Réf. Tinner & al. 1999 - A

Référence bibliographique complète

TINNER, W, HUBSCHMID, P, WEHRLI, M, AMMANN, B, CONEDERA, M. 1999. Long-term forest-fire ecology and dynamics in southern Switzerland. Journal of Ecology, Vol. 87, 273–289. PDF

Abstract:
1. Pollen and charcoal analysis at two lakes in southern Switzerland revealed that fire has had a prominent role in changing the woodland composition of this area for more than 7000 years.

2. The sediment of Lago di Origlio for the period between 5100 and 3100 BC cal. was sampled continuously with a time interval of about 10 years. Peaks of charcoal particles were significantly correlated with repeated declines in pollen of Abies, Hedera, Tilia, Ulmus, Fraxinus excelsior t., Fagus and Vitis and with increases in Alnus glutinosa t., shrubs (e.g. Corylus, Salix and Sambucus nigra t.) and several herbaceous species. The final disappearance of the lowland Abies alba stands at around 3150 BC cal. may be an example of a fire-caused local extinction of a fire-intolerant species.

3. Forest fires tended to diminish pollen diversity. The charcoal peaks were preceded by pollen types indicating human activity. Charcoal minima occurred during periods of cold humid climate, when fire susceptibility would be reduced.

4. An increase of forest fires at about 2100 BC cal. severely reduced the remaining fire-sensitive plants: the mixed-oak forest was replaced by a fire-tolerant alder–oak forest. The very strong increase of charcoal influx, and the marked presence of anthropogenic indicators, point to principally anthropogenic causes.

5. The authors suggest that without anthropogenic disturbances Abies alba would still form lowland forests together with various deciduous broadleaved tree taxa.

Mots-clés
Abies alba -Charcoal analysis - Fire history - Pollen analysis - Southern Alps - Vegetation history

Organismes / Contact

• Institute of Geobotany, Section Palaeoecology, University of Bern, Altenbergrain 21, CH-3013 Bern, Switzerland - Correspondence: Willy Tinner, tinner@sgi.unibe.ch
• Swiss Federal Institute for Forest, Snow and Landscape Research, Sottostazione Sud delle Alpi, PO Box 57, CH-6504 Bellinzona, Switzerland


(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
  Vegetation Forest fire  

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Southern Switzerland Canton of Ticino • Lago di Origlio
• Lago di Muzzano
  • 416m a.s.l.
• 337m a.s.l.
The Holocene

(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

There was more than 4m of minerogenic sediment below the deepest dated sample at Lago di Origlio [14 520 80 BP (uncal.)], but the authors assumed lake sedimentation started at 16 500 BP (deglaciation in this area occurred between 18 000 and 15 000 BP; Wohlfarth 1993).

VEGETATION HISTORY

The vegetation history of the area around Lago di Origlio is summarized in Table 1. A more detailed description of the vegetation history of Lago di Origlio and Lago di Muzzano will be published subsequently. The correlation of the two sites, based on radiocarbon datings [Fig. 2], suggests very similar vegetation histories [Fig. 4]. The only major difference is that afforestation started (at around 14 200? BC cal., 13 500 BP) with Pinus cembra being the dominant tree at Lago di Origlio but with Betula at Lago di Muzzano. At the beginning of the Holocene (c. 9200? BC cal., 10 000 BP) the Late Glacial woodlands were replaced by continental forest and shrublands (Table 1). About 2000 years later, Abies and Alnus glutinosa t. expanded (7200 BC cal., 8200 BP), probably indicating a shift to more oceanic conditions. Regular findings of Vitis and Hedera pollen, as well as the more or less stable representation of Quercus (deciduous), Tilia and Ulmus, testify to the thermophilous character of these lowland Insubrian forests, where Abies alba was mixed with deciduous broadleaved trees.

