Late Quaternary Vegetation Climate and Fire Dynamics

IN sOUTHERN bRAZIL

Several palaeoecological studies from the Araucaria forest and Campos regions of the southern Brazilian highlands have been carried out in the last decades (Behling 2002) (Figure 2). Data from the states of Paraná (Serra Campos Gerais: Behling 1997), Santa Catarina (Serra do Rio Rastro, Morro da Igreja, Serra da Boa Vista: Behling 1995) and Rio Grande do Sul (Aparados da Serra: Roth and Lorscheitter 1993; Sao Francisco de Paula: Behling et al. 2001; Cambará do Sul: Behling et al. 2004) have proved that extensive areas of Campos vegetation existed on the highlands through glacial, early and mid Holocene times. The dominance of Campos vegetation was attributed to cold and dry glacial, and warm and dry early Holocene climates.

Figure 2. Photo showing a mosaic of Araucaria forest and grassland on the southern Brazilian highland (Photo Behling).

A dry season lasting probably about three months per year was characteristic for the early and mid Holocene period (Behling 1997, 2002). Initial expansion of Araucaria forests started by migration from the gallery forests along the rivers about 3210 calibrated radiocarbon years before present (cal yr BP), which indicates a turn to somewhat wetter climates. A marked expansion of Araucaria forests started on the highlands, replacing Campos vegetation in Santa Catarina State about 930 cal yr BP ago, and in Paraná State (Serra Campos Gerais) about 1400 cal yr BP ago, reflecting a very humid climate without a marked seasonal dry period.

Pollen data are also available from the lowland Campos (grassland) region near the town of Sao Francisco de Assis in the western Rio Grande do Sul State (Behling et al. 2005). The region was naturally covered by Campos throughout the recorded glacial and Holocene period under cold and relatively dry and warm and dry condition, respectively. Initial expansion of gallery forest after 5170 cal yr BP indicates a change to wetter climatic conditions. Maximum extent of gallery forest after 1550 cal yr BP reflects the wettest recorded period.

Detailed high resolution data are available from a peat core from a shallow basin about 7 km distance to the village Cambará do Sul (Figure 3) spans the last 42,840 uncalibrated radiocarbon years before present (uncal yr BP). A more complete palaeoenvironmental data set and reconstruction, including records of selected single pollen and spore taxa have been published in Behling et al. (2004). The pollen record from Cambará do Sul also documents the dominance of a diverse Campos vegetation between 42,840 uncal and 11,500 cal yr BP. The highland region of northwestern Rio Grande de Sul State was probably almost treeless.

The evidence of a few single pollen grains representing Araucaria forest and Atlantic rain forest vegetation in the late Pleistocene sediments were most likely wind transported over some distance from possible forest refugia in deep and protected valleys in the highlands or from the 6-7 km distant slopes of the Serra Geral mountains or from the lowland vegetation of the coastal region, respectively.

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Figure 3. Percentage (a) and summary (b) pollen percentage diagram for Cambará do Sul, including the radiocarbon dates, age scale (cal yr BP), records of different taxa and ecological groups (Campos, Araucaria forest, Atlantic rainforest, Ferns), concentration and influx of carbonised particles, and pollen zones. For more details see Behling et al. 2004).

Figure 3. Percentage (a) and summary (b) pollen percentage diagram for Cambará do Sul, including the radiocarbon dates, age scale (cal yr BP), records of different taxa and ecological groups (Campos, Araucaria forest, Atlantic rainforest, Ferns), concentration and influx of carbonised particles, and pollen zones. For more details see Behling et al. 2004).

The widespread treeless Campos vegetation, indicate cold and dry climate conditions. Repeated frosts events and minimum austral winter temperatures below -10 °C, which do not permit the growth of Araucaria on the highlands is suggested. Mean annual temperature depression was probably similar to the southeastern Brazilian highland which was in the range of 5-7 °C between about 26,000 and 17,000 uncal years ago during the Last Glacial Maximum (LGM) period (Behling and Lichte 1997). During the LGM, the driest period recorded, pollen of Eryngium was abundant indicating drier conditions, and the shallow lake at that time was not permanent, indicating a seasonally dry climate. Based on the Cambará do Sul record, it is suggested that seasonal climatic conditions developed after 26,900 uncal years ago. Seasonal climate with a long annual dry period since the LGM period prevailed until the late Holocene.

