Woodland trees modulate soil resources and conserve fungal diversity in fragmented landscapes
Introduction
Resource islands are a popular model for many semiarid and arid ecosystems (Ewing et al., 2007). These biogeochemical ‘hotspots' are often associated with woody vegetation in a herbaceous matrix, and are the result of plant nutrient accessions in throughfall, litter and roots, and of physical interception of wind- and water-borne material (Belnap et al., 2005, van der Valk and Warner, 2009). Relative to the herbaceous inter-patches, resource islands have an ameliorated microclimate and have greater stores of soil water and nutrients; in turn, these conditions favour plant growth and promote feedbacks between plant and soil (Ludwig et al., 2005). Indeed, it is thought that the nature and strength of these feedbacks ‘define the structure and function of arid ecosystems' (Belnap et al., 2005).
While patchiness of plants and soil resources in dry environments has been well documented, there has been little examination of the associated patterns in soil biota. A few studies have found that soil microorganisms ‘follow’ resources and are more abundant and more active in vegetated patches than inter-patches (Herman et al., 1995, Camargo-Ricalde and Dhillion, 2003, Ewing et al., 2007, Goberna et al., 2007). However, with the exception of some recent studies (e.g. Ewing et al., 2007, Orlando et al., 2007), the extent to which patch/inter-patch differences in microbial numbers also represent differences in microbial community composition remains largely unexamined in dry environments (Herman et al., 1995). This gap limits our knowledge of critical linkages between plants and soil biota, and of the likely environmental impacts of human-induced change, such as vegetation clearing, in semiarid and arid ecosystems (Wardle et al., 2004).
In addition to patch-scale factors, spatial patterning of soil microbial populations is influenced by population processes such as reproduction and dispersal (Ettema and Wardle, 2002). A long-held assumption has been that microbes have cosmopolitan distributions due to their large populations, short generation times, and capacity for long-distance dispersal (Green and Bohannan, 2006). However, recent studies based on molecular approaches provide strong evidence for spatial scaling of soil microbial diversity (Green and Bohannan, 2006, Zhou et al., 2008). For example, two studies measured medium-scale (i.e. <2 km) genetic structure in ectomycorrhizal fungi, which in both cases was related to limitations in spore dispersal (Peay et al., 2007, Carriconde et al., 2008). These findings support predictions that fungal populations will be particularly susceptible to broad-scale habitat loss due to less efficient dispersal and colonising abilities than bacteria (Hedlund et al., 2004), and lend support to the notion that conserving fungal genetic diversity requires conservation of host habitats over their entire geographic range (Gehring et al., 1998).
Recent studies have also indicated a dominant role for fungi in key processes within dryland soils. For example, Collins et al. (2008) presented evidence for fungal involvement in decomposition, nitrogen transformations, and nutrient translocation between plants and biological soil crusts in semiarid grasslands of New Mexico, USA. They argued that fungi assume particular functional importance in dryland soils because they can metabolize at higher temperatures and lower water potentials than bacteria (Collins et al., 2008). Molecular analyses of fungal communities in these grasslands indicated high functional diversity, with roots of a dominant grass colonised by at least 10 different orders, including endophytic, mycorrhizal, saprophytic, coprophilous, and plant pathogenic fungi (Porras-Alfaro et al., 2008). This high fungal diversity was largely attributed to ecosystem heterogeneity resulting from the above-described patchiness of plant and soil resources in dry environments.
We assessed soil fungal community composition in open-woodland remnants of semiarid north-western Victoria, Australia. These savannah-like systems are dominated by the evergreen tree ‘buloke’ (Allocasuarina luehmannii (R.T. Baker) L.A.S. Johnson), and are habitat for several endangered and threatened floral and faunal species. Nonetheless, they are subject to a serious ongoing threat from clearing for agriculture (Maron and Fitzsimons, 2007). We used the DNA-based fingerprinting technique Terminal Restriction Fragment Length Polymorphism (T-RFLP) for its benefits over other DNA-based techniques and its strength in comparative analyses (Thies, 2007). Multivariate ordination techniques were used to compare patterns in soil fungal community composition both within (canopy versus non-canopy) and between remnants, and to examine relationships with sample location and a range of soil properties. Our specific hypotheses were: (1) individual buloke trees would form resource islands within a broader herbaceous matrix; (2) buloke ‘islands' would support distinct soil fungal communities; and (3) soil fungal community composition would be distinguished on the basis of remnant location. More broadly, we aimed to describe the role of remnant trees in conserving soil fungal diversity and to highlight potentially unseen consequences of further tree loss in highly fragmented landscapes.
Section snippets
Study sites
Study sites were two remnant open-woodlands (Specht, 1981) located within a cropping and sheep station near Ninyeunook in north-western Victoria, south-eastern Australia (36°0′S, 143°24′E). The landform is flat to mildly undulating Quaternary alluvial plains of low elevation (<100 m above sea level). The soils have not been classified in detail but are broadly classified as red duplex soils or ‘Sodosols', with a strong texture contrast between the A horizon and sodic B horizon (Isbell, 2002).
Soil physicochemical properties
Canopy soils had significantly lower bulk density and significantly greater total C, total N, and available P than inter-canopy and disturbed soils of the same remnant (Table 1). In addition, canopy soils were moister and more acidic than inter-canopy but not disturbed soils (Table 1). Electrical conductivity (EC) was significantly greater in canopy than associated inter-canopy and disturbed soils of the south-east remnant, but was similar in canopy and disturbed soils of the north-west remnant
Buloke trees formed resource islands
We found clear evidence to support our first hypothesis that individual buloke trees form resource islands within a broader herbaceous matrix. Threefold concentrations of total C and total N in canopy versus inter-canopy soils (Table 1) are consistent with ratios in dryland woody systems both in Australia (Facelli and Brock, 2000, Tongway and Hindley, 2004), and elsewhere (Ewing et al., 2007, Goberna et al., 2007). The strength of the resource island effect reflected the high cover of canopy
Acknowledgements
This study was supported by The University of Melbourne and the North Central Catchment Management Authority (NCCMA) through a University of Melbourne Collaborative Research Grant. LTB and SK also acknowledge ongoing support from the Victorian Government Department of Sustainability and Environment. Special thanks to Geoff Park and Malory Weston (NCCMA) for support with study establishment, Gerd Bossinger for access to laboratory facilities, Trevor Meers for field assistance, and the Ninyeunook
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