Despite marked fluctuations in abundance, a more or less stationary floristic composition characterizes the period between 5000 and 3000 BC cal. (6100-4400 BP). Five Abies declines are detectable at Lago di Origlio, when Abies and other taxa were temporarily replaced mainly by Corylus and Alnus glutinosa t. (Table 1). Regular findings of Plantago lanceolata and Cerealia pollen point to the anthropogenic nature of these disturbances and the presence of various sub-mediterranean and mediterranean plants (Ostrya, Fraxinus ornus, Vitis, Staphylea pinnata, Buxus, Quercus ilex t., Olea, Phillyrea, Pistacia) and of evergreen taxa partly preferring (sub)atlantic conditions (Ilex, Viscum, Hedera) suggests further warming of the climate. After the local disappearance of Abies (last stomata recorded c. 2800 BC cal., 4200 BP). Fagus finally had a chance to expand (> 5%) at about 2700 BC cal. (4100 BP), although this late variant only lasted until c. 2150 BC cal. (3750 BP) before replacement with oak-alder forests (Table 1). When Castanea sativa was introduced by the Romans (AD 50–100 cal., 1800 BP), it spread with surprising success, becoming the dominant tree of the lowland forests. High values of anthropogenic indicators and of non-arboreal pollen suggest continuous rather intensive land use since 2150 BC cal. (3750 BP).

Age BC/AD cal.

Age BP uncal.

LPAZ

Vegetation type

Important taxa (decreasing relevance)

Thermophilous plants

1994-200 AD

Present-1800

OR 21-23

Chestnut  forests and farm lands

Castanea  sativa, Quercus (deciduous), Alnus glutinosa, herbs, cultivated  plants, neophytes          

+ + +

200 AD-2150 BC

1800-3750

OR 16-20

Oak-alder forests and farm lands

Alnus glutinosa, Quercus (deciduous), Pteridium aquilinum, Betula, Calluna vulgaris, herbs, cultivated plants

+ + (+)

2150-3000 BC

3750-4400

OR 15

Late Insubrian forests                      

Alnus glutinosa, Fagus sylvatica, Tilia, Quercus "deciduous#, Fraxinus excelsior, Ulmus, Corylus avellana

+ + (+)

3000-5000  BC

4400-6100

OR 9-14

Disturbed  Insubrian forests

Abies alba, Tilia, Fraxinus excelsior, Ulmus, and Hedera helix were regularly replaced  by Corylus avellana, Alnus glutinosa and in part Betula

+ + +

5000-7200 BC

6100-8200

OR 7-8

Insubrian forests

Abies alba, Tilia, Alnus glutinosa, Quercus "deciduous#, Fraxinus  excelsior, Ulmus, Corylus avellana, Hedera helix

+ + (+)

7200-9200? BC

8200-10 000

OR 5-6

Temperate continental forests and shrublands

Pinus sylvestris, Quercus "deciduous#, Corylus avellana, Ulmus, Betula,Fraxinus excelsior, Tilia

+ +

9200?-12 700 BC

10 000-12 500       

OR 3-4

Late Glacial forests

Pinus sylvestris, Betula

+

12 700?-14 200? BC

12 500-13 500

OR 2

Open Late Glacial forests

Pinus cembra, Betula, P. sylvestris

14 200?-17 500? BC

13 500-16 500?

OR 1

Late Glacial tundra  steppes

Artemisia, Poaceae and other herbs. Since 14 700 BP Juniperus is locally present

Table 1. The vegetation history of Lago di Origlio:
LPAZ = Local pollen assemblage zones; OR = Origlio.
+ = Thermophilous plants were present but not dominant, mixed oak forest < 5%.
+ + = Thermophilous plants were dominant.
+ + (+) = Thermophilous plants were dominant, submediterranean and mediterranean were occasionally present.
+ + + = Thermophilous plants were dominant, submediterranean and mediterranean plants were regularly present.


FIRE HISTORY AND VEGETATION CHANGES DURING THE HOLOCENE
...

