Phreatophytic vegetation response to climatic and abstraction-induced groundwater drawdown: Examples of long-term spatial and temporal variability in community response

Ecological Engineering 36 (2010) 1191–1200

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Phreatophytic vegetation response to climatic and abstraction-induced groundwater drawdown: Examples of long-term spatial and temporal variability in community response Ray Froend ∗ , Bea Sommer Centre for Ecosystem Management, Edith Cowan University, 270 Joondalup Drv, Joondalup 6027, WA, Australia

a r t i c l e

i n f o

Article history: Received 20 August 2009 Received in revised form 5 November 2009 Accepted 26 November 2009

Keywords: Phreatophyte Vegetation Groundwater management Climate change Drawdown Monitoring

a b s t r a c t The influence of climatic drought and groundwater abstraction on phreatophytic vegetation dynamics was investigated in the southwest of Western Australia. Two contrasting examples of long-term phreatophytic plant community response to reduced water availability are presented. Multivariate analysis of vegetation and hydrological parameters determined depth to watertable as the dominant biophysical driver of floristic spatial and temporal patterns. Under lower rates of watertable drawdown (9 cm year−1 ), a progressive change in floristic composition was observed over a 33-year period. The abundance of species with a preference for wetter sites was significantly reduced, whereas that of more drought-tolerant species increased. Higher rates of drawdown (50 cm year−1 ) where groundwater abstraction exacerbated climatic drought resulted in a threshold response in vegetation and 33% dissimilarity to pre-abstraction floristics in 12 years. In the context of an ecohydrological state and transition conceptual model, it is suggested higher rates of groundwater drawdown result in a threshold breach and subsequent transition to an alternative ecohydrological state, whilst lower rates result in a progressive floristic transition. © 2009 Elsevier B.V. All rights reserved.

1. Introduction An important consideration in water resource management is the potential impacts of groundwater abstraction on dependent ecosystems. In more recent years the additional impact of climatic drought on groundwater-dependent ecosystems has become of increasing concern. In effect, climatic drought exacerbates groundwater abstraction impacts and there is increasing concern amongst water resource managers and environmental regulators about how groundwater-dependent vegetation will respond over the short and long term. Although there is an increasing understanding of variation in phreatophyte physiological response to ‘natural’ seasonal fluctuations in groundwater levels (reviewed in Naumburg et al., 2005), questions still remain about the integrated effects of natural and anthropogenic alterations in groundwater regime on the composition and health of phreatophytic plant communities. Groundwater drawdown is of obvious importance to phreatophytic vegetation as reduction of water tables may sever these plants from one of their key water sources. However, there are few accounts of long-term alteration of native phreatophytic vegetation

∗ Corresponding author. Fax: +61 8 63045509. E-mail address: [email protected] (R. Froend). 0925-8574/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2009.11.029

attributed to the interaction of climatic drought and exploitation of groundwater. The majority of previous studies relate short-term response to the direct influence of abstraction in phreatophytic riparian vegetation from the arid and semi-arid western regions of western North America accessing alluvial aquifers (Stromberg and Patten, 1990; Stromberg et al., 1992, 1996; Scott et al., 1999, 2000; Shatfroth et al., 2000; Lite and Stromberg, 2005; Naumburg et al., 2005). The broader ecohydrological range occupied by terrestrial phreatophytes, in regards to groundwater depth, suggests that unlike riparian phreatophytes, not all communities of terrestrial phreatophytes will respond similarly to groundwater declines. At greater depths to groundwater, response may be mediated by soil moisture reserves accessible to the extensive root systems established in the absence of shallow aquifers (Zencich et al., 2002). Results of a study conducted by Shatfroth et al. (2000) suggest the antecedent water table conditions experienced by a plant is also of importance in influencing phreatophyte response to groundwater drawdown. Shatfroth et al. (2000) proposed it is the change in groundwater depth experienced by a tree relative to the previous groundwater regime under which it has established that determines response. For example phreatophytes associated with a formerly shallow, stable groundwater source are likely to be more sensitive to its decline than trees formerly exposed to a more variable groundwater regime.

