Holocene environmental history at the treeline in the Northern Apennines,Italy: A micromorphological approach

Abstract

The aim of this work is to investigate paleoenvironment at treeline quotas through the help of soil micromorphology. It also assesses how the micromorphological approach can provide information in paleoenvironmental studies where paleosols are available as proxy archives. Nine soil profiles are described across the treeline, between 1723 and 1860 m, on Mt Cusna slope (2121 m a.s.l., Northern Apennines, Italy). Undisturbed samples from selected soil horizons are collected in Kubiëna boxes. From field observations, all the profiles appear to be composed of two main pedogenetic units: the upper one, composed of a recent soil of colluvial origin, and the deeper one, consisting of an underlying more developed buried paleosol. Thin sections give detailed information about the nature and the origin of both pedological units. Four principal phases of paleosol development are detected. A first period of temperate climate with forest cover and contrasted seasons is followed by a phase of change and then by a successive decrease of forest cover. In a last phase, the soil experiences frost action under the effect of a colder climate. The final deposition of colluvium seems to be very recent (historical time). Moreover, multiple colluvial layers are identified. Colluvial material of different origins could be identified as part of dismantled soils similar to the preexisting paleosol. In conclusion, with the help of soil micromorphology, it is possible to assess the existence of past stable forest at least 100 m above the present treeline. The micromorphological approach represents a powerful tool in multidisciplinary paleoenvironmental studies due to its high level of resolution in outlining the individual and successive phases of soil evolution.

Keywords

colluvium micropedology mountain soils paleoenvironment paleosols treeline fluctuations

Introduction

Fluctuations of the treeline (the uppermost or northernmost limit of tree growth form) through time represent the expansion or regression of tree species in response to large-scale variations in environmental conditions. As such, they are considered a reliable sign of environmental change (Holtmeier and Broll, 2007). Paleoenvironmental research in treeline areas, although particularly important for its high diagnostic value, is not always easy. Proxy archives are not usually available and, if present, may vary greatly in nature and techniques required to investigate them. Among these archives, paleosols can give accurate interpretations both for single sites and on a wider scale. The fundamental role of paleopedology in Quaternary studies is indeed widely acknowledged (Fedoroff et al., 1990; Kemp, 1999; Yaalon, 1971). The area of the Northern Apennines (Italy) investigated in this paper is one of those uncommon cases when a stable treeline is characterized by the presence of well-preserved Holocenic paleosols. The importance of this site lies in four aspects. First, it was glaciated until the beginning of the Holocene. Therefore, reconstructions can be easily circumscribed into a defined time frame. Second, fluctuations of the treeline during the Holocene have been documented (Cremaschi et al., 1984; Compostella et al., 2012). Third, the present treeline is static, a factor linked to local constraints opposed to the global temperature increase (Harsch et al., 2009). And fourth, it is also presently at an altitude below its climatic potential, an observation that has been made elsewhere in the Apennines and still not clearly explained (Körner, 2012). Investigating its history would possibly help in understanding what causes its peculiar behavior: hence the need for detailed analyses of the paleosols of this area in order to reconstruct the environment under which they were formed.

Micromorphological analysis of soil thin sections has proven to be a valuable instrument in detecting proxy data in soils and recognizing pedogenetic processes. This is true for different disciplines such as sedimentology (see Bertran and Texier, 1999; Van der Meer and Mendies, 2011), hydrology (Clegg et al., 1999), mineralogy of weathering (Mulyanto and Stoops, 2003), and so on. In archeological research, thin-section analysis of paleosols and sedimentary deposits is consolidated practice for both the characterization of a single site (e.g. Cremaschi and Trombino, 1999; Goldberg and Berna, 2010) and the reconstruction of human influence on larger areas (e.g. Delhon et al., 2009; Sageidet, 2009; Trombino, 2007). In Quaternary studies, micromorphology is applied to both terrestrial sedimentary sequences (such as lacustrine varves or peat levels: Cruise et al., 2009; Ringberg and Erlström, 1999) and paleosols, sometimes in combination (as in soil-loess sequences; see Kemp, 1999). Reconstructions can either focus on a single diagnostic feature linked to specific phenomena (e.g. deposition of calcite or gypsum; Khokhlova et al., 2001; Mees, 2003; Retallack, 2005) or consider thin sections as a whole. In any of these cases, the final purpose is to outline the history of a site: usually from a geomorphological point of view (e.g. Cremaschi and Negrino, 2005; Kemp, 1998; Kühn, 2003; Zerboni et al., 2011), sometimes in more ecological terms. In paleoenvironmental reconstructions, micromorphology is considered a useful tool (Fedoroff, 1991). Nevertheless, the literature provides few examples of its application to real cases (Cremaschi and Trombino, 1998; Magliulo et al., 2006; Muggler and Buurman, 2000; Scarciglia et al., 2003; Srivastava et al., 2010; Tsai et al., 2007), mainly because of the already-cited rarity of suitable paleosols in areas of interest.

