Effects of soil water potential on germination of co-dominant Brigalow species: Implications for rehabilitation of water-limited ecosystems in the Brigalow Belt bioregion

Ecological Engineering 70 (2014) 35–42

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Effects of soil water potential on germination of co-dominant Brigalow species: Implications for rehabilitation of water-limited ecosystems in the Brigalow Belt bioregion Sven Arnold a,∗ , Yolana Kailichova a , Jürgen Knauer b , Alexander D. Ruthsatz c , Thomas Baumgartl a a

Centre for Mined Land Rehabilitation, The University of Queensland, Brisbane 4072, Australia Department of Geography, Ludwig-Maximilians-Universität München, Munich 80333, Germany c Department of Earth Sciences, Freie Universität Berlin, Berlin 12249, Germany b

a r t i c l e

i n f o

Article history: Received 4 November 2013 Received in revised form 13 April 2014 Accepted 19 April 2014 Keywords: Ecohydrology Hydropedology Soil water potential Seed germination Ecosystem rehabilitation Ecosystem reclamation Brigalow Belt bioregion

a b s t r a c t Rehabilitation of disturbed and degraded land is a critical legal and ecological requirement to achieve stable and non-polluted ecosystems. In some semi-arid climates, such as the Brigalow Belt Bioregion in Eastern Australia, extensive areas have been affected by open-cut mining. With erratic rainfall patterns and clayey soils, the Brigalow Belt is a unique biome which is representative of other water-limited ecosystems worldwide. Direct seeding and native plant germination on post-mining land may be an effective and economically viable solution to re-establishing plant communities. Germination is governed by the amount of water the seed can imbibe, which is a function of the soil water potential and hydraulic soil properties rather than soil water content. The question remains how soil water potential triggers germination of Brigalow Belt plant species. We used six replicates of 50 seeds of three co-dominant native species from the Brigalow Belt Bioregion – Eucalyptus cambadgeana, Eucalyptus populnea, and Casuarina cristata – to investigate germination in relation to water potential as environmental stressor. Solutions of polyethylene glycol (PEG 6000) were applied to expose seeds to nine osmotic water potentials ranging from soil water saturation (0 kPa) and field capacity (−10–30 kPa) to the permanent wilting point (−1500 kPa). We measured germinability (number of germinated seeds relative to total number of seeds per lot) and mean germination time (mean time required for maximum germination of a seed lot) to quantify germination. Further, we employed Hydrus-1D to simulate daily values of soil water potential and water content at 1 mm depth of a Black Vertosol under the climatic conditions of Emerald in Central Queensland. While E. cambadgeana demonstrated the fastest and most prolific germination across the widest range of water conditions with germination being observed even at water potentials as low as −1500 kPa, E. populnea and C. cristata required wetter conditions over longer periods to achieve maximum germination rates. For the 11 years of meteorological data examined in this study, soil water conditions were most favourable for germination of E. cambadgeana; this was primarily due to its high tolerance of water deficit in soils of lighter texture than that of the denser Black Vertosols. In contrast, for the seed-soil combination of C. cristata and Black Vertosol, modelled climatic conditions proved favourable to triggering germination only 25% of the time. The empirical data on seed germination can be used to parameterise and calibrate germination models. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Successful rehabilitation of post-mining land is a requirement for best practice land use and is often regulated through

∗ Corresponding author. Tel.: +61 7 3346 4190. E-mail address: [email protected] (S. Arnold). http://dx.doi.org/10.1016/j.ecoleng.2014.04.015 0925-8574/© 2014 Elsevier B.V. All rights reserved.

legislation. Mining activity, especially large scale open-cut mining, dramatically impacts almost every component of the landscape, particularly landform, hydrology, and ecosystem structure. Ecosystem rehabilitation is an iterative process of progressive refinement of concepts and empirical data. The grey box modelling approach (Arnold et al., 2012c, 2013) is the cornerstone to achieve landscape-scale restoration (Menz et al., 2013) and to develop effective and measurable criteria of restoration

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S. Arnold et al. / Ecological Engineering 70 (2014) 35–42