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Relatively little, however, is understood of how vegetation responds over the long term at the community and/or ecosystem scale, particularly the rate and spatial variability of community compositional change. Research by Scott et al. (1999) and Shatfroth et al. (2000) indicates that the rate, magnitude and duration of water table drawdown determine the short and long-term impacts on dependent vegetation. Rapid declines in groundwater levels and separation from the water table result in the acceleration of impacts and likely threshold responses (Segelquist et al., 1993; Lite and Stromberg, 2005). Gradual reductions, on the other hand, provide greater opportunity for recharge to occur and mitigate the effects of water stress, for plants to adapt (i.e. root growth, physiological adjustments) in the shorter term, and/or for progressive vegetation change to occur over the long term. An opportunity to assess climatic and abstraction impacts on phreatophytic plant communities exists in the southwest of Western Australia. Underlying the northern Swan Coastal Plain (SCP) and near the city of Perth is the Gnangara Groundwater Mound (GM), a shallow aquifer resource that is developed for public and private groundwater abstraction. The Mediterranean climate of the region results in warm, dry summers during which phreatophytes access the shallow water tables to alleviate water stress until break-ofseason rainfall occurs in winter (Zencich et al., 2002). Of primary concern in the region are the effects of groundwater drawdown on plant communities that are dependent on this groundwater. Long-term changes in the hydrology of the GM are likely to be reflected in changes to the plant communities that grow in these areas. To facilitate adaptive management of groundwater resources and conservation of native phreatophytic vegetation, knowledge of vegetation response to changes in groundwater levels is required. As part of the water resource management agency’s compliance with environmental regulations (Yesertener, 2007; Bekesi et al., 2009), monitoring of vegetation response has occurred for up to 40 years at selected locations on the GM. Although previous research has shown that groundwater abstraction and reduced rainfall in recent decades have impacted phreatophytic vegetation on the GM (Groom et al., 2000, 2001; Froend and Drake, 2006), there has yet to be a published account of the long-term variability in phreatophytic plant community responses to varying rates of watertable decline. Ongoing research by the authors using the long-term vegetation monitoring dataset of the GM, has yielded some preliminary results indicating differences in long-term vegetation response relative to rate of drawdown. This paper aims to present two examples of long-term response of phreatophytic vegetation to climatic and abstraction induced groundwater drawdown on the GM. Firstly, an example of progressive vegetation response in which the concept of gradual shifts in indicator species distribution and importance, with increasing depth to groundwater, is assessed. Secondly, an example of a rapid response to acute drawdown will be presented in which the high rate of change in the hydrological habitat has negated the possibility of a gradual vegetation response. Both case studies will contribute to understanding ecohydrological dynamics, especially the rate and nature of vegetation change in response to a drying climate and increased groundwater use.

2. Study site The SCP consists of a series of distinct landforms parallel to the west coast of south-western Australia. It is distinguished from adjoining regions by its geological history, soils, climate and vegetation (Seddon, 1972). The SCP experiences a Mediterranean-type climate with warm to hot, dry summers and mild, wet winters (Gentilli, 1972). Based on the length of the summer dry season, the

Fig. 1. The Gnangara Mound in Western Australia showing the location of the two vegetation monitoring transects ‘P50’ and ‘Neaves’.

climate type of the SCP can be further classified as Warm Mediterranean, with 5–6 dry months between November and April. During this period rainfall is low and evaporation and temperatures are higher than at other times of the year. The rainfall records from the Perth meteorological station show that the long-term (1876–2008) average is 850 mm annually. The records also show that Perth and the GM area (Fig. 1) are experiencing an extended period of below average rainfall. The reduction in mean annual rainfall for the Perth area for the period 1966–2008 (Fig. 2) is almost 14% of the long-term average, resulting in a more recent (1966–2008) annual average of 734 mm. Eighty-five percent of annual rainfall throughout the SCP occurs between May and October with June to August being the wettest months. Rainfall only exceeds evaporation between June and August with most evaporation occurring in the summer months. Winter is the only period when rainfall recharges the soil profile and the groundwater. The SCP overlies the Perth Basin, which consists of sedimentary rock several kilometres thick of Mesozoic and earlier age (McArthur and Bettenay, 1960). Younger formations overlie this sediment and are composed almost entirely of aeolian and alluvial marine deposits up to 100 m thick. These deposits, consisting of unconsolidated sand and limestone with discrete beds of silt and/or clay, were formed in the Late Tertiary and Quaternary period and are referred to as the superficial formations (Allen, 1976). Significant volumes of groundwater pervade both the superficial formations beneath the SCP and the underlying geological formations of the Perth Basin. The aeolian and alluvial deposition of the superficial formations resulted in an undulating sandplain of low relief composed

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Fig. 2. Climate data centred on the Gnangara Groundwater Mound and groundwater levels from the closest bore to the Neaves transect. White line is depth to groundwater.

