Plant Soil (2006) 289:103–121 DOI 10.1007/s11104-006-9075-6
ORIGINAL PAPER
Evidence of active biotic influences in pedogenetic processes. Case studies from semiarid ecosystems of south-west Western Australia
W. H. Verboom Æ J. S. Pate
Received: 7 February 2006 / Accepted: 22 June 2006 / Published online: 26 October 2006 Springer Science+Business Media B.V. 2006
Abstract Soil profiles and rooting morphologies contemporary lateral facies changes and verti- were examined under an ecotone where open cally-stacked paleosol formations in the study woodland of multi-stemmed, small, lignotuberous region provided corroborating evidence of similar eucalypts (mallee) graded into proteaceous heath. profile attributes, including presence of Fe- or Soils under the mallee showed a Solonetz-type Si-lined root channels, overprinting phenomena seal which separated, hydrologically, the upper and consistency in occurrences of ferricrete and acidic horizon of bleached sand from lower calcrete as expected of each class of vegetation. alkaline horizons rich in calcrete, silcrete, finely Observations were related to the concepts of divided carbonates and clay. Seal composition bioengineering of soil profiles through activity of appeared to vary consistently with overlying macroflora and associated micro-organisms as set species of mallee. The generally acidic lateritic out more generally in our companion review. profiles under heath were rich in pisolithic ferri- cretes and displayed Fe-coated root channels. Keywords Ecotone Æ Lateral facies changes Æ Both sets of taxa exhibited dimorphic rooting Laterite Æ Niche-building Æ Solonetz Æ Woody patterns, with ectomycorrhizal roots and seal-pe- plants netrating, second-order tap roots developed on the extensive lateral roots of mallee versus a dominance of primary tap roots and cluster root Introduction development on laterals of Proteaceae. Over- printing of ferricrete by clays and silicified mate- It is increasingly apparent that organisms of all rial was evident where mallee appeared to have taxonomic groupings engage in niche-building invaded areas of heath. Examination of other activities which not only support their own and each others survival but also determine the trajectory of their evolution and that of the ecosystems in which they occur (see Day et al. W. H. Verboom (&) Department of Agriculture and Food, 10 Doney 2003). In a companion review of the literature Street, Narrogin, WA 6312, Australia (Verboom and Pate, 2006) we support the e-mail: [email protected] hypothesis that higher plants and their associated micro-organisms carry an inherent capacity to J. S. Pate Æ W. H. Verboom School of Plant Biology, The University of Western actively modify the physical and chemical Australia, Crawley, WA 6009, Australia characteristics of their rooting environments. We
123 104 Plant Soil (2006) 289:103–121 use the term ‘bioengineering’ to describe one of the above communities to the other, and such processes and introduce the concept of a relate certain soil profile characteristics to pat- ‘phytotarium’ to connote the major role players terns of root distribution of woody macroflora involved and the collective outcomes of their occupying end members and intervening zones of functioning in an ecosystem. the ecotone. Secondly, we use this background When first pursuing the above line of thinking, and earlier work to describe and interpret we found evidence (Pate et al. 2001; Verboom and selected examples of lateral and vertical facies Galloway 2000) of a role of cluster-root bearing changes in soil profiles encountered in the region. taxa and microbes in the laying down of ferric Some of the facies described embody contempo- layers in oligotrophic soil profiles, and suggested rary differentiation of direct relevance to what that such activities had probably been in place for we describe for our ecotone, others comprise millions of years and been continuously associated paleosols incorporating vertically-stacked chro- with and driven by plant-based acquisition and nosequences representing successive epochs of cycling of limited resources of phosphorous. pedogenetic activity of the same or several dif- Ranging more widely in our companion review ferent types. We discuss the information obtained (Verboom and Pate, 2006), we further fledged this against the broader background of bioengineering hypothesis by using a number of examples of bio- of soil profiles and the ‘phytotarium’ concept. engineering principles at work and their overall effects on contemporary ecosystem functioning. Dominant woody species and associated micro- organisms were then regarded as prime architects Materials and methods for fashioning the autochthonous elements of regoliths, with an end result likely to optimize Location of study areas command and effective utilisation of nutrients and water. Figure 1 provides a map pinpointing the location We recently combined airborne radiometric of the principal study site (E) at which an ecotone and ground-based surveys in a south-west between proteaceous and myrtaceous vegetation Australian setting to uncover complex dynamical was examined. The transect used in this investi- patterns between vegetation types across land- gation is close to Lake King (WGS 84 starting S scapes, with sharp breaks clearly evident between 33 12.12¢ E 119 48.09¢ and ending S 33 11.81¢ E communities and soils (Verboom and Pate 2003). 119 47.77¢) and traverses a gently undulating In particular, our examinations of soil landscapes upland within a large (866 ha), near-pristine and vegetation in semiarid environments of the patch of native vegetation typical of the region. mallee and wheatbelt regions demonstrated par- Figure 1 also gives locations of the two lateral ticularly great contrasts in pedogenetic processes facies changes (Belka (B) and Merredin (M)) and between heathlands dominated by Proteaceae a vertical facies change (Kalannie (K)) which are and Casuarinaceae on acidic lateritic sandplains, described in the second part of the paper. Major and myrtaceous woodlands on alkaline texture- bioclimatic and laterite zones are given for the contrast (duplex-type) soils (see vegetation types study region. defined and listed by Beard 1984, 1990). These contrasting ecosystems feature prominently in Examination of the ecotone this paper. The objectives of this paper are twofold. The ecotone lies in a climatic region classified as Firstly, we set out to examine in detail the Extra Dry Mediterranean (see Beard 1984). The edaphic features and associated formative effects mean annual rainfall at the site is 343 mm and the of functioning of myrtaceous versus proteaceous mean diurnal temperature ranges from 3 to16 C communities in a contemporary setting. We con- in mid-winter and 12 to 30 C in mid-summer. Two centrate particularly on lateral changes displayed thirds of the annual rain is typically received across a soil profile where an ecotone grades from between May and September.
