Treeline advances along the Urals mountain range – driven by improved winter conditions?

Abstract

High-altitude treelines are temperature-limited vegetation boundaries, but little quantitative evidence exists about the impact of climate change on treelines in untouched areas of Russia. Here, we estimated how forest-tundra ecotones have changed during the last century along the Ural mountains. In the South, North, Sub-Polar, and Polar Urals, we compared 450 historical and recent photographs and determined the ages of 11,100 trees along 16 altitudinal gradients. In these four regions, boundaries of open and closed forests (crown covers above 20% and 40%) expanded upwards by 4 to 8 m in altitude per decade. Results strongly suggest that snow was an important driver for these forest advances: (i) Winter precipitation has increased substantially throughout the Urals (~7 mm decade(-1) ), which corresponds to almost a doubling in the Polar Urals, while summer temperatures have only changed slightly (~0.05°C decade(-1) ). (ii) There was a positive correlation between canopy cover, snow height and soil temperatures, suggesting that an increasing canopy cover promotes snow accumulation and, hence, a more favorable microclimate. (iii) Tree age analysis showed that forest expansion mainly began around the year 1900 on concave wind-sheltered slopes with thick snow covers, while it started in the 1950s and 1970s on slopes with shallower snow covers. (iv) During the 20th century, dominant growth forms of trees have changed from multistemmed trees, resulting from harsh winter conditions, to single-stemmed trees. While 87%, 31%, and 93% of stems appearing before 1950 were from multistemmed trees in the South, North and Polar Urals, more than 95% of the younger trees had a single stem. Currently, there is a high density of seedlings and saplings in the forest-tundra ecotone, indicating that forest expansion is ongoing and that alpine tundra vegetation will disappear from most mountains of the South and North Urals where treeline is already close to the highest peaks.

Species interactions slow warming-induced upward shifts of treelines on the Tibetan Plateau

“…The average rate over the eastern TP also was much less than the rapid upward shift of 91-119 m reported since the 1960s for the Green Mountains in Vermont (7). It also is somewhat less than the upward shift of 4-15 m per decade in High Asia (southern Siberia and along the Urals mountain range) (48)(49)(50). The average treeline shift rate we found for the TP, however, was close to that reported for other alpine treelines in the Spanish Pyrenees and northern Siberia (9, 51), With a warming in summer and annual mean temperature of around 1.2-1.5°C in the past 100 y on the TP (Fig.…”

Experimental soil warming and cooling alters the partitioning of recent assimilates: evidence from a 14C-labelling study at the alpine treeline

“…for almost one growing season. In the longer term, warming effects on C dynamics will also depend on factors we did not investigate, including aboveground biomass turnover and changes in vegetation composition (Walker et al 2006;Myers-Smith et al 2011;Sistla et al 2013;Hagedorn et al 2014). Our experiment suggests that rhizodeposition did not increase with warming, despite increased net 14 C assimilation.…”

Population Spatial Dynamics of Larix potaninii in Alpine Treeline Ecotone in the Eastern Margin of the Tibetan Plateau, China

“…Spatial analysis showed that individuals which were established in the early period tend to be clustered when compared with the young individuals both on north-and south-facing slopes. Similar to the Ural Mountains [20], old trees in our study region have two main growth forms: a large proportion of single-stemmed trees with one upright stem and a small part of crippled trees or multi-stemmed trees-like individuals, which indicates that the site-specific environment is stressful here. Individuals with a clustered pattern have the ability to improve the harsh environment by providing shade to shelter from wind desiccation and reduce diurnal temperature range [47].…”

“…For the north-facing slopes in eastern TP, treeline dynamics and recruitment appears to be limited by lower soil temperature and a short growing period [34]. In winter, the thicker snow cover on north-facing slopes could prevent winter stress by insulating root and plant tissues from wind damage, frost, and abrasion, which provides snow protection for individuals in winter [20]. It could also lead to higher soil temperatures in winter, prolonging the availability of soil moisture for the vertical growth of individuals during the growing season [47].…”