of a series of distinct units parallel to the present coastline. The major geomorphic unit on which the study sites are located is the Bassendean Dune System (McArthur and Bettenay, 1960) consisting of undulating bands of sand dunes 115,000–429,000 years of age. These deposits, thought to have originated from an era when sea water was less alkaline than at present (Kendrick et al., 1991), have almost completely leached all of the original carbonate material, leaving the soils consisting entirely of quartz sand. The groundwater pervading the superficial formations of the SCP originates mainly from direct rainfall recharge due to the porous nature of the sediments, with a small component being derived from local run-off from the Darling Scarp. Groundwater in the Quaternary formations is referred to as the superficial aquifer (Allen, 1976) and extends unconfined throughout the SCP. The upper surface of the superficial aquifer is the water table, which at any particular position reflects the general topography in the region, the hydraulic conductivity (permeability) of the sediments and the direction of groundwater flow. Of significance to this study is the GM which is the major superficial formation north of Perth on the SCP. The mound is recharged directly by rainfall infiltration and developed as a result of the rate of vertical rainfall infiltration being greater than the rate of horizontal groundwater flow through the aquifer (Davidson, 1995). Horizontal groundwater flow on the mound occurs continuously; however, when little or no recharge occurs (summer–autumn), the rate of horizontal groundwater flow exceeds that of vertical infiltration and the mound begins to subside, resulting in lowered water table levels. This seasonal cycle results in seasonal fluctuations in water table levels (about 0.5–3 m) over the SCP. The water table is at its highest elevation during September–October and lowest during April–May. Evapotranspiration by native vegetation of the SCP is a significant factor in water table decline, particularly during the seasons of low recharge. Regional water balance investigations on the SCP have estimated that over three quarters (77–88%) of the annual infiltrated rainfall is discharged through evapotranspiration by native vegetation, the balance being available for net recharge to the groundwater (Allen, 1976; Farrington et al., 1989, 1990). Current land use on the GM includes conservation of flora and fauna, horticulture, silviculture, urban development and recreation. The metropolitan area of Perth, is unique in Australia in that a large proportion of its total water usage (∼55%) is obtained from groundwater resources (∼70% from the Gnangara Mound) (Loh and Coghlan, 2003) which have been used to supply domestic, industry and horticultural water requirements since the early 1960s (Davidson, 1995). More recently, it has been recognised that this resource also sustains a range of important ecosystems including large areas of native woodlands. Where climatic drought

and land and water management activities have reduced water availability through lowering both local and regional watertables, this has contributed to incidences of loss, contraction, or severe ecological responses of some of the dependent ecosystems (Water Authority of Western Australia, 1992; Groom et al., 2000). The most severe event occurred in late summer of 1991 where large-scale tree mortalities occurred both close to public and private groundwater abstraction activities (notably at a site known as ‘P50’, see Fig. 1) (Groom et al., 2000). The plants most severely affected were overstorey Banksia species with mortalities of over 70% recorded in some areas on the Gnangara mound (Water Authority of Western Australia, 1992). These threshold responses occurred after 2 years of significantly below average rainfall and high summer temperatures in 1991. The difference in recharge between 1990/91 and a normal rainfall year was over 1.2 m. Although low recharge was considered to be a key factor, threshold responses were more extensive where abstraction exacerbated reductions in groundwater levels. More progressive impacts of groundwater decline on overstorey species have also been recorded (Groom et al., 2001). In particular, decreases in abundance and distribution of sensitive overstorey species that fringe wetlands or inhabit low-lying areas. Swan Coastal Plain vegetation characteristics are strongly related to physical factors such as landform, soil type and climate conditions (Havel, 1968; Speck, 1952). Vegetation of the Bassendean Dune system is adapted to the nutritionally impoverished soils, with the gently undulating topography dictating community structure and floristic composition through its influence on soil moisture availability (Beard, 1990; Havel, 1968). The major vegetation type formed across much of the Bassendean Dune System is low woodland or forest dominated by the proteaceous tree Banksia. Structure of the vegetation ranges from a low open forest to an open woodland and low, open woodland. The woodland has a diverse understorey 1–1.5 m in height consisting mainly of sclerophyllous shrubs from the families Myrtaceae, Proteaceae, Epacridaceae, Papilionaceae and Dilleniaceae. 3. Methods 3.1. Long-term vegetation monitoring dataset Analysis of long-term response of phreatophytic vegetation to climatic and abstraction induced groundwater drawdown is only possible with consistent monitoring over a number of decades. Transects were initially established on the GM to assess the use of native species as indicators of suitable areas for growing pine

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Fig. 3. Layout of a vegetation transect. This sequence of plots continues until the end of the transect. The shaded area represents the area used for the Neaves transect to represent a single sampling unit. For P50, entire transect data were used.

plantations (Havel, 1968), and were positioned where both existing pine plantation and relatively undisturbed native vegetation occurred side by side in a topographical gradient. The potential for groundwater abstraction impacts on vegetation was recognised and in 1976 the Havel transects were incorporated into regular vegetation monitoring on the GM (Heddle, 1980) as they provided pre-abstraction floristic data, with some transects in close proximity (<2 km) to groundwater abstraction bores whilst others were over 25 km from the nearest production bore. From 1976, transects were monitored every 2–3 years during September and October (spring). All transects occur within conservation reserves or on crown land and were positioned along a topographical gradient, starting at a localized depression and ending at a high point in the landscape, usually a dune crest. Transects varied from 200 to 520 m in length, and were subdivided into two (20 m wide) parallel lines down the length of the gradient (Fig. 3). Each line was further subdivided in 20 m × 20 m plots for overstorey assessment; within each of these were two 4 m × 4 m quadrats used to monitor the understorey. For overstorey and understorey plots/quadrats, species composition, foliage cover and abundance were recorded. The two monitoring transects analysed in this paper, Neaves and P50, represent contrasting vegetation responses to altered water availability. The sites are within 10 km of each other (Fig. 1)

