نوع مقاله : مقاله پژوهشی

نویسندگان

1 دانشکده جغرافیا، دانشگاه تهران، تهران، ایران

2 مرکز پژوهش های کاربردی، سازمان زمین شناسی و اکتشافات معدنی کشور، البرز، ایران

چکیده

در مورد شرایط اقلیم دیرینه ایران، اطلاعات جامع و فراگیری وجود ندارد.غالبا مطالعات به صورت پراکنده بوده و عمدتا از نظر مکانی در مناطق غرب و شمال غرب ایران تمرکز دارند. در این پژوهش تعداد 110 اثر علمی که در زمینه اقلیم گذشته ایران با استفاده از روشهای مختلف از جمله رسوب شناسی، گرده ­شناسی و حلقه­ های درختی و ... انجام شده است، مورد بررسی قرار گرفت. با توجه به اینکه اخیرا شواهد جدیدی از تغییر پوشش گیاهی کواترنری در ایران به دست آمده ، این مقاله به اختصار به بررسی نتایج حاصل از مطالعات و تحقیقات مربوط به پوشش گیاهی کواترنری ایران پرداخته و چشم اندازهای ناشی از آب و هوای دیرینه را شناسایی نموده است. مباحث و بررسی های پوشش گیاهی دیرینه بر اساس بقایای گیاهی، حلقه های درختی، اسپور و گرده می باشد. طبق مطالعات انجام شده در شمال و غرب ایران در دوره های یخچالی شرایط اقلیمی سرد و خشک و در دوره های بین یخچالی آب و هوای گرم و مرطوب حاکم بوده است. پوشش گیاهی استپی نشان دهنده شرایط سرد و خشک و افزایش گیاهان درختی مؤید افزایش رطوبت در دوره بین یخبندان می باشد. به طور کلی چرخه های اقلیمی و رخدادهای شناخته شده در نواحی مختلف دنیا در ایران بندرت مطالعه و شناسایی شده و شناخت ما از شرایط آب و هوای گذشته و به تبع آن شرایط اکولوژی و پوشش گیاهی کشور بسیار اندک و ناچیز می باشد. برای رسیدن به شناختی دقیق و جامع از شرایط اکولوژیکی دیرینه ایران به داده های ژئوکرونولوژیکی و استفاده از شواهد و پروکسی های مختلف به ویژه برای نواحی مرکزی و جنوب ایران نیاز است. اطلاعات گرده گرفته شده از مغزه های رسوبی دریاچه ها و حلقه های درختی پتانسیل خوبی جهت شناسایی وضعیت اکولوژی دیرینه کشور را دارند.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Holocene Vegetation and Climate Changes in Iran

نویسندگان [English]

  • Ghasem Azizi 1
  • Sahar Maleki 1
  • Mostafa Karimi 1
  • Reza Shahbazi 2
  • Homa Rostami 1

1 Faculty of Geography, University of Tehran, Tehran, Iran

2 Applied Research Center, Iran Geology and Mineral Exploration Organization, Alborz, Iran

چکیده [English]

