Search

The tree-ring width index (RWI) and satellite-derived vegetation indices, such as the Normalized Difference Vegetation Index (NDVI), are used as long-term indicators of the past forest carbon uptake. However, fundamental questions remain with respect to what is represented by the RWI and NDVI at the ecosystem level. To address this question, we compared tree-ring parameters (RWI and the carbon isotope ratio: δ13C) and NDVI products with forest ecosystem CO 2 fluxes estimated using the eddy covariance method, at a larch forest in eastern Siberia. The RWI and tree-ring δ13C correlated well with the ecosystem gross primary production (GPP), and their temporal stabilities were high during 2004–2014. However, the NDVI products did not show any temporally stable relationship with the GPP. This could be ascribed to significant changes in the understory vegetation in this forest, i.e., from dense cowberry to shrubs and moisture-tolerant grasses, because of an excessively moist environment during 2007–2008. Changes in the understory vegetation could be reflected by the NDVI products but not by the GPP. Our results indicate that it is more feasible to study forest carbon uptake using tree-ring parameters than using satellite-derived vegetation indices in such larch-dominated forest ecosystems in eastern Siberia. [ABSTRACT FROM AUTHOR]

Circumboreal forest ecosystems are exposed to a larger magnitude of warming in comparison with the global average, as a result of warming-induced environmental changes. However, it is not clear how tree growth in these ecosystems responds to these changes. In this study, we investigated the sensitivity of forest productivity to climate change using ring width indices (RWI) from a tree-ring width dataset accessed from the International Tree-Ring Data Bank and gridded climate datasets from the Climate Research Unit. A negative relationship of RWI with summer temperature and recent reductions in RWI were typically observed in continental dry regions, such as inner Alaska and Canada, southern Europe, and the southern part of eastern Siberia. We then developed a multiple regression model with regional meteorological parameters to predict RWI, and then applied to these models to predict how tree growth will respond to twenty-first-century climate change (RCP8.5 scenario). The projections showed a spatial variation and future continuous reduction in tree growth in those continental dry regions. The spatial variation, however, could not be reproduced by a dynamic global vegetation model (DGVM). The DGVM projected a generally positive trend in future tree growth all over the circumboreal region. These results indicate that DGVMs may overestimate future wood net primary productivity (NPP) in continental dry regions such as these; this seems to be common feature of current DGVMs. DGVMs should be able to express the negative effect of warming on tree growth, so that they simulate the observed recent reduction in tree growth in continental dry regions. [ABSTRACT FROM AUTHOR]

Summary We present a tree-ring reconstruction of Palmer Drought Severity Index (PDSI) in Ust-Maya region (60°00′N, 133°49′E), central part of eastern Siberia using total ring (TR) widths and latewood (LW) δ 13 C chronologies from larch trees (1850–2008 AD). Summer (JJA) PDSI was correlated positively and negatively with the TR widths and LW δ 13 C, respectively. Using a multiple liner regression approach, we reconstructed summer PDSI using the time series of TR widths and LW δ 13 C. The reconstruction showed an interannual to decadal wet/dry fluctuation with several moist periods before 1950s and a severe drought event from 1991 to 1993. Comparison of the reconstruction with reconstructed July PDSI for the Yakutsk region, 300 km northwest of Ust-Maya, showed heterogeneous changes in the mean states of soil moisture, but synchronous year-to-year changes. These results indicate that regional studies are quite important to precisely depict the spatio-temporal variability of hydrological changes in the central part of eastern Siberia. [ABSTRACT FROM AUTHOR]

We present a large-scale dendroclimatic reconstruction of July temperatures from 42–52°N to 140–145°E in the Northwest Pacific region for the period from 1800 to 1996. A multiple regression model with principal components (PCs) of a tree-ring chronology network was used for the reconstruction, which accounted for 31.7% of the temperature variance in the calibration period (1901–1996). The reconstructed spatially-averaged July temperatures show large fluctuations, which are comparable to previously published dendroclimatic reconstruction of spring temperatures in northeast Asia. It also shows stable relationships with other datasets, notably sea surface temperatures (SSTs) in a wide area of the North Pacific and the Pacific Decadal Oscillation (PDO), indicating atmospheric–oceanic interaction in the Northwest Pacific region since AD 1800. [ABSTRACT FROM AUTHOR]

Tree-ring chronologies of ring width and stable carbon isotope ratios (δ13C) over the past 160 years were developed using living larch trees at two forest sites, each with different annual precipitation, in eastern Siberia: Spasskaya Pad (SP) (62°14′N, 129°37′E); and Elgeeii (EG) (60°0′N, 133°49′E). Intrinsic water-use efficiency (iWUE) was derived from tree-ring δ13C. The physiological responses of the larch trees to climate varied between these sites and over time. Ring widths correlated negatively with summer temperatures at SP, where summer precipitation is lower than at EG, probably due to temperature-induced water stress. Since the 1990s, however, the negative effect of warming has been more severe at EG, where the productivity of larch trees is higher than at SP. A greater reduction of larch tree growth and higher increase rate of iWUE at EG reflects greater temperature-induced water stress, which is incident to the larger forest biomass. Our results suggest that effect of increase in atmospheric CO2 on larch tree growth is not sufficient to compensate for temperature-induced water stress on larch growth in eastern Siberia and differences in precipitation and forest productivity largely affect the larch tree response to changing climate in eastern Siberia. [ABSTRACT FROM AUTHOR]

Abstract: We present a dendroclimatic reconstruction of Palmer Drought Severity Index (PDSI) in Yakutsk (62° 14′N, 129° 37′E), East Siberia using a larch tree-ring (latewood) δ13C chronology (1780–2008 AD). The correlation function analysis showed significant positive correlations between earlywood, latewood and total ring width and July PDSI of the previous summer. On the other hand, tree-ring δ13C from earlywood, latewood and total ring showed significant negative correlations with July PDSI in current summer, of which latewood δ13C showed highest correlation with PDSI. A moving-window correlation analysis demonstrated that the temporal stability in correlation between tree-ring variables and PDSI was highest when July PDSI was compared with latewood δ13C. Tree-ring δ13C values were corrected by taking account of both the changes in the isotope ratio of atmospheric CO2 (δ13Ca) and the plant physiological response to changes in atmospheric CO2 concentration (Ca). This correction significantly improved the sensitivity of larch tree-ring δ13C to PDSI, demonstrating that the correction is useful in building a reliable model to estimate past changes in PDSI. Our study shows that larch tree-ring δ13C has enough potential to reconstruct July PDSI series. Using a liner regression model, July PDSI was reconstructed from latewood δ13C over the last 229 years. The reconstruction showed an interannual to decadal wet/dry fluctuation with a decreasing trend from 1780 to 1960, suggesting that aridification progressed at least after the late Little Ice Age in East Siberia to the modern period of global warming. [Copyright &y& Elsevier]