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It is well established, that tree growth at high elevations is mainly limited by low temperature during the growing season and climate warming was frequently found to lead to more growth and expansion of trees into alpine tundra. However, dendroclimatological studies revealed contradictory growth response to recent climate warming at the upper elevational limit of tree growth, and transplant experiments unveiled that high elevation tree provenances are not adequately benefiting from higher temperatures when planted at lower elevation. We therefore re-evaluated growth response of trees to recent climate warming by developing tree ring series of co-occurring conifers (Swiss stone pine (Pinus cembra), European larch (Larix decidua), and Norway spruce (Picea abies)) along several altitudinal transects stretching from the subalpine zone to the krummholz-limit (1630–2290 m asl; n=503 trees) in the Central European Alps (CEA). We evaluated whether trends in basal area increment (BAI) are in line with two phases of climate warming which occurred from 1915–1953 and from mid-1970s until 2015. We expected that BAI of all species shows an increasing trend consistent with distinct climate warming during the study period (1915–2015) amounting to >2 °C. Although enhanced tree growth was detected in all species in response to climate warming, results revealed that at subalpine sites (i) intensified climate warming since mid-1970s did not lead to corresponding increase in BAI, and (ii) increase in summer temperature primarily favored growth of Norway spruce, although Swiss stone pine dominates at high altitude in the CEA and therefore was expected to mainly benefit from climate warming. At treeline BAI increase was above the determined age trend in all species, whereas at the krummholz-limit only deciduous larch showed minor growth increase. We explain missing adequate growth response to recent climate warming (i) by strengthened competition for resources (primarily nutrients and light) in increasingly denser stands at subalpine sites leading to changes in carbon allocation among tree organs, and (ii) by frost desiccation injuries of evergreen tree species at the krummholz-limit. Our findings indicate that tree growth response to climate warming at high elevation is possibly nonlinear, and that increasing competition for resources and the influence of climate factors beyond the growing season impair stem growth. more...

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&ndash, 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&ndash, 1953 and from 1975&ndash, 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. more...