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Fig. 1 Large-scale map of the south-west region of Western Australia, indicating locations of study sites. Site E locates the ecotone grading between mallee and proteaceous heath near Lake King examined in detail in the first section of text. Sites B and M locate lateral facies changes examined at Belka and Merredin, respectively. Site K locates a profile at Kalannie where several vertical facies changes were investigated. Major bioclimatic and lateritic zones are delineated
An area of 30 km2 surrounding the ecotone Upland soil of the study region consists mostly was examined using aerial photography draped of sandy upper layers overlying either ferricretes over a relatively high-resolution digital elevation set in an acid sandy matrix or calcretes set in an model (DEM) of the same area (see Caccetta alkaline loamy matrix. The former is typically et al. 2000 for DEM specifications). This was vegetated by kwongan-type proteaceous open combined with GPS-based ground verification to woodlands or shrub-heath, the latter by mallee distinguish types of native bush, cleared land and and associated myrtaceous taxa (see Beard 1984, other features. 1990). Other intergrading areas of mixed vegeta- A total magnetic intensity map overlain on the tion and soil type were found to be scattered same DEM provided information on relative size throughout the area. Inventories of species were and positioning of playa lakes, associated hydro- made across the area by comparing tentative field aeolian structures and topographical expression identifications of specimens against voucher of certain bed rock structures and outcroppings in specimens lodged in herbaria. Names of species the region. and authorities were as listed by Green (1985), Complementary information on relative abun- with recent species naming within the Melaleuca dances of elements (thorium (Th), uranium (U) uncinata complex made using the descriptions of and potassium (K)) in surface soils was obtained Craven et al. (2004). Classification of soil types by airborne radiometric survey at a flying height and descriptions of their properties were as given of 60 m and a line spacing of 400 m (UTS Geo- by the FAO (1998), McArthur (1991) and Stace physics Pty Ltd. 2005). Data were interpreted, et al. (1968). essentially as recently described for similar land- Water retention characteristics of samples of scapes of the region by Verboom and Pate (2003). pisolithic, blocky and reticulate ferricrete from
123 106 Plant Soil (2006) 289:103–121 the region were determined by courtesy of Broad scale excavations such as conducted Professor Graham Aylemore at the University of above, deployed the nozzle of the air-spade at an Western Australia. Our study also amplified the angle of 45 and held 10 cm or so away from the earlier work of Nulsen et al. (1986), on the fate of soil surface. However, by directing the air stream rainfall under mallee, by examining the water vertically downwards and closer to the substrate it relations across a texture-contrast profile in early proved possible to slowly pierce specific parts of a summer, using measurements of water contents seal in the upper B horizon and determine whe- and corresponding matric potentials. ther a specific tap root had successfully penetrated The 750 m long, 15 m wide transect selected down into the less compacted substrate below. for study included opposing end members of ei- ther pure mallee woodland or pure proteaceous Observations of lateral and vertical facies shrub heath and an intervening region of mixed changes in exposures vegetation. Pit excavations, supplemented by deep augering were conducted at nine selected Using our field experience of contemporary soils, stations along the transect to provide general and information from Retallack (2001) for information on soil layering and rooting archi- recognition of paleosol formations, we photo- tectures of principal species. Listings of taxa and graphed and described morphological features of their extents of relative dominance were made for two major exposures of lateral facies traversing each of the stations. ecotones (B & M, Fig. 1) and one involving a Soil profiles were also examined using an air- vertically-stacked paleosol sequence (K, Fig. 1) spade (model 150 scfm/90 psig, Air-Spade Tech- exposed during the silcrete investigations of nology, Verona, PA, USA), activated by an Bennett et al. (2005). Rooting morphologies of Ingersoll-Rand compressor supplying air at 3 –1 extant taxa were described and possible bioengi- 0.8 m s at a pressure of 0.6 Mpa (see Nadezh- neering effects discussed. dina and Cermak 2003). When deployed on dry soil profiles during summer, the air spade readily blew away loose sand particles and dead fine-root material from upper sandy horizons, thereby Results revealing the architecture of residual networks of major lateral roots of adjacent trees and shrubs. Study of the ecotone between mallee Exposing some of these lateral roots along a and proteaceous shrub-heath complete length of up to 9 m, it proved possible to assess patterns of production and positionings of Figure 2 presents information on topography, laterally-extending feeder and vertically- vegetation, soils and underlying geological struc- descending sinker (tap) roots. Air spade excava- tures of the general area in which our study tions below 0.5 m proved difficult in lateritic transect (E) is located. profiles under proteaceous vegetation, due to The DEM-aligned aerial photograph (Fig. 2a ferricrete gravel falling back into excavations. with a 20 fold vertical exaggeration) depicts a low Similar excavations in texture-contrast soils under relief landscape with areas of remnant vegetation, myrtaceous vegetation terminated abruptly on cleared land, major granitic outcrop (OC) and reaching a dense intractable B horizon. some of the larger salty playa lakes (SL) typical of By extending such excavations over areas of the region. The lunettes (LN) radiating out from a several square metres, we were able to examine large (9 km2) playa lake in the bottom left hand the surface topographies of whole regions of the (south-western) corner of the image are inter- exposed ‘sodic’ B horizon under mallees and thus preted as being formed from material blown out of determine patterns of growth and penetration by the lake during hydro-aeolian activity of the sinker roots as they descended vertically down- Pleistocene (see Bowler 1976). The yellow line (E) wards from an overlying platform of parent lat- marks the transect traversing remnant vegetation eral roots. typical of the region.