Vegetation is very sensitive to climate change and is an important factor for identifying the changes of climate and environment. In 12000 to 6000 years ago there was extreme climatic conditions in the southwest Asia with hotter and longer summers than the current conditions. As a result of such climatic conditions, many shallow lakes and seasonal rivers have been disappeared and vegetation altered. Plants that were more adapted to environmental conditions and dry seasons like annual plants includes (peas, lentils, wheat ) by the  changes in legume and shape of the grain, completed their annual life cycle. Studies have shown that the climate has changed between cold and dry conditions during glacial and hot and humid periods in interglacial in north and west of Iran. The lake sediments and paleo sol layers in loess deposits indicate there was an increase humidity during period between the last interglacial and pre end glacial period (Kehl, 2009). The Palynological results of paleo ecological sites in west of Iran have shown that there were steppes of Artemisia and chenopodiaceaes in glacial periods that represent cold and dry climate in this region. About 6500 years ago, Zagros oak forests have replaced by pistachio-oak vegetation. Studies in parts of Iran such as Zaribar Lake, Urmia Lake, Mirabad Lake and comparison with the Van Lake in Turkey showed that they have almost the same vegetation changes. In all of these studies, the end of the Yongerdryas and the beginning of the Holocene by a sudden increasing in chenopodiaceae vegetation (which represents cold and dry weather) and gramineaes ( which represents warm and dry weather) are characterized (Davoodi et al,2014). Generally, the global climate cycles and events in different parts of the Iran are rarely studied and our understanding of paleo climate and paleo ecological conditions is very insignificant. Pollen data from lakes or tree rings data have the potential for identify the paleoecology conditions in Iran.
Result and Discussion:
Climate and vegetation changes in Iran in Holocene:
Palynological studies in Komishan wetland (located on the southeastern part of Caspian Sea) has provided significant results about vegetation changes and sea level fluctuations. Delay in forest growth at beginning of the Holocene, which is in areas of eastern Turkey, there was also on the plateau of Iran and the south of the Caspian sea and in the northeastern slopes of the Alborz mountain (Gomishan). Pollynological studies and radiocarbon dating in Neur lake showed about 12800 years ago, in the late glacial ratio of trees pollens has been negligible,in the transition to the early Holocene pollens of trees and shrub vegetation such as Ephedra gradually increased. During Younger Deryas in Neur,the grasslands replaced with trees.In early Holocene at 9800 years ago Artemisia increased in Neur because it seems that there was a more milder phase in the north of Iran.  An increase in Oak and herbaceous palnts pollens such as artemisia showed that between 8000 and 9000 years ago there was wet phase in the  Neur basin. The presence of  high  amounts Alnus  pollen in the north of Iran  has shown that there were humid climate in the northern parts of Iran at 3000 years ago. Evidences suggests that highest levels of Caspian sea occurred between 1900 and 2100 years ago.

vegetation and climate in Holocene in the west of Iran

In the west of Iran during the 40000 to 20000 years ago, there was scattered vegetation (shrubs and trees). In late Pleistocene (22000 to 14000 years ago) there was arid and semi arid steppes in this region. In interglacial period in Holocene (10000-6000 years ago) precipitation increased Gramineae replaced with chenopodiaceae and Almond and Pistacia atlantica grew up such a change in vegetation indicates an increase in precipitation.In the Zagros mountain 7000 years ago Pistachio trees have existed which represent dry climate in early Holocene in this area. In mid Holocene moisture increased and vegetation was changed to Oak-Pistachio forests.In the early Holocene in western regions of Iran winter rainfall has increased and have been dry conditions in summers,In 6000 years ago in mid Holocene there was warm and humid climate in this region and Oak forests expanded. In the west of Iran, Almond and pistachio forests were more than Oaks, it indicated that humidity in spring and summer was more than other seasons in early Holocene. 
In the Last glacial  Period The Ephedraceae shrub steppes has been developed(Djamali et ak,2008).In middle of the last glacial period the water level of Lake Urmia increased, In transition between last glacial to Holocene in the west of Iran, dominant plants were ephedraceae,pistachio,Oak, Juniperus excels and Betula alba.Untill 9000 years ago dominant plants in Urmia basin was Artemisia steppes,In this basin, forests has been developed between 8000-9000 years ago.The results of Studies showed that Pollens in the west of Iran decreased in 2500 to 1500 years ago and replaced with Artemisia pollens (Talebi et al,2015). In Late Holocene Pollens such as Oak pollens has icreased (1600-1200 years ago) and there was a humid climate in the west of Iran.Also the level of Urmia lake was higher than present and the water salinity was lower.Presence of Reilla spores and low magnetic sensivity and calcium carbonate values confirmed this fact. in 1200 to 900 years ago water level in  Urmia lake decreased,  on the edges of lake ,Halophytes increased. The semi desert steppe developed in area. Pollen of Artemisia and Chenopodiaceae, Senecio vulgaris, chamomile and  Acantholimon has increased that indicate the dry and cold climate. At 650 to 450 years ago lavel of lake increased. Pollens of Juniperus represents a cold and dry climate in this region.
In the late Pleistocene and Younger Deryas period, the south Zagros had a Cold climate with steppe vegetation(Davoodi et al,2014). Davoodi et al(2014) concluded that bigining of Holocene in Parishan lake was about 10200 years ago and lasted to 8170 years ago. In this period Artemisia and Chenopodiaceae have decreased and Geraminea have increased. So we can concluded that the humidity in spring and Summer have increased but it was not enough to grow Oak trees, of course Almond and Pistachio trees grew well. Precipitation required for the growth of almond and pistachio forests is about 300 mm but for Oak forest it is 500 mm.Evidences indicated that climate in early Holocene in the southern Zagros Mountains was dry and hot. In 8170 to 7570 years ago Gramineae vegetation decreased    around Parishan Lake and cold resistant plants such as Cousinia, Umbelliferae and Cichorioideae plants has increased (والتر،1971). In this period dry climate has dominated and summer precipitation has decreased. In 7570 to 5600 yaers ago Oak forests expanded also Almond trees and Asparagus trees increased around the southern Zagros.in this period humid climate has dominated.  In western and southern parts of Iran, In 5600 to 2700 years ago Climate was warm and humid.
North west-West of Iran
The vegetation and climate of southern and southeast of Iran  in Holocene:
Distribution of grain size,petrographic characteristics and sedimentary cores chemical measurements show that:

