Abstract
BackgroundRecent warming is affecting species composition and species areal distribution of many regions. However, although most treeline studies have estimated the rates of forest expansion into tundra, still little is known about the long-term dynamic of stand productivity at the forest-tundra intersection. Here, we make use of tree-ring data from 350 larch (Larix sibirica Ledeb.) and spruce (Picea obovata Ledeb.) sampled along the singular altitudinal treeline ecotone at the Polar Urals to assess the dynamic of stand establishment and productivity, and link the results with meteorological observations to identify the main environmental drivers.ResultsThe analysis of stand instalment indicated that more than 90% of the living trees appeared after 1900. During this period, the stand became denser and moved 50 m upward, while in recent decades the trees of both species grew faster. The maximum afforestation occurred in the last decades of the twentieth century, and the large number of encountered saplings indicates that the forest is still expanding. The upward shift coincided with a slight increase of May–August and nearly doubling of September–April precipitation while the increase in growth matched with an early growth season warming (June + 0.27 °C per decade since 1901). This increase in radial growth combined with the stand densification led to a 6–90 times increase of biomass since 1950.ConclusionTree-ring based twentieth century reconstruction at the treeline ecotone shows an ongoing forest densification and expansion accompanied by an increased growth. These changes are driven by climate change mechanism, whereby the leading factors are the significant increase in May–June temperatures and precipitation during the dormant period. Exploring of phytomass accumulation mechanisms within treeline ecotone is valuable for improving our understanding of carbon dynamics and the overall climate balance in current treeline ecosystems and for predicting how these will be altered by global change.
Highlights
Recent warming is affecting species composition and species areal distribution of many regions
Many studies focusing on ongoing dynamics of woody vegetation at the limit of their distribution (e.g. Harsch et al 2009; Holtmeier 2009; Körner 2012) have documented that treelines are experiencing an extension of seedling establishment, densification of forest structures and a stimulation of radial growth and biomass (e.g. Kearney 1982; Wardle and Coleman 1992; Nicolussi et al 1995; Kullman and Engelmark 1997; Canadell et al 2007) that are consistent with observed rates of recent (Suarez et al 1999; Gamache and Payette 2005; Shiyatov et al 2007) and past (Foley et al 1994; Lloyd and Graumlich 1997; Grace et al 2002; Körner and Paulsen 2004; Hoch and Körner 2009) temperature changes
Our study showed that the forest-tundra intersection at the Polar Urals has been undergoing important changes during the twentieth century
Summary
Recent warming is affecting species composition and species areal distribution of many regions. Other studies suggest that winter temperature and precipitation affect on the survival of treeline trees as they play a role on soil hydrological and thermal regimes (Harsch et al 2009; Hagedorn et al 2014). In contrast to low latitudes and altitudes, where forest distribution is mainly limited by lack of moisture (Woodward et al 2004), forests from cold regions can grow with less than 400 mm of yearly precipitation, which is usually sufficient to cover trees’ transpiration needs (Pan et al 2013)
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