapter devoted to policy and principles underpinning water resource management and their application to water and vegetation management. A chapter on modelling brings together the ecophysiological and hydrological domains and compares a number of models that are used in ecohydrology. For the sustainable management of water in Australia and elsewhere, this important reference work will assist land managers, industry, policy makers, students and scientists achieve the required understanding of water in landscapes.
Chapter
This book contains 15 chapters dealing with the integration of ecology with hydrology at the river basin scale. The patterns and processes in the catchment including nutrients, lotic vegetation and aquatic faunal processes are covered. In addition, the benefits and risks of ecological modelling to water management and nutrient budget modelling for lakes and watershed restoration are discussed. The roles of ecohydrology in managing the savannah ecosystem, agricultural landscape, and watershed development are explored.
Article
Since 1969, watertable and wetland levels in the Gnangara Groundwater Mound have declined in response to a 10 – 15% lower rainfall, increased groundwater abstraction for Perth and horticulture, and establishment of pine plantations. A relationship between the cumulative departure from mean rainfall (CDFM) and watertable levels has been applied to 200 monitoring bore hydrographs to separate the effects of climate, abstraction and landuse change. Rainfall is shown to cause a maximum 4m decline, with localised atertable declines around borefi elds and mature plantations. Clearing for plantations, thinning, and bush fi res are shown to have a positive impact on water table levels
Article
The magnitude of climate changes forecast for the next century is comparable to the magnitude of warming during the last deglaciation. No climate change of similar magnitude has occurred since that event. The palaeoecological evidence of the response, especially of plants, to past climate change indicates that evolutionary adaptation has played no more than a minor role and that migration is the usual response of organisms to climate change. The individualism of response has important implications with respect to changes in the nature of vegetation and ecosystems. The maximum realized rates of migratory response by trees, although perhaps matching the maximum potential rates, are close to the maximum that it is believed can be achieved by such long-lived sessile organisms. The rate of climate change forecast for the future is 10–100 times faster than the rate of deglacial warming. Unless steps are taken to facilitate the migratory response of organisms to the forecast changes, then widespread extinction is likely. Artificial dispersal of trees and other organisms of limited dispersal and/or migratory capacity, the general extension of the legal protection currently afforded to some threatened organisms only within designated reserves, and the integration of wildlife habitat requirements and of wildlife corridors into human landscape utilization are all likely to be necessary. Stringent measures to limit the extent of future climate change by limiting emissions of greenhouse gases will also be necessary if the possibility of widespread and even catastrophic extinction is to be avoided.
Article
In order to prescribe effective management for arid rangelands, the effects of management must be separable from other factors. Climate, grazing and fire interact to cause complex changes in vegetation and multivariate analysis can separate the relative influence of each factor. It is also important that the causes of vegetation change are easily interpretable for the practical manager. This study outlines a multivariate procedure for reducing a large number of species to a small number of readily recognizable "functional groups", using the vital attributes of the species to form the groups. The relative proportions of the functional groups at monitoring sites spread throughout a particular range type are used to classify the sites in terms of their "condition state". Thus, sites in a poor condition state might have predominantly "unpalatable forbs" and few "palatable grasses". The groups are easily recognized but species have been assigned to them by a method that is repeatable and minimizes subjectivity. Initially, tests of correlation were used to help eliminate attributes that did not provide independent information. The remaining attributes were used to classify species into functional groups whose relative proportions were then used to classify monitoring sites into condition states. The procedure is effective even when knowledge of species autecology is imperfect but it can refined as knowledge increases. Until autecology is very well documented, it would be wise to collect field data at species level. Future data collection at functional group level could increase speed and efficiency and reduce the amount of data. The value of the method lies in the ease of interpretation of complex vegetation change for both land management agencies and pastoralists.