[Cf. below]

Observations

The present dynamics of the vegetation of southern Switzerland are influenced by complex changes of both anthropogenic and natural origin. Extensification of land use has occurred during the last 30 years, for example with the abandonment of forest use. This has led to increased accumulation of dead biomass and increased fire risk, and to the invasion by various tree species of lowland forests (up to about 900m a.s.l.) that had long been dominated by Castanea sativa] the chestnut that is still characteristic of the cultural landscape has become less dominant or even displaced (Conedera et al., in press). At the same time the abandonment of farmland has led, via several successional stages, to natural reforestation. The invasion of former farmland by shrubs also produces an accumulation of easily burned material, such as dead wood, dry leaves and herbaceous litter. A sudden rise, starting in 1960s, has been noted in the number of forest fires (Conedera et al., 1996a; Tinner et al., 1998), so that at the present time fire is one of the most important disturbance factors in lowland forests of the Swiss southern Alps (Delarze et al., 1992; Conedera et al., 1996b; Hofmann et al., 1998).

About 15% of the forested areas of southern Switzerland have burned more than once during this century and every year about 3% of the forest surface is affected by fire. The vegetation shows distinct patterns of response to fire, which from the study of chrono-sequences appears to lead to the dominance of a few fire-adapted species (Delarze et al., 1992; Hofmann et al., 1998). Because fire affects species composition at scales of years to centuries (Sugita et a., 197), direct observation of the full range of post-fire vegetational changes is hardly possible (Finegan 1984). Palaeoecological tools, such as high-resolution pollen and charcoal analysis, can overcome this difficulty, and so provide important evidence for the long-term effects of fire upon ecosystems (Birks 1997; Bradshaw et al., 1997).

Modélisations

 

Hypothèses

 


Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

[See below]


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

FIRE HISTORY AND VEGETATION CHANGES DURING THE HOLOCENE

Table 2 shows the pattern of charcoal influx for five major zones that describe the fire history of southern Switzerland. Zones FS-1 to FS-5 can be identified in the profiles at Lago di Origlio and Lago di Muzzano [Fig. 4] and influx and concentration values are similar at the two sites. Detailed comparison shows that fire history is related to vegetation changes with, for example, Pteridium aquilinum accompanying the charcoal oscillations throughout the Holocene and Calluna vulgaris doing so after about 2150 BC cal. (3000 BP) [Fig. 3]. A decrease of the charcoal values at the transition from FS-4 to FS-5 coincides with the introduction of Castanea sativa [Fig. 4] and follows a decrease in Alnus glutinosa t. at the last charcoal peak of zone FS-4. This led Tinner & Conedera (1995) to conclude that fire was probably used for clearance to allow introduction of Castanea, but was then abandoned once woodlands were cultivated.

RCZ

Age BC/AD cal.

Age BP

Average charcoal influx (mm² cm² year-1)

FS-5

1960-100 AD

Present-1900

Medium (c. 10)

FS-4

100 AD-1400 BC

1900-3200

High (c. 40, maximum of 120)

FS-3

1400-5000 BC

3200-6100

Medium (c. 10)

FS-2

5000-8300 BC

6100-9300

Low (c. 5)

FS-1

8300-13 000 BC

9300-12 700

Very low (c. 1)

Table 2. The regional fire history of southern  Switzerland (Lago di Origlio and Lago di Muzzano)


FIRE ECOLOGY AND VEGETATION DYNAMICS BETWEEN 5000 AND 3100 BC CAL.

The pronounced vegetation fluctuations between 5000 and 3100 BC cal. (6100–4450 BP) ended with the regional extinction of Abies alba [Fig. 4]. The few natural stands of Abies alba in southern Switzerland today are restricted to altitudes between 8000 and 1700m a.s.l. (Ellenberg & Klötzli 1972; Cotti et al. 1990), at least 400m higher than Lago di Origlio and Lago di Muzzano. It has been suggested the disappearance of the lowland thermophilous Abies alba stands might be due to the drier climate around 5150 BC cal. (6200 BP), which could have dis! advantaged Abies alba (Schneider & Tobolski 1985; Auguadri et al. 1986), a pest attack (Wick Olatunbosi 1996), disease (Wick Olatunbosi 1996) or increasing anthropogenic pressure (Wick 1989). However, a preliminary study (Tinner & Conedera 1995) suggested that the fluctuations and final disappearance of Abies, as well as the transition from mixed-oak to oak forests\, could have been caused by forest fires: the present paper presents the results of a more detailed study in which the sample interval is approximately 10 instead of 300 years. Cross-correlations were carried out between charcoal influx and pollen percentages for taxa that showed significant fluctuations.