Fig. 1. Location of the Brigalow Belt bioregion and Emerald.

success (Hobbs and Harris, 2001). This is particularly critical where water is scarce or erratic (Rodriguez-Iturbe and Proporato, 2004). Within water-limited ecosystems, climate and soil properties set the boundaries of water availability for plant development. Soil water balance then plays a pivotal role for ecosystem rehabilitation (Arnold et al., 2013). Large areas worldwide are affected by mining activities, such as the open-cut mines in the Bowen Basin in Eastern Australia. Located within the Bowen Basin (Fig. 1), the Brigalow Belt is the predominant bioregion and is globally unique (Hutchinson et al., 2005). This bioregion is representative of other water-limited ecosystems, which are similarly impacted by anthropogenic interventions such as agricultural land use and mining activities. Its name is derived from prevalent stands of Acacia harpophylla F. Muell. ex

Benth. (aka. Brigalow) (Johnson, 1980), typically associated with Eucalyptus cambageana Maiden (aka. Dawson gum), Eucalyptus populnea F. Muell. (aka. Poplar box), or Casuarina cristata Miq. (aka. Belah) as well as several other co-dominant species associated with semi-evergreen vine thickets (Dwyer et al., 2009; Johnson, 1984). Some of these species (e.g., Acacia harpohylla and C. cristata) are able to reproduce vegetatively through suckering or sprouting of basal shoots from parent roots to recover from severe damage to the aboveground parts (Dwyer et al., 2009, 2010; Fensham and Guymer, 2009; Johnson, 1964; Ngugi et al., 2011; Reichman et al., 2006; Seabrook et al., 2006). While A. harpophylla is by far the dominant species, co-dominant species may play a critical role in water balance regulation, particularly with respect to evaporation from bare soil (Arnold et al., 2012c).

S. Arnold et al. / Ecological Engineering 70 (2014) 35–42

This interplay between climate and hydropedology (Gunn, 1984) is driven by erratic rainfall patterns of short and intense storm events during summer and low rainfall during winter (Cowie et al., 2007; Lloyd, 1984). The predominant clayey Vertosols and Dermosols (Isbell, 2002) (equivalent to Vertisols and Lixisols, respectively (Rees et al., 2010; WRB I.W.G, 2006)) have a high water holding capacity, relatively high fertility, and moderately high levels of salt (Bui and Henderson, 2003). Based on these physical and chemical attributes, only a small proportion of the stored soil water is usually available to plants (Gunn, 1984; Isbell, 2002). Since the 1950s, the natural Brigalow Belt has contracted significantly following decades of land clearing, initially for agriculture and subsequently for coal mining (Arnold et al., 2012a; Lindenmayer and Burgmann, 2005; Reichman et al., 2006). The latter also involved substantial disturbance and deterioration of native soils—the critical component for rehabilitation of water-limited ecosystems (Arnold et al., 2013). Consequently, the bioregion has been listed as endangered under both state (Vegetation Management Act 1999) and national legislation (Environment Protection and Biodiversity Conservation Act 1999) (DEWHA, 2010). The legislative requirement to rehabilitate disturbed land to create stable, safe and non-polluted ecosystems (DRET, 2006; Grant, 2006; Tisdell, 1998) requires land managers and planners to consider native Brigalow plant communities for rehabilitation of post-mining areas. Native ecosystems have inherent adaptive mechanisms against environmental stressors such as water-limited conditions (Bowen et al., 2007; Fensham and Guymer, 2009; Lamb et al., 2005), and they provide crucial habitat for native fauna (McAlpine et al., 2002; Reichman et al., 2006). Re-establishment of vegetation typically comprises passive regeneration as well as transplanting seedlings and mature stands (Musselman et al., 2012), tube stocking, or direct seeding. Direct seeding may be an effective and economically viable method to reestablish Brigalow plant communities (Engel and Parrotta, 2001; Lamb et al., 2005; Reichman et al., 2006), but this can be risky in water-limited environments. It is therefore crucial to understand the soil water conditions that trigger germination and survival of Brigalow Belt species. In this regard, germination is governed by the amount of water the seed can imbibe, which is a function of the soil water potential and other soil hydraulic properties (Bradford, 2002; Evans and Etherington, 1990; Williams and Shaykewich, 1971) such as soil texture, structure, compaction, and organic matter. Although the critical hydration level of seeds that triggers germination is species-specific (Bell, 1999; Hadas and Russo, 1974a, 1974b; Hunter and Erickson, 1952), germination ceases abruptly for many species if soil water potential drops below −1500 kPa (e.g., Evans and Etherington, 1990; Masin et al., 2005; Norsworthy and Oliveira, 2007). The complex relations of the seed-soil interface underpin the critical role physical soil reconstruction plays for germination at post-mining areas and under erratic rainfall conditions. Therefore, the objective of this study was to quantify the germination response to soil water potential of three representative co-dominant tree species from the Brigalow Belt (Eucalyptus cambadgeana, E. populnea, and C. cristata). For the germination trials two metrics were quantified (germinability, mean germination time), which are critical for the parameterisation of ecohydrological models and ecosystem rehabilitation at post-mining areas. Based on weather observations from Emerald in central Queensland we evaluated the suitability of selected soils to provide water potentials that allow germination of the three species of interest. We then outlined the implications for post-mining land rehabilitation in the Brigalow Belt Bioregion.

