Abstract
Tree growth at high elevation in the Central European Alps (CEA) is strongly limited by low temperature during the growing season. We developed a tree ring series of co-occurring conifers (Swiss stone pine, Norway spruce, European larch) along elevational transects stretching from the subalpine zone to the krummholz limit (1630–2290 m asl; n = 503 trees) and evaluated whether trends in basal area increment (BAI) are in line with two phases of climate warming, which occurred from 1915–1953 and from 1975–2015. Unexpectedly, results revealed that at subalpine sites (i) intensified climate warming in recent decades did not lead to a corresponding increase in BAI and (ii) increase in summer temperature since 1915 primarily favored growth of larch and spruce, although Swiss stone pine dominates at high elevations in the Eastern CEA, and therefore was expected to mainly benefit from climate warming. At treeline, BAI increases in all species were above the level expected based on determined age trend, whereas at the krummholz limit only deciduous larch showed a minor growth increase. We explain missing adequate growth response to recent climate warming by strengthened competition for resources (nutrients, light, water) in increasingly denser stands at subalpine sites, and by frost desiccation injuries of evergreen tree species at the krummholz limit. To conclude, accurate forecasts of tree growth response to climate warming at high elevation must consider changes in stand density as well as species-specific sensitivity to climate variables beyond the growing season.
Highlights
It is well established that tree growth at high elevation and in boreal regions is mainly temperature-limited [1,2,3,4] and temperatures below 5 ◦ C during the growing season impair tissue formation in woody plants [5,6]
The study sites are located along the Eastern Central European Alps (CEA) (Table 1) in the area of the Tuxer and Stubaier Alps (Tyrol, Austria) and Ötztaler Alps and encompass elevational transects including the subalpine zone (1630–2290 m asl), treeline (1950–2130 m asl) and the krummholz-limit
We found significant elevational and species-related trends in basal area increment (BAI) of widespread coniferous species of the Eastern CEA, but no consistent growth response that is in parallel with the rapid rise of summer temperature in recent decades
Summary
It is well established that tree growth at high elevation and in boreal regions is mainly temperature-limited [1,2,3,4] and temperatures below 5 ◦ C during the growing season impair tissue formation in woody plants [5,6]. Forests 2020, 11, 132 models predict even further temperature increases in the future [9] This phenomenon is especially pronounced in the European Alps, showing almost twice increase of air temperature compared to the global rate [10,11]. Climate warming was found to increase radial growth at high elevation and in boreal regions [12], radial growth did not consistently track recent temperature increase [13]. In the European Alps no unusual late 20th century divergence problem was found by [12], several authors [15,16] have found that recent temperature increase had a negative effect on tree growth at high elevation.
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