The aim of this work is therefore to apply micromorphological analysis on paleosols in order to provide a full pedogenetic and environmental history of the study area. This research will also outline the contribution of soil micromorphology in multidisciplinary studies where paleosols are the primary source of proxy data. This study is in fact part of a larger multidisciplinary project on Holocene dynamics in the Northern Apennines (Italy) in which techniques from soil science, dendrochronology, entomology, and anthracology are employed (Compostella et al., 2012).

Materials and methods

Study area

The study was conducted on the northern slope of Mt Cusna (2121 m a.s.l.; Figure 1), the second highest peak of the Northern Apennines (Italy), facing Mt Bagioletto (1758 m a.s.l.). The climate is sub-Mediterranean with abundant and well-distributed precipitations (2000 mm/yr; Filippi and Sbarbati, 1994; Rossetti, 1988), showing a summer minimum. Mean annual temperatures range from 8.8°C (Ligonchio, 928 m a.s.l.; 44°31′N; 10°35′E) to 2.2°C (Mt Cimone, 2165 m a.s.l.; 44°21′N; 10°70′E, for both stations, observation period of 1961–1990).

Figure 1.

Study area and location of the investigated profiles.

The bedrock is mainly composed of claystones and siltstones, which originated from terrigenous deep-sea turbiditic deposits. Rare limestones can also appear (Bortolotti, 1992; Servizio Geologico, Sismico e dei Suoli della Regione Emilia Romagna (SGSS), 2007a, 2007b). During the last glacial maximum, the Mt Cusna glacier covered the entire study area (Losacco, 1949); its progressive disappearance during the transition to the Holocene left abundant glacial and periglacial deposits, which form most of the present landscape (Panizza et al., 1982). The main active processes today are mass movements and gully/stream erosion. Anthropic activity, especially livestock grazing, also has a significant impact on landscape (Panizza et al., 1982).

Vegetation is composed of deciduous beech (Fagus sylvatica) forest with the treeline located at 1750 m. The areas above are occupied for the most part by Vaccinium myrtillus–dominated heathland and acid pastures with Nardus stricta and Brachypodium genuense (Tomaselli, 1997).

Entisols, Spodosols, and Inceptisols (sensu Soil Survey Staff, 2010) can be observed up to 1900 m (Panizza et al., 1982); in the uppermost area, weakly developed humiferous and desaturated soils occur (Filippi and Sbarbati, 1994). The recently published ‘Soil Map of Italy’ (Costantini et al., 2012) identifies one unit for the Apennines watershed composed of Haplic and Leptic Umbrisols (Humic); Rendzic Leptosols; Calcaric, Calcaric Leptic, Eutric, and Dystrict Skeletic Cambisols; and Haplic Podzols.

Colluvium-buried paleosols characterize a former surface, the Mt Cusna paleosurface, extensively glacialized during the glacial maxima, then subjected to soil development, and finally buried by colluvial phenomena during the Holocene (Bernini et al., 1978; Compostella et al., 2012; Panizza et al., 1982).

The first traces of human settlements in the area belong to Mesolithic hunters, between the Boreal and Atlantic periods (Biagi et al., 1980; Castelletti and Cremaschi, 1975; Cremaschi et al., 1984; Panizza et al., 1982). After this interval, evidence is found of occasional frequentation during the Sub-Boreal period, Iron Age, and Roman Age. The first written documents of stable activities date back to the High Middle Ages (Panizza et al., 1982).

Methods

Nine soil profiles were opened and described (according to Food and Agriculture Organization (FAO), 2006) across the treeline, in a range of altitudes between 1723 and 1860 m (Table 1 and Figure 2). Sites were chosen to comprise different conditions for vegetation, lithology, and geomorphological settings (slope inclination in particular). In all, 26 undisturbed samples from selected horizons were collected in Kubiëna boxes. Thin soil sections were prepared on covered glass slides from undisturbed samples through impregnation with polystyrene. Thin sections were described with the help of a petrographic microscope (Olympus BX41) at 20–400× magnifications, in plane/cross polarized and incident light. The description follows Stoops (2003); interpretation is mostly based on Douglas and Thompson (1985) and Stoops et al. (2010).

Table 1.

Main features of the investigated soil profiles.