Decreasing taxa

The major taxon to decline was Abies, and Fig. 5 shows that decreases (LPAZ 9, 11, 13, 14b, 15) were regularly accompanied by increases in charcoal influx and in both influx and percentage of Corylus. Similar patterns were found at Lago di Muzzano, although with events occurring around 100–200 years later (data not shown). The cross-correlogram of charcoal influx vs. Abies percentages (Fig. 6) shows significant negative correlation coeficients at most intervals, but most marked lag 0, probably reflecting fire sensitivity and the subsequent lack of regeneration in this taxon.

Pollen types of other taxa known to be reduced after forest fire (Delarze et al. 1992; Hofmann et al. 1998) showed similar patterns; of these Fraxinus excelsior t., Tilia, Ulmus and Hedera (Fig. 7) declined during periods with high charcoal influx, and their cross-correlograms [Fig. 6] show significant negative correlation coeficients. Although Ilex aquifolium pollen is extremely rare in lake deposits (Faegri & Iversen 1989), it was found at Lago di Origlio where the charcoal values were minimal [Fig. 7]. Fagus values remained very low (< 2%) although there was a slight expansion between 5100 and 3100 BC cal. The correlogram of charcoal vs. Fagus shows significant negative correlation coeficients, indicating that Fagus itself is fire sensitive and could therefore not benefit from declines in the dominant Abies. A final pollen type showing significant negative correlation coeficients was Vitis, probably the now rare V. sylvestris, the wild grape (Hegi 1975).

Plant taxa had different recovery rates: Fraxinus excelsior seems to have been the fastest of all, followed by Hedera and then Tilia, Ulmus and Abies, Fagus and Vitis showed delayed and hesitant restorations. Forest plants with a slow recovery rate tend to decrease when fire frequency increases (Delarze et al. 1992) in contrast with species that are able to recover rapidly after fire, including the short-term opportunists.

Increasing taxa

Corylus increased simultaneously with the charcoal particles and regularly replaced Abies [Fig. 5]. The highest positive correlation coeficient between charcoal particles and Corylus [Fig. 6] was observed with one positive lag interval (c. 12 years after the charcoal peak).

Other taxa showing increases together with charcoal particles were: Alnus glutinosa t. (albeit with a lag of 30 years), Humulus t., Salix and Sambucus nigra t. [Figs 6 and 8]. The significant positive cross-correlation coeficients of Nymphaea alba t. and Typha latifolia t. are probably due to eutrophication caused by anthropogenic activities shortly before the fire (see fre precursors), inwashed ash and soil erosion during and after the forest fires (Wright 1981; Birks 1997; Bradshaw et al. 1997).

Short-term opportunists

For Cichorioideae, Rosaceae, Trifolium repens t. and Mentha t. (including Thymus, Origanum and other Lamiaceae), positive cross-correlations are restricted to a very short time period (observed at lag 10; [Fig. 6]). This temporary increase seems to be typical of most upland herbs, as indicated by the correlogram charcoal vs. herbs (all upland taxa combined, Poaceae excluded). Anemone nemorosa shows a similar correlogram type but with a longer-lasting positive cross-correlation [Fig. 6].

