Structural Development of Vegetation On
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Audet et al. Ecological Processes 2013, 2:20 http://www.ecologicalprocesses.com/content/2/1/20 RESEARCH Open Access Structural development of vegetation on rehabilitated North Stradbroke Island: Above/belowground feedback may facilitate alternative ecological outcomes Patrick Audet*, Amanda Gravina, Vanessa Glenn, Phill McKenna, Helen Vickers, Melina Gillespie and David Mulligan Abstract Introduction: This study depicts broad-scale revegetation patterns following sand mining on North Stradbroke Island, south-eastern Queensland, Australia. Methods: Based on an ecological timeline spanning 4–20 years post-rehabilitation, the structure of these ecosystems (n = 146) was assessed by distinguishing between periods of ‘older’ (pre-1995) and ‘younger’ (post-1995) rehabilitation practices. Results: The general rehabilitation outlook appeared promising, whereby an adequate forest composition and suitable levels of native biodiversity (consisting of mixed-eucalypt communities) were achieved across the majority of rehabilitated sites over a relatively short time. Still, older sites (n = 36) appeared to deviate relative to natural analogues as indicated by their lack of under-storey heath and simplified canopy composition now characterised by mono-dominant black sheoak (Allocasuarina littoralis) reaching up to 60% of the total tree density. These changes coincided with lower soil fertility parameters (e.g., total carbon, total nitrogen, and nutrient holding capacity) leading us to believe that altered growth conditions associated with the initial mining disturbance could have facilitated an opportunistic colonisation by this species. Once established, it is suspected that the black sheoak’s above/belowground ecological behaviour (i.e., relating to its leaf-litter allelopathy and potential for soil-nitrogen fixation) further exacerbated its mono-dominant distribution by inhibiting the development of other native species. Conclusions: Although rehabilitation techniques on-site have undergone refinements to improve site management, our findings support that putative changes in edaphic conditions in combination with the competitive characteristics of some plant species can facilitate conditions leading to alternative ecological outcomes among rehabilitated ecosystems. Based on these outcomes, future studies would benefit from in depth spatio-temporal analyses to verify these mechanisms at finer investigative scales. Keywords: Eucalypts; Mono-dominance; Forest regeneration; Ecological restoration; Mineral sands; Mining * Correspondence: [email protected] Centre for Mined Land Rehabilitation, Sustainable Minerals Institute, The University of Queensland, Brisbane (St. Lucia), QLD 4072, Australia © 2013 Audet et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Audet et al. Ecological Processes 2013, 2:20 Page 2 of 17 http://www.ecologicalprocesses.com/content/2/1/20 Introduction suarina and Acacia spp.) has been documented for reha- Located off the south-eastern coast of Queensland, bilitated coastal sites following mining elsewhere in Australia, North Stradbroke Island is the World’s second Australia (Grant and Loneragan 1999, 2001; Norman largest sand island (~27,500 ha) and the setting for and Grant 2005), these outcomes elicited fundamental wide-spread mining activity over much of the last half- questions as to the overall development of rehabilitated century due to its richness in silica and heavy mineral ecosystems on the Island and, more specifically, the po- ores (e.g., ilmenite, rutile, and zircon; Morley 1981). A tential mechanisms underlying the black sheoak’s weedy requisite corollary of this activity has been the progres- proliferation. sive rehabilitation of its disturbed lands (~15% The reconstruction of essential abiotic landform area footprint) towards the post-mining land-use of elements represents a critical first step in the re- self-sustaining native subtropical mixed-Eucalypt com- habilitation of highly disturbed post-mining ecosys- munities (EMRC 2006). Since 2005, environmental mon- tems (Doley et al. 2012). Thereafter, more subtle itoring has been conducted to document the Island’s environmental drivers appear to be responsible for annual development of rehabilitated forest blocks within the convergence or divergence of rehabilitation out- the Amity, Bayside, Gordon and Ibis mining areas comes relative to desired natural analogues (Grant (Figure 1) by comparing the structure and composition 2006; Suding et al. 2004). These factors include the of revegetated sites in relation to remnant ecosystems. life-history strategies of the species used