Soil profile	Elevation (m a.s.l.)	Aspect (°N)	Slope (°)	Parent material	Vegetation	Coordinates
CUS1	1723	297	10	Claystones	Natural forest	44°17′45.25″N; 10°22′57.42″E
CUS3	1745	–	0	Claystones	Grassland and shrubs	44°18′11.63″N; 10°23′3.84″E
CUS4	1752	290	22	Claystones	Grassland	44°17′46.11″N; 10°23′4.61″E
CUS12	1760	37	11	Claystones	Grassland and shrubs	44°18′2.33″N; 10°23′25.46″E
CUS10	1762	314	9	Claystones	Grassland and shrubs	44°18′8.02″N; 10°23′11.66″E
CUS6	1765	128	6	Claystones	Grassland and shrubs	44°18′9.14″N; 10°23′17.47″E
CUS11	1767	16	7	Claystones	Grassland and shrubs	44°18′8.41″N; 10°23′16.67″E
CUS7	1804	10	29	Claystones	Grassland and shrubs	44°17′44.98″N; 10°23′29.40″E
CUS8	1860	19	18	Marlstones	Grassland and shrubs	44°17′40.90″N; 10°23′26.15″E

Figure 2.

Sketches of the investigated soil profiles.

Soil classification of paleosols is hardly accurate (Krasilnikov and Calderón, 2006; Nettleton et al., 1998) according to the available soil nomenclature codes (e.g. FAO, 1998; Soil Survey Staff, 2010; WRB, 2006). In fact, most of the key soil attributes have a low probability of being preserved in paleosols without major modification or destruction (James et al., 1998). Furthermore, in many cases, the classification of paleosols is not useful in paleoclimatic studies, as most of the diagnostic criteria depend on the present-day climate. Notwithstanding that, for the sake of clarity, analogies were made between the described paleosols and soil categories defined by the current international nomenclature (WRB, 2006).

Results

Total thickness of all studied profiles range between 70 and 200 cm (Figure 2 and Appendix 1, available online). From field observations, it is possible to divide every profile into two main units (Figure 3): the upper unit, composed of the superficial (i.e. recent) soil of colluvial origin, and the lower unit, constituted of the underlying buried (i.e. older) more developed soil (i.e. paleosol).

Figure 3.

Typical situation: profile composed of two units (profile CUS6, 1765 m a.s.l.).

Field observations

The upper unit has a variable thickness ranging from 30 to 110 cm (Figure 2). It is composed by one or more A horizons, usually underlain by AB horizons (absent in profiles CUS8 and CUS11). In the profile located at the lowest altitude (CUS1, 1723 m), just below the treeline, a weakly developed B horizon is also observed. Field descriptions of the horizons of the upper unit show recurring characteristics (Appendix 1, available online). They contain few weakly weathered rock fragments exceeding 2 mm (around 10%, sometimes less) and have very low chroma (2–3 wet). Their texture is silty/loamy and usually more clayey with depth.

The lower unit is characterized by a sequence of 2B horizons with an increasing degree of development. At its top there are one or more moderately developed 2Bw horizons (absent in profiles CUS4, CUS6, and CUS8), silty/loamy or clayey, with rare coarse fragments. Structure is always blocky but variable in size and development. Color shows a higher chroma than the upper unit (3–4 wet). The lowest part of the unit is composed of well-developed 2Bt horizons with clay illuviation features, which are well visible in the field. Texture is always at least partly clayey and structure always blocky. Coarse material is generally rare, and color has the highest chroma (usually 4 wet, sometimes up to 6). The base of the profile is usually marked by a clear contact with the bedrock, often fragmented in centimetric blocks. In some profiles (CUS1, CUS3, CUS11), the boundary between soil and bedrock is a gradual clayey saprolite-like level of transitional 2BC horizons.

In four profiles located in the flatter areas (CUS3, CUS6, CUS10, CUS11), a 2Ab horizon is found at the top of the buried unit. It shows a very loose fine structure with the absence of rock fragments. Its color is characterized by low value (2–3 wet), which gives the horizon a blackish appearance. It also contains a few macroscopic pieces of charcoal (519 mg/kg).

