Long-term conditioning of soil by plantation eucalypts and pines does not affect growth of the native jarrah tree
Introduction
Plant–soil feedbacks (PSF) can influence the restoration of native forests where soil has been conditioned by non-native species for long periods of time (i.e. agricultural, forestry activities, invader species; Kulmatiski et al., 2006, Jordan et al., 2008, Kulmatiski and Beard, 2008). Specifically, non-native species can condition soil properties in ways that lead to reduced growth of native species (van Grunsven et al., 2007, Perkins and Nowak, 2012, Suding et al., 2013), due to alterations of nutrient cycling processes (Bezemer et al., 2006, Kulmatiski et al., 2006, Kardol and Wardle, 2010, Weidenhamer and Callaway, 2010), imbalance of microbial communities (i.e. free-living organisms, plant-pathogens and root symbionts; Inderjit and van der Putten (2010)), and negative effects of toxic allelochemicals (Bertin et al., 2003, Badri and Vivanco, 2009, Inderjit and van der Putten, 2010, Weidenhamer and Callaway, 2010). Plant-conditioned properties of soil can persist for long periods of time even after the removal of the non-native species (Kardol and Wardle, 2010).
Soil conditioning operates through two mechanisms: (1) leaf litter deposition, and (2) root deposition. In particular, litter quality is a species-specific plant trait (i.e. C:N ratio, lignin and phenolic content) that directly influence the chemical and biological properties of soil. Elements released from litter during decomposition alter the concentrations of soil macro and micronutrients in a species-specific manner (Hobbie, 1992, Guo and Sims, 1999, Berg and McClaugherty, 2008, Witt and Setälä, 2010, Uselman et al., 2012). As well, litter deposition drives changes in the microbial and microfauna (i.e. nematode) community, including its biomass, composition, and physiological properties (Berg and McClaugherty, 2008). Generally, plant species that produce labile litter (i.e. low C:N ratio) such as legumes, promote a microbial community dominated by bacteria and their consumers (i.e. protozoa, bacterial-feeding nematodes). In contrast, plant species that produce recalcitrant litter (i.e. high C:N ratio) such as pines and eucalypts promote a microbial community dominated by fungi and their consumers (i.e. fungal-feeding nematodes, collembolans, oribatid mites; Moore and Hunt, 1988, Scheu et al., 2005, Moore et al., 2007, Holtkamp et al., 2008, Witt and Setälä, 2010). Thus, in this manner soil conditioning occurs via differences in leaf litter quality.
Bauxite has been mined in the jarrah forest of south-west Western Australia since 1964. Initial attempts to restore mine sites post-bauxite mining consisted of commercial plantations of fast-growing, exotic softwoods, mainly pines (Pinus radiata) and cypresses (Cupressus sp.), as well as eastern Australian hardwood eucalypts such as Sydney blue gum (Eucalyptus saligna; Koch, 2007a, Koch, 2007b). To date there are 3600 ha of plantations of non-native species within the jarrah forest ecoregion that will be harvested for timber in the near future and then restored to native jarrah forest (Conservation Commission of Western Australia, 2012). At these sites non-native trees have been conditioning the soil for 35 years, causing species-specific changes to soil properties. Here, we tested species-specific conditioning of soil by pines (P. radiata), Sydney blue gums (E. saligna; both non-native, plantation trees), and jarrah (Eucalyptus marginata; dominant native tree). Was expected that the conditioning process of soil by the three plant species would be primarily driven by litter deposition, thus influenced by litter characteristics of each species in particular. Pines and eucalypts both produce recalcitrant litter; nonetheless, differences in conditioning processes occur between the two genera associated with changes in the soil chemistry and biology owing to differences in litter quality.