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2. Materials and methods We used seeds of three co-dominant native species from the Brigalow Belt Bioregion – E. cambadgeana, E. populnea, and C. Cristata – to investigate germination in relation to water potential as environmental factor. Six replicates of 50 seeds per treatment (i.e., water potential) were placed at equal distance on an absorbent substrate (Wettex® ). Materials (e.g., tweezers, Wettex® , glassware) were autoclaved for 20 min at 121 ◦ C and preparation of treatments and monitoring took place in a laminar flow cabinet. All treatments and replicates were then placed randomly within a germination cabinet under constant temperature (25 ◦ C) and a 12 h day and night cycle. Seeds were removed from petri dishes once a perceptible radicle emerged. The experiment ceased 5 days after no germination occurred. No pre-treatment of seeds was required for any of the selected species to break dormancy (Schmidt, 2000; Turnbull and Martensz, 1982). Prior to the experiment, we X-rayed three replicates of 50 seeds for each species to estimate the physical condition of seeds (Gustafsson and Simak, 1963), which indicates their germination potential. The digital X-ray machine was used with an exposure time of 6.5 s, with 28 kV effect. All batches of seeds were considered to be equally viable or had equal germinability potential.

2.1. Water potential For controlled experiments on seed germination the osmotic water potential can be used to simulate soil matric potential (Mcwilliam and Phillips, 1971). For these purposes we used solutions of polyethylene glycol (PEG 6000) to expose seeds to nine osmotic water potentials between soil saturation and the classical permanent wilting point: 0, −10, −30, −100, −250, −500, −750, −1000, and −1500 kPa. The empirical equation derived by Michel and Kaufmann (1973) and revised by (Wood et al., 1993) was used to establish the required water potential ( in kPa): = (6.3 10−5 T − 0.02196)O2.2357 ,

(1)

where T is the temperature in K (here 298.15 K), and O denotes the osmolality in g 1000 g−1 of water. Solutions measuring 15 ml of PEG 6000 were added to the seeds on the Wettex® substrate within a 90 mm petri dish and wrapped with Parafilm® to minimise evaporation. The relationship between water potential and water content of soils is highly non-linear and a function of the soil specific water retention curve (Van Genuchten, 1980). These curves can be used to determine the water potential of selected soils at given depth, rainfall, and potential evaporation. We employed the water flow and solute transport model Hydrus-1D (Simunek et al., 2005) to simulate daily values of soil water potential and water content at 1 mm depth of a Black Vertosol (equivalent to Vertisol according to Rees et al. (2010)). We obtained historical rainfall and evaporation data for Emerald Airport (Bureau of Meteorology station No. 035264) (Fig. 1) for the period from February 1996 to August 2007 (Bureau of Meteorology, 2013) (Fig. 2). The van Genuchten parameters of the Black Vertosol in Emerald were based on the APSoil database (http://www.apsim.info/wiki/APSoil.ashx). For comparison, we also considered further three soil types (based on the internal Hydrus-1D database): clay loam, silt loam, and loam. Detailed information regarding soil hydraulic parameters and model configuration in Hydrus-1D is provided in Table 1 and Supplement A, respectively (Fig. 4).

