Tree recruitment above the treeline and potential for climate‐driven treeline change

Abstract

Questions: How do population structure and recruitment characteristics of Betula saplings beyond the treeline vary among climatic regions, and what is the potential for development into tree-sized individuals with interacting grazing pressure?Location: Scandes Mountains.Methods: Sapling characteristics of Betula pubescens subsp. tortuosa, their topographic position above the treeline, growth habitat and evidence of recent grazing was investigated in three areas with a long continuous grazing history, along a latitudinal gradient (62-691N).Results: Saplings were common up to 100 m above the treeline in all areas. The northern areas were characterised by small (o30 cm) and young (mean 14 years old) saplings in exposed micro-topographic locations unfavourable to long-term survival. In the southern area, broad height (2-183 cm) and age (4-95 years; mean 32 years) distributions were found in sheltered locations. Age declined with altitude in all areas. Sapling growth rate varied within and between areas, and the ageÂheight interaction was significant only in the southern area. Growth rates decreased from south to north and indicated a considerable time required to reach tree size under prevailing conditions. Conclusions: Regional differences can be attributed to climatic differences, however, interacting biotic and abiotic factors such as micro-topography, climate and herbivory, mutually affect the characteristics of birch saplings. In view of the long time needed to reach tree size, the generally expected evident and fast treeline advance in response to climate warming may not be a likely shortterm scenario. The sapling pool in the southern region possesses strongest potential for treeline advance.

Do Himalayan treelines respond to recent climate change? An evaluation of sensitivity indicators

“…In recent years, an increasing number of studies have investigated the response of treeline seedlings to changing environmental factors such as soil temperature, soil moisture, soil nutrients, light conditions, herbivory, and competition/facilitation (e.g. Maher and Germino, 2006;Anschlag et al, 2008;Batllori et al, 2009;Hofgaard et al, 2009;Kabeya, 2010;Munier et al, 2010); some studies have also explicitly addressed the relative importance of multiple factors (Barbeito et al, 2012;Zurbriggen et al, 2013).…”

“…Locally increasing numbers of tree seedlings above the current upper tree limit, occasionally far above, are reported from many mountain ranges (e.g. Kullman, 2008;Hofgaard et al, 2009).…”

Spaceborne potential for examining taiga–tundra ecotone form and vulnerability

“…The way in which spaceborne data are integrated and scaled may be a key part of identifying structural patterns and TTE form. Fine-scale data can resolve individual trees that, when grouped to patches, may reveal ecotone forms (Danby and Hik, 2007;Hansen-Bristow and Ives, 1985;Hofgaard et al, 2009Hofgaard et al, , 2012Holtmeier and Broll, 2010;Mathisen et al, 2013). Without resolving groups of individual trees, coarse studies of the land surface may misrepresent ecotone form, be less frequently corroborated with ground data, and disguise the structural heterogeneity of discontinuous forests.…”

The Response of Alpine Salix Shrubs to Long-Term Browsing Varies with Elevation and Herbivore Density

“…Stem basal discs were cut and air dried (for at least two months) prior to preparation for dendrochronological analyses. The upper surface of each disc was cut smooth with a scalpel, and zinc cream was applied to increase the contrast between early and late wood (Hofgaard et al, 2009). Ring widths were measured to the nearest 0.001 mm using a Leica MZ16-A microscope and Leica DFC420 digital camera with Leica Application Suite 3.2 imaging and analysis software (Leica Microsystems, Wetzlar, Germany).…”