TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 52, 2018

Isoscapes of plant δ13C in the northern forests: addressing the question of inter-tree and -site variability in Pinus sylvestris L. tree rings from Finnish Lapland

Samuli Helama1,*, Laura Arppe2, Juha Hyvönen1, Kari Mielikäinen3 and Markku Oinonen1

1Natural Resources Institute Finland, Rovaniemi, Finland
2Laboratory of Chronology, Finnish Museum of Natural History, University of Helsinki, Finland
3Natural Resources Institute Finland, Vantaa, Finland

(Received February 17, 2016; Accepted October 24, 2017)

Abstract: Stable carbon isotope values (δ13C) in tree rings have been increasingly used as indicators of climatic and ecological processes. However, compared to inter-annual δ13C variability, the factors related to inter-tree and -site variability (i.e. dendroclimatic noise) have attracted less attention. Here we present a statistical study quantifying the inter-tree and -site variability in a tree-ring δ13C dataset in the context of biogeographical, climatic, and forestry variables. This dataset originates from sixty-seven Scots pine (Pinus sylvestris L.) trees growing in seven sites in the north boreal forests and near the subarctic timberline in Finnish Lapland. Using estimates from a linear mixed model, we identified a set of three constraints, possibly behind the inter-tree and -site δ13C variations. First, a latitudinal constraint was found with more negative δ13C values in colder (timberline) conditions, in comparison to trees in south of timberline. Second, a longitudinal constraint appeared to result in more negative δ13C levels than in the eastern sites. Third, an altitudinal constraint led to in less negative δ13C levels at higher altitudes. Additional source of inter-tree and -site δ13C variations attributable to tree size variables resulted in less negative δ13C values than in taller trees with larger crowns. As the tree size and age are closely connected in our data, these results supported earlier findings that Pinus δ13C data may exhibit age-related changes. These results were consistent with mechanistic links between environmental drivers and carbon isotope fractionation. The extent of inter-tree and -site variability affected the length of confidence intervals around the mean δ13C value. The use of statistical power analyses was highlighted for assessing the required sample size to estimate the mean value.
Key words: biogeochemistry, carbon isotope discrimination, dendrochronology, power analysis, stable isotope chronology

*Corresponding author E-mail: samuli.helama@luke.fi