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Fig. 2. Observations of (a) rainfall depth and (b) potential evaporation at Emerald from February 1996 to August 2007 (Bureau of Meteorology, 2013).

Table 1 Hydraulic parameters of selected soils. Soil parameter −3

 r (m m )  s (m3 m−3 ) ˛ (mm−1 ) n (−) Ks (mm d−1 ) 3

with a probit scale) and various transformations (arcsine, square root transformations) but these either did not fit the data or failed homogeneity of variance and normal distribution of error (p > 0.05).

Black vertosol

Clay loam

Silt loam

Loam

0.062 0.434 0.001 1.29 48

0.095 0.41 0.0019 1.31 62.4

0.067 0.45 0.002 1.41 108

0.078 0.43 0.0036 1.56 249.6

2.2. Measuring germination For this study, we used germinability G (%) and mean germination time t¯ (days) as measurements to quantify germination (Ranal and Santana, 2006). While G simply represents the number of germinated seeds (g) relative to the total number of seeds per replicate (n): G=

g  n

100,

3. Results For all species germinability G was highest at water potential corresponding to saturated soil water conditions (0 kPa) and G decreased significantly with decreasing water potential (p < 10−4 ) (Fig. 3). However, maximum G values were variable across the three species: 89% for E. cambadgeana, 57% for E. populnea, and 12% for C. cristata. Remarkably, for E. cambadgeana germination was still

(2)

t¯ denotes the mean length of time required for maximum germination of a seed lot (Czabator, 1962):

k t¯ =

gt i=1 i i

k

g i=1 i

,

(3)

where ti is the time elapsed from initiation of the experiment to the ith observation day until germination ceases on the kth day. We applied a generalised logistic regression model (GLM) on a logit scale and simple linear regression for G and t¯ , respectively, to test the significance of the relationship between germinated seeds or time required for maximum germination and decreasing water potential. Assumptions were tested via diagnostic plots, which showed no violation of homogeneity of variance and normal distribution for the residuals. Separate GLM models were adopted for each species as this provided the best fit for the data. We also employed other models (linear regression and logistic regression

Fig. 3. Germinability G of selected co-dominant species from the Brigalow Belt in relation to the water potential . Error bars indicate the standard deviation across 6 replicates of 50 seeds.

S. Arnold et al. / Ecological Engineering 70 (2014) 35–42

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Fig. 4. Soil water content  of selected soils in relation to the water potential . The van Genuchten parameters of the water retention curves are based on Hydrus-1D (Simunek et al., 2005) and the APSoil database (http://www.apsim.info/wiki/APSoil.ashx). The grey lines illustrate the soil water content of a Black Vertosol with a soil depth of 1 m at water potentials of −1500, −1000, and −750 kPa, respectively.

observed at a water potential as low as −1500 kPa (permanent wilting point), whereas E. populnea and C. cristata did not germinate at water potentials below −1000 kPa and −750 kPa, respectively. For a Black Vertosol with a soil depth of 1 m, these values correspond to water amounts of 149 mm, 160 mm, and 168 mm, respectively (Fig. 4). More importantly for restoration efforts, the mean time required for maximum germination (Fig. 5) was lowest for E. cambadgeana: for water potentials above −250 kPa, t¯ was below 5 days and then increased significantly (p = .005) with decreasing water potential. Likewise, for E. populnea and C. cristata, t¯ increased significantly for water potentials below −250 kPa (p = 10−5 and

Fig. 5. Mean germination time t¯ of selected co-dominant species from the Brigalow Belt in relation to the water potential . Error bars indicate the standard deviation across six replicates of 50 seeds.