Book
From the reviews of the First Edition."An interesting, useful, and well-written book on logistic regression models . . . Hosmer and Lemeshow have used very little mathematics, have presented difficult concepts heuristically and through illustrative examples, and have included references."—Choice"Well written, clearly organized, and comprehensive . . . the authors carefully walk the reader through the estimation of interpretation of coefficients from a wide variety of logistic regression models . . . their careful explication of the quantitative re-expression of coefficients from these various models is excellent."—Contemporary Sociology"An extremely well-written book that will certainly prove an invaluable acquisition to the practicing statistician who finds other literature on analysis of discrete data hard to follow or heavily theoretical."—The StatisticianIn this revised and updated edition of their popular book, David Hosmer and Stanley Lemeshow continue to provide an amazingly accessible introduction to the logistic regression model while incorporating advances of the last decade, including a variety of software packages for the analysis of data sets. Hosmer and Lemeshow extend the discussion from biostatistics and epidemiology to cutting-edge applications in data mining and machine learning, guiding readers step-by-step through the use of modeling techniques for dichotomous data in diverse fields. Ample new topics and expanded discussions of existing material are accompanied by a wealth of real-world examples-with extensive data sets available over the Internet.
Article
Tree and shrub species of the Banksia woodlands on the sandplains of northern Swan Coastal Plain, Western Australia possess a range of strategies to avoid or tolerate soil water deficits during the annual summer drought. Shallow-rooted shrub species (< 1 m rooting depth) inhabit a range of locations in the landscape, from top of dune crests to wetland embankments. These are the most drought-tolerant of all sandplain species, surviving extremely low summer soil water potentials (< –7 MPa) and tissue water deficits by significantly reducing their transpirational water loss (< 0.2 mmol m–2 s–1). This is in contrast to the few shallow-rooted species restricted to low-lying or seasonally waterlogged areas which are reliant on subsurface soil moisture or groundwater to maintain their relatively high summer water use. Recent studies of water source usage of selected Banksia tree species have shown that these deep-rooted species access groundwater up to a maximum depth of 9 m depth during the summer months, or soil moisture at depth when groundwater was greater than maximum rooting depths, depending on the species. Medium- and deep-rooted (1–2 m and > 2 m, respectively) shrub species cope with the summer soil drying phase and related decrease in groundwater levels by conserving leaf water loss and incurring predawn water potentials between –1 and –4 MPa, enabling them to occur over a range of topographic positions within the sandplain landscape.
Article
Annual evaporation from a site within a Banksia woodland on a groundwater mound near Perth, Western Australia, was estimated from measurements of daily evaporation by ventilated chambers on fourteen occasions during a 12-month period. The total evaporation for this period was estimated to be 666 mm (77% of annual rainfall). About two-thirds of the total evaporation came from the ground flora, one-fifth from Banksia trees, and the remainder from the tall shrub Adenanthos cygnorum. Depth to water table, which ranged from 4 to 12 m over the site, had little effect on total evaporation. This work suggests that regular reduction in ground flora foliage, for example, by controlled burning could increase recharge.
Article
THIS REVIEW EXPLORES BOTH ECOLOGICAL THEORY AND THE BEHAVIOR OF NATURAL SYSTEMS TO SEE IF DIFFERENT PERSPECTIVES OF THEIR BEHAVIOR CAN YIELD DIFFERENT INSIGHTS THAT ARE USEFUL FOR BOTH THEORY AND PRACTICE. THE RESILIENCE AND STABILITY VIEWPOINTS OF THE BEHAVIOR OF ECOLOGICAL SYSTEMS CAN YIELD VERY DIFFERENT APPROACHES TO THE MANAGEMENT OF RESOURCES. THE STABILITY VIEW EMPHASIZES THE EQUILIBRIUM, THE MAINTENANCE OF A PREDICTABLE WORLD, AND THE HARVESTING OF NATURE'S EXCESS PRODUCTION WITH AS LITTLE FLUCTUATION AS POSSIBLE. THE RESILIENCE VIEW EMPHASIZES DOMAINS OF ATTRACTION AND THE NEED FOR PERSISTENCE. BUT EXTINCTION IS NOT PURELY A RANDOM EVENT: IT RESULTS FROM THE INTERACTION OF RANDOM EVENTS WITH THOSE DETERMINISTIC FORCES THAT DEFINE THE SHAPE, SIZE AND CHARACTERISTICS OF THE DOMAIN OF ATTRACTION. THE VERY APPROACH, THEREFORE, THAT ASSURES A STABLE MAXIMUM SUSTAINED YIELD OF A RENEWABLE RESOURCE, MIGHT SO CHANGE THESE CONDITIONS THAT THE RESILIENCE IS LOST OR IS REDUCED SO THAT A CHANCE AND RARE EVENT THAT PREVIOUSLY COULD BE ABSORBED CAN TRIGGER A SUDDEN DRAMATIC CHANGE AND LOSS OF STRUCTURAL INTEGRITY OF THE SYSTEM. A MANAGEMENT APPROACH BASED ON RESILIENCE, ON THE OTHER HAND, WOULD EMPHASIZE THE NEED TO KEEP OPTIONS OPEN, THE NEED TO VIEW EVENTS IN A REGIONAL RATHER THAN A LOCAL CONTEXT, AND THE NEED TO EMPHASIZE HETEROGENEITY. THE RESILIENCE FRAMEWORK DOES NOT REQUIRE A PRECISE CAPACITY TO PREDICT THE FUTURE BUT ONLY A QUALITATIVE CAPACITY TO DEVISE SYSTEMS THAT CAN ABSORB AND ACCOMMODATE FUTURE EVENTS IN WHATEVER UNEXPECTED FORM THEY MAY TAKE.