and have experienced the same extended period of below average rainfall however they differ in their proximity to groundwater abstraction bores. The P50 transect starts 50 m from a production bore and extends to over 300 m away, whereas the Neaves transect is over 1 km from the nearest production bore. The Neaves transect was selected as an example of long-term, progressive response of vegetation where the influence of groundwater abstraction is moderate, relative to P50. The P50 transect represents a threshold response to a higher rate of groundwater drawdown due to the proximity of a production bore. Both sites include similar plant assemblages within transects although the P50 transect has a more subdued topography with (initially) shallower water tables and therefore is dominated by a single vegetation type. Monitoring commenced at Neaves and P50 in 1976 and 1988, respectively. Relationships between the watertable surface and topography were determined by hand-augering to saturated soil at 30 m intervals along the length of each transect (Fig. 3). Groundwater levels from monitoring bores in the vicinity of the transects were then related to water table levels at the time of sampling to allow extrapolation of historic groundwater data levels along each transect. Historic water table records were taken from the monitoring bore nearest to each transect (within 100 m in both cases).

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3.2. Numerical analysis Prior to numerical analyses, the transect data were divided into discrete plots relative to elevation (and depth to groundwater). Data from the two longitudinal parallel lines that formed each transect were pooled, therefore data from each pair of adjacent 20 m × 20 m plots at the same topographic position (and line distance) were combined to form one 20 m × 40 m plot. Each 20 m × 40 m plot therefore contained overstorey (obtained from combining the two 20 m × 20 m plots) and understorey (obtained from combining four 4 m × 4 m quadrats) data (Fig. 3). Each species present in sampling plots was classified according to four site water preference categories first proposed by Havel (1968). Havel reported that many SCP species inhabiting damplands and swamps were able to tolerate periods of waterlogging and excessive wetness, with species able to tolerate moist (but not waterlogged) sites fringing these depressions. These species have proven to be highly susceptible to non-seasonal decreases in soil moisture availability (Heddle, 1980). These categories include (A) tree and shrub species tolerant of excessive wetness; (B) tree and shrub species of optimum moist sites, but intolerant of extremes in moisture conditions; (C) tree and shrub species with wide tolerance, but with maximum development on dry sites; (D) tree and shrub species without clear cut site preference. For the Neaves transect with multiple, apparent vegetation types, a resemblance matrix (using Bray–Curtis distances) was computed for all plots (10 in total) and all monitoring years (13 years between 1976 and 2008). Species abundance data were first transformed (ln(y + 1)). A principal coordinate analysis (PCoA) was then carried out in order to detect variability in floristic composition relative to depth to groundwater. Indicator species, coded according to the water requirement categories of Havel (1968), were correlated with the first two ordination axes and overlain onto the ordination. The relative significance of floristic changes over time was assessed by considering the proportion of variability in the data set explained by the first two ordination axes. Next, we wished to ascertain to what extent hydrological and climatic factors could explain the variation in species composition over time and space. For this we conducted a canonical redundancy analysis (RDA) on (transformed) species abundance data grouped into three identified vegetation hydro-communities (based on the previous PCoA analysis). As we wished to preserve the Bray–Curtis distances used in the previous analysis, we first computed additional Bray–Curtis dissimilarity matrices (one for each hydro-community) and performed PCoAs on these. The RDAs (now distance-based, or db-RDAs) were performed on 5 climatic and hydrological variables and 5 principal coordinates (which together explained between 83% and 84% of the variation in the PCoAs). Because plotting the scores of the principal coordinates would make no sense, we plotted instead correlations of species abundances with the first two ordination axes. These analyses were performed in XLStat version 2009.2.1 (Addinsoft® ). The effect of groundwater depth on species composition was further evaluated by plotting total number of plants, and total abundance of species of wet sites, moist sites, dry sites and with no preference (categories of indicator species as per Havel, 1968), against the range of depths to groundwater that have occurred between 1976 and 2008. In order to detect floristic change over time at the P50 transect, a PCoA was performed as described for the Neaves transect above; however because this transect is relatively flat with a single dominant community type, analyses were performed on wholetransect data. Raw abundance data from 1988 to 2005 were used

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as this explained a greater proportion of the variability in the PCoA. A bubble plot of groundwater depth overlain onto the PCoA ordination was produced in order to assess its effect on floristic change.