.05, respectively) and t¯ was significantly greater than for E. cambadgeana, ranging between 7 and 11 days. The cumulative distribution and associated exceedance probability of soil water potential at 1 mm depth based on long term average daily observations of rainfall and potential evaporation in Emerald (Fig. 2) are plotted in Fig. 6. For clay loam, silt loam, and loam, the soil water potential did not drop below −1201 kPa, −909 kPa, and −354 kPa, which would mean the probability of exceeding soil water potentials of −1500 kPa was 100%. For the Black Vertosol, a soil water potential of −1500 kPa was exceeded only for 68% of the simulated time period. Likewise, the exceedance

Fig. 6. (a) Cumulative distribution and (b) exceedance probability of simulated soil water potential for selected soils (see water retention curve in Fig. 4) in 1 mm depth based on daily observations of rainfall and potential evaporation from February 1996 to August 2007. Simulations of soil water potential are based on Hydrus-1D (Simunek et al., 2005).

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probability was 39% and 25% for soil water potentials of −1000 kPa and −750 kPa, respectively. 4. Discussion 4.1. Ecohydrology and hydropedology under water limitation The seed germination trials revealed a contrasting response to water-limited conditions across three selected co-dominant species of the Brigalow Belt Bioregion. E. cambadgeana demonstrated the fastest and most prolific germination across the widest range of water conditions with germination being observed even at water potentials as low as −1500 kPa (Fig. 3). Similar results were found for the A. harpophylla (Brigalow)—the pre-dominant species in the Brigalow Belt Bioregion (Arnold et al., 2014). By contrast, E. populnea and C. cristata required wetter conditions (Fig. 3) over longer periods to achieve maximum germination rates (Fig. 5). These distinct germination responses are most likely regulated by ecophysiological adaptations rather than dormancy mechanisms (Schmidt, 2000), i.e., germination only proceeds if seed hydration achieves some species-specific level (Bell, 1999). At plant community scale, the interplay between biosphere and hydrosphere (i.e., ecohydrology (Li et al., 2012)), as well as interactions between pedosphere and hydrosphere (i.e., hydropedology (Li et al., 2012)) are critical for the structure and function of ecosystems (Arnold et al., 2013; Lin, 2003; Lin et al., 2006a, 2006b). Therefore, the soil water retention characteristics of a typical soil in the Brigalow Belt – the Black Vertosol in Emerald (Fig. 4) – quantify the amount of soil water required to meet the threshold values of water potential above which germination is triggered. During the period of 11 years considered in this study, soil water conditions were most favourable for germination of E. cambadgeana (due to its high tolerance of water deficit) at soils with clay content lower than that of Black Vertosol (due to higher values of soil water potential). By contrast, for the seed–soil combination of C. cristata and Black Vertosol only 25% of the time period was favourable to trigger germination. Indeed, C. cristata tend to be more prevalent on sandy and loamy horizons (Gunn, 1984) of the southern area, whereas E. cambadgeana predominates the central and northern region of the Brigalow Belt (Johnson, 1984). However, as with other Casuarina species, C. cristata can also propagate vegetatively by root suckers (Chesterfield and Parsons, 1985; Husain and Ponnuswamy, 1980), which might compensate for the lower germinability during periods of water deficit, when E. cambadgeana is the stronger competitor. Water stress tolerant species are more likely those that require less canopy cover and thus may be ideal pioneer species for revegetation (Daws et al., 2002). Given the obligation of reconstructing stable and sustainable ecosystems (Grant, 2006), developing knowledge of species-specific germination thresholds has critical implications for timing and conditions of seeding. 4.2. Implications for post-mining land rehabilitation This study provides implications for the physical restoration of soils, where soil texture and depth denote critical soil attributes that translate atmospheric water availability (primarily rain) into seed available soil water (Audet et al., 2012, 2013; Hutchinson et al., 2005). Therefore, species requiring high values of water potential, i.e., moist conditions (e.g., E. populnea and C. cristata), benefit from soils with high water holding capacity, which can be encouraged by high clay contents, deep soils, or amendment with organic matter (Karhu et al., 2011; Lax et al., 1994; Leu et al., 2010; Mbagwu, 1989; Metzger and Yaron, 1987; Zebarth et al., 1999), hydrogels (Agaba