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Fig. 2 Characteristics of the general area near Lake King at which the 750 m transect of the ecotone (E) between proteaceous and myrtaceous woodland was located. (a) relief photograph showing distribution of remnant vegetation and cleared land; locations of principal granite outcrops (OC) and salty playa lakes (SL) are indicated. A series of lunettes (LN) radiate out from the largest playa lake. (b) image of DEM aligned total magnetic intensity showing structure of underlying geology. (c) ternary radiometric image of soils draped onto corresponding digital elevation model
The relief total magnetic intensity image outcrop, red ‘fresh’ granitic sands and black (Fig. 2b) of the granitic terrain underlying the denotes oligotrophic sands. ecotone indicates a complex pattern of deforma- It should be noted that the captured gamma tion along a predominantly NW to SW axis, rays on which the images are based derive prin- running parallel to the line of the transect. cipally from the top 30 cm of land surface and are However, there is no indication of major geo- therefore of limited diagnostic value where an logical change along or in close proximity to the area is carrying a thick mantle of quartz sand. ecotone (E). Note that the signal recorded for the ecotone The DEM-aligned ternary radiometric image suggests surfacing of ferricrete towards its north- (Fig. 2c) merges maps of potassium, uranium and west where proteaceous heath predominates. thorium into a single element ratio map. As in our Conversely, progressive darkening of the image earlier publication (Verboom and Pate 2003), towards the south-east indicates submergence of intense green colouration depicts high concen- this horizon of ferricrete below a mantle of sand. trations of thorium (Th), intense blue denotes Further south-east down the transect, weaker high concentrations of uranium (U) and intense signalling is interpreted as deriving from clays red indicates zones high in potassium (K). Black blanketed by a sandy layer, that is as expected of locates low concentrations of all three elements texture-contrast soils under mallee. while white records where all are at high con- The transect included end members represen- centration. Highest concentrations of U and Th tative of the two diagnostic soil:vegetation types (brightest blue/green) typically denote exposed for the area, namely mallee-form eucalyptus ferricretes where K is low. Pink denotes granitic woodland (MW) occupying Calcic Solonetzes or
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Sodic Calcisols, and proteaceous shrub-heath of the ecotone are given below, using information (PH) restricted to Petric Plinthosols. The above from soil pits, augerings and air-spade excava- designations are given in accordance with the tions at representative sampling stations. FAO (1998) WRB classification system. When referring generally to proteaceous heath soils we Mallee:Calcic Solonetz association also employ commonly used terms such as later- (Station 1, Fig. 5) itic and laterite. Soil:vegetation units are delineated in the This involved a Calcic Solonetz under Eucalyptus GPS-verified air-photo map of the site (Fig. 3)on flocktoniae (Maiden) Maiden (Fig. 5), and other which simplified boundaries between mixed (MV) mallee eucalypts (see Table 1) with understorey and pure vegetation types (PH, MW) are given, occupying less than 10% ground cover. relative to positioning of the more or less equally The top of the B horizon in the Calcic Solonetz spaced sampling stations along the transect. was located at about 25 cm depth and overlain by Figure 4 provides a schematic representation of a thin, bleached, clay-depleted layer carrying a distributions of vegetation and soil profile char- mildly acidic, structureless A2 horizon. The A1 acteristics along the ecotone. layer above this was also sandy and mildly acidic
The cutaway drawings, Figs. 5–7, provide pic- (pHw values of profile shown in Fig. 5) with a torial representations of principal canopy and weakly developed massive structure. The texture rooting architecture characteristics of vegetation at the top of the B horizon was a sandy loam and arrangement of soil horizons for pure mallee exhibiting strongly developed massive structure. (Fig. 5), proteaceous shrub-heath (Fig. 6) and the No reactive aluminium was detectable in this part mixed vegetation of the mid-region of the ecotone of the profile using the method of Fieldes and (Fig. 7). Listings of major species typical of each Perrott (1966). The B horizon graded down of these three zones are provided in Table 1. through 20 cm to a calcareous, highly alkaline, More detailed accounts of the situation dull brown, clay loam. Hard carbonate segrega- encountered in end-members and the mid-region tions were evident in upper parts of the B hori- zon, particularly in association with roots. Surfaces of these segregations were dull white and spot tested positive for reactive aluminium. The seal at the top of the B horizon was of neutral reaction with its surface bleached and its pores blocked by kaolinitic and micro-crystalline silica deposits (see Brewer et al. 1983). It effec- tively sealed off, hydrologically, all lower parts of the profile from the A horizon. This property is well demonstrated in the very different hydraulic properties of the A and lower calcareous B horizons, see graphs of Fig. 8 showing (a) water content with depth and (b) water held between wilting point and field capacity. Visually-based testings of water permeability showed high transmissivity in both the A and lower B horizons, whereas the sealed top of the B horizon proved to Fig. 3 Plan view showing location of study transect of be only sparingly permeable- an observation ecotone (E) and the sampling stations at which vegetation consistent with water perching in such soils under and soil profiles were examined in native vegetation near agricultural conditions (see Hatton et al. 2002). Lake King. Boundaries between pure mallee woodland Rooting architectures of a seedling or juvenile (MW), intergrading mixed vegetation (MV) and pure proteaceous heath (PH) are given for the surrounding specimen of each dominant species of mallee area, authenticated by ground verification using GPS showed dimorphic characteristic typical of the
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MW MV PH
b
2m
Station 134568297 Number
Fig. 4 Schematic cross-section across the ecotone between proteaceous shrub-heath and mallee woodland showing locations of sampling stations and changes in characteristics growth form (see Wildy and Pate 2002), namely a top of the B horizon in the Calcic Solonetz. lignotuberous root–stock bearing from its sides a Passage of sinkers through the B horizon showed number of superficial laterals (see Fig. 5) and avoidance of hard calcrete. Noticeable ‘paper- from its base one or more dominant sinker (tap) bark-like’ coverings of exfoliating periderm roots. In virtually all adult plants at our site, the developed on roots as they penetrated any form mid-central regions of lignotubers and associated of seal-like layer. primary tap (sinker) roots had rotted away or Our air-spade excavation at Station 1 revealed been destroyed by termites. The resulting central an abrupt junction (dotted line in (Fig. 9a) region in the lignotuber and disintegrated primary between the Calcic Solonetz associated with tap root might then act as a major avenue for Eucalyptus flocktoniae and the Sodic Calcisol percolation of water running down stems after encountered under an adjacent grouping of the rainfall events. At the same time trees which had less common Eucalyptus scyphocalyx F. Muell. ex lost their primary tap root (see Fig. 5) would be Benth. (Maiden & Blakely) (B, Fig. 9a). Sodic forced to rely on the accessory tap roots formed Calcisol incursions into the Calcic Solonetz were on their laterals to abstract water from deep in evident (Fig. 9b), some with root-like morpho- the profile. Consistent with this, some of the logies (Fig. 9c) closely enveloping living roots larger of these sinkers were of two centimetres or (Fig. 9d). more in diameter and descended for several Air spade excavations carried out along the metres down into the profile (see also Wildy and complete 5–9 m lengths of major laterals of Pate 2002). Other smaller sinkers on the same certain mallees confirmed the presence of regu- laterals were much narrower and penetrated to larly-spaced sinker roots of similar or lesser lesser depth, albeit well through the seal at the diameter to the laterals on which they were
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borne. Measurements of widths of major laterals along their 5–8 m lengths showed little attenua- tion in diameter in proximal regions bearing sin- ker roots, but a noticeable decrease in diameter distally from the outermost sinker to the end of its parent lateral root. Our excavations during summer bore no evi- dence of live fine-root material nor of mycorrhizal investments in the fully desiccated A horizon, yet thicker parts of horizontally-extending fine later- als were alive and well hydrated. Roots of this kind were subsequently shown to initiate dense new seasonal sets of mycorrhizal feeding roots once the A horizon had rewetted with the onset of autumn and winter rain.