Fire precursors

The correlograms of the Poaceae, Caryophyllaceae, Pteridium aquilinum, Plantago lanceolata, Artemisia, Quercus and Betula show significant cross-correlations ahead of the charcoal peaks. This may be due to the fact that, for different reasons, all are known to be anthropogenically favoured. The strongest cultural indicator among them is Plantago lanceolata, an adventive plant occurring on anthropogenically disturbed places and in meadows (Behre 1981; Lang 1994). Poaceae, Caryophyllaceae, Pteridium aquilinum, Artemisia, Quercus and Betula suggest an increase in light and might indicate grazing in slightly open forests (forest pasture). It is, however, remarkable that compared with the charcoal influx the Cerealia do not show significant cross-correlation coeficients (data not shown).

Floristic diversity

Rarefaction analysis was used to estimate pollen diversity (Birks & Line 1992). Figure 9 shows that the pollen diversity (pollen sum = 618) had major ~uctuations from 5100 to 3100 BC cal. and normally did not reach values typical for the 19th century (45–58; Tinner et al. 1998). The decrease of the floristic richness after forest fire is confirmed by the significant negative correlations between charcoal influx and pollen diversity (lags 3–12 [in Fig. 6; and by Fig. 9]). Pollen diversity seems more or less to follow the Abies curve [Fig. 9], a sign that the unburned fir stands were relatively rich in species, as shown by the correlogram Abies vs. pollen diversity [Fig. 6].

Observations

The present dynamics of the vegetation of southern Switzerland are influenced by complex changes of both anthropogenic and natural origin. Extensification of land use has occurred during the last 30 years, for example with the abandonment of forest use. This has led to increased accumulation of dead biomass and increased fire risk, and to the invasion by various tree species of lowland forests (up to about 900m a.s.l.) that had long been dominated by Castanea sativa] the chestnut that is still characteristic of the cultural landscape has become less dominant or even displaced (Conedera et al., in press). At the same time the abandonment of farmland has led, via several successional stages, to natural reforestation. The invasion of former farmland by shrubs also produces an accumulation of easily burned material, such as dead wood, dry leaves and herbaceous litter. A sudden rise, starting in 1960s, has been noted in the number of forest fires (Conedera et al., 1996a; Tinner et al., 1998), so that at the present time fire is one of the most important disturbance factors in lowland forests of the Swiss southern Alps (Delarze et al., 1992; Conedera et al., 1996b; Hofmann et al., 1998).

About 15% of the forested areas of southern Switzerland have burned more than once during this century and every year about 3% of the forest surface is affected by fire. The vegetation shows distinct patterns of response to fire, which from the study of chrono-sequences appears to lead to the dominance of a few fire-adapted species (Delarze et al., 1992; Hofmann et al., 1998). Because fire affects species composition at scales of years to centuries (Sugita et a., 197), direct observation of the full range of post-fire vegetational changes is hardly possible (Finegan 1984). Palaeoecological tools, such as high-resolution pollen and charcoal analysis, can overcome this difficulty, and so provide important evidence for the long-term ffects of fire upon ecosystems (Birks 1997; Bradshaw et al., 1997).

Modélisations

 

Hypothèses

 


Paramètre de l'aléa
Sensibilité des paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

Controlling factors
Frequency

 

The present paper primarily addresses two questions: (i) whether and how fire changed the composition of the lowland woodlands of the Swiss southern Alps, and (ii) whether fire during the Holocene was of natural or anthropogenic origin.

Two parallel cores 1m apart were taken with a piston corer from the deepest point of both lakes; 19.55m of lake sediment were cored at Lago di Origlio, and 16.45m at Lago di Muzzano. The core sections analysed consisted of a uniform silty finedetritus gyttja at Lago di Muzzano and of discernible layers of gyttja and silt at Lago di Origlio. Pollen identification, charcoal analysis and radiocarbon datings of terrestrial macrofossils were performed. Lag effects of one variable on another one (e.g. pairs of pollen types, charcoal particles vs. pollen) were studied with cross-correlations. [See details and references in the paper]


(4) - Remarques générales

Cf. Discussion p. 284–287.


(5) - Syntèses et préconisations

 

Références citées :

Cf. References p. 287–289.