Soil Moisture Water Equivalent Research Articles

Historical variation in the normalized difference vegetation index compared with soil moisture in a taiga forest ecosystem in northeastern Siberia

Abstract. The taiga ecosystem in northeastern Siberia, a nitrogen-limited ecosystem on permafrost with a dry climate, changed during the extreme wet event in 2007. We investigated the normalized difference vegetation index (NDVI) as a satellite-derived proxy for needle production and compared it with ecosystem parameters such as soil moisture water equivalent (SWE), larch foliar C/N ratio, δ13C and δ15N, and ring width index (RWI) at the Spasskaya Pad Experimental Forest Station in Russia for the period from 1999 to 2019. Historical variations in NDVI showed a large difference between typical larch forest (unaffected) and the sites affected by the extreme wet event in 2007 because of high tree mortality at affected sites under extremely high SWE and waterlogging, resulting in a decrease in NDVI, although there was no difference in the NDVI between typical larch forest and affected sites before the wet event. Before 2007, the NDVI in a typical larch forest showed a positive correlation with SWE and a negative correlation with foliar C/N. These results indicate that not only the water availability (high SWE) in the previous summer and current June but also the soil N availability likely increased needle production. NDVI was also positively correlated with RWI, resulting from similar factors controlling them. However, after the wet event, NDVI was negatively correlated with SWE, while NDVI showed a negative correlation with foliar C/N. These results indicate that after the wet event, high soil moisture availability decreased needle production, which may have resulted from lower N availability. Foliar δ15N was positively correlated with NDVI before 2007, but foliar δ15N decreased after the wet event. This result suggests damage to roots and/or changes in soil N dynamics due to extremely high soil moisture. As a dry forest ecosystem, taiga in northeastern Siberia is affected not only by temperature-induced drought but also by high soil moisture (led by extreme wet events) and nitrogen dynamics.

Reconstruction of soil moisture for the past 100 years in eastern Siberia by using δ13C of larch tree rings

AbstractA stable carbon isotope ratio (δ13C) chronology for the past 100 years was developed from larch tree rings in eastern Siberia (near Yakutsk, 62°14′N, 129°37′E), to reconstruct past soil moisture water equivalent (SWE). Based on the correlation analyses between SWE and tree ring δ13C, we developed a linear regression model for SWE in the late growing period (LGP: 15 July to 31 August) using annual tree ring δ13C, which was calculated from the combination of latewood in a current year and earlywood in the following year, and then reconstructed SWE (LGP) for 1908–2007. Reconstructed SWE was compared with factors such as the output of the land surface model, annual precipitation, and Palmer Drought Severity Index for July. From the results, the reconstructed SWE appears reasonable and shows a large variation, including repeated occurrences of severe drought and an unprecedented high soil moisture event in 2006–2007 during the past 100 years. The reconstruction also captured a past documented record of severe drought in the 1940s. Despite the generally good performance of the reconstruction, by the 1930s the estimated SWE was higher than that expected from the annual precipitation. Tree ring width and δ13C were negatively correlated in most periods. However, the negative correlation was weaker for the period from 1919 to 1925, when relatively low air temperature was observed. This result suggests that the rate of photosynthesis, together with the degree of stomata opening, also affected the tree ring δ13C during cool periods.