4. Results 4.1. Variability in floristic composition relative to groundwater depth The Neaves transect PCoA results indicate distinct variability in floristic composition relative to landscape position and depth to groundwater (Fig. 4). The floristic variability was evident as 3 communities spatially organised along the elevational gradient represented by the transect. Community A plots at Neaves were associated with down-slope positions on the gradient, whereas Communities B and C plots were found at mid-slope and dune crest positions, respectively. Communities A, B and C represent a floristic series associated with increasing depth to groundwater as inferred by landscape position. All three communities appear to remain floristically distinct in spite of over 3 m of groundwater decline from 1976 to 2008. The down-slope Community A displayed the least variability. The topographical organisation of flora also appears to reflect indicator species habitat preferences as represented by the Havel (1968) species moisture preference classes. Water requirements of strong (r > 0.90 correlation with PCoA F1 axis) indicator species are generally associated with the appropriate community and landscape position (and groundwater depth). Four of the 5 Havel A species with a preference for ‘wet’ sites are associated with the Community A (down-slope and shallower water table) space of the PCoA ordination. Of the species with moist site preferences 6 of the 9 indicator species were associated with the mid-slope Community B. All 6 indicator species with dry site preferences were associated with dune crest Community C. Species classified as having no clear preferences were spread across the F1 axis of the PCoA.

Fig. 4. Principal coordinate analysis (PCoA) ordination of vegetation abundance data of the Neaves transect from 1976 (black symbols) to 2008 (lightest grey-shaded symbols). Symbols are individual plots for each sampling year along the transect. Community A = down-slope, Community B = mid-slope, Community C = dune crest; Havel A = species of wet sites, Havel B = species of moist sites, Havel C = species of dry sites, Havel D = species with no clear preferences. All species (110) were used for the analysis but only indicator species (generally with correlations >0.90 with F1) are shown.

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Fig. 5. Distance-based redundancy analysis (db-RDA) triplots of vegetation abundance data of the Neaves transect from 1976 to 2008. Community and Havel class groups as described for Fig. 4. Species with an occurrence of at least 4 in the individual community types (total of 25, 69 and 81 for types A, B and C, respectively) were used for the analysis but only indicator species (i.e. having significant correlations with F1; see Table 2) are shown. Arrows from coordinates 0.0 to the species have been omitted in order to simplify the chart. Environmental variables: DTGW = 3-year mean depth to groundwater; DR = 3-year mean duration of recharge; RR = 3-year mean recharge rate; rain = 3-year mean total winter rainfall; temp. = 3-year mean maximum summer temperature.

4.2. Floristic response to change in groundwater level: progressive change in community composition A db-RDA analysis of the Neaves transect revealed that throughout 1976–2008 the three communities (A, B and C), representing a spatial hydrological gradient from the shallow water table downslope end of the transect to the deep water table dune crest, remained distinct across the F1 axis (graph not shown). In order to better represent the progressive changes within each community, individual db-RDAs were produced for each community (Fig. 5). These yielded strong multi-temporal floristic patterns associated with both rainfall and groundwater decline. Although the 5 environmental variables were together significantly linearly related to the floristic assemblages in the individual community types (p < 0.005, p < 0.01 and p < 0.01 for Community A, B and C; 999 permutations), only the 3-year mean depth to groundwater (DTGW) contributed significantly to the variation explained by the F1 axis (see also Table 1). Depth to groundwater as a significant driver of

Table 1 db-RDA scores of the environmental variables presented in Fig. 5 for the three Neaves Community types A, B and C. Code

Environmental variable

F1

F2

DTGW RR DR Rain Temp

3-Year mean depth to groundwater 3-Year mean recharge rate 3-Year mean duration of recharge 3-Year mean total winter rainfall 3-Year mean maximum summer temperature

−0.934 0.782 −0.262 0.437 −0.133

−0.130 −0.269 −0.216 −0.323 0.519

difference between landscape position (along the elevational gradient) as well as change over time (drawdown), supports the notion of a floristic pattern of communities along a spatial and temporal hydrological gradient. The arrangement of the monitoring years in Fig. 5 approaches the shape of an inverted arch, a mathematical artefact that is indicative of progressive (floristic) change over time (Legendre and Legendre, 1998). Community A shows a floristic shift from high abundances of predominantly Havel A species with a preference for wet sites in the ‘early years’ when DTGW was shallowest, to low abundances of Havel A type species and higher abundances of Havel B (notably Kunzea ericifolia), C (notably Banksia attenuata) and D type species in the later years when DTGW was greatest (see also Table 2). Likewise, the mid-slope Community B ordination shows that a large number of predominantly Havel B type species (moist site preference) were associated with the early years, while higher abundances of certain Havel C type species were associated with the later years (Table 2). The most obvious feature of the Community C ordination is the comparatively large number of Havel B, C and D type species with high abundances associated with the ‘early years’ and only a few predominantly Havel C type species (including B. attenuata) that have increased in abundances in the later years, suggesting a ‘thinning out’ of the transect. The association between indicator species (Havel classes) abundance and increasing depth to groundwater over time is more clearly represented in Fig. 6. All species classed as having a preference for wet sites (shallow depth to groundwater) decreased in abundance with declining water tables. From an initial 55% of total abundance these species have gradually decreased to 10% asso-