Shrub-heath: laterite association (Station 9 Fig. 6)
Vegetation of this association comprised a diverse assemblage of typical shrub-heath species domi- nated by Proteaceae (see Table 1). The soil pro- file exhibited a structureless, grey, sandy A horizon of variable thickness. The sesquioxide- Fig. 5 Cutaway drawing illustrating the relationship rich B layer consisted of pisolitic nodular and between rooting architecture and the arrangement of soil horizons under the mallee Eucalyptus flocktoniae at conglomeritic ferricrete in a sandy matrix sited Station 1. Numbers within horizons refer to pH. Symbols above an iron-rich reticulite layer (Fig. 6). The for profile layers as given in Fig. 4 latter overlay, at about 1 m, a pallid clayey layer with noticeable kaolin. Reactive Al was present
Fig. 6 Cutaway drawing of an idealised proteaceous shrub at Station 9. Numbers within horizons refer to pH. Symbols for profile layers as given in Fig. 4
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encountered below the reticulite. All horizons above bore visual evidence of rapid infiltration, except where affected by localised hydrophobicity. Rooting morphologies of common Proteaceae at the site were as recorded earlier for other members of the family (see Pate et al. 1995; Verboom and Pate 2006). Each tree possessed one or occasionally several major tap (sinker) roots penetrating the ferricrete pavement down to the reticulite layer. Some tap roots had clearly taken a convoluted course when avoiding large fragments of ferricrete. A number of horizontally-extending laterals radiated out above the ferricrete layer. Fine, mostly upwardly-extending roots borne from these laterals developed superficially-located mats of cluster roots during the wet season (Fig. 6). Unlike the situation in eucalypts, development of mini sinkers on lateral roots tended to be sparse in proteaceous species and the primary tap root of most mature trees remained alive to presumably function as the major, if not the only provider of deeply located available water. Numerous Fig. 7 Cutaway drawing of mixed mallee heath vegetation instances were recorded where distinctive sets of at Station 4. Numbers within horizons refer to pH. downwardly-directed cluster roots were encoun- Symbols for profile layers as given in Fig. 4 tered where minor roots were ramifying through the lowermost parts of a tree’s rooting profile (see on the dull brown ferricrete surfaces, particularly Fig. 6). under large shrub and tree species of Proteaceae. Mild acidity occurred throughout the profile and Mixed vegetation: ferricrete/texture-contrast soil there was no evidence of the abrupt downward association (Station 4, Fig. 7) change in moisture content recorded above for mallee. This intergrading region of the ecotone supported Water-holding characteristics of the sands in scattered mallee, sheoaks (Casuarinaceae), hak- the upper part of the soil profile were similar to eas and grevilleas and an understorey of shrubs of those of the A horizon of soil under mallee. certain Myrtaceae, alongside members of other Porous pisolithic-ferricretes encountered further families (Table 1). The soil profile was basically a down the A horizon exhibited similar water stor- hybrid (Fig. 7) between the Calcic Solonetz and age capacity yet contained much greater quantities the lateritic-podsolic soil types mentioned above of tightly-bound water than in surrounding sand for the two pure vegetation types. The acidic A1 (Table 2). In an earlier paper (Pate et al. 2001)we horizon comprised a brownish-grey structureless suggested that such ferricrete might provide refu- sand, invested with 5–10% ferricrete, and carry- gia for citrate consuming and other bacteria. ing the same dull-brown coatings as seen in the Reticulate layers below the pisoliths possessed proteaceous shrub-heath at Station 9. Below this similar water-storage potential to that of the sand, was a bleached acidic A2 horizon, 20 cm thick but showed relatively more tightly-bound water. and packed densely with pale-brown pisolitic This was presumably due to a combination of ferricretic materials. Some of the latter sported porous ferricrete and increased clay content. A bleached deposits on their upper surfaces layer of poorly-permeable pallid clay was (Fig. 10a). Testing negatively for reactive Al,
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Table 1 Listings of major woody shrub and tree species encountered in the study transect between proteaceous shrub-heath and mallee woodland, near Lake King Western Australia Zone Family Relative Major species abundance present
Mallee (Fig. 5) (Stations 1–3) Myrtaceae Dominant Eucalyptus flocktoniae (Maiden) Maiden, Eucalyptus scyphocalyx F. Muell. ex Benth. (Maiden & Blakely). Eucalyptus pileata Blakely Eucalyptus eremophila (Diels) Maiden Melaleuca scalena Craven & Lepschi Proteaceae Rare Grevillea huegelii. Meisn Grevillea pectinata R.Br. Other Rare Astroloma epacridis. (DC.) Druce Acacia intricata S. Moore Daviesia benthamii Meissner Dillwynia acerose S. Moore Dianella revoluta R. Br. Phebalium filifolium. Turcz. Mixed vegetation (Fig. 7) (Stations 4–5) Myrtaceae Sub-dominant Eucalyptus perangusta Brooker Eucalyptus phaenophylla Brooker & Hopper Eucalyptus eremophila (Diels) Maiden Melaleuca scalena Craven & Lepschi Proteaceae Sub-dominant Grevillia oligantha F. Muell. Hakea lissocarpha R.Br. Hakea marginata R.Br. Other Sub-dominant Callitris tuberculata R. Baker & H.G. Smith Hibbertia nutans Benth. Leptospermum erubescens Schauer Leucopogon conostephioides DC. Proteaceous heath (Fig. 6) (Stations 6–9) Proteaceae Dominant Banksia elderiana F. Muell. & Tate. Grevillea cagiana McGill. Grevillea paniculata Meisn Hakea pandanicarpa R.Br. Isopogon teretifolius R.Br Isopogon scabriusculus Meisn Petrophile.seminuda Lindl Casuarinaceae Sub-dominant Allocasuarina microstachya (Miq.) L.A.S. Johnson Myrtaceae Sub-dominant Melaleuca subtrigona Schauer Verticordia roei Endl. Beaufortia micrantha Benth these surfaces were again interpreted as being under pure mallee, the horizon was mildly acidic composed of poorly crystalline Si. Overprinting of and bleached by what was again interpreted as this nature was encountered only where mallee overprinting by microcrystalline silica deposits. type myrtaceous vegetation overlapped protea- Further down, the profile became increasingly ceous heath. clayey and alkaline with hard carbonate segrega- The underlying alkaline B horizon at Station 4 tions clearly evident. was of massive structure and encapsulated both Rooting characteristics of the scattered mallee pisolitic and nodular ferricrete. As at Station 1, a eucalypts and proteaceous shrubs were essentially moisture gradient was detected in places where a as described above for counterpart associations in seal-like layer occurred. Reactive aluminium was which they happened to be dominant (see earlier absent and, as in the Calcic Solonetz situation descriptions of Stations 1 and 9 respectively).