Micromorphology

Thin sections increase the level of complexity of the profiles (Table 2; Figures 4 and 5; and Appendix 2, available online). In the upper unit, the main constituents of the fabric are weakly sorted coarse rock fragments (Figure 4a), which in one case (CUS4) shows a horizontal orientation pattern. This can possibly indicate the presence of a stoneline inside the unit. Allocthonous soil rounded fragments (i.e. pedorelicts sensu Brewer, 1976) with a higher degree of pedogenesis are other recurring elements of the upper unit (Figure 4b). They can be found isolated or represent the dominant fabric unit in the groundmass (CUS6, CUS10). In the second case, their morphological characteristics are similar to those of the buried unit horizons (e.g. CUS10 2Bw; Figure 5c). Proportions between these two main components vary at different depths and profiles, forming various microstructures: intergrain microaggregate when rock fragments prevail, granular to subangular blocky (in AB horizons) in the other case (Figure 4c). Fine material generally shows a brown or more grayish color, with a speckled limpidity and a weak stipple speckled b-fabric. Porosity is high (voids are usually common or frequent) as an effect of both transport and biological activity (Figure 4d and e). Bioturbation is common, with passage features represented by fabric hypocoatings and matrix infillings (Figures 4f and 5f). Star-shaped vughs appear in the more developed AB horizons (CUS1, CUS6). Other pedofeatures are quite rare, usually limited to small and possibly anorthic Fe–Mn nodules with sharp, rounded boundaries produced by transport. Fragmented clay coatings (i.e. papules sensu Brewer, 1976) are also present (CUS4, CUS6).

Table 2.

Micromorphological features of the analyzed thin sections.

Profile	Horizon	Microstructure	Coarse mineral fraction and weathering	Textural coating limpidity and grain size/internal fabric	Fabric pedofeatures	Pedorelicts (sensu Brewer, 1976)
CUS1	AB	Granular/subangular blocky	Common weak./mod. w. claystones	–	–	–
	2Bw	Granular/subangular blocky	Few mod. w. sandstones/claystones	–	–	–
	3Bw1–3Bw2	Subangular blocky/channel	Frequent str. w. claystones	Dusty clay microlaminated	–	–
	3Btg	Subangular blocky	Few mod. w. sandstones/claystones	Limpid/dusty clay microlaminated	Fe–Mn depletion hypocoatings	–
	3BC1	Channel	Few str. w. sandstones/claystones	Limpid/dusty clay microlaminated	–	Clayey aggregates
	3BC2	Intergrain microaggregate	Common weak./mod. w. claystones	Limpid/dusty clay laminated	–	–
CUS3	AB–2Ab	Granular	Frequent weak. w. claystones	–	Compaction hypocoatings	Clayey aggregates
	2Bw	Granular/subangular blocky	Very few weak./mod. w. sandstones/claystones	Dusty clay microlaminated	–	–
	2Bt	Subangular blocky	Few var. w. claystones	Impure clay nonmicrolaminated	Reorientation hypocoatings	–
CUS4	A	Intergrain microaggregate	Frequent weak. w. horizontal sandstones/claystones	–	–	Clayey aggregates
	2Bt1–2Bt2	Vughy/channel	Few weak./mod. w. claystones	Limpid/dusty clay nonlaminated	Reorientation hypocoatings	–
	2Bt3	Subangular blocky/channel	Few mod. w. claystones	Limpid/dusty clay microlaminated	Reorientation hypocoatings	–
CUS6	BA	Granular	Frequent weak. w. claystones	–	Compaction hypocoatings	Fragmented clay coatings
	2Ab	Granular	Very few str. w. sandstones/claystones	–	–	–
	2Bt–2BC	Subangular blocky/channel	Very few weak./str. (lp) w. sandstones/claystones	Clay and silt nonmicrolaminated	–	–
CUS7	2Bt1–2Bt2	Granular/subangular blocky	Very few var. w. sandstones/claystones	Impure clay nonlaminated	–	Clayey aggregates, fragmented clay coatings
CUS7	2Bt3–2BtC	Subangular blocky channel (lp)	Common str. w. sandstones	Limpid/dusty clay microlaminated	Reorientation hypocoatings	–
CUS8	2Bt	Channel	Very few mod. w. sandstones/claystones	Clay and silt nonlaminated, dusty clay microlaminated	Reorientation hypocoatings	–
CUS10	A1–A2	Granular (up) subangular blocky	Common/few (lp) mod. w. claystones	–	–	–
	AB–2Ab	Granular	Very few mod. w. claystones (up)	–	Compaction hypocoatings	–
	2Bw1–2Bw2	Granular/subangular blocky	Very few mod. w. claystones	Clay and silt nonlaminated	–	–
	2Bt	Subangular blocky	Few mod. w. claystones	Clay and silt nonlaminated, dusty clay microlaminated	–	–
CUS11	2Bw–2Bt	Granular	Frequent mod. w. sandstones/claystones	–	Compaction hypocoatings	–
CUS11	2Bt–2BC	Subangular blocky	Few var. w. claystones (lp)	Dusty clay microlaminated	Reorientation hypocoatings	–
CUS12	2Bw1–2Bw2	Intergrain microaggregate	Common mod. w. horizontal claystones	–	–	–
CUS12	3Bw–3Bt	Granular (up) subangular blocky	Common/very few (lp) weak. w. claystones	Impure clay microlaminated	Reorientation hypocoatings	–

lp: lower part; up: upper part; abundance: very dominant – >70%; dominant – 50–70%; frequent – 30–50%; common – 15–30%; few – 5–15%; very few – <5%; mod.: moderately; str.: strongly; var.: variably; w.: weathered; weak.: weakly.

