Understanding the effects a cold climate poses upon the Earth’s flora is vital. It allows us to estimate how certain plant species will react and to what sensitivity they can tolerate a frozen environment. As the prior glaciation of the Earth can be largely attributed to its predictable astronomical cycles, we can assume that our planet is likely going to enter another glacial period similar, or indeed, colder than that of the ‘Little Ice Age’. This is especially so if current CO2 concentrations are controlled and therefore drop. How our flora and ultimately, our ecosystems will react to this change in climate can often be discovered, among other methods, through proxy records of tree-ring analysis; otherwise known as dendroclimatology. This blog aims to recognise where, when and to what extent a future glacial advancement could affect our planet’s vegetation. As the glaciation period discussed covered vast areas of the planet when within an ice age, analysis of locations from its furthest extents are necessary. Investigation of living trees is generally incapable of giving a long-term timescale of glacial events; however fossilised wood can be used much to the same extent and can provide considerably older data, especially when aided by other proxy records. A combination of both of these methods allows a wider spectrum of information to be analysed.
It is important to understand exactly when dendroclimatology measurements begin. Live trees can rarely store more than a thousand years in their timeline, however cross sections recovered from sediments and morass have been found to date back through the entirety of the Holocene; which is currently around 11,700 years into its duration. As the late glacial maximum (LGM) ended around this time, it is possible to study events succeeding from that point onwards up to the present. It can then be interpreted what the surrounding vegetation was and how it had reacted to this glaciation. The extent to which the LGM reached is largely undisputed. The majority of Canada and the top portion of the USA were covered by the Laurentide Ice Sheet – its maximum extents were reached 15,000 years ago. It was only after this retreat of the glacier that boreal forests began to repopulate the continents affected areas, albeit in some places sparsely. It is thought that the 3 Great Lakes were formed as the receding glacier melted. Dendroclimactic studies conducted by Nishimura P. (2009) in the territories of Labrador, Canada found that various species of coniferous trees responded sensitively to relatively minor annual temperature changes. These species were also found to react differently when in a maritime or continental location, regardless of latitude. It could be assumed that the freezing temperatures of the Laurentide Ice Sheet would have cleared almost all the area of coniferous species; however it is possible that abundant numbers of trees could have survived, particularly upon east and west coast islands such as Ile d’Anticosti, which were not fully engulfed in ice. Pollen analysis in Washington State, during the time of the glacial maximum, finds that plants such as sage and grass are considerably more abundant than trees, of which only pine and spruce had any significant percentages. This area, however, falls just short of the furthest extend of Laurentide. The interglacial period after the ice sheet within British Columbia was thought to have warmer summers and colder winters than that of today’s climate (Walker, I. R.; Pellatt, M. G., 2001). These continental land masses could then have possibly seen the regeneration of trees first. Mountainous areas with high runoff from the ice sheet have increased evaporation potential which may have resulted in more rain. In most cases for these areas, increased precipitation results in enhanced tree growth (Luckman, H. B., Watson, E; 2001).
Europe during the LGM was of a very different topographic layout to that of today’s scenario. The 120 metre drop in sea level, caused by the absorption by the ice, meant that land usually submerged was now visible. The continent was much less affected by the LGM in terms of area covered, but temperatures were still considerably lower – steppe/tundra covered a significant portion of Eurasia. The main areas affected were Fennoscandia, upper Russian territories and the majority of the British Isles. As large parts of these regions are rich in boreal, taiga forestry, a vast expanse of dendroclimactic data is available. The oldest tree species in the area are Scots Pine, capable of reaching 800 years old in extreme circumstances (Haggstrom, 2005). An extensive history of tree-ring analysis in the region has been compiled by Linderholm, H. W., et al. (2010). They describe the three multi-millennial chronologies of tree-ring widths, which implement subfossil specimens, preserved in peat bogs and mountainous lakes – Tornetrask, Finnish Lapland and Jamtland. The ADVANCE-10K project in Finnish Lapland dates some samples back 7400 years and shows, at the furthest end of the scale, tree-ring widths (TRW) slightly higher than that of today’s scale, with a peak in the tree-ring index at around 4700 years ago (Timonen, 2005). It is likely that this increase is in part a result of trees recovering from the LGM and, if it were possible to see further back, even higher mean index numbers could be found. These numbers are somewhat difficult to rely on as sub-fossil and regular wood can provide divergent statistics.
Updated Advance-10K data for Northern Finland: 5633 BC – 2004 AD (Timonen M., 2005)
It has been often agreed that an arid climate arose after the LGM began to recede. Moir A. K. (2008) found overwhelming evidence for a drier to wetter climate in Northern Scotland, necessary for growth and preservation of sub-fossil pine. He finds that for the sub-fossil sites (c. 3500-2300 14C cal. yr. BC), “synchronicities of germination, growth rates…from the nine sites in this analysis across northern Scotland, provide strong evidence of climatically induced response”. These trees’ measurements document a very cold period within the last 10,000 years and provide a good proxy for the earlier Holocene, which was around 2oC colder on average. Vegetation responds sensitively to variables in climate, especially in terms of dendroclimatology. Assuming that temperatures rose quite slowly succeeding the LGM, the time taken for trees to return to expected growing standards would have been long. It could be argued that most trees never had time to recover due to the Younger Dryas (YD): another cold period occurring almost exactly after; and likely caused by, the LGM. Most agree that the glacial melting at the time reduced the salinity and temperature of northern hemispheric oceans, resulting in climate feedbacks that affected the entire planet (NOAA, 2008). Analysis of pollen on Lake Moon in China by Wu J., et al. (2016) showed that forest steppe began to dominate over meadows during the warmer LGM-YD interval (the Bølling-Allerød), but that a distinct 1000-year temperature drop was found at about 2ka after this period. Schaub et al., (2008) combined Swiss tree-ring data with radiocarbon dating to produce, at the time, the “…oldest absolutely dated wood samples worldwide”. They find “increased re-generation around years 14,000, 13,700 and 13,300 cal. yr. BP”, highlighting the improved germination/growing conditions in the transitional period between the LGM and the YD – up to 20oC increases at certain areas across the globe (Lesage A.,2010). Additionally, the segment length (SL) of samples at the onset of the YD is clearly reduced, revealing that the colder, nutrient-poor conditions affected tree width. Compared to today’s statistics, the pine trees in this period have a 40% lower average growth rate. Combinations of “temperature, precipitation, competition and/or aggradation processes” are thought to be the cause.
When our planet faces another glacial expansion; as is very likely, if not certain, it is clear that the surrounding vegetation will de detrimentally affected. Hypothetically assuming Milankovitch cycles remained identical to that of the LGM, polar latitudes in the Northern Hemisphere would be worst affected. Much of the remaining vegetation would be converted to coniferous, boreal forest and steppe-like conditions would carpet large areas of the North American and Eurasian land masses. Trees that do survive would face soils with poor nutritional content and suffer from emaciation; however, currently uncommon plants and shrubs may take hold in some regions. Worldwide sea levels and temperature would dramatically fall, resulting in mass migration of numerous species across new land and land bridges. Fluctuations in climate in all manners would be certain. As is concluded in many dendroclimatology papers, further and more extensive research is required in this field. The almost insurmountable expanse of available data across the Earth makes samples easy to find and acquire, however a broader range of knowledge in terms of time-spans is unlikely to increase unless technology improves. As well as this, living trees can only be so old, and sub-fossil samples are ultimately the key to understanding past climate variations in trees. This area of study paired with other proxy indicators are our best chance of understanding more about our planet’s ever-altering climate, and the effects it is capable of.
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