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a 0 Observations on facies changes 5 in contemporary and paleosol formations 10
) 15 The four examples of lateral facies changes shown 20 in Fig. 11 relate to man-made exposures of Depth (cm 25 contemporary soil profiles in regions where 30 vegetation changed abruptly from proteaceous 35 shrub heath (PH) to myrtaceous woodland 40 0268 4 10 12 (MW). In each case, the junction between vege- Moisture % (v/v) tation types marked a change in underlying soil b profile from one containing a ferricrete layer (Fe) to one with a prominent calcrete pavement (Ca). Figure 11a shows a profile exposed in construc- tion of a drain through an alluvial valley near Belka (WGS 84: 31 40.81¢ S, 118 15.21¢ E) where original vegetation graded from proteaceous shrub heath into open woodland of salmon gum (Eucalyptus salmonophloia F. Muell.). The wedging-out of the ferricrete layer (Fe) under the shrub-heath terminated sharply to be replaced by a layer of calcrete (Ca) starting at commencement Fig. 8 Water relations of a texture-contrast soil under of what was originally eucalypt woodland. Redder mallee, showing (a) steep gradient in fractional water colours of the general soil matrix in the calcrete content across the seal in early summer, (b) retentivity of region indicated diminished Fe segregation than topsoil and subsoil determined on semi-disturbed samples by D. Hall using gravimetric moisture and filter-paper in regions of ferricrete. techniques prescribed by Klute (1986) Figure 11b–d feature a rail cutting near Merr- edin (WGS 84: 31 22.42¢ S, 118 35.49¢ E). Just as at Belka, the lateral facies change at this site Targeted air-spade excavations and augerings (Fig. 11b) bore evidence of a pronounced showed that mini-sinkers and tap roots of mallees discontinuity between abutting facies of calcrete and closely associated proteaceous shrubs went (Ca) and reticulate ferricrete (Fe). There was also through a sodic layer into the underlying B hori- evidence of overprinting (arrowed) of the ferri- zon. Lateral roots of all species radiated out crete by carbonate and silica below nodular through upper parts of the mildly acid A horizon, calcrete, while similar reticulate ferricrete in the with proteaceous species developing cluster roots right of picture did not exhibit overprinting. The mostly within the upper ferricrete, while mallee facies thus replicated the overprinting phenom- eucalypts developed prominent mini sinkers ena described earlier for the ecotone at our throughout their proximal lengths and numerous primary study site. The photographs shown in mycorrhizal feeding roots (Fig. 10b, c), essentially Fig. 11c and d were taken, respectively, a few as already described for Station 1. hundred metres to the left and right of the face Large scale air-spade excavations on rooting shown in b. Figure 11c shows an enclave of catchments of myrtaceous species typically ferricrete intercalated within a broad region of showed a bleached A2 horizon containing sili- calcrete while Fig. 11d shows a reciprocal phe- ceous overprinting of ferricrete between an upper nomenon involving a pocket of calcrete. Closer platform of lateral roots and an underlying seal at examination of 11c and d revealed instances of the top of the B horizon (Fig. 10b, c). These calcrete overprinting lateritic fabrics. In addition features were consistently absent from adjoining lateritic material was found to be overprinting the areas of shrub heath devoid of mallee-type siliceous remnants of the calcrete in the profile Myrtaceae. shown in Fig. 11c.