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Table 2 Species shown in Fig. 5 with respective Havel classification and correlations with RDA axes F1 and F2. (bold = at least p < 0.01 on F1, and at least p < 0.05 on F2). Species

Havel class

Astartea fascicularis Calothamnus lateralis Euchilopsis linearis Hypocalymma angustifolium Patersonia umbrosa Pericalymma ellipticum Adenanthos obovatus Chordifex microcodon Kunzea ericifolia Stylidium brunonianum Acacia pulchella Anigozanthos humilis Dasypogon bromeliifolius Gompholobium confertum Melaleuca seriata Phlebocarya ciliata Adenanthos cygnorum Banksia ilicifolia Lyginia barbata Patersonia occidentalis Xanthorrhoea preissii Banksia attenuata Beaufortia elegans Boronia ramosa Calytrix fraseri Conostephium minus Conostephium preissii Dampiera linearis Gompholobium tomentosum Hibbertia racemosa Petrophile linearis Allocasuarina humilis Boronia purdieana Eremaea pauciflora Hibbertia helianthemoides Hypocalymma robustum Jacksonia floribunda Lechenaultia floribunda Leucopogon nutans Leucopogon sprengelioides Hypochaeris glabra Conostephium pendulum Gonocarpus pithyoides Philotheca spicata Bossiaea eriocarpa Calytrix flavescens Conostylis aculeata Hibbertia hypericoides Lepidosperma squamatum

A A A A A A B B B B B B B B B B B B B B B C C C C C C C C C C C C C C C C C C C D D D D D D D D D

Comm. A F1 0.678 0.668 0.776 0.771 0.335 0.815 0.724 0.945 −0.697 −0.083 – – – – – – – – – – – −0.908 – – – – – – – – – – – – – – – – – – −0.119 – – – – – – – –

ciated with a 3 m drawdown over 33 years. Conversely, species of moist sites (Havel B) have increased in abundance during this time. This is accounted for by the aforementioned increase in the importance of K. ericifolia in Community A plots (down-slope) with progressive drawdown of the watertable. Dry site species also increased in abundance with increasing depth to groundwater. There was no significant relationship between drawdown and abundance of species with no clear site preference. Total abundance (all species) decreased with increasing depth to groundwater. 4.3. Floristic response to change in groundwater level: threshold response The PCoA ordination of the P50 transect (Fig. 7) also shows the ‘arch effect’ that typically occurs when community composition changes progressively along an environmental gradient (Legendre and Legendre, 1998). This implies the true dissimilarities between sampling years are not adequately represented in the ordina-

Comm. B F2 −0.231 0.675 0.498 −0.410 −0.629 0.352 −0.197 0.069 −0.114 −0.564 – – – – – – – – – – – −0.138 – – – – – – – – – – – – – – – – – – −0.574 – – – – – – – –

F1 – – 0.718 – – – 0.923 0.911 −0.844 – 0.284 0.753 0.788 0.893 0.785 0.784 – – – – – −0.854 −0.869 −0.838 0.892 0.858 0.805 0.076 −0.868 0.881 0.885 – – – – – – – – – −0.104 0.911 −0.777 0.776 – – – – –

Comm. C F2 – – 0.458 – – – −0.119 0.086 0.268 – 0.714 0.447 −0.299 0.129 −0.472 −0.292 – – – – – −0.200 −0.119 0.274 0.357 −0.202 −0.147 −0.779 −0.218 −0.262 −0.291 – – – – – – – – – −0.723 0.087 0.190 −0.171 – – – – –

F1 – – – – – – – – −0.794 – – 0.883 – – – 0.877 0.815 0.921 0.774 0.786 −0.321 −0.852 – – 0.918 0.963 – 0.880 – – 0.912 0.821 0.804 0.898 0.855 0.898 0.962 −0.788 0.822 0.926 – 0.903 – 0.866 0.767 0.851 0.877 0.870 0.791

