Search

Shrub dendrochronology is gaining increasing momentum in temperate high mountain regions to decipher climatic controls on current shrub expansion. Yet, a lack of consensus still persists in terms of sampling protocols, thus hampering comparability of results from different studies. For instance, serial sectioning, i.e. the sampling of multiple sections along the same shrub stem is recommended as it increases the detection of partial and missing rings, but has only been employed in few studies as it is time-consuming. Similarly, as a result of serial sectioning, chronologies frequently combine sections sampled at different positions along the stem and at the root collar which hinders the detection of climatic signals. Here, we used cross-sections sampled on 21 Rhododendron ferrugineum shrubs from the French Pyrenees to define a parsimonious protocol enabling detection of partial and missing rings while increasing the strength of the climate signal in the shrub ring chronology. We demonstrate that partial and missing rings are almost evenly distributed along Rhododendron ferrugineum stems and that they can be detected optimally using two sections on which growth rings are measured along three radii. Our results also evidence that chronologies which include only ring-width series from basal sections more strongly integrate summer temperature fluctuations than stem-based or mixed chronologies. Noteworthy, the snowpack signal is stronger in chronologies with individuals from the upper stem sections. Overall, our results confirm that sampling design - serial sectioning and caution in ring-width series aggregation - is key to ensure robustness of dendroecological studies on dwarf shrubs in alpine environments.

International audience; 21 Enhanced shrub growth in a warming alpine climate has potential far-reaching implications, 22 including soil nutrient cycling, carbon storage, or water and surface energy exchanges. Growth 23 ring analysis can yield mid-to long-term, annually resolved records of shrub growth, and 24 thereby offer valuable insights into how growth is driven by interannual climate variability. In 25 the European Alps, dendroecological approaches have shown that dwarf shrub productivity is 26 influenced by interannual variations of growing season temperature but results also point to a 27 negative effect of winter precipitation on radial growth. However, as past work lacked snow 28 cover data, links between snow cover duration, growing season length, energy availability and 29 inter-annual shrub growth remain poorly understood. 30 In this paper, we combined multi-decadal shrub-ring series from 49 individuals sampled at three 31 sites along a 600-m elevational gradient in the Taillefer massif, located in the French Alps to 32 assess growth sensitivity of long-lived and widespread Rhododendron ferrugineum shrubs to 33

International audience; Premise: Mountain ecosystems are particularly sensitive to climate change. However, only a very small number of studies exist so far using annually resolved records of alpine plant growth spanning the past century. Here we aimed to identify the effects of heat waves and drought, driven by global warming, on annual radial growth of Rhododendron ferrugineum.Methods: We constructed two century‐long shrub ring‐width chronologies from R. ferrugineum individuals on two adjacent, north‐ and west‐facing slopes in the southern French Alps. We analyzed available meteorological data (temperature, precipitation and drought) over the period 1960–2016. Climate–growth relationships were evaluated using bootstrapped correlation functions and structural equation models to identify the effects of rising temperature on shrub growth.Results: Analysis of meteorological variables during 1960–2016 revealed a shift in the late 1980s when heat waves and drought increased in intensity and frequency. In response to these extreme climate events, shrubs have experienced significant changes in their main limiting factors. Between 1960 and 1988, radial growth on both slopes was strongly controlled by the sum of growing degree days during the snow free period. Between 1989 and 2016, August temperature and drought have emerged as the most important.Conclusions: Increasing air temperatures have caused a shift in the growth response of shrubs to climate. The recently observed negative effect of high summer temperature and drought on shrub growth can, however, be buffered by topographic variability, supporting the macro‐ and microrefugia hypotheses.

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVIN; International audience; Rhododendron ferrugineum L. is a widespread dwarf shrub species growing in high-elevation, alpine environments of the Western European Alps. For this reason, analysis of its growth rings offers unique opportunities to push current dendrochronological networks into extreme environments and way beyond the treeline. Given that different species of the same genus have been successfully used in tree-ring investigations, notably in the Himalayas where Rhododendron spp. has proven to be a reliable climate proxy, this study aims at (i) evaluating the dendroclimatological potential of R. ferrugineum and at (ii) determining the major limiting climate factor driving its growth. To this end, 154 cross-sections from 36 R. ferrugineum individuals have been sampled above local treelines and at elevations from 1800 to 2100 m asl on northwest-facing slopes of the Taillefer massif (French Alps). We illustrate a 195-year-long standard chronology based on growth-ring records from 24 R. ferrugineum individuals, and document that the series is well-replicated for almost one century (1920–2015) with an Expressed Population Signal (EPS) > 0.85. Analyses using partial and moving 3-months correlation functions further highlight that growth of R. ferrugineum is governed by temperatures during the growing season (May–July), with increasingly higher air temperatures favoring wider rings, a phenomenon which is well known from dwarf shrubs growing in circum-arctic tundra ecosystems. Similarly, the negative effect of January–February precipitation on radial growth of R. ferrugineum, already observed in the Alps on juniper shrubs, is interpreted as a result of shortened growing seasons following snowy winters. We conclude that the strong and unequivocal signals recorded in the fairly long R. ferrugineum chronologies can indeed be used for climate–growth studies as well as for the reconstruction of climatic fluctuations in Alpine regions beyond the upper limits of present-day forests.