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Fig. 9 (a) Top of texture-contrast layer in mallee vegeta- foreground typically under A and Sodic Calcisol under B; tion after removal of sandy A horizon using an airspade. (b) higher magnification view of box within boundary Note the overlying platform of intermixed lateral roots of shown in (a); (c) magnification of the box in (b) showing two mallee species, A- Eucalyptus flocktoniae subsp. tubular Sodic Calcisol intrusions; (d) section through seal flocktoniae, and B-Eucalyptus scyphocalyx). A dotted line with white arrows designating bleached surface and root marks the boundary between bleached Calcic Solonetz in passing through horizontally
Table 2 Water retention characteristics of pisolithic, blocky and reticulate ferricrete from a site between Narrogin and Lake King Sample (Horizon) Bulk Vacuum Naturally Water Water Plant densitya saturated saturated contentb contentb available mg/m3 water water content at –10 kPa at –1500 kPa water V/V% content V/V% V/V% V/V% V/V% Mean SE Mean SE Bulk Sample Mean SE Mean SE Mean SE
Pisoliths (A2) 2.23 0.14 8.87 Blocks (A3) 2.03 0.02 19.53 1.47 13.23 15.39 1.16 11.17 1.26 4.26 0.64 Reticulite (B1) 2.05 0.01 23.51 1.13 18.96 18.66 0.74 10.66 1.23 7.93 0.85 a Determined on three separate individual structural units after wax coating and mass loss on immersion b Determined by UWA soil physics laboratory and based on 3 individual, vacuum-saturated structural units after equili- bration in pressure plate apparatus. Connection between gravel and plate was established by seating in diatomaceous earth
Figure 12 displays a stacked paleosol formation numberings of prefixes in Fig. 12 relate to layer- revealed in an exploratory pit through aeolian ings within each of these sequences. deposits flanking a playa lake system near According to the farmers who had cleared Kalannie (WGS 84: 30 10.48¢ S, 117 21.94¢ E). We the land, the upper contemporary formation interpret it as a chronosequence involving three (Formation 1 in Fig. 12) was vegetated until very successively buried soil formations, namely: (1) an recently by wodjil-type acacias such as Acacia uppermost, yellow, lateritic sand; (2) an interme- coolgardiensis Maiden, Acacia neurophylla diate alkaline Calcic Solonetz, and; (3) a lower W.Fitzg. and Acacia assimilis S. Moore. Similar lateritic profile possibly of Tertiary age. The remnant native vegetation can still be encoun-
123 Plant Soil (2006) 289:103–121 115
The lowermost, formation (Formation 3 in Fig. 12), is interpreted as an ancient lateritic- podsolic soil. Although overprinted with silcrete, the arrangement of its pisolithic, blocky and reticulate ferricretes closely resembles that found under present day proteaceous heath. Remains of vertical-descending root channels are visible in all horizons of this exposure.
Discussion
This paper provides consistent evidence support- ing our concept that major woody taxa of open woodland communities of semiarid ecosystems of south-west Australia have an intrinsic capacity to modify the characteristics of a soil profile, and that each class of vegetation accordingly leaves its characteristic imprint in the rooting profile which it occupies. In the case of ecosystems dominated by Proteaceae, we reinforce our earlier findings (see Pate et al. 2001; Verboom and Pate 2003)of highly definitive correlations between the devel- opment of cluster roots and the formation of Fe- rich pisoliths and iron-coated root channels. These outcomes are evident regardless of whether Proteaceous taxa occupy deep sandy podzols or Fig. 10 (a) Overprinting of white amorphous Si on the lateritic profiles. The mobilisation and eventual pisolithic (A2), blocky (A3) and reticulate ferricrete (B) of a profile typical where myrtaceous species are invading deposition of Fe and other chelatable elements is proteaceous shrub-heathland; (b) airspade excavation of generally regarded as being causally related to boundary region between mallee woodland and protea- carboxylate-secreting activities of roots (see ceous shrub-heath showing distribution of sinker (minor Hinsinger 1998; Jones 1998; Lambers et al. 2002; tap) roots (positions marked by red dye) borne on lateral roots of the mallee; (c) close-up view of similar sinker Shane and Lambers 2005), whereas eventual entering seal at the top of the B horizon in the Calcic precipitation of the elements in question is pre- Solonetz. Note occluded ferricrete gravel within the seal sumed to follow from consumption of the organic moiety of the complexes concerned as energy tered near the site. The top layer (1A1) is a pale- source by specialised rhizosphere organisms (see yellow, humus-rich sand. The next layer (1B1) is a Pate et al. 2001). highly acidic, high-chroma yellow, sandy earth For texture-contrast soils in which Myrtaceae overlying a weakly-indurated pisolithic layer are dominant we conclude that the principal (1B2). The lowest layer of this uppermost bioengineering activity of major mallee taxa is in formation (1B3) is a reticulate ferricrete that the construction of a strong texture-contrast soil appears to have overprinted a texture-contrast with a depleted A horizon and ‘sodic’ B horizon. sodic layer of an earlier profile (Formation 2 in The former houses a platform of lateral roots Fig. 12). The lowermost layer of this second which we suggest to be at least partly instrumental formation at 2 m depth comprised a horizon of in construction of the underlying seal, possibly calcrete and silcrete similar to that encountered in through the agency of exudates from their the Calcic Solonetz under myrtaceaeous species seasonally-produced mycorrhizal symbionts. The of the ecotone described in the previous section. highly impervious qualities of this seal result in a
123 116 Plant Soil (2006) 289:103–121
Fig. 11 (a) Drain in flat PH MW valley alluvium exposing lateral changes in soil a profile under an ecotone involving original proteaceous shrub-heath (PH) abutting on myrtaceous (salmon gum) woodland (MW), with MW PH overlying ferricrete (Fe) and calcrete (Ca) typical b Ca for the two classes of vegetation respectively; Fe (b) Similar discontinuity involving original proteaceous shrub-heath and myrtaceous (mallee) woodland and their respective underlying layers of ferricrete and c Fe calcrete, (c) Pocket of ferricrete bearing soil Ca (Fe) in a broader region Ca of calcrete bearing soil (Ca), (d) enclave of calcrete bearing soil (Ca) in a broader region of laterite (Fe)
d
Fe Ca Fe
distinct hydrological separation of the A horizon wide range of water potentials so that in almost from lower parts of the profile. The graphical any situation, the proportional decline in K representations of Fig. 8 thus demonstrate quite through the A horizon to the seal may span several dramatic changes in moisture content across orders of magnitude (see Williams 1983). the seal with little change in matric potential in the Our studies consistently demonstrate that each upper profile during early summer. Given that the mallee tree develops a multiple series of vari- driver of water flow is the gradient in water ously-sized sinker (tap) roots arising sequentially potential (w), and that hydraulic conductivity from a superficial platform of expanding lateral (K (w)) is often represented as a power function of roots. Virtually all of these sinkers are sited the volume wetness (h), only minimal movement within 3 m out from a tree bole and successfully of water is likely to occur across the seal at this penetrate the seal at the top of the B horizon. time of year. Furthermore, it is a characteristic of They might thus act as major vehicles for redis- such texture-contrast soils that plots of the K (w) tributing water upwards or downwards between function for the seal layer are positioned graphi- upper and basal regions of the profile, presumably cally below the K (w) function for the A horizon. by utilising the reversible hydraulic ‘up lift’ and As shown in Fig. 8, this effect is evident across a ‘down lift’ mechanisms now known to widely oc-