Conditioning of soil by pines via litter deposition includes acidification of soil, alteration of mineral concentrations such as N, K, Mg, and Ca, and low microbial activity (Feller, 1983, Lavelle and Spain, 2003, Ehrenfeld et al., 2005). In contrast, allelopathy has been described as a key mechanism driving soil conditioning by eucalypts. Litter extracts from eucalypts inhibit the germination of a diversity of plant species as well as the growth of microorganisms such as nitrifying bacteria (Bezuidenhout and Laing, 2006, Kohli et al., 2006). There is the potential for pines and eucalypts to condition soils differently but no tests of the influence of the conditioned soils on plant growth, and in particular how conditioning of forest soil by non-native species will influence the growth of native species such as jarrah via PSF are available. Jarrah displays a high degree of growth plasticity in response to resource availability and the soil environment (Bleby et al., 2009). This plasticity suggests that jarrah is capable of producing constant biomass even under variable soil conditions.
The aims of this research were to determine what differences (if any) occurred in the soils after 35 years of conditioning by non-native species, and to determine the effect of conditioning on the subsequent growth of jarrah (E. marginata) seedlings (i.e. the net effect of the PSF). We predicted that the plant species (pine, jarrah and blue gum) would condition the soil in a species-specific manner (Aponte et al., 2013), however, differences in soil conditioning would be more marked after pine than eucalypt growth since the former presents litter with composition that is different from that of the eucalypts. In contrast, differences in conditioning between blue gums and jarrah would be less distinct due to similar litter quality given that both belong to the same genus. The second specific aim was to test the net PSF associated with jarrah plants grown in soils conditioned by non-native plants by comparing their growth in soils conditioned by jarrah. We predicted that species-specific conditioning of soil by non-native plants would not affect the biomass produced by jarrah seedlings (i.e. a neutral PSF would be observed).
Section snippets
Sampling site and experimental design
Soils for the experiment were collected from a site that had been cleared of jarrah forest, mined for bauxite, and then planted with diverse native and non-native eucalypt species, pines and cypresses to create an arboretum in 1976 (i.e. 35 years prior to sampling). Soil samples were collected in June 2011. At the site, six pines (P. radiata: Pinaceae), six jarrah (E. marginata: Myrtaceae) and six blue gums (E. saligna: Myrtaceae) were selected at random and four intact soil cores (0.785 L;
Properties of soils conditioned in the field by pines, jarrah, and blue gums
The physico-chemical properties of soils conditioned in the field were significantly different according to plant species (F2,26 = 2.83, P = 0.003; Fig. 1a). Pair-wise comparisons showed that the physico-chemical properties of soils conditioned by jarrah differed from those conditioned by blue gums (t = 1.74, P = 0.031), and pines (t = 1.91, P = 0.034), and that soils conditioned by blue gums and pines were not significantly different from each other (t = 1.36, P = 0.06). The biological properties of field
Discussion
Conditioning of soil properties was observed in field soils collected beneath pines, jarrah and blue gums. A first prediction was that both eucalypt species would similarly condition the properties of soil and that these species would condition soils differently from pines. The results supported the prediction for the biological properties (i.e. CLPPs, numbers of fungal-feeding nematodes, omnivorous nematodes, and nematode channel ratio), these soils were similarly conditioned by both eucalypt
Conclusion
Non-native plant species can condition soils in ways that differ to native species, which has important implications for ecosystem functioning. This is critical for the restoration of human-modified ecosystems, since the ultimate goal of most restoration activities is a complete functional ecosystem. In restored jarrah forest post-bauxite mining, despite alterations to soil properties are caused by non-native species, jarrah seedlings were able to establish in these conditioned soils, without
Acknowledgements
We thank The Mexican National Council for Science and Technology (CONACYT), John Koch (ALCOA World Alumina Australia) and Tim Morald (University of Western Australia – UWA) for their support and help with field work, Michael Smirke (UWA) for his technical support during laboratory work, Deborah Lin, Khalil Kariman and Alonso Calvo Araya (UWA) for their help during the experiment, Vivien Vanstone, Sarah Collins and staff (DAFWA) for help with nematode extraction as well as Jackie Nobbs (SARDI)
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