Campos vegetation still dominated the landscape during early and mid Holocene (11,500 to 4320 cal yr BP). Species of the Araucaria forest increased slightly but were still rare, indicating that populations migrated into the study region, probably along small streams. Atlantic rain forest taxa became more common, suggesting an expansion on the Serra Geral coastal slopes closer to the study site. Extensive grasslands coupled with the rare occurrence of Araucaria forest taxa, suggest a dry climate. Changes in the composition of the Campos vegetation, reflect a change to a warm and dry climate. The annual precipitation must have been lower than 1400 mm and the climate was seasonal, with a dry season lasting about 3 months. Apparently the climatic conditions did not favor expansion of Araucaria forest in the study area during the early and mid Holocene.

Only during the first part late Holocene period (4320 to 1100 cal yr BP), Araucaria forests expanded in the region around Cambará do Sul, forming a net of gallery forests along streams, while regionally grassland vegetation dominated. Araucaria forests included populations of Myrsine, and less frequently individuals of arboreal such as Mimosa scabrella, Myrtaceae, Podocarpus and Ilex. Dicksonia sellowiana, a tree fern was already common in the gallery forests. Since the late Holocene the Atlantic rain forest and probably also cloud forest species were well established on the upper coastal slopes, located about 6 - 10 km distance to the study site. During the second period of the late Holocene (1100 - 430 cal yr BP), a remarkable strong expansion of Araucaria forest taxa took place, primarily Araucaria angustifolia and Mimosa scabrella, within 100 years replacing Campos vegetation. During the uppermost late Holocene period (430 cal yr BP - modern) still Araucaria forests kept expanding, reducing the Campos area in the study region. The expansion of Araucaria forests, including tree ferns, since 4320 cal yr BP and specially after 1100 cal yr BP is apparently related to a change to a wetter climate, with higher rainfall rates and a shorter annual dry season, or no marked dry season.

The results from Cambará do Sul indicate a complete replacement of the original grassland by forest in the surrounding of the site. Other above mentioned records such as Serra Campos Gerais or Serra da Boa Vista document the formation of a mosaic of forest and grassland during the late Holocene.

The results of pollen and charcoal analysis at lowland site at Morro Santana (Behling et al. 2007) indicate the occurrence of a mosaic of taxonomically highly diverse forest and Campos vegetation that were under the influence of fire during the late Holocene period. Between 1230 and 580 cal yr BP, Campos taxa were well represented, while forest taxa were relatively less abundant. The Campos vegetation was primarily composed of species of the Poaceae, Cyperaeceae and Asteraceae families, with the genera Eryngium and Eriocaulon also being important taxa. Some Cyperaceae may also originate from the studied swamp. Forest taxa, such as Moraceae/Urticaceae, Myrtaceae and Dodonaea, were rare. At about 580 cal yr BP, there is a change in the composition of the Campos vegetation by the increase of Baccharis and the decrease of Eryngium, among others. This trend is indicated by phase transitions in the vegetation composition around the same period and as well between 380 and 300 cal yr BP. This can be interpreted as an indicator of a decrease in fire frequency.

Baccharis, which was negatively correlated to charcoal concentration, is the most important genus of grassland shrubs in the present-day vegetation, leading to development of shrubland in the absence of fire over longer periods (Müller et al. 2007). Species of the genus Eryngium, positively correlated to charcoal concentration, have been characterized as disturbance specialists profiting from frequent fires in abundance and reproductive output (Fidelis et al. 2008), and thus should diminish in importance with greater fire intervals or - as grassland species - with forest encroachment. During the following period until the present, forests expanded continuously, especially by the increase of the pioneer Myrsine. The marked higher representation of Myrsine, which was negatively correlated to charcoal concentration, may indicate a change in the disturbance regime.

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