Growth-ring variations of dwarf shrubs reflect regional climate signals in alpine environments rather than topoclimatic differences

Aim Our main aim is to determine if ring-width variations in Empetrum hermaphroditum reflect regional or local topoclimate signals in an alpine environment. In the case that topoclimate provides the dominant signal, a secondary aim is to link these to spatial distribution patterns of different vegetation types. Location The study area is situated in the middle alpine belt in the Vagamo region, Central Norwegian Scandes. Sampling sites cover different topoclimates: ridges, north-facing slopes and south-facing slopes. Methods We constructed ring-width chronologies of E. hermaphroditum for each type of microsite for the common period 1951-2004. Climate data were prepared on an hourly, daily and growing-season time scale. Climate-growth relationships were evaluated using bivariate correlations and regression tree methods for continuous time-series analyses. In addition, extreme growth anomalies (pointer years) were compared with the climate conditions in those years. The impact of water supply on wood anatomy was determined by correlating the conductive area (percentage of vessel per growth ring) with a running mean (sum) of 10-day intervals for temperature and precipitation. Results This study indicates that mean summer (June-August) temperatures determine the width of the growth rings of E. hermaphroditum irrespective of topoclimate. The length of the growing season, which is the most differentiating climatic factor between microsites, does not substantially alter the anatomical ring structure. Microsite differences in mean growth rates are attributed to the higher frequency of warm days. Extremely warm days limit ring-width development at south-facing slopes, while plants at ridges and north-facing slopes still benefit from higher temperatures. As a consequence, pointer years are not developed synchronously at all microsites. Vessel formation is affected by available moisture, especially in the later part of the growing season. Main conclusions Topoclimate induces slight modifications of annual growth-ring increments of E. hermaphroditum at different microsites. In contrast to the distribution patterns of vegetation types that are determined by snow cover, growth-ring variations are related to summer temperature conditions, and the prominent regional climate signal is still reflected at all microsites. This offers the opportunity to reconstruct climatic change in alpine regions from dwarf shrub ring-width chronologies.