There was a  full of water lake in Sistan basin, in the late glacial to Early Holocene. MLW(Mid Latitude Westerly currents) and ISM (Indian Monsoons) precipitations has fed the area.
In Early Holocene to Mid Holocene  ITCZ  moved to  the south so ISM weakened, as a result, dry periods started in Sistan basin. High pressure gradient between Sistan depression and Hindu Kush Mountains caused severe and persistent dust storms.
In mid Holocene to late Holocene hydroclimate regime in Sistan basin has controlled by MLW precipitations. The frequent fluctuations iin the water level of Lake Hamoon represent the unsustainable climate  in early Holocene to mid Holocene.
There are not enough information about pollens in the south-southeast of Iran and we can only reconstruct vegetation 1900 years ago. Results show that 910 years ago in this region plains turned into deserts.
Climatic anomalies in Medieval in 1145-910 years ago xerophytes grew in south and southeast of Iran. Impagidinium paradoxum in gulf of Oman Indicated the absence of ocean water drainage.

 By using pollen data and sedimental cores Miller et al (2016) reconstructed vegetation in south of Iran and reconstructed vegetation areas:
The overall dominance of Poaceae, at the expense of Amaranthaceae and Asteraceae and the decrease in clay abundance throughout the LIA suggests a shift from desert to grassland vegetation, implying wetter conditions during the LIA in S Iran. Modern ecoregions within close proximity to the coring locality where grass is a dominant component of the vegetation composition include the Zagros Mountain Forest Steppe and the Baluchistan Xeric Woodlands, where annual precipitation exceeds more than 150 mm per year, Additionally, the abundance of I. paradoxum and Dubridinium sp. is low suggesting an increased amount of freshwater discharge into the ocean. The high abundance of S. ramosus a species particularly observed where the upper water salinity conditions are reduced permanently, again suggests more freshwater discharge into the ocean near the coring vicinity. The low pollen to dinocyst (PD) ratio values during the LIA indicates more marine organic flux, likely due to increased nutrient supply promoting an increase in dinocyst populations. A decrease in Betula and a slight increase in Amaranthaceae abundance are evidenced during the last c. 100 years, which may
 
Conclusion:
Climatological and vegetative evidences suggests that cold periods in Iran have been accompanied with increase in air aridity. Probably in cold and dry periods Siberian anticyclone was strengthened and led to weakening and southward shift of monsoons. Perhaps at that time cyclones of westerlies were also weakened. In northern Iran prevailing winds were from north or northeast, have originated from central Asian deserts. Evidence of these winds are Quraqom desert sand dunes and the spatial distribution of loess deposition in Kopet Dag and north of Iran. During the glacial periods direction of prevailing winds have been northwest to northeast for tropical currents (Kehl, 2009). In the Quaternary period in Iran climate has changed several times, glacial periods has dry and cold climate than now, also many of studies concentrates on west and north parts of Iran and there is no enough data and information for the east and central parts

کلیدواژه‌ها [English]

  • Pollen
  • Sedimentary Core
  • Tree Ring
  • Paleoclimate
  • Geochronology