Figure 4.

Photomicrographs of some micromorphological features from upper unit and 2Ab horizon. (a) Dominant coarse rock fragments in A horizon of superficial soil unit (CUS4 − 20×, PPL); (b) coarse rock fragments and pedorelicts (p) in AB horizon of superficial soil unit (CUS1 − 20×, PPL); (c) granular/subangular blocky microstructure in AB horizon (CUS1 − 20×, PPL); (d) excrement infilling (ei) in A horizon of superficial soil unit (CUS10 − 20×, PPL); (e) same field as previous in XPL; (f) matrix infilling (mi) in BA horizon of superficial soil unit (CUS6 − 20×, PPL); (g) typical pattern of 2Ab horizons, with planes (pl) and vertical wedges (w) (CUS10 − 20×, PPL); (h) very fine granular microstructure of 2Ab horizons (CUS10 − 100×, PPL); and (i) charcoal fragment (ch) in 2Ab horizon (CUS3 − 100×, PPL).

Figure 5.

Photomicrographs of some micromorphological features from buried paleosol (deeper unit). (a) microlaminated limpid clay coatings (cc) in 3Btg horizon (CUS1 − 100×, PPL); (b) same field as previous in XPL; (c) blocky microstructure in 2Bw horizon (CUS10 − 20×, PPL); (d) clay and silt coatings (c) in 2Bt horizon (CUS8 − 100×, PPL); (e) same field as previous in XPL; (f) matrix infilling (mi) in 2Bt horizon (CUS10 − 20×, PPL); (g) argilliturbation in 2Bt horizon (CUS7 − 20×, PPL); (h) same field as previous in XPL, with striated b-fabric; and (i) Fe−Mn nodules (n) in 3Btg horizon (CUS1 − 100×, PPL).

In thin section, the deep unit is more complex than the upper one. The 2Ab horizon is micromorphologically characterized by a very fine granular microstructure (40–80 µm; see Figure 4h) and a general scarcity of coarse constituents other than charcoal and nodules (Figure 4i). The most important features are related to porosity (Figure 4g), in particular the pattern of parallel-perpendicular planes and the presence of vertical wedges at the upper interface (CUS6). In one case (CUS3), this pattern of planes is expressed enough to form a secondary angular blocky microstructure. Moreover, clayey pedorelicts (sensu Brewer, 1976) with fabric similar to deeper horizons (e.g. CUS6 2Bt) are found in one of the biggest fissures (a few millimeters wide). Subrounded Fe–Mn nodules with sharp boundaries are the other pedofeature of the 2Ab horizon. These features, and particularly the absence of coarse rock fragments, mark a strong discordance from both the upper unit and the rest of the buried unit. This suggests a different origin for this horizon, which shall be discussed separately.

The 2Bw horizons sampled in the deep unit present a range of microstructures from granular/blocky and rich in star-shaped vughs to more expressed blocky (Figure 5c). Fine material turns to a more reddish or yellowish color and to a cloudy limpidity; b-fabric is better expressed, often granostriated or porostriated. In some cases, very few charcoal fragments are found (CUS1, CUS3, CUS10). Fe–Mn nodules share the same characteristics of the horizons below.

These features become more emphasized in the 2Bt horizons. Thin sections show blocky or channel microstructures usually with few or very few voids. Fine material shows a high degree of pedogenesis, with well-expressed reddish-brown or yellowish-brown colors and cloudy limpidity. B-fabric is usually striated and associated with argilloturbation (shrink and swell) features such as fabric hypocoatings (Stoops, 2003; Figure 5g and h). These b-fabric characteristics are lacking in CUS6 and CUS10. Instead, CUS1 shows a striated b-fabric associated with redoximorphic features: concentrations of Fe–Mn nodules (Figure 5i), changes in color and Fe–Mn depletion hypocoatings. The 2Bt horizons are in general rich in pedofeatures. Fe–Mn nodules occur more frequently (although they are always very few) and have different shapes, from subrounded or rounded to more irregular. Their shape changes from one profile to another but not at different depths within the single profile. Their boundary is in general more gradual than in the upper unit and in the 2Ab horizons. Textural pedofeatures show high variability in many profiles. Clay coatings are found in every profile. Microlaminated ones (Figure 5a and b) are always present in the lowest part of the profiles. Nonlaminated coatings (Figure 5d and e; absent in CUS1, CUS11, and CUS12) are located above them or at the same depth of the profile, but never below. In CUS1, the deeper 2BC2 horizon has coatings with wider laminations. Nonlaminated coarse coatings also appear in some profiles (CUS6, CUS8, CUS10), always located in horizons above fine coatings. In every thin section, at least part of the coatings has a gradual boundary with the groundmass: this can indicate a process of incorporation into the groundmass.

