Réf. Caracaillet & Brun 2000 - A

Référence bibliographique complète

CARCAILLET, C., BRUN, J.-J. 2000. Changes in landscape structure in the northwestern Alps over the last 7000 years: lessons from soil charcoal. Journal of Vegetation Science, Vol. 11, 705–714. [PDF]

 

Abstract: Current land-use abandonment and the current rise in temperature in the Alps both suggest that tree limits may change. When it is assumed that the climate of the early mid-Holocene between 8000 and 5000 yr before present is analogous to that of the predicted climate of the late 21st century, palaeo-ecological studies of the early Holocene may provide data for the prediction of the vegetation pattern in a century from now. It appears that mid-Holocene charcoal assemblages can be used to reconstruct the spatial patterns of the vegetation before, or during, the practice of slash-and-burn.

Correspondence analysis (CA) of charcoal assemblages shows that an important ecological gradient is determined by elevation. However CA also shows that charcoal assemblages in profiles between 1700 and 2100 m a.s.l. are roughly stratified: the more recent assemblages from the topmost centimetres of soil are intermediate between the lowermost assemblages and assemblages from higher elevations. This suggests that the woody communities at the highest elevation were located at lower elevations at a later date. The taxonomic diversity of the soil charcoal assemblages has been compared to that of present-day phytosociological relevés after transformation to charcoal-equivalent data. This comparison revealed that the vegetation pattern along the altitudinal gradient in the mid-Holocene was different from that at present. The assemblages indicate that some communities disappeared, that Picea is a late-Holocene invading species, and that there is no strict modern analogue for the vegetation structure prior to that of 3000 yr ago. The past structure of the woody vegetation was also different from that of today. Although past vegetation is not a good analogue for predicting future vegetation patterns, it still has potential as an indicator for the potential presence of tree species where there is none today. If we assume a temperature rise, and take into account current trends of landscape use abandonment, then we can expect strong vegetation dynamics at the upper tree line in the future: Abies alba may expand to occupy elevations of ca. 1800–2000 m in mixed communities with Picea abies, Pinus sylvestris and hardwood species, and Pinus cembra may expand up to 2500–2700 m a.s.l.

Mots-clés

Altitudinal gradient - Present-day vegetation - Holocene - Paleo-ecology - Pedo-anthracology - Savoy - Taxonomic richness - Tree limit - Vegetation pattern

 

Nomenclature: Tutin et al. (1968-1993); the French nomenclature of soils follows Anon. (1992); Brethes et al. (1995) for humus type.

 

Abbreviations: CA = Correspondence Analysis; Cal yr BP = calendar years before present, fixed at AD 1950.

 

Organismes / Contact

• Institut Méditerranéen d’Écologie et de Paléoécologie (CNRS), Faculté des Sciences et Techniques de St Jérôme, Université Aix-Marseille III, F-13397 Marseille, France;
• Cemagref-Grenoble, Écosystèmes et Paysages de Montagne, Domaine Universitaire BP 76, F-38402 Saint-Martin d’Hères, France

 

(1) - Paramètre(s) atmosphérique(s) modifié(s)

(2) - Elément(s) du milieu impacté(s)

(3) - Type(s) d'aléa impacté(s)

(3) - Sous-type(s) d'aléa

 

 

 

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

 

 

 

 

 

 

 

(1) - Modifications des paramètres atmosphériques

Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

 

 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

 

(2) - Effets du changement climatique sur le milieu naturel

Reconstitutions

Introduction

In Europe, most ecological changes over the last 6000 yr resulted from human impact (Frenzel 1979; Berglund 1991; Pons & Quézel 1998). Because paleoecological tools are able to reconstruct woody vegetation through time, they appear well suited for determining vegetation composition and species distribution patterns before human impact. In the Alps, many palynological studies have been carried out which provide a detailed vegetation history at a subcontinental and regional level. However, one question remains, i.e. how was vegetation composition organised and structured at the landscape level? Because soils are abundant, and charcoal is well preserved in soil, soil charcoal studies are useful for fine scale spatial reconstructions at the landscape level (Carcaillet 1997). Charcoal identification details the woody plant composition at the first time of deforestation by fire, because slash and burn is a universal and historical deforestation process and because charcoal results from the incomplete combustion of wood.