123 Plant Soil (2006) 289:103–121 117
be of significance in mallee species growing and reproducing in summer although the significance of reserves only a few meters below the seal is difficult to evaluate in absence of a properly constructed budget for the tree species in ques- tion. For instance the studies of Wildy et al. (2004) on alley planted oil mallees on deep sands show how trees can exploit principally upper subsoil reserves during spring and early summer and then subsist mainly on ground water when engaging in peak growth and reproduction. In the situation described in this paper, ground water resources may not be available and there is the added presence of a Calcic Solonetz possibly bioengineered by the particular mallee species. We then view the role of sinker roots in downward distribution of water as conducive to restricting water access by understorey species incapable of penetrating the Calcic Solonetz’s seal. This fits well with a general paucity of competing herbaceous ground flora across many mallee systems particularly where the seal is close Fig. 12 Stacked paleosol formations (numbered) in aeo- to the surface. lian deposits near Kalannie. Formation 1 is interpreted as Mallee vegetation at our study site showed the contemporary profile. Its high chroma, highly acid highly localised evidence of Calcic Solonetz yellow sands, weakly indurated ferricrete and silcrete layers are typically found under wodjil-acacia: grevillea development under certain species of mallee, communities. The texture-contrast layer underlain by whereas a Sodic Calcisol was present under other carbonate nodules in formation 2, is associated with mallee species. While the above mentioned spe- myrtaceous vegetation and would typically occur at a cies-specific differences relate to our observations depth of 10–20 cm in a contemporary settings under certain eucalypts. The lowest formation is interpreted as within a single mallee ecosystem, they may well a Tertiary laterite, presumably vegetated by proteaceous apply widely. For instance, B. Nicholas (pers. flora comm.) has recorded parallel examples of inti- mate associations between mallee species and cur in woody root systems (see accompanying specific subsoil pavement types across landscapes review by Verboom and Pate, in press). Activities neighbouring our study site. of this kind would then explain the notable The idea that species of woody plants in differences in water contents of soil above and semiarid Australian ecosystems induce definitive below the seal, as commonly seen at certain times and highly localised variations in soil properties of the year. Thus, the drying out of the sandy A such as clay content, water repellency, pH, total horizon 10 m out from the boles of trees into dissolved solids and bulk density is by no means winter grown annual crops, and the annular 6 m new. Indeed, as farmers involved in land clearing diameter wetting up of the B horizon, demon- may know, highly distinctive features within a soil strated by Nulsen et al. (1986), corresponds ex- profile can be identified as being consistently actly to the radial extent of the lateral roots and associated with boles and rooting catchments of the 6 m diameter catchment exploited by sinkers certain woody species. As Field (2003) points out, in the mallees at our study site. the significance of such observations has largely Patterns of topsoil drying and subsoil storage been ignored by regolith scientists. during winter and utilisation during summer seen We regard the above idiosyncrasies between in uncleared mallee by Nulsen et al. (1986) may species of mallee and the imprints which they
123 118 Plant Soil (2006) 289:103–121 appear to make on the soil profile, as ultimate mallee species, one finds much higher electrical hallmarks of bioengineering. For example, the conductivities below the texture-contrast seal, sharp transition which we observe from the Calcic where 90% of such conductivity would typically Solonetz under one mallee to the Sodic Calcisol involve NaCl, with MgCl accounting for most of under another species of mallee might then result the remainder. Much of the Ca in this salt-enriched from highly specific effects on regolith structure, zone has precipitated out of solution, with a peak possibly executed by specific suites of mycorrhiza carbonate concentration below the seal at about and other microbia associated with each species 30–70 cm depth (Griffin et al. 2002; McArthur of host mallee. 1991; Stace et al. 1968). This is to be expected from Turning generally to our ecotone we find yet the sparing solubility of carbonate at high soil pH, again evidence of proteaceous and myrtaceous the low levels of sulphate relative to carbonate, players contributing in highly localised fashion and a generation through root and microbial res- their own highly individualistic imprints on pro- piration of partial pressures of CO2 well exceeding files, and thereby collectively forming a complex those in the atmosphere (see McFadden et al. and fascinating facies transition between end 1991). members of the ecotone. As concluded earlier by A most intriguing aspect of the comparisons Anand and Paine (2002) for regoliths across the which we have made for opposing end members of wheatbelt region of Western Australia, we were our ecotone concerns the intrinsic ability of unable to find evidence that such changes in soil mallee woodland to compete so effectively against profile characteristics were related to underlying other species and thereby subjugate its understo- geological or colluvial features. rey to a relatively sparse scattering of only a few At fine scale within the rooting media of our specialised taxa. On the other hand, one finds high ecotone we encountered examples of overprinting plant density and great biodiversity of understo- of pisolithic ferricrete-gravels by Si and clays. In rey vegetation, and, indeed, a general lack of some cases such phenomena occurred to the ex- dominance amongst the floras of shrub-heath. As tent of fully encapsulating the original gravels to be expected, the intergrading mid-zone of our into what we interpret as a more recently formed ecotone transect exhibits intermediate qualities in seal. Finding that such overprinting was strictly the above respects while also exhibiting a capacity confined to rooting catchments carrying live to support niches for floristic elements not found myrtaceous components, we concluded that such in either of the pure communities. species were apparently establishing on profiles The two examples of lateral facies change de- previously occupied by Proteaceae. scribed in this paper bore evidence of consistent A highly definitive feature of the syndrome of repercussions in contemporaneous layers of a soil effects displayed by our ecotone is the high pH, profile in terms of development of discrete ferri- high Ca, high clay and high amorphous Si status of crete or calcrete layers, precisely as to be ex- the