F2 – – – – – – – – 0.400 – – −0.025 – – – −0.232 −0.346 0.260 −0.329 −0.247 −0.812 −0.056 – – 0.089 0.017 – −0.226 – – −0.152 0.266 0.213 −0.248 0.259 0.023 0.053 0.260 0.270 0.251 – −0.060 – −0.209 0.480 −0.337 −0.283 −0.351 −0.269

tion space. To address this, the Bray–Curtis dissimilarity matrix between years is shown in Table 3. A marked floristic shift, as represented primarily by the F1 axis, is most pronounced between 1993 and 2004 and is associated with a rapid increase in the depth to watertable (Fig. 8). In the course of 21 years (1988–2005), a 44.1% dissimilarity in floristics has developed, with most of this change (76%) occurring over 12 years (1993–2004) when depth to groundwater increased by 6 m. Interestingly, higher abundance of Havel A (wet), B (moist) and C (dry) species (Beaufortia elegans being the only exception) are strongly correlated with the early phase of this floristic transition whereas Havel D species (no site preference) are most abundant during the later (drier) phase (Fig. 7 and Table 4). 5. Discussion Analysis of the Neaves transect showed a distinct ordering of unique plant communities relative to a spatial gradient in hydrological habitat (as inferred from depth to groundwater). This

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Fig. 7. Principal coordinate analysis (PCoA) ordination of vegetation abundance data of the P50 transect from 1988 to 2005. Havel A = species of wet sites, Havel B = species of moist sites, Havel C = species of dry sites, Havel D = species with no clear preferences. Forty-five species were used for the analysis but only indicator species (generally with correlations >0.90 with F1; see table below) are shown. Principal coordinate axes were scaled (2×) in order to improve the clarity of the graph. The direction of maximum correlation of depth to groundwater (DTGW) with the PCoA axes is shown.

reduction in more susceptible species (Havel A) and increases by more drought-tolerant ones (Havel B and C), than by replacement. Previous work (Shatfroth et al., 2000) on phreatophyte predisposition to drawdown vulnerability would suggest that the communities that developed over shallower depths to a water table (e.g. Community A at Neaves) may be more responsive (alter composition, abundance) to hydrological change relative to those occupying positions higher in the landscape. However, in the analysis presented in Fig. 4, this did not appear to be the case, or was not detectable given the methodology employed. Community A appeared to vary the least; however, the difference in the rate Fig. 6. Neaves transect. Relationship between depth to groundwater (lowest transect mean value for each year) and number of plants, and with groups of indicator species (Havel classes: Havel A = species of wet sites, Havel B = species of moist sites, Havel C = species of dry sites, Havel D = species with no clear preferences).

ecohydrological gradient represents species habitat preferences and is corroborated by the selective distribution of Havel indicator species. One could hypothesise therefore that with a gradual change in the hydrological habitat (water table drawdown) across the spatial gradient, the floristic composition of each community would change, making each less distinct with time. This was observed not to be the case on the Neaves transect. Although there were compositional changes within each community, the difference remained between communities over the 33 years of groundwater drawdown (9 cm year−1 ). Therefore, despite the temporal shift in the hydrological gradient, the majority of the compositional and structural attributes that define each community has remained. Shifts in the floristic character of each community appears to be driven more by a change in species abundance, i.e.

Fig. 8. Bubble plot of the magnitude of depth to groundwater at the P50 transect overlain on the PCoA ordination shown in Fig. 7.

Table 3 Bray–Curtis dissimilarity matrix of the P50 vegetation (raw abundances).

1988 1993 1996 1999 2002 2004 2005

1988

1993

1996

1999

2002

2004

0 0.151 0.211 0.279 0.347 0.398 0.441

0 0.131 0.239 0.277 0.336 0.378

0 0.174 0.220 0.292 0.346

0 0.185 0.276 0.307

0 0.140 0.200

0 0.129

R. Froend, B. Sommer / Ecological Engineering 36 (2010) 1191–1200 Table 4 Species shown in Fig. 7 with respective Havel classification and correlations with PCoA axes F1 and F2. (bold = at least p < 0.001 on F1). The correlation of depth to groundwater with the PCoA axes F1 and F2 are also shown (p < 0.01). Species

Havel class

F1

F2

Hypocalymma angustifolium Regelia ciliata Verticordia drummondii Verticordia nitens Actinotus glomeratus Banksia ilicifolia Leucopogon conostephioides Acacia barbinervis Jacksonia floribunda Banksia menziesii Banksia attenuata Beaufortia elegans Calytrix flavescens Philotheca spicata Hibbertia spicata Nuytsia floribunda Stylidium brunonianum Bossiaea eriocarpa Anigozanthos humilis Comesperma calymega Dampiera linearis Conostylis juncea Gonocarpus cordiger

A A B B B B C C C C C C D D D D D D D D D D D

0.938 0.921 0.953 0.931 0.765 0.609 0.960 0.887 0.814 0.593 0.422 −0.926 0.889 0.857 0.782 −0.696 −0.833 −0.913 −0.971 0.802 −0.768 −0.788 −0.823

0.226 0.252 0.189 −0.104 0.071 0.525 0.268 0.425 0.330 0.699 0.698 −0.234 0.245 0.193 0.518 0.256 0.373 0.220 0.070 0.137 0.489 0.331 0.412