Study site

Holocene vegetation history

Between 8000 and 5000 cal yr BP and during the deforestation period, the vegetation above 1900 m was mixed forest, with Abies alba, Acer spp., Pinus cembra and Pinus sylvestris below 1900 m, and Pinus cembra woodlands above 2000 m (Wegmüller 1977; David 1995; Carcaillet & Thinon 1996; Talon et al. 1998). Between 4300 and 3000 cal yr BP, the upper treeline was at ca. 2700 m and was mainly composed of Pinus cembra, Ericaceae and Juniperus (Carcaillet et al. 1998). Picea abies, which for economic reasons is currently a key species in the sub-alpine forest, immigrated ca. 2000 yr ago into the High-Maurienne Valley (Wegmüller 1977; David 1995). Picea abies has probably never spread in the catchment area of Saint-Michel-de-Maurienne where cultivation and livestock grazing has inhibited the woody vegetation for at least the past 2000 yr (Carcaillet 1998).

 

Holocene fire history and charcoal assemblages frame time

18 charcoal samples from St-Michel-de-Maurienne have been AMS 14C dated (Carcaillet 1998). They reveal a fire history which coincides with the time span of the burned vegetation as determined from charcoal identification. The fire periods are similar throughout the transect. The fires occurred all during the mid-Holocene between 7500 and 3000 cal yr BP.

Results

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Past versus modern distribution of richness along the elevation gradient

The charcoal-equivalent taxonomic richness of shrubs and trees is shown for the mid-Holocene and for the 20th century (cf. Fig. 5). It appears that the taxonomic distribution along the elevation gradient has changed during the last millennia. The most important change in tree richness distribution appears to be a decrease in number of taxa with increasing elevations up to 2700 m, while no significant distribution of tree richness occurs at present. The past tree richness between 2000 and 1700 m was higher than today at similar elevation. This more elevated abundance of tree taxa is apparent from the presence of charcoal of deciduous broad-leaved taxa, e.g. Acer opalus, A. cf. platanoides, Acer pseudoplatanus, Corylus avellana, Fraxinus excelsior and Sambucus racemosa, which are today located at lower elevations (Table 2). However, Picea abies which presently occurs in mountain and sub-alpine forests up to 2300 m where it was previously absent, contributes to this increase in tree taxa richness. The authors found that shrub richness was independent of elevation during the mid-Holocene, while a slight rise with increasing elevation occurs for current vegetation. Maximum richness occurs around the modern tree line. The lower abundance of shrubs in the past compared to that in recent assemblages may result from the spatial representation of pedo-anthracological profiles, which is smaller than that of phytosociological relevés.

 

The charcoal-equivalent richness for the main forest is shown (cf. Fig. 6), as well as for the assemblages of the most abundant tree taxa. No data are available for past Picea abies and modern Abies-Pinus sylvestris because these forest types did not exist in the High-Maurienne valley during the mid-Holocene and the present day, respectively. Statistical significance was not tested because these data have been recorded by different methods. However, it appears that total richness was similar for Pinus sylvestris forests during the mid-Holocene and the present day, despite a clear decrease in tree richness. The Abies-Pinus sylvestris assemblages had a high total richness due to increased tree richness, which was characterized by broad-leaved taxa. The modern Pinus cembra forest currently has a higher total richness characterised by shrubs than that from the mid-Holocene. Furthermore, overall tree richness has been modified bythe presence of Picea abies. Finally, Picea abies, Pinus cembra, Larix decidua and occasionally Pinus uncinata regenerate actively at the present-day upper treeline (Didier 1998), contributing to the observed increased tree taxa richness.

 

Discussion

 

Changes in landscape structure

The difference between mid-Holocene charcoal assemblages and present-day phytosociological relevés points to a switch in landscape pattern. The treeline dropped from ca. 2700 to 2400 m over that time and the pattern of tree richness changed. This difference in landscape structure between the initial mid-Holocene and the present day woody vegetation is reflected in a higher richness at elevations occupied by mixed Abies alba and Pinus sylvestris communities (Figs. 5 and 6). This mid-Holocene richness results from the occurrence of broad-leaved trees, which are today rather rare and generally located at lower elevations (Table 2). Moreover, Abies alba is currently absent from communities from this area, although Abies individuals are still scattered and rare in the Pinus sylvestris forest below 1950m.

