Straight lines or eccentric eggs? A comparison of radial and spatial ring width measurements and its implications for climate transfer functions

Shrub dendrochronological investigations are recently gaining more and more importance within the dendro-scientific community. As being a rather young discipline, many means of shrub dendrochronology lean on established methods that have been developed for trees. Although shrubs as trees are woody plants, it seems likely that they express differing growth characteristics due to their often multi-stemmed and prostrate stature. Yet, the majority of shrub dendrochronological investigations have measured shrub ring widths along two radii within one (sometimes several) stem disk(s) per individual. To our knowledge only one study so far has undertaken the approach to measure complete area increments (e.g. basal area increments, if applied to the basal stem disk of a shrub), however not focusing in detail on a comparative evaluation of this new approach with respect to radial measurements. To fill this knowledge gap our study focuses on the comparison of stem disk area increment measurements with radial measurements in the context of shrub growth representation and response- and transfer-function analyses. Our results indicate that for eccentric shrubs a minimum of four radial measurements per stem disk should be obtained for a good representation of the average stem disk growth. Inter-stem-disk comparisons showed that growth differences between individuals were often misestimated when only based on one or two radial measurements per stem disk. Response- and transfer-function analyses suggested, that the investigated shrubs reflect different environmental signals within different sectors of stem discs. This implies to carefully select radial measurements and individuals to increase the strength of environmental signals within transfer functions.