Article
A recent debate among ecologists has focused on mechanisms by which species diversity might affect net primary productivity. Communities with more species could use a greater variety of resource capture characteristics, leading to greater use of limiting resources (complementarity) and therefore greater productivity (overyielding). Recent experiments, however, have shown a variety of relationships between diversity and productivity. In an experiment on serpentine grassland communities spanning 8 years, we found that overyielding increased several years after plot establishment. Overyielding varied greatly depending on the functional characteristics of the species involved and the biotic and abiotic environment (particularly water availability). While functional differences among species led to strong complementarity and facilitation, these effects were not sufficient to cause significant transgressive overyielding or consistent increases in productivity with increased plant diversity. These results suggest that greater absolute production with greater diversity may be restricted to particular species combinations or environmental conditions.
Article
Habitat loss and increasing landscape fragmentation are known to be key forces driving the ongoing loss of plant species diversity. While the combined effects of increasing isolation and decreasing population size have been studied intensively; it is less understood how plant population performance in heterogeneous landscapes is affected by changes in fragmentation alone.To test whether general rules exist to describe the complex response of plant species to fragmentation, a model is developed to simulate plant populations in a spatially realistic context. The performances of six ruderal plant functional types (PFT), as defined by Grime's CSR scheme, are compared among landscapes varying in their level of fragmentation.In general, increasing fragmentation has a negative but varying effect on the measured set of regional fitness parameters of the PFTs. For several output variables, it is not only the main functional types that differ in their relative competitive, ruderal and stress-tolerance features, but also their subtypes.Strictly ruderal types react most strongly to fragmentation intensity showing high regional extinction vulnerability. In contrast, competitive types are less affected.Some specific traits like having dormant seeds have a positive impact on some regional output variables measured. However, this positive impact is not valid for all output variables simultaneously, suggesting a trait syndrome centred view of PFT's behaviour.Our simulation experiments show that a thorough categorisation based on plant functional types provides a suitable approach for improving our understanding of complex plant species responses in dynamic heterogeneous landscapes.
Article
1] The interaction of vegetation with the groundwater is one of the key mechanisms affecting the dynamics of wetland plant ecosystems. The main feature of these interactions is the feedback between the downward shift of the water table caused by riparian vegetation and the emergence of soil aeration conditions favorable to plant establishment, growth, and survival. We develop a conceptual framework to explain how vegetation– water table feedbacks may lead to the emergence of multiple stable states in the dynamics of wetland forests and riparian ecosystems. This framework is used to investigate the sensitivity of these ecosystems to vegetation disturbances and changes in water table depth. As a result of these feedbacks, such ecosystems are prone to catastrophic shifts to an unvegetated state. Because of their competitive advantage, water-tolerant and shallow-rooted species can replace the original vegetation, contributing to the occurrence of vegetation succession in riparian zones and to the existence of alternative vegetation states between areas with shallow and deep water tables.