 

Above the present Pinus sylvestris dominated forest the Picea dominated-forests correspond to the lower sub-alpine belt. Picea is also abundant in the upper subalpine belt dominated by Pinus cembra. The late-Holocene expansion of Picea had a major impact in terms of overstorey richness by changing the competitive relationship between tree species and certainly also by changing ecosystem properties through altering humus and soil qualities. The current landscape structure is not older than 2000 - 2500 yr, as a result of fire suppression (Carcaillet 1998) as well as expansion by Picea abies (Wegmüller 1977; David 1995).

 

Tree limits and trends of change

Tree limits have changed since 5000 - 7000 yr ago. Indeed, most tree taxa recorded in charcoal assemblages currently have lower upper tree limits than they had previously. This includes Pinus cembra which occupied the highest elevation both previously and presently, and which shows a drop of almost 300 m (Table 2) in upper tree limit over time. Many species showing a decrease in upper altitudinal tree limits are broad-leaved species, i.e. Acer spp., Alnus type incana, Betula, Corylus avellana, and Sambucus racemosa. Several processes can explain these changes. Firstly, these taxa are broadleaved and more susceptible to browsing by livestock and wildlife than are conifers (Eiberle & Nigg 1987; Güthorl 1994; Saint-Andrieux & Klein 1995). The abundance of domestic herbivores during summer time on mountains since the Neolithic may have played an important role in the regeneration of broad-leaved trees. Furthermore, broad-leaved species are not resistant to winter damage due to disturbances, e.g. avalanches, or to frost and thickness of snow cover. A change in tree density induces a change in snow accumulation (Plamodon et al. 1984). In open woodland, conditions for survival of saplings are less favourable. As a result, both summer livestock grazing and forest cutting at high elevations may have caused a decrease of the uppermost tree limit of most broad-leaved trees and also some conifers, i.e. Pinus cembra and Larix decidua.

 

Secondly, climatic change may also have had an impact on tree limits and upper tree lines. It is generally accepted that in Europe the upper tree lines have progressively been lowered over the past 5000 - 8000 years in mountain areas where human impact is low or insignificant (Kullman 1995) as well as in North America (LaMarche 1973; Carrara et al. 1991). The effect of this climate forcing can not be excluded. However, analysis of the various different trends of tree limit changes shows that some species have currently a higher limit, others a lower, while some have remained similar (Table 2). The range of trends observed in our study suggests that different mechanisms were important with regard to different species, and that possible effects of climate change cannot be the result of a shift in a single parameter such as temperature. Furthermore, the diversity in magnitude of drop in tree limits across tree species strongly speaks in favour of processes which do not act globally on plants. A single change in temperature should have had more or less similar effects on all tree species, rather than the range of responses across species that we observed.

 

A combination of temperature- and precipitation changes may have triggered the kind of pattern that we observed (Kullman 1995). During the mid-Holocene, Abies was more abundant and occurred in communities below 1900 m with Pinus sylvestris and P. cembra. Currently these forests have disappeared and individualsof Abies are scattered in the abundant P. sylvestris forests. The process causing these changes could result from climatic conditions in the late Holocene being drier than they are at present; Abies is less adapted to drought than P. sylvestris (Aussenac 1980; Tan & Bruckert 1992; Guicherd 1994). However, the Pinus cembra treeline has also been lowered, and P. cembra is better adapted to dry conditions than P. sylvestris (Hättenschwiller & Körner 1995). Consequently a drier climate can not be held responsible for the vegetation change.

 

Climate or human forcing?

Neither a decrease in temperature nor a drier climate are adequate in explaining, either individually or in combination, the changes in landscape vegetation that occurred since the mid-Holocene. However, plant strategies are relevant in this context. Pinus sylvestris, Populus and Fraxinus excelsior, which show increase in tree limits since the mid-Holocene, are adapted to recurrent disturbances, while less abundant species or species at a lower elevation are generally not so well adapted to these disturbances, e.g. Acer pseudoplatanus, A. platanoides, Pinus cembra and Abies alba. Moreover Abies alba, Acer pseudoplatanus and Pinus cembra are more abundant than other species in old forests relative to young forest, because they are shade-tolerant (Rameau et al 1993) and sensitive to disturbances (Delarze et al. 1992; Güthorl 1994; Saint-Andrieux & Klein 1995).

