Climate change and forest distribution

Treeline questions have been studied from a number of different perspectives, but nearly all are at least implicitly connected with the question of how cold temperatures limit tree growth and survival. Because of this overriding interest in temperature limits, and because of the ubiquitous occurrence of treelines across the Arctic and the high state of preservation of dead trees in the cold environment of the Arctic, treeline studies have much to offer on the specific question of climate warming and cooling.
Holocene wood deposits in the southern Yamal Peninsula include a large quantity of subfossil tree remains, including stems, roots, and branches. This is the result of intensive accumulation and conservation of buried wood in permafrost. In these conditions tree remains conserved in peat sediments were direct evidence that the northern treeline in the warmest period of the Holocene reached the central regions of the Yamal Peninsula (up to 70° N). Today the polar treeline is considerably further south on the Peninsula (67°30’ N).
Systematic collection of subfossil wood samples started in 1982 in the watersheds of the Khadyta, the Yadayakhodiyakha, and the Tanlova Rivers in the southern Yamal Peninsula. River flow in this area is from north to south, excluding the possibility that wood was transported to the collection site from more southerly locations; thus, there is very high confidence that the region experienced a considerably warmer summer climate in the relatively recent past.

Radiocarbon dating of the subfossil wood (53 dates) was cross dated with ring series from the samples, allowing the construction of a continuous tree-ring chronology 7,314 years in length. Absolute dates can generally be assigned to the recovered wood remains. The result makes it possible to reconstruct the dynamics of tree limits on the Yamal Peninsula during the Holocene, as the following figure displays.

From at least 10,000 to 9,000 years BP, trees grew across most of the Peninsula. The most favorable conditions for tree growth (almost certainly warm summers) occurred from 7200 to 6000 BC. From about 6000 to 5600 BC, climatic conditions became less favorable for tree growth (almost certainly cooler summers), but trees persisted and did not retreat to the south. Beginning about 5400 BC, trees were displaced southward. The forest stand density also greatly decreased during this period, and it can be considered as a transition to the next stage of the Holocene. From 5400 until 1700 BC, the polar treeline was still located at 69° N, well north of the present-day position. In unfavorable periods (4500–3900 BC and 3600–3400 BC), tree survival was mainly restricted to the river corridors, but in climatically more favorable periods (5200–4500 BC, 3900–3600 BC, and 3400–1800 BC), forests grew on hills and raised surfaces beyond the rivers.

ne of the most important periods of displacement of polar treeline to the south and major reduction in the density and productivity of forest stands occurred about 1700 BC. This stage can be regarded as the end of the Middle Holocene and the beginning of the modern stage of treeline evolution on the Yamal Peninsula. During the last 1,700 years, forest–tundra and forest associations have been primarily restricted to river valleys in the southern part of the Peninsula. Somewhat more favorable conditions occurred from 1200 to 900 BC, from 100 BC to AD 200 and during the Medieval Warming Period (MWP) (AD 700–1400).

Treeline dynamics for the last 4,000 years of the Holocene were reconstructed with even greater precision using more than 500 cross-dated tree stems with known coordinates of their burial places in valleys of different rivers on the southern-Yamal Peninsula.

Treeline displacements northward and southward were relatively small and less important during the last 3,600 years than those that occurred in the previous few millennia. Treeline generally moved at the most 5 km to the south of the present-day treeline, and subsequently northward only to the present boundary of open woodland in the river valleys. However, one particularly noteworthy major displacement of treeline to the south occurred during the second half of the 17th century BC. In this relatively short period (not exceeding 100 years), the boundary of larch open woodland moved southward nearly 15 to 20 kilometers (km), and the treeline retreated a further 8 to 10 km during the next 700 years. This major displacement of treeline in the 17th century BC appears to have been driven by strongly inclement climatic conditions (cold summers), representing the lowest reconstructed summer temperatures in the entire series. It was in the years immediately after 1657 BC that the temperature decreased sharply. Fourteen years during the interval 1630 to 1611 BC appear to have been extremely cold, reaching a nadir in two specific years, 1626 and 1625 BC. No other period during the reconstruction is even close. Moreover, it is clear that in 1625 BC, a severe freeze occurred in the middle of the summer (as indicated by characteristic anatomical structures of freeze injury in the tree rings). It is very probable that this short-term extreme climate event represented climate cooling following one of the largest volcanic eruptions of the last few millennia, which happened in about 1628 BC (possibly the eruption of the Santorini volcano in the eastern Mediterranean). The cooling appears to have reinforced another closely spaced cooling event that preceded it. The earlier of the two periods of extreme cold temperatures began sometime after 1657 BC, but in this case, it is difficult to determine the cause. These events were the final circumstances that resulted in the most significant southward retreat of treeline during at least the last 4,000 years. In spite of extremely favorable summer warmth that returned afterward and even persisted at various intervals, the treeline never returned to its previous boundary.

Another important result of dendrochronological dating of large samples of subfossil wood is the ability to calculate the relative abundance of Siberian spruce in forest stands of the area, which is an index or proxy for the degree of continentality of the climate. The figure presented below shows the change in the proportion of spruce in forest stands (the remaining part is all larch). In the first six centuries, from AD 900 to 1500, the proportion of spruce decreased from 22% to 3–5%. After that, the percentage of spruce stabilized in the range of 7-10%. The 20th century is characterized by an increasing percentage of spruce in forest stands in the valley of the River Khadytayakha, and a weak northward advance of the polar treeline.

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