et al., 2010), or biopolymers (Patil et al., 2011). Thus, it is highly recommended that seeds be mixed with topsoil (no deeper than twice the depth of the seed’s diameter (Dalton, 1993)), which is also advantageous to mitigating evaporation, predation, and the inhibitory processes of light (Dalton, 1993; Scott et al., 1984). However, as shown in this study, seeds which require high values of water potential, also typically require this water exposure over prolonged periods (Fig. 5). Therefore, rehabilitation plans for post-mining land would certainly benefit from examining the optimal timing of seeding for species of interest. Apart from historic climatic conditions and possible future climate scenarios (Hughes, 2003) these plans should also consider soil properties such as depth and texture. The empirical findings of this study are critical within the grey box modelling framework, which seeks to integrate empirical data across a wide range of investigative scales (Arnold et al., 2013). For example, the species-specific relations of soil water potential and germination can be used to calibrate the hydrotime model (Bradford, 2002; Gummerson, 1986) to simulate initial ecosystem development at post-mining areas such as those located in the Brigalow Belt bioregion (Arnold et al., 2012a, 2013). In conclusion, given these findings among the three selected codominant species, E. cambadgeana may be the strongest competitor for A. harpophylla (Brigalow) with respect to re-establishment on post-mining lands. This conclusion still needs verification by ecohydrological model simulation and/or control-comparison studies of field and glasshouse experiments (Arnold et al., 2012b, 2013). The latter is even more critical with regard to environmental conditions that facilitate the subsequent stages of seedlings, such as availability of water, nutrients, and light. Acknowledgements This work was kindly supported by the Postdoctoral Fellowship Scheme and the Early Career Research Grant of The University of Queensland to S.A., and by the German Academic Exchange Service (DAAD) to A.D.R. and J.K. The authors would like to thank Steve Adkins and David Doley for constructive comments on the experimental design and data analysis. We also thank the two anonymous reviewers for constructive suggestions to improve the manuscript. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ecoleng. 2014.04.015. References Agaba, H., Baguma Orikiriza, L.J., Osoto Esegu, J.F., Obua, J., Kabasa, J.D., Hüttermann, A., 2010. Effects of hydrogel amendment to different soils on plant available water and survival of trees under drought conditions. Clean: Soil, Air, Water 38, 328–335, doi: 10.1002/clen.200900245. Arnold, S., Audet, P., Doley, D., Baumgartl, T., 2013. Hydropedology and ecohydrology of the Brigalow Belt, Australia: opportunities for ecosystem rehabilitation in semi-arid environments. Vadose Zone J., doi: 10.2136/vzj2013.03.00. Arnold, S., Kailichova, Y., Baumgartl, T., 2014. Germination of Acacia harpophylla (Brigalow) seeds in relation to soil water potential: implications for rehabilitation of a threatened ecosystem. PeerJ 2, e268, doi: 10.7717/peerj.268. Arnold, S., Knauer, H., Baiquni, H., Baumgartl, T., 2012a. Effect of water potential on germination of seeds in ecosystems restoration, Brigalow Belt, Queensland, Australia. In: Burkitt, L.L., Sparrow, L.A. (Eds.), Proceedings of the Fifth Joint Australian and New Zealand Soil Science Conference: Soil Solutions for Diverse Landscapes. Hobart. Australian Society of Soil Science Inc., pp. 43–46. Arnold, S., Lechner, A., Baumgartl, T., 2012b. Merging modelling and experimental approaches to advance ecohydrological system understanding. In: Webb, A.A., Bonell, M., Bren, L., Lane, P.N.J., McGuire, D., Neary, D.G., Nettles, J., Scott, D.F., Stednick, J.D., Wang, Y. (Eds.), Revisiting Experimental

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