−0.907

−0.256

Environmental variable Depth to groundwater

and magnitude of change between communities may have been masked by within-community variability. db-RDA analysis (Fig. 5) corroborated these findings with all three community types displaying similar rates and magnitudes of transition with time. The progressive change in phreatophytic community composition at Neaves is clearly associated with a gradual increase in depth to groundwater. In contrast, the rapid change in the hydrological habitat at the P50 transect (water table drawdown of up to 50 cm year−1 ) is associated with a marked floristic response. It appears the higher rate of change in water availability due to groundwater abstraction at this site has not permitted a gradual change in abundance of vulnerable species (Havel A species) only. Decreases in abundance with increasing groundwater depth were noted for all indicator species (Havel A, B and C) except those with no obvious hydrological habitat preference, and has resulted in a marked decrease in floristic similarity in a relatively short period of time. It is apparent that this shift in the ecohydrological state of the P50 site is largely via a threshold response of the vegetation, i.e. mass mortality of a high proportion of all species present (Groom et al., 2000) followed by a recovery characterised by an increased abundance of facultative, xerophytic species and decreased abundance of drought-susceptible species. The contrasting nature of phreatophytic vegetation response to different rates of groundwater drawdown depicted in the examples presented in this study, support the suggestion made by Scott et al. (1999) and Shatfroth et al. (2000) that the rate, magnitude and duration of water table drawdown determine impacts on dependent vegetation. The decrease in total abundance of plants with increasing depth to water table/rainfall reduction observed at the Neaves transect may represent a gradual shift in the carrying capacity of the hydrological resources available to vegetation. Although analysis of change in foliage cover/transpirational area per unit area is needed, the trend in decreasing abundance may represent thinning due to a shifting ecohydrological equilibrium as hypothesised by Eagleson (1982) and Rodriguez-Iturbe et al. (2001). In both examples presented here, Neaves and P50, the change in ecohydrological state over time is driven by both climatic drought

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and groundwater abstraction. At Neaves the lower rate of drawdown is largely a reflection of greater distance from production bores and therefore the corresponding change in floristics has been gradual. In contrast, the higher rate of drawdown at P50, induced by close proximity to a production bore, has led to a threshold response resulting in an elevated rate of vegetation change and greater dissimilarity in composition relative to the pre-abstraction vegetation. It is expected that in the absence of groundwater abstraction, the pathway of vegetation response at P50 would have been similar to Community A at Neaves. Indeed, with the restoration of shallower groundwater levels, a gradual increase in Havel A and B species abundance may reverse the observed floristic dissimilarity relative to pre-abstraction vegetation states. However, despite the potential to manage groundwater abstraction to prevent threshold responses or to allow rehabilitation, continuation of the trend to lower rainfall in the southwest of Western Australia (Timbal, 2004) suggests reversal of the floristic trends observed at both P50 and Neaves is unlikely in the foreseeable future. On the basis of observed floristic and hydrologic change at P50, it could be argued the site has breached a threshold (high rate of water table decline) and undergone a transition to an alternative ecohydrological state. Re-establishment and recovery of vegetation at the P50 site reflects a different hydrological state with a deeper water table. One can hypothesise that this alternative ecohydrological state may be less vulnerable to further watertable declines given the shift in floristics towards more drought-tolerance species. Conversely, the Neaves site has not breached a threshold (or one that is defined by mass mortality) but has undergone a progressive floristic transition in response to a lower rate of water table decline. Such a model would recognise the transient conditions in which the vegetation does not persist but develops to another persistent state, with transitions occurring once a threshold has been overcome, and progressing either rapidly or more gradually depending on the disturbance and management regimes (Westoby et al., 1989; George et al., 1992). Management thresholds, in this case an excessive rate of groundwater decline, can be considered a tipping point where past this threshold, management intervention to restore the original community type may be extremely difficult or in fact no longer possible (Hobbs, 1994). The consideration of state and transition models for groundwater dependent ecosystems may provide the theoretical underpinning for the development of a framework that will enable environmental water requirements to be applied to these systems. This will assist the management of these systems through the recognition of changing states and transitions and the appropriate management procedures in the face of changing disturbance regimes, particularly through the effects of a changing climate. Acknowledgements The authors wish to acknowledge the support of the Western Australian Department of Water (DoW) for their financial support of ongoing research on this topic. Support was also provided through Australian Research Council Linkage Project LP0669240 and the Water Corporation of Western Australia. Access to the long-term vegetation monitoring dataset was granted by the DoW and vegetation monitoring during the period 1976–2008 was conducted by Mattiske Consulting Pty Ltd. References Allen, A.D., 1976. The Swan Coastal Plain: hydrogeology of superficial formations. In: Carbon, B.A. (Ed.), Groundwater Resources of the Swan Coastal Plain. Commonwealth Scientific Industrial and Research Organisation, Perth, Australia, Perth, pp. 12–33. Beard, J.S., 1990. Plant life of Western Australia. Kangaroo Press, Kenthurst.

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