Discussion

Upper unit

In the field, this unit has a very uniform appearance, being weakly structured and with significant quantities of coarse rock fragments. The latter decrease with depth, in ‘more developed’ AB horizons having an increase of chroma and more clayey textures. According to these features, these soils can be regarded as Haplic Regosols or in some cases Haplic Cambisols (WRB, 2006). These characteristics seem to indicate the nature of this unit as an event of colluvium, which was influenced after its deposition by in situ pedogenesis, apparently the same in the whole area. This unit is similar in appearance to ‘cover-beds’ (Kleber, 1997), even if the latter are more strictly defined as slope deposit related to periglacial dynamics, as described in Austria and Germany (Semmel and Terhorst, 2010; Terhorst, 2007).

In thin section, however, all horizons, including the ‘more developed’ AB ones, show the characteristics of a mass-transported soil (Fedoroff et al., 2010) with only very weak signs of pedogenesis acting after deposition. Color of thin sections, degree of porosity, and b-fabric indicate very weak pedogenetic development. Star-shaped vughs in AB horizons are not diagnostic since they can be attributed both to mechanical compaction and pedogenesis (Aurousseau et al., 1985; Fedoroff et al., 2010). On the contrary, the groundmass appears to be composed of a high proportion of subrounded to subangular pedorelicts (sensu Brewer, 1976). Their internal fabric is comparable with the underlying 2Bw horizons: this can indicate their origin from dismantled soils similar to those found in the buried unit. Consequently, the presence of pre-altered material (Stoops, 1989) inherited from the slope could explain the apparent development of some horizons at the field observation level. In this light, present-day pedogenesis on the upper unit appears less effective than thought, because of its weakness or its short time of action, possibly both.

Moreover, in two profiles (CUS4, CUS12), the upper unit seems to be constituted by multiple colluvial layers deposited through time, which present different development due to both provenience of material and duration of pedogenesis. The key features to recognize these events are related to the coarse fraction, particularly the presence of stonelines at the interface between two layers (CUS4), and abrupt changes in lithology at different depths (CUS12). These features are practically invisible at a macroscopic scale, so in this case, micromorphology is essential. From these observations, it is also possible to assume two main depositional patterns of colluvium in the area. One involves large quantities of material in single events, and the other is based on accretion of colluvial layers in multiple events.

Lower unit

Micromorphology provides better detail of this unit and shows a higher complexity than hypothesized in the field, with more than a single linear pedogenetic history. For this reason, its main components – the buried paleosol and the 2Ab horizon overlying it – shall be treated separately.

Buried paleosol

All features found both at macroscopic (color, texture, aggregation) and microscopic level (fabric, microstructure, color, b-fabric, pedofeatures) are more strongly developed in this unit in comparison with the one overlying it. Such paleosols can be tentatively classified as Luvisols (WRB, 2006). Nonetheless, being composed by a truncated unit, it is not possible to add the prefix Cutanic with certainty. Haplic Luvisols (Siltic) can be a more general way to consider them. This complies with other studies that recognized similar paleosols in this area as buried Alfisols (Panizza et al., 1982). Its characteristics, and above all clay translocation features, lead us to infer the nature of this soil as product of a strong brunification with clay illuviation process (Duchaufour, 1983).

Clay illuviation in the deep horizons not only is useful for soil genesis and classification but also helps in reconstructing the history of the paleosol. In fact, three successive illuviation phases can be recognized, likely related to three different environmental stages. The first phase is represented by microlaminated clay coatings in the deepest part of profiles. Their presence is compatible with stable, continuous vegetation cover and a strong seasonality in climate (e.g. Fedoroff, 1997, described for Mediterranean soils). At similar depths or above this first phase is found a second phase of nonlaminated clay coatings, which is compatible with climatic conditions lacking strong seasonal contrasts, or is indicative of the final phase of clay illuviation (Kühn et al., 2010; Miedema et al., 1999; Rogaar et al., 1993). Illuviation of coarser fractions (dusty clay and silt coatings) indicates loss of stable vegetation cover and marks a final phase of soil erosion. After that, no more evidence of translocation of fine material are present. On the contrary, there is ample evidence of a process of incorporation of these coatings into the soil, where sometimes a striated b-fabric remains as the only visible trace. This phenomenon demonstrates that coatings are no longer in equilibrium with the surrounding conditions and outlines the presence of an environmental change after the ones described above.