Article
Rangelands have undergone—and continue to undergo—rapid change in response to changing land use and climate. A research priority in the emerging science of ecohydrology is an improved understanding of the implications of vegetation change for the water cycle. This paper describes some of the interactions between vegetation and water on rangelands and poses 3 questions that represent high-priority, emerging issues: 1) How do changes in woody plants affect water yield? 2) What are the ecohydrological consequences of invasion by exotic plants? 3) What ecohydrological feedbacks play a role in rangeland degradation processes? To effectively address these questions, we must expand our knowledge of hydrological connectivity and how it changes with scale, accurately identify ''hydrologically sensitive'' areas on the landscape, carry out detailed studies to learn where plants are accessing water, and investigate feedback loops between vegetation and the water cycle.
Article
The Gnangara groundwater system (Gnangara system) is an important source of groundwater for Perth, Western Australia: in the order of 350GL of groundwater is abstracted annually. The Gnangara system also sustains groundwater dependent ecosystems (GDEs), mostly wetlands and native vegetation. Declining groundwater levels across the system have led to impacts on a number of key GDEs. Western Australia’s Department of Water recently prepared a Water Management Plan for the Gnangara system. Allocation limits were reviewed as part of the plan preparation. To assist in reviewing allocation limits, an adaptive Groundwater Level Response Management (GWLRM) methodology was developed and implemented. This paper describes the methodology and its application to the Gnangara system. The methodology was developed to be used as a corrective tool for the short- and medium-term, to assist in achieving long-term sustainability of groundwater management in the context of changing climate and declining groundwater levels. The GWLRM methodology is based on groundwater storage depletion and can be applied to existing allocation limits as an interim tool to assist in making management decisions aimed at recovering groundwater resources. The key to the GWRLM correction is that it will direct water allocation towards sustainable levels on the basis of measured trends. Allocations corrected through application of the GWRLM would therefore represent interim and improved water allocation figures. GWLRM can also identify potential problem areas where the principles or calculations used for long-term sustainable groundwater allocation would need to be reviewed. For the Gnangara system, the calculated storage changes or GWLRM corrections were considered together with results of predictive modelling as part of an expert panel process to derive a more sustainable interim groundwater allocation regime while further research is being completed.
Article
In areas such as parts of Britain where many closely spaced sites have been investigated, between-site diversites prevent any simple correlation between vegetation response and climatic variables. These diversities reveal the influence of other factors in modulating this response. Analysis of profiles of allochthonous lake sediments has provided evidence for the importance of soil factors. The otherwise inexplicable spatial variation in the response of trees, in Britain and neighbouring parts of the European mainland, to the climatic warming (inferred from faunal evidence) at the most recent glacial termination, can be explained by the postulated spatial differentiation of raw skeletal soils. Differences in particle-size, composition and drainage, consistent with the spatial differentiation of pre-arboreal vegetation, would affect the water-retaining capacity of immature late-glacial soils and hence the reproductive success of tree birches. The lag in response of these to climatic amelioration appears to have varied from 500 to 1500 yr, and it seems likely that a similar lag may have characterised the early warming period of each interglacial. The coclusion must be that in this situation the degree of success of trees measured by pollen values is not a reliable indicator of palaeotem-peratures. Secondly, it appears from evidence in northern Scotland that at an ecotone between forest types, the relation between vegetation and climate is likely to be obscured except in regions of uniform soils. In areas of differentiated bedrock, the vegetation pattern is likely to have been influenced most by the effects of soil maturation processes in developing a soil mosaic.
