Effects of Climate Change at Treeline: Lessons from Space-for-Time Studies, Manipulative Experiments, and Long-Term Observational Records in the Central Austrian Alps

Abstract

This review summarizes the present knowledge about effects of climate change on conifers within the treeline ecotone of the Central Austrian Alps. After examining the treeline environment and the tree growth with respect to elevation, possible effects of climate change on carbon gain and water relations derived from space-for-time studies and manipulative experiments are outlined. Finally, long-term observational records are discussed, working towards conclusions on tree growth in a future, warmer environment. Increases in CO2 levels along with climate warming interact in complex ways on trees at the treeline. Because treeline trees are not carbon limited, climate warming (rather than the rising atmospheric CO2 level) causes alterations in the ecological functioning of the treeline ecotone in the Central Austrian Alps. Although the water uptake from soils is improved by further climate warming due to an increased permeability of root membranes and aquaporin-mediated changes in root conductivity, tree survival at the treeline also depends on competitiveness for belowground resources. The currently observed seedling re-establishment at the treeline in the Central European Alps is an invasion into potential habitats due to decreasing grazing pressure rather than an upward-migration due to climate warming, suggesting that the treeline in the Central Austrian Alps behaves in a conservative way. Nevertheless, to understand the altitude of the treeline, one must also consider seedling establishment. As there is a lack of knowledge on this particular topic within the treeline ecotone in the Central Austrian Alps, we conclude further research has to focus on the importance of this life stage for evaluating treeline shifts and limits in a changing environment.

Larch Cellulose Shows Significantly Depleted Hydrogen Isotope Values With Respect to Evergreen Conifers in Contrast to Oxygen and Carbon Isotopes

“…In cembran pine heterotrophism increase with the altitude (Wieser et al, 2019). The finding that the δ 2 H difference between larches and evergreen conifers is lower in Bern (∼500 m elevation) than at >2000 m of tree-line (40‰ in Figure 4 vs 52‰ in Figure 3) support the hypothesis that heterotrophism affect δ 2 H but the difference between the two metabolisms is maintained.…”

Treeline Research—From the Roots of the Past to Present Time. A Review

“…In addition, historical displacement of treeline by humans and a delay of treeline response to climate change also play a major role in the respect. Hence, at finer scales, treeline position is often out-of-phase with climate (e.g., [1,[161][162][163][164][165][166][167][168][169][170][171][172][173][174][175][176]). Global overviews, necessarily disregarding the local differences, may easily overemphasize coarse drivers such as temperature [167,177,178].…”

Forward Modeling Reveals Multidecadal Trends in Cambial Kinetics and Phenology at Treeline

“…This lack of long-term data on trends in cambial dynamics is especially unwelcome in areas where forests have shown significant trends in tree growth. For instance, most stem increment chronologies from mountain treelines in Central Europe show significant increases in growth coupled with increasing temperatures ( Büntgen et al, 2006 ; Ponocná et al, 2016 ; Björklund et al, 2019 ; Wieser et al, 2019 ). Without sufficiently long series of intra-annual growth dynamics it is not possible to determine whether this increase is driven by longer growing seasons, higher growth rates or both.…”