Abstract
AbstractAimTo quantify tree biomass and stand productivity of treeline ecotones and identify driving factors.Locationtreeline ecotones of seven regions from the South to Polar Urals, spanning a latitudinal gradient of 1,500 km.TaxaPicea obovata, Betula pubescens, Larix sibirica.MethodsStand biomass and productivity were estimated across 18 elevational transects from the tree species line to the closed forest line based on allometric measurements of 326 trees (including roots for 53 trees), stand structure assessments and demographic patterns of 20,600 trees. Stand growth data were linked to (a) temperatures monitored in situ for five years in the South and Polar Urals, (b) climate variables extrapolated from nearby climate stations and (c) measures of nutrient availability in soils and tree foliage.Resultstreeline position along the latitudinal gradient occurred at a similar mean growing season temperature. Despite the common cold limitation of tree distribution along the Ural mountain range, stand biomass and productivity within the treeline ecotone decreased by a factor of three and five from the South to the Polar Urals, mainly due to a declining stand density. Among climatic variables, growing season length decreased by 20% and winter temperatures declined by 4°C towards the Polar Urals, whereas growing degree days > 5°C remained similar, averaging 554 ± 9°C. Soil development was poorer in the Polar than in the South Urals, and plant‐available N and P in the soil were 20 and 30 times lower, respectively, probably due to lower winter temperatures.Main conclusionsOur results suggest that once the thermal limitation for tree growth is relieved, soil fertility—restricted by permafrost and low soil temperatures during winter—plays a key and yet underexplored role for stand productivity in treeline ecotones. The observed latitudinal decline in stand productivity is important for above‐ and belowground diversity and functioning.
Highlights
Climate warming during the past century has repeatedly been shown to have profound effects on the productivity, distribution and diversity of vegetation throughout the world (Walther et al, 2002)
Our study is one of the first assessments of stand biomass and productivity at multiple treeline sites, variables that serve as the basis for estimating and modelling various ecosystem functions
Our results demonstrate that treeline positions are located at similar temperature regimes during the growing season along the entire 1500-km latitudinal gradient in the Ural mountains
Summary
Climate warming during the past century has repeatedly been shown to have profound effects on the productivity, distribution and diversity of vegetation throughout the world (Walther et al, 2002). The treeline ecotone, where closed forest transitions to alpine or arctic tundra, is both a boundary and a cold ecosystem and is valuable as a bio-indicator of climate change (Holtmeier, 2003). The expansion of forests into alpine and arctic tundra can impact plant productivity and diversity (Gazol, Moiseev, & Camarero, 2017) and has important implications for carbon and nutrient cycling (Hagedorn, Gavazov, & Alexander, 2019; Kammer et al, 2009; Speed et al, 2015), as well as snow accumulation and albedo (de Wit et al, 2014; Schwaab et al, 2015), which can in turn affect ecosystem functioning and result in further vegetation change. It is highly relevant to understand how environmental conditions influence current treeline ecosystems and contribute to future shifts in treeline position and characteristics
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