B horizon under mallee, versus the precise pected of present or prior occupancy by opposite of these features under proteaceous proteaceous or myrtaceous vegetation. Further- shrub-heath. Yet again, we suggest that these more, sharp lateral transitions were observed differences partly derive from biogenic outcomes between such layers as one moved from one of competition between and within alliances of vegetation type to the other. Additional features macro-flora and micro-organisms. Speaking more observed within the facies included ferric linings generally, the literature records that soil profiles to root channels and overprinting of earlier- vegetated by Proteaceae exhibit electrical formed ferricrete by calcrete, essentially as de- conductivities reflecting low soluble salt concen- scribed above for the ecotone at Lake King and trations except in close proximity to, and within for parts of the rail cutting at Merredin. (Fig. 11b, the pallid clay layer. In this case, exchangeable d). One hundred metres further along the same cations are dominated by Mg and Na rather than cutting, we encountered the reverse situation by Ca and precipitated carbonate is absent from where a pocket of ferricrete overprinted calcrete the rooting zone. By contrast, in soil under certain (Fig. 11c). We have difficulties in interpreting
123 Plant Soil (2006) 289:103–121 119 such highly localised phenomena without re- descriptions made by Anand and Paine (2002) coursing to biologically mediated effects. One for regolith arrangements characteristic of a might relate such overprinting to local changes in range of semiarid parts of the inland plateau of moisture regimes brought about by a sequence of southwest Western Australia. climatic change, but in absence of a consistent One might of course argue that presence of sequence of overprinting such an interpretation is vertically-arranged soil formations in any of the difficult to sustain. We also point out that the above cases might merely reflect complex histories morphology and setting of the pocket of laterite, of erosion, deposition and weathering. However, shown in Fig. 11c, is in many ways akin to the where one encounters stacked paleosols in peri- basket podsol situation found under Kauri trees odically-accreting, essentially uniform aeolian in New Zealand and Tasmania (see Fanning and materials, it becomes difficult to suggest any form Fanning 1989). Lateritic enclaves might of course of origin other than a biological one. In such cases result from localised redox reactions on wetter the sedimentary components concerned presum- parts of the hillslope or within paleo-channels ably originated allochthonously from dusts and that were subsequently raised by differential sandy material blown in from nearby playa-lakes erosion (see Pain and Ollier 1995). However, this during periods of general aridity (see Bowler 1976; view becomes problematic when cast against their Glassford and Semeniuk 1995). Subsequent spiralling plan forms (See Verboom and Pate autochthonous development of pedogenetic 2003) and, in the case of the latter scenario, one structures would then have taken place during also needs to explain how laterites of presumed periods of semi-aridity (Glassford and Semeniuk Tertiary age came to flank valley systems with 1995). Biologically-based origins are then easily sediments from the same and earlier vintage. envisaged as paramount, especially where rele- Incidentally, we have discovered many addi- vant pedogenetic structures presumably devel- tional cases of overprinting of ferricrete with sil- oped outside the influence of regional water crete or calcrete in uncleared land throughout the tables, and where successive episodes of deep wheatbelt and found that all such phenomena burial are likely to have limited transgressive underlie or are in very close proximity to extant (overprinting) effects. myrtaceous vegetation. We would suspect that Arguing along the above lines, especially com- phenomena of this kind created difficulties for pelling evidence implicating episodes of soil scientists attempting to classify profiles using proteaceous vegetation as biotic agents in podzol conventional criteria. formation comes from the finding of stacked Our example of a vertically-stacked paleosol paleosol sequences comprising ferricrete/humi- is interpreted as embodying sequential bioen- crete layers alternating with deep bleached sand gineering effects of three different vegetation layers in the Gnangarra Mound north of Perth, types, namely an upper stratum currently Western Australia (information provided by R. occupied by an acacia community, a mid stra- Froend and based on deep corings undertaken by tum formed under a predominantly myrtaceous the Metropolitan Water Board, Perth, Western community and a lower-most stratum resulting Australia). We interpret such layerings as com- from prior occupancy by ancient proteaceous prising residual imprints of phases of proteaceous shrub-heath. There is also evidence of extensive woodland successively buried during intervening overprinting, with transgressions evident in the phases of deposition of wind-blown coastal sand. two older strata. Other large open pits on the In a similar vein, the development of Banksia same slope, and involving episodic deposition ecosystems on sand incursions on the Queensland of similar aeolian material, indicated lateral coast studied by Walker et al. (1981) show laterally discontinuities and variable degrees of over- separated chronosequences involving giant podz- printing with silcrete, essentially as we have ols. The above authors interpret these as resulting recorded here and elsewhere (Verboom and from biogenic activity, with each successive com- Pate 2003) for contemporary facies. Indeed our munity of vegetation becoming restricted in func- observations are again in full accord with the tioning and longevity through limitations in
123 120 Plant Soil (2006) 289:103–121 resources of P. Thus, ferric pavements in the oldest Bowler JM (1976) Aridity in Australia: age origins and formations tend to occur at greatest depth, with expression in aeolian landforms and sediments. Earth- Sci Rev 12:279–310 evidence of collapse of the ecosystem as deep- Brewer R, Sleeman JR, Foster RC (1983) The fabric of rooted trees become replaced by shallow rooted Australian soils. In Soils an Australian Viewpoint. components which succumbed on being unable to CSIRO Division of Soils, Melbourne, pp. 439–476 benefit from deeply-located reserves of P. Caccetta PA, Allan A, Watson I (2000) The land monitor project. Proceedings of the Tenth Australasian Re- In conclusion we note that the evidence pre- mote Sensing Conference, Adelaide, Australia sented here extends and substantiates what may be Craven LA, Lepschi BJ, Broadhurst L, Byrne M (2004) traced to century old propositions by Russian Taxonomic revision of the broombush complex in scientists (cited in the review of Verboom and Pate Western Australia (Myrtaceae, Melaleuca uncinata s.l). Aust Syst Bot 17:255–271 (in press)) that soil formation is a process broadly Day RL, Laland KN, Odling-Smee FJ (2003) Rethinking linked to the geography and physiology of higher adaptation: the niche-construction perspective. Per- plants and their microbial associates. While this idea spect Biol Med 46:80–95 may have languished in face of increasing support of FAO (1998) The world reference base for soil resources (WRB), World Soil Resources Report No. 84. 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