HOLARCTIS: The Holocene Development of the Caspian Forests: A Palynological Study with Silvicultural Applications
Zusammenfassung der Projektergebnisse
Many tree species that were widespread over the Northern Hemisphere during the warm Pliocene period became extinct in Europe and northern Asia during the Pleistocene ice ages, but can still be found in the Hyrcanian forests of Iran and Azerbaijan and the Colchic forests of Georgia. Whereas these areas are generally called ‘refugia’, it was completely unknown how and where precisely the species survived the last ice age, as the lack of fossil bearing deposits prohibited palaeo-ecological studies in this area. The work in and associated with this DFG project has changed this situation fundamentally. Not only could several suitable deposits from the Caspian coastal plain and from various altitudes in the adjacent Elbrusz mountains be found and studied. The biggest surprise was undoubtedly the discovery of Pay Hassal, a continuous 12 m deep deposit of peats and lake sediments that appeared to cover the last 19,000 years up to the present, including the Late Glacial Maximum (LGM), the coldest phase of the last Ice Age. The study of Pay Hassal revealed that before the LGM (19,500-16,900 years ago), sparse stands of beech, oak and birch, associated with elm and hornbeam surrounded the study site. High abundances of herbs and buckthorn suggest that at higher elevations a dry or cold environment prevailed. The period from 16,900 to 14,500 years ago was characterized by an expansion of oak and a strong decrease of beech. Pollen of herbs still dominated the pollen assemblage and indicated, together with the continuous presence of the typical steppe species Eremurus, the prevalence of dry and cold steppe. 14,500-13,150 years ago a substantial expansion of trees (oak, hornbeam, elm) and a strong decrease of herbs took place, reflecting the Allerød interstadial, a warm phase at the end of the last Ice Age. From 13,150 to 12,100 years ago the decline of forests and pronounced expansion of herbs persuasively reflect the Younger Dryas cold episode. The early Holocene (12,100-8,100 years ago) shows increased values of oak, elm and hornbeam, abundant willows and strongly diminished occurrences of herbs. Over the last 8,500-8,100 years, oak has been replaced by beech and hornbeam indicating the establishment of contemporary, i.e. temperate, climatic conditions. Alder pollen whose values were hitherto moderate, turned around 3,200 year ago into one of the main pollen types. Pterocarya started to expand around the same time, but experienced a dramatic decline around 800 AD, a feature also observed in other pollen studies from northern Iran and Georgia. The occurrence of walnut pollen since 1,350 years ago may point to the cultivation of walnut in the Caspian region of N Iran. The study of Pay Hassal, two other sites, and a pollen/vegetation surface transect has added substantially to understanding the development, natural composition and dynamics, human impact and current character of the Hyrcanian forests. With the perspective of climatic variability and human impact increasing over the next 50–100 years, it is essential to inform forestry and conservation policies on their natural dynamics, so that the forests can be managed within the range of natural variability. As the bottom of the sediment at Pay Hassal has not been reached, future research can extend deeper into the last Ice Age. Its long time coverage and high temporal resolution will make Pay Hassal to one of the key sites for paleo-environmental studies in Southwest Asia. The discovery of Pay Hassal and the other deposits during this project shows that – with ample field knowledge - further promising deposits at higher and lower elevations may be found. Their study will greatly improve our understanding of vegetation responses to climate changes in the past … and in the future.
Projektbezogene Publikationen (Auswahl)
- 2008. The late-Holocene vegetation history of the Central Caspian (Hyrcanian) forests of northern Iran. The Holocene 18, 305–319
Ramezami, E., Marvie Mohadjer, M.R., Knapp, H.-D., Ahmadi, H. & Joosten, H.
