Climate warming does not adequately translate to increased radial stem growth of coniferous species along the Alpine treeline ecotone

Abstract

<p>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 (<em>Pinus cembra</em>), European larch (<em>Larix decidua</em>), and Norway spruce (<em>Picea abies</em>)) 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 (<em>i</em>) intensified climate warming since mid-1970s did not lead to corresponding increase in BAI, and (<em>ii</em>) 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 (<em>i</em>) 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 (<em>ii</em>) 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. </p>

Tree growth at the limits: the response of multiple conifers to opposing climatic constraints along an elevational gradient in the Alps

“…The most noticeable trends in BAI were an increase in growth since the 1970s which was found at most high-elevation and forest line LD sites at the North Slope high-elevation PA site but not for PC sites. Such high-elevation growth increments in recent decades for PA and LD but not for PC were also observed in the Central Eastern Alps (Oberhuber et al, 2020) and in the French Alps (Rolland et al, 1998;Rozenberg et al, 2020). Additionally, a growth increase in PA near the forest line in recent decades was also found in the Northern Limestone Alps in Germany and Austria (Hartl-Meier et al, 2014b) and in the Sudetes and Carpathians (Ponocna et al, 2016).…”

“…Climate warming was found to reduce the sensitivity of tree growth to summer temperature at cold-dry sites worldwide (Babst et al, 2019) and, specifically, in the Alps (Büntgen et al, 2012;Coppola et al, 2012;Oberhuber et al, 2020). This effect may be attributed to an earlier start of the growing season or growth depression due to lower water availability in late summer caused by earlier snowmelt (Saderi et al, 2019).…”