Article
Fossil pollen in sediments documents vegetation responses to climatic changes in the past. Beech (Fagus grandifolia), with animal-dispersed seeds, moved across Lake Michigan or around its southern margin, becoming established in Wisconsin about 1000 years after populations were established in Michigan. Hemlock (Tsuga canadensis), with wind-dispersed seeds, colonized a 50,000 km2 area in northern Michigan between 6000 and 5000 years ago. These tree species extended ranges northward at average rates of 20–25 km per century. To track climatic changes in the future, caused by the greenhouse effect, however, their range limit would need to move northward 100 km per C warming, or about 300 km per century, an order of magnitude faster than range extension in the past. Yet range extension in the future would be less efficient than in the past, because advance disjunct colonies have been extirpated by human disturbance, and because the seed source is reduced due to reductions in tree populations following logging. Many species of trees may not be able to disperse rapidly enough to track climate, and woodland herbs, which have less efficient seed dispersal mechanisms, may be in danger of extinction.
Chapter
Drylands are regions encompassing hyper-arid, arid, semi-arid or sub-humid climatic conditions (see also, Chapter 1). They include cold and warm subtropical deserts, savannas, and the Mediterranean environments. Our focus is here on warm drylands, which are generally characterized by the existence of a well-defined dry season dominated by subtropical high pressure (Malanson, 1993), and a rainy season with average precipitation of less than 700 mm/year. Such regions cover approximately 50% of the continents, with about 20% of the world's population living in these areas (Le Houerou, 1982; Nanson et al., 2002). This explains the growing scientific interest in the study of drylands. Here we focus on the interactions between fluvial geomorphology and riparian vegetation. These interactions act at different spatial and temporal scales, suggesting the existence of an intrinsic and remarkable sensitivity of riparian ecosystems to hydrological and geomorphological modifications. Riparian ecosystems have often been affected by heavy anthropogenic disturbances, with great reductions in spatial extent (up to 80%, as in certain U.S.A. sites) with respect to presettlement times (Smith et al., 1991; Tooth, 2000a,b; Salinas et al., 2000; O'Connor, 2001; Pettit et al., 2001).
Chapter
Drought is a meteorological term which indicates a long period when there is not enough rain for the successful growing of crops or replenishment of water supplies. The expression water stress is frequently used to indicate the complex series of effects that are triggered in plants by drought. The term drought stress is more appropriate to specify when the stress status occurs only over a long period of time. However, because it is often difficult to separate the two phenomena, the definitions of water stress, drought stress and water deficit are frequently used interchangeably. Drought leads to water deficit in the soil and plant tissues, which in turn alters physiological processes and can have ultimate consequences for growth, development and survival of plants. Among the many biochemical and developmental processes that are affected by water stress, decrease of photosynthesis (Bradford and Hsiao, 1982; Flore and Lakso, 1989; Hsiao, 1973), changes in water relations (Brough et al., 1986; Olien and Lakso, 1986), reduction of both cell division and expansion (Hsiao and Acevedo, 1974), abscisic acid (ABA) synthesis (Davies and Zhang, 1991; Zeevaart and Creelman, 1988), and accumulation of sugars (Wang et al., 1995; Wang and Stutte, 1992) play a fundamental role in reducing productivity.