Increasing disturbance appears to be a better explanation than climate shifts for the observed landscape disorganisation and community change since the mid-Holocene. Lack of synchronization of fire history, revealed by AMS 14C datings from this study site compared with a site 10 km apart in the same valley, suggests that local processes controlled fire ignition during the Holocene (Carcaillet 1998). Fire history is roughly synchronous with human occupation (Bocquet 1997). Although it is impossible to demonstrate clearly that human activity is the sole ecological mechanism operating, our results do not support the hypothesis that current forest landscape organisation along the elevation gradient is natural. Moreover, palynological studies carried out in the Maurienne highlight severe changes of forest composition ca. 2000 yr ago, resulting in the expansion of Alnus viridis, Picea and Larix (Wegmüller 1977; David 1995). Several millennia of fire, livestock grazing, and selective cutting of trees have triggered the present-day pattern of vegetation along the elevation gradient.

Observations

Introduction

By the turn of the 20th century, large-scale land-use abandonment triggered vegetation changes in the Alps(Brun et al. 1994; Delcros 1994). The previous agricultural landscape had been invaded by woody communities dominated by either deciduous broad-leaved or coniferous species depending on the presence of species at the time of the land-use abandonment and on natural ecological forcing (Dasnias 1987; Bozon et al. 1994; Delcros 1994; Didier & Brun 1998). Since that time the cover of scrub and wood has increased, and the upper tree limit has moved up in altitude. These vegetation changes may have resulted from change of land use, but may also have been caused by climate. By the mid-19th century, a period of global warming had occurred (Overpeck et al. 1997) together with associated changes in precipitation (Flannigan et al. 1998). (…)

Study site

Present-day vegetation pattern

At the turn of the 18th century, the catchment area of St-Michel-de-Maurienne was almost totally deforested and used for agriculture (Delcros 1994). Current landuse abandonment is triggering an intense vegetation change. Between 1500 and 2000 m, plant communities are dominated by the conifers Larix decidua, Picea abies and Pinus sylvestris. Heathland and shrubland occurring above 1500 m is composed of Alnus viridis, Arctostaphylos uva-ursi, Betula pendula, Cotoneaster integerrima, Hippophae rhamnoides, Juniperus communis, Rhododendron ferrugineum, Salix spp., Sorbus aria, S. aucuparia, Vaccinium myrtillus and V. uliginosum. Above 2000 m, the area is continually grazed by livestock from late June to early October. Finally, areas above 2300 m are covered with boulders, rocks, and short-grass meadows dominated by Carex curvula and Nardus stricta.

Modélisations

Introduction

(…) Future landscape structure and community composition can be predictably applying the present-day relationship between plant distribution and climate to the most probable future climate conditions (Prentice et al. 1992; Velichko et al. 1993). However, because the present-day plant communities in the Alps are highly dynamic, the vegetation is probably not in equilibrium with climate. Consequently, the present-day pattern of vegetation is not the best analogue for future predictions. The expected temperatures of the late-21st century may be similar to those of the early mid-Holocene between 8000-6000 yr ago (COHMAP Members 1988). The study of this period is thus crucial for a sustainable management of areas subjected to land-use abandonment. Paleo-ecological studies reveal changes in biota over the last millennia, and thus provide the possibility to predict plant community structure and landscape organization for the late 21st century. The issue of predicting “the effects of global change on ecosystem composition and diversity”, is one of the most important for ecology (Tilman 1998).

(…)

Hypothèses

 

 

Sensibilité du milieu à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

This study is based on the charcoal analysis of 15 soil profiles along an elevation transect in a small catchment area in the northwest Alps, St-Michel-de-Maurienne, France. The aim was to focus on (1) the landscape structure along the altitudinal gradient given the global importance of altitude, which is one of the most important in affecting natural processes of vegetation organisation (von Humboldt 1807), and (2) comparisons between initial and modern vegetation. For both approaches a qualitative approach has been used to estimate the power of soil charcoal analysis in revealing the structure of the vegetation along the altitudinal gradient, and to compare the past with the modern landscape structure described by various methods. A qualitative analysis is more able to describe vegetation in terms of species presence and species limits than is charcoal abundance, which is dependent on flammability and combustibility of component species as well as on the biomass of species. The comparison between past and modern vegetation patterns is based on number of taxa, a basic ecological proxy of biodiversity. Saint-Michel-de-Maurienne has been chosen because the fire period has been dated by AMS 14C measurements from soil charcoal fragments (Carcaillet 1998). Fire history is similar over the whole altitudinal transect, between 7500 and 3000 cal. yr BP.