Relationships with parent material (parent material effect; e.g. Kooijman et al., 2005) also seem to have a distinct effect on the morphology of these soils. The substrate is usually composed of weathering claystones that enrich soil groundmass with great quantities of clay minerals. Above a certain percentage, clay is subject to shrink and swell movements, which tend to mix the whole groundmass slowly obliterating and incorporating coatings. These movements can also be responsible of a more rounded shape and sharper boundaries in nodules. Over time, pressure of the different aggregates swelling against each other forms in time clay reorientation hypocoatings (Kovda and Mermut, 2010, formerly defined stress argillans: Nettleton and Sleeman, 1985). Only in three cases do these features not appear. CUS6 and CUS10 showed a less expressed b-fabric, probably related to an inferior quantity of clay into the groundmass. This was possibly caused by undetected differences in weathering or in field conditions. The other case, CUS1, represents the effect of water saturation (i.e. pseudogley) caused by clay illuviation and shrink and swell movements, which tend to close porosity.

Water saturation also affects Fe–Mn nodules. When it is more significant, nodules should get irregular and gradual as an effect of oxide deposition (Lindbo et al., 2010). In the lower unit, they always tend to get more gradual boundaries with depth as expected with normal water percolation dynamics. However, their shape does not change between horizons in a single profile, changing instead between different profiles. This links conditions of water saturation to local/site effects and not to a general process affecting the whole area.

In all the described profiles, the buried paleosol of the deep unit is truncated, and composed by either 2Bw and 2Bt horizons (CUS1, CUS3, CUS10, CUS11, CUS12) or only by 2Bt horizons (CUS4, CUS6, CUS7, CUS8). It is therefore evident that erosion has acted differently on different profiles. Causes for these differences are difficult to find: profiles in these two groups do not seem to share common features in topography, slope, or other relevant parameters. However, charcoal found in the upper portion of this unit can highlight what probably triggered soil erosion. Its presence most likely indicates a period of fire events, which are usually followed by phases of removal of soil material.

2Ab horizon

This dark horizon found in four topographically close profiles (CUS3, CUS6, CUS10, CUS11) shows features very different from both the colluvium above and the paleosol below. As such, it is most important to define its nature in order to understand the processes which produced it. According to recent Italian soil literature (e.g. Iamarino and Terribile, 2008; Mileti et al., 2013), soils with andic features are widespread in the Italian mountains ecosystems, and their occurrence provides a further help in describing past environments. In this light, due to the peculiar field and micromorphological characteristics of the 2Ab horizon, samples of the profiles CUS3, CUS6, CUS10, and CUS11, have been tested for the allophane presence by the determination of the pH in NaF (Fieldes and Parrott, 1966). Only one sample (CUS11) showed pH values slightly greater than 9.4 (i.e. pH 9.47), which is the lower limit for the allophane occurrence. Thus, at present, we can exclude the occurrence of andic features in the described profiles, but further and rigorous analyses are planned in the near future.

On the contrary, granules show a regularity in shape and dimensions that can identify them as excrements from invertebrate activity, which was especially abundant here. In combination with the very dark color of the micromass (caused by large quantities of organic carbon as highlighted by soil analyses), these features suggest the origin of this part of the soil as a surface A horizon. In favor of this hypothesis, there is also the presence of numerous charcoal fragments, which in natural conditions accumulate on the ground’s surface and are then incorporated only in minor quantities into the soil (Carcaillet, 2001). Void patterns in thin sections are the result of frost action, which is also related to surface proximity (Van Vliet-Lanoë, 2010). Since similar voids do not appear on the present soil surface, it can be argued that the old surface was exposed to a colder climatic regime than present. Pedorelicts inside the wedges suggest mass movements after void formation, which very likely correspond to the deposition of the colluvial unit. Finally, scarcity of coarse particles shows a discontinuity not only with colluvial deposits but also with the rest of the buried unit. Consequently, the origin of this A horizon can be explained by the accumulation of a new material, made by organic matter scarcely mineralized. This hypothesis does not contrast with the supposed colder climate.

