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Arctic shrubs have a strong potential for climate and environmental reconstructions in the chronically understudied regions of the high northern latitudes. The climate dynamics of these regions are important to understand because of large-scale feedbacks to the global climate system. However, little is known about other factors influencing shrub ring growth, possibly obscuring their climate signal. For example, as of yet we are not able to differentiate between herbivory or climatically induced growth depressions. Here, we use one of the most common Arctic shrubs, Alnus viridis as a test case to address this question. We sampled Alnus in Kobbefjord, Greenland, measured shrub-ring width and cell wall thickness and built site chronologies of each parameter. We analysed climate-growth relationships, tested their stability over time and employed a pointer-year analysis to detect growth depressions. We employed bootstrapped transfer function stability tests (BTFS) to assess the suitability of our shrub chronologies for climate reconstruction. Correlations with climate data showed strong significantly positive and stable correlations between summer temperature and ring-width with the exception of the recent decade. A climate reconstruction model failed stability tests, when the complete period of record was used for calibration and verification. Wood anatomy analysis uncovered the occurrence of unusual cell structure (very thin cell walls) in the exceptionally narrow ring of 2004, a recorded insect outbreak year in other parts of Greenland. When excluding the affected ring and a recovery period, the reconstruction model passed all tests, suggesting that the unusual 2004 ring was not climate driven, but rather the result of an insect attack. When combining anatomical analysis with traditional ring-width measurements, we move a step further in potentially distinguishing small rings caused by insect attacks from small rings formed in climatically challenging years. While this study does not provide unambiguous evidence, it does provide potential useful methodological combinations to enable more robust climate reconstructions in areas where climatic records are extremely sparse. [ABSTRACT FROM AUTHOR]

Northern and elevational treelines are classic sites for dendroclimatological studies. At these marginal sites only one climate parameter is usually considered growth limiting and trees from these sites are therefore used to reconstruct that parameter back in time. Marginal sites are also those sites within a species range, where clonal reproduction is most frequent. Clonal growth can ensure plant species survival and growth under stressful conditions or if the environmental conditions do not allow sexual reproduction, e.g. by layering, by stems resprouting after damage, or through the exchange of resources between different clone ramets (“stems”). We literally stumbled across clonal and non-clonal growth forms of White spruce growing intermingled with each other at two Alaskan treeline sites. The two growth forms could not be distinguished a priori in the field. After sampling and detection of clones we thus asked whether clonal ramets and non-clonally grown trees (singletons) showed similar growth patterns. Clones were identified by identical multilocus genotypes in a SSR microsatellite genotyping analysis and radial growth was analyzed using traditional tree ring width methods High-frequency growth patterns were very similar between singletons and clonal ramets in Alaskan treeline White spruce, thus posing no problem in including both reproductive strategies in a classic dendroclimatological investigation. [ABSTRACT FROM AUTHOR]

Trees do not respond to climatic conditions uniformly, but instead show individualistic growth responses. The extent of and causes behind this within-stand variability represents significant uncertainty in predictions of how forests will respond to future climate change. We analyzed patterns of individualistic tree growth within two types of conifer stands of Central Europe – high-elevation Picea abies and low-elevation Pinus sylvestris forests. We quantified the relative effect of age, size, and topographic conditions on variability in growth patterns and climate-growth responses using principal component analysis and linear models, considering both year-to-year and decadal growth variability. Our results show that Picea abies stands with dominant temperature limitation exhibit greater growth coherency than Pinus sylvestris stands characterized by drought-limited growth. Growth variability and individual climate-growth responses in both forest types were mainly driven by tree size and age, while the effect of topographic conditions was less important. The effect of size and age variables was dominant considering decadal growth trends, whereas intermediate importance of topographic variables was observed for high-frequency growth variability and climate-growth responses. Our results highlight that between-tree variability in climatic signal and growth trends also reflects the specific distribution of the age/size and topographic parameters within the stand. We suggest careful selection of datasets used for large-scale assessments of growth trends and climate-growth responses which should consider the age and size representation of sampled trees. [ABSTRACT FROM AUTHOR]