Stable carbon isotope ratios of latewood cellulose in Pinus sylvestris from northern Finland: variability and signal-strength

Abstract

Stable carbon-isotope ratios were measured on the latewood cellulose of 25 Pinus sylvestris trees at five sites from the Arctic Circle to the northern limit of pine in Finland. A correlation-based analysis of variance was used to define within and between-site signal strengths. Expressed Population Signals (EPS) from samples of five trees are very high (0.89 to 0.95). Combining 15 trees from the three northernmost sites yields an EPS of 0.97. Despite the very strong common signal, different trees yield absolute δ13C values offset by >2%, which is similar in magnitude to the variability within individual series that includes the effect of climate. Samples of. 10 trees are necessary to obtain mean absolute values with confidence limits <1% wide (p < 0.05). The common practice of pooling four cores from four trees to yield absolute δ13C values is thus inadequate for palaeoclimate research. Because the δ13C values are homeoscedastic they can be indexed by differencing from the mean of each tree series, retaining all of the information on relative differences within each tree but yielding much narrower confidence limits when series are averaged. When five trees are used, 80% of annual confidence bands are, < ± 0.5%(P<0.05). Indexing effectively masks low-frequency variations when overlapping tree series are combined to produce long chronologies.