Article
During drought periods, sugar maple (Acer saccharum) demonstrates hydraulic lift; nocturnal uptake of water by roots from deep soil layers that is released from shallow roots into upper soil layers. Using standard water relations methods and stable hydrogen isotope analysis of both source-water and plant-water, I investigated (1) the magnitude and radial extent of hydraulic lift by mature, relatively open-grown trees, of A. saccharum, (2) the proportion of hydraulically-lifted water (HLW) used by shallow-rooted neighbors growing at different distances from target trees, and (3) the influence that this water source had on stomatal conductance to water vapor (g), water balance and growth of these neighbors. Soil water potentials (s) at –20 and –35 cm showed a distinct diel fluctuation. Soil pits dug beneath three mature trees revealed a distinct hard-pan (e.g. fragipan) layer at a depth of approximately 50 cm. Examination of root distributions obtained from soil cores and soil pits revealed that some larger diameter roots (1.9–3.7 cm) did penetrate the fragipan and were established in the ground water table. The presence of the fragipan indicated that the rewetting of the upper soil layer during the night could not be explained by capillary rise from the shallow water table; it was the trees that were taking up ground water and then redepositing it at night into the upper 35 cm of soil, above the fragipan. The greatest fluctuations in s occurred within 2.5 m of trees and only extended out to approximately 5 m. Application of a two-end-member linear mixing model which used stable hydrogen isotopic data obtained from environmental water sources and xylem-sap demonstrated that all neighbors used some fraction (3–60%) of HLW supplied by sugar maple trees. Plants that used a high proportion of HLW (e.g. rhizomatous or stoloniferous perennials) maintained significantly higher leaf water potentials and g, and showed greater aboveground growth when compared with (i) neighbors that used little or no HLW or (ii) conspecifics found growing at distances greater than about 3 m away from maple trees. Three important conclusions can be drawn from the results of this investigation that have not been demonstrated before: (1) hydraulic lift need not only occur in arid or semi-arid environments where chronic water deficits prevail, but can be important in relatively mesic environments when subjected to periodic soil water deficits, (2) that plants neighboring trees which conduct hydraulic lift can use a significant proportion of this water source, and (3) that the HLW source can effectively ameliorate the influence of drought on the performance and growth of neighboring vegetation. The results are also discussed in terms of their influence on plant nutrient relations (including plant-mycorrhizal associations), the nature of plant-plant interactions and the water balance of individuals, communities and floristic regions.
Article
This series of four papers studies the complex dynamics of water-controlled ecosystems from the hydro-ecological point of view [e.g., I. Rodriguez-Iturbe, Water Resour. Res. 36 (1) (2000) 3–9]. After this general outline, the role of climate, soil, and vegetation is modeled in Part II [F. Laio, A. Porporato, L. Ridolfi, I. Rodriguez-Iturbe, Adv. Water Res. 24 (7) (2001) 707–723] to investigate the probabilistic structure of soil moisture dynamics and the water balance. Particular attention is given to the impact of timing and amount of rainfall, plant physiology, and soil properties. From the statistical characterization of the crossing properties of arbitrary levels of soil moisture, Part III develops an expression for vegetation water stress [A. Porporato, F. Laio, L. Ridolfi, I. Rodriguez-Iturbe, Adv. Water Res. 24 (7) (2001) 725–744]. This measure of stress is then employed to quantify the response of plants to soil moisture deficit as well as to infer plant suitability to given environmental conditions and understand some of the reasons for possible coexistence of different species. Detailed applications of these concepts are developed in Part IV [F. Laio, A. Porporato, C.P. Fernandez-Illescas, I. Rodriguez-Iturbe, Adv. Water Res. 24 (7) (2001) 745–762], where we investigate the dynamics of three different water-controlled ecosystems.
Article
To meet conservation goals it is necessary to assess vegetation status and to be able to monitor effects of management and environmental change. In northern Europe grazed grasslands are one of the most threatened habitat in the rural landscape and thus in focus for conservation plans. At present managers use species indicator list to assess past and present management status of grassland and succession stages in particular, as well as effects of the environment. However, these indicators have rarely been scientifically tested.In this study we discuss if plant functional traits may be a key to select suitable indicator species for monitoring land-use change in Swedish rural landscape. The suitability of two possible monitoring tools: (i) plant species selected from functional traits (PFTs) and (ii) indicator species commonly used today to assess grassland management status, were tested along two gradients, a succession gradient and a wetness gradient. We found no association between successional change and plant functional traits, but a response in plant functional traits was found along the wetness gradient. However, the more common “non-scientific” indicator species responded fairly well to the varying gradient categories along both gradients. We believe that there is a need to further validate the ecological mechanisms behind the present-day indicators and to place them in a geographical context.