Field methods

15 soil profiles were collected along an altitudinal transect (cf. Fig. 1). Soils which are eroded, hydromorphic or disturbed by human activities were avoided, as well as soils located at the foot of steep or long slopes (Carcaillet & Thinon 1996). Sampling was conducted at various depths (10 - 15 l of dry soil per ca. 20 cm in depth) in a soil trench dug down to the bedrock when possible. Each dried profile weighed 30 to 100 kg depending on the profile depth and the organic and sand content of the horizon. This sampling method is based on the preliminary assumption that soil is stratified and displays an age/depth relationship with the oldest charcoal occurring in the lower part of the profile and the youngest in the topmost few cm.

 

Laboratory methods

A flotation procedure with a defloculant (e.g. Na4P2O7 or NaP2O4) was used to extract charcoal from the total soil sample. Flotation, with or without an ascending water flow, and with sieving followed by manual sorting under a binocular microscope, allowed for the separation of charcoal fragments from other soil particles (Carcaillet & Thinon 1996). Only charcoal particles larger than 400 ìm were extracted, because fragments smaller than 400 ìm might have been transported from other locations from up to ca. 1000 m away (Wein et al. 1987; Clark et al. 1998), and therefore resulted from a source area outside the stand. Moreover, the identification of charcoal particles smaller than 400 μm is very difficult and time consuming.

 

Identification of wood charcoals

All fragments that passed through a mesh sieve larger than 2 mm were used for identification. However, charcoal fragments of this size range are rare and comprise few taxa, and are generally derived from these species which contain the most standing biomass, e.g. Pinus cembra. Hence, 200 smaller fragments were identified per soil depth levels, i.e. 100 from 0.8 - 2.0mm and 100 from 0.4 - 0.8 mm. When charcoal fragments are not abundant, e.g. at elevations of above 2000 m, all fragments were identified. Charcoal particles were observed by using an incident light microscope (× 200, × 500, × 1000), with reference to published anatomical descriptions of wood (e.g. Schweingruber 1990), wood charcoal (Talon 1997), and a charred wood reference collection.

 

Numerical analysis of assemblages

The data were analysed by Correspondence Analysis (CA), which allows measurement of differences in composition of charcoal assemblages between samples on a statistical basis. These data analyses were performed using ADE-4 (Thioulouse et al. 1997). Levels without charcoal or with non-identifiable fragments were not taken into account. The interpretation of axes is based on the taxa and assemblages having a large contribution. In this study, an elevated contribution corresponds to a score above the average contribution of all taxa or all, assemblages for a given axis.

 

Comparison of past and modern landscape

The comparison of past and modern vegetation at thelandscape level is possible by comparison of taxonomic assemblages from different sites in a uniform landscape, as well as soil, climate, elevation and valleys. Presentday vegetation is inferred from phytosociological relevés collected in the High-Maurienne Valley (Bartoli 1966; Didier 1998), exclusively located at southern expositions and on acidic soils derived from carboniferous schists and sandstones (Table 1). Charcoal identification does not always enable resolution at the species level, but rather at the level of groups of species, genera and occasionally only family. For this reason, it has been necessary to reduce phytosociological relevés to charcoal-equivalent assemblages, followed by a comparison of past and modern charcoal taxonomic richness.

 

The resulting comparison is then established on the charcoal taxonomic richness. The richness has been calculated for two life forms proposed by Raunkiaer (1934), i.e. trees and shrubs. Because life forms take into account biological features of organisms and attributes linked to functional strategies (e.g. Floret et al. 1990; Tatoni & Roche 1994; Weiher et al. 1999), it is possible to model the landscape structure and to interpret it in terms of ecological change.

 

Conclusion

The past structure of the woody vegetation was different from that of today. The landscape pattern during the mid-Holocene cannot be a proper model the prediction of the future climate, because the late-Holocene experienced an invasion of a very competitive species, Picea abies, which is also strongly selected by forest management for economic use. However, the past landscape organisation shows evidence of the potential of trees in the area. With a warming climate and ongoing land-use abandonment, we may expect an important development at the upper tree line, similar to the previous tree line which was located around 2650 – 2700 m in the mid-Holocene. Moreover, Abies alba could be expected to expand at elevations between 1700 and 2000 m in mixed communities together with Picea abies, Pinus cembra and Pinus sylvestris.

 

(3) - Effets du changement climatique sur l'aléa

Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

 

 

Paramètre de l'aléa

Sensibilité des paramètres de l'aléa à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

 

 

 

(4) - Remarques générales

 

 

(5) - Syntèses et préconisations

 

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