in revegeta- Despite mostly satisfactory indications of post-rehabilita- tion and their potential for above- and belowground tion development, preliminary field observations noted feedback relationships (Baer et al. 2002; Bonebrake that black sheoak (Allocasuarina littoralis)—a regionally et al. 2011; Kardol and Wardle 2010; Siddique et al. native tree species that was not highly prevalent among 2008). Together, such environmental drivers may rep- any of the reference communities—may be expressing resent key mechanisms underpinning species mono- unexpectedly ‘weedy’ behaviour among some of the dominance (or ‘invasiveness’)atthewhole-ecosystem rehabilitated sites (Gravina et al. 2011). Since similar be- scale (Peh et al. 2011; Richardson et al. 2000; Rout haviour by native woody species (e.g., Alloca- and Callaway 2009; Wardle et al. 2004). Hence, Figure 1 Distribution of long-term monitoring (LTM) plots on North Stradbroke Island. Aerial image (inset) was captured by Consolidated Rutile Ltd. in 2002. Audet et al. Ecological Processes 2013, 2:20 Page 3 of 17 http://www.ecologicalprocesses.com/content/2/1/20 understanding of these relationships within the con- and relatively infertile due to the illuviation of clay, text of post-mining ecosystems would directly benefit organic matter and nutrient components (Isbell 2002; rehabilitation planning and management. Thompson 1981, 1992). To address these aspects, the monitoring record for North Stradbroke Island provides a relevant oppor- Mineral extraction and rehabilitation procedures tunity for determining the structural development of According to Smith and Nichols (2011), the ongoing native vegetation among reconstructed ecosystems. mineral extraction procedures (conducted by Consoli- Based on a depiction of land revegetation spanning a dated Rutile Ltd., now Sibelco Australia Ltd.) have timeline of 4–20 years post-rehabilitation, this ana- generally consisted of clearing all vegetation ahead of lysis seeks to describe broad-scale revegetation pat- theminepathandstrippingthetopsoillayers(0– terns (i.e., whether rehabilitated sites are mostly 20 cm) to enable mining of the underlying mineral converging toward or diverging from natural ana- sands. An artificial dredging pond was then created logues) and stipulate some of the factors potentially within the bare sand dunes to enable mining of ‘wet’ underlying this development. This depiction of eco- sands and extraction of mineral ore concentrates logical rehabilitation is unique insofar that, while the (~1% of the total volume of sand). The rehabilitation mining activity is entirely destructive toward the nat- process consisted of pumping the ‘cleaned’ residual ural ecosystems, the removal of mineral ore from the underlying sand is not detrimental to the reconstruc- Table 1 Rehabilitation monitoring campaigns tion of the essential landforms (Smith and Nichols Site Chronosequence Notable No. Sample Rehab. Plots 2011). In fact, the sand mining process does not pro- Practice duce or expose any toxic or biogeochemically un- Rehab. Monitoring Time Year Year Elapsed stable tailings that could otherwise significantly alter Amity 1987 2007 20 A 2 physicochemical growth conditions within the post- disturbance environment (Brooks 2000). Under these 1992 2010 18 A 12 circumstances, targeting the veritable restoration of Bayside 1987 2007 20 A 2 highly disturbed ecosystems following sand mining 1988 2008 20 A 5 can be achieved within the range of feasible biological 1990 2005 15 A 4 and socioeconomic outcomes—which is not equally 1995 2005 10 B 5 applicable to many mine sites and alternative forms Gordon 1987 2007 20 A 3 of mining (Doley and Audet 2013; Tibbett 2010). 1988 2008 20 A 6 Since the purpose of land rehabilitation on North 1990 2010 20 A 9 Stradbroke Island is the development of a sustainable 1994 2009 15 A 8 ecosystem,a such analyses will provide direct insight when 1998 2008 10 B 3 formulating appropriate environmental management strat- 1999 2009 10 B 17 egies toward the conservation of native species among transitional ecosystems. 2000 2010 10 B 4 1998 2005 7 B 3 Bioregional description Ibis 1998 2008 10 B 3 The Island (located at approximately 27.30°E and 1998 2005 7 B 3 153.25°S) holds a total of 18 regional ecosystems and 1999 2006 7 B 5 7 distinct vegetation assemblages (Stephens and Sharp 2000 2007 7 B 9 2009), which can be grouped into three generic bio- 2001 2008 7 B 8 logical areas: (1) freshwater swamps, (2) beaches and 2001 2005 4 B 8 fore-dunes, and (3) inland high dunes. Representing 2002 2006 4 B 9 the main target of the mining activities, the high 2003 2007 4 B 7 dunes cover >50% of the island and are characterised by primary