Article
Consumptive water use from riparian evapotranspiration is a large component of many semiarid basins’ groundwater budgets — comparable in magnitude to mountain front recharge and surface water discharge. In most long-term groundwater studies the amount of water used by phreatophytes is estimated by empirical formulae and extrapolation of measurements taken elsewhere. These approaches are problematic due to the uncertainties regarding the vegetation’s water source (e.g., groundwater or recent precipitation) and its magnitude. Using micrometeorological techniques in this study, surface energy and water fluxes were measured for an annual cycle over two dominant types of vegetation in the riparian floodplain of the San Pedro River in southeastern Arizona. The vegetation communities were a perennial, floodplain sacaton grassland (Sporobolus wrightii) and a tree/shrub grouping composed largely of mesquite (Prosopis velutina). These measurements are compared with estimates from previous studies. Additionally, measurements of soil water content and water table levels are used to infer the dominant sources of the evaporated water. The results indicate that the grassland relied primarily on recent precipitation, while the mesquite obtained water from deeper in the soil profile. Neither appears to be strongly phreatophytic, which suggests that the dominant, natural groundwater withdrawals in the Upper San Pedro Basin are mainly confined to the narrow cottonwood/willow gallery that lines the river.
Article
Water limited ecohydrological systems (WLES), with their broad extent, large stores of global terrestrial carbon, potential for large instantaneous fluxes of carbon and water, sensitivity to environmental changes, and likely global expansion, are particularly important ecohydrological systems. Strong nonlinear responses to environmental variability characterize WLES, and the resulting complexity of system dynamics has challenged research focussed on general understanding and site specific predictions. To address this challenge our synthesis brings together current views of complexity from ecological and hydrological sciences to look towards a framework for understanding ecohydrological systems (in particular WLES) as complex adaptive systems (CAS). This synthesis suggests that WLES have many properties similar to CAS. In addition to exhibiting feedbacks, thresholds, and hysteresis, the functioning of WLES is strongly affected by self-organization of both vertical and horizontal structure across multiple scales. As a CAS, key variables for understanding WLES dynamics are related to their potential for adaptation, resistance to variability, and resilience to state changes. Several essential components of CAS, including potential for adaptation and rapid changes between states, pose challenges for modelling and generating predictions of WLES. Model evaluation and predictable quantities may need to focus more directly on temporal or spatial variance in contrast to mean state values for success at understanding system-level characteristics. How coupled climate and vegetation changes will alter available soil, surface and groundwater supplies, and overall biogeochemistry will reflect how self-organizational ecohydrological processes differentially partition precipitation and overall net metabolic functioning. Copyright © 2011 John Wiley & Sons, Ltd.
Article
The possibility that different species assemblages may represent persistent alternative community states remains largely unexplored by experimental ecologists because of a variety of conceptual and experimental problems. We discuss some of the conceptual roadblocks to experimentation and propose several avenues for attacking the problem experimentally. We address the conceptual issues involved in (1) the blurring of the distinction between the processes that initiate the switch among alternative states and the positive feedback processes that maintain those states, and (2) the role of spatial scale in initiating the switch. We suggest that the switch between alternative states requires, first, a disturbance that removes species involved in the positive feedbacks needed for maintenance and, second, the arrival of other individuals that initiate the switch to the alternative assemblage. The removal of the species that maintain the system must be large enough and over a long enough time to allow the arrival and establishment of members of the alternative assemblage, and so we hypothesize that the switch among alternative states is scale dependent. This scenario suggests that the switch among alternative states can be investigated experimentally through the manipulation of the scale of the disturbance and of the arrival of members of the alternative state. Small-scale disturbances should consistently fail to initiate a switch, while larger-scale events should initiate a switch at least part of the time. We also note that in some cases the scale of disturbance and/or the arrival of recruits cannot be manipulated or controlled and suggest that several approaches other than factorial experiments with ANOVA, such as spatial autocorrelation methods, may be useful. We illustrate the potential and the difficulties of various approaches by discussing two systems in eastern North America that may contain alternative states. Mosaics of mussel beds and algal beds occupy rocky coasts from New England northward, and patchworks of forests and heathlands occur in eastern Canada and in the Appalachian highlands. While the study of alternative states in the marine system can be approached experimentally, the scale of disturbance required to switch forests to heathlands is too large for experimentation and must rely on the use of other approaches.