The anthracological assemblage in 2Ab horizons shows the presence of Laburnum sp., Abies alba, and Vaccinium sp. In comparison, in 2Bw horizons, Vaccinium sp. is completely absent (Compostella et al., 2012). The two different assemblages suggest two different vegetation covers, hence two successive phases of fire events. The 2Ab horizon was thus produced by a process of secondary pedogenesis after the first fire event. In this light, its development at the surface of the truncated paleosol took place before or during a colder climate phase, and then, a second fire event produced the charcoal found in the micromass. After this, a period of slope instability caused colluvial deposits which buried the horizon and the whole unit. These mass movements could also be responsible for the preservation of 2Ab horizons only on more stable slopes.

Environmental history

From all observations above, it is possible to draw a general diachronic framework of environmental changes (Figure 6). The 2B horizons of the deep unit indicate 2–3 genetic phases:

an initial stable period of temperate climate, possibly with contrasted seasons, allowing the formation of soils subject to brunification and clay illuviation (Duchaufour, 1983) characterized by microlaminated clay coatings;

a second stable period (or the end of the first one) still allowing the formation of the same soils but showing a different stage of illuviation;

a successive loss of vegetation (attested by coarser coatings and charcoal) with evidence of erosive events that truncate the paleosol itself.

Figure 6.

Schematic soil profile showing main features and related environmental phases.

Then, the formation of the 2Ab horizon takes place during or just before a period of colder climate (frost action). After that, a second phase of fires (again attested by charcoal) marks a subsequent loss of vegetation cover. This loss probably triggers the mass movements, which bury the old soil under a colluvial layer deposited in one or more episodes. The deposition of this colluvium, mainly constituted by material eroded from paleosols in upper topographic positions, is probably very recent considering the weakness of pedogenesis acting on this unit.

A suitable temporal sequence for these events can be inferred from external data. Soil formation starts after the late Glacial Period with the meltdown of the glaciers covering the area (Losacco, 1949). This phase of rising temperatures marks the formation of the paleosol and lasts probably for most part of the early and middle Holocene. The latter is characterized by wetter conditions, which could correspond to a period without strong seasonal contrasts. This period is also characterized by the prolonged presence of a temperate forest cover (Panizza et al., 1982) with A. alba and Laburnum sp. (Compostella et al., 2012) at least in the last part. These species are only occasionally found in the present-day forest. Radiocarbon dating of charcoal (Compostella et al., 2012) places the removal of the forest and upper part of the paleosol into the climatic recrudescence at the beginning of the late Holocene. An increase of landslide frequency is also documented for other parts of the Northern Apennines (Bertolini, 2007). The old woodland cover is replaced by a woody heathland dominated by Vaccinium and Laburnum species (Compostella et al., 2012), forming the 2Ab horizon in response to the cooler climate of the late Holocene. Charcoal suggests again the removal of vegetation by fires around the beginning of Modern Age (Compostella et al., 2012). Some authors relate these fire events to the expansion of human activity in the nearby valleys (Panizza et al., 1982). The insufficient plant cover and the effect of the ‘Little Ice Age’ are most likely the trigger of the new period of slope instability; there is evidence of repeated colluvial events until present day.

Conclusion

This study not only led to the identification of substantial treeline shifts, contrasting with the present-day apparent static behavior (Compostella et al., 2012), but also allowed to link them to specific environmental drivers. A stable forest cover occurred well above the present treeline (at least 100 m), throughout two distinct climatic phases, both warmer than present, as assessed by brunification in thin sections. Tentatively, they should correspond to the early and middle Holocene. The successive lowering of the treeline appears as the result of a complex combination of climatic forcing and disturbance events related to late Holocene and Modern Age dynamics. All these events left distinctive traces in the soil as microscopic pedofeatures, which could be recognized and interpreted. The combination with other disciplines (macroremain analysis, pedoanthracology) could place such events in a suitable temporal framework and provide information about vegetation cover and plant species assemblages. Such a multidisciplinary approach may help the comprehension of the present dynamic pattern of the treeline and the modeling of its future response to a changing climate.

Through these findings, the value of micromorphological analysis within the context of paleoenvironmental research becomes evident. An important contribution comes from its higher level of resolution in outlining different phases of soil development compared with field observation only. In this way, it provides a fundamental role in designing a framework where extremely various data from different disciplines can be placed. Micromorphology also allows the extrapolation to wider areas of data available only for single sites, such as plant species characterization provided by charcoal assemblages. This multidisciplinary approach is essential to fully investigate and date environmental dynamics.

Footnotes

Acknowledgements

The authors are grateful to Marco Caccianiga and Arcangelo Matarrese for their assistance on the field and in the laboratory and for their